TABLE 12.4

ABMA (American Boiler Manufacturer’sAssociation)Standard Boiler Water Concentrations for Minimizing Carryover

DrumPressure

(psig)

Boiler Water

TotalSilica*

(ppm SiO2)

Specific**Alkalinity

(ppm CaCO3)

Conductance(micromhos/cm)

0-300 150 700 7000

301-450 90 600 6000

451-600 40 500 5000

601-750 30 400 4000

751-900 20 300 3000

901-1000 8 200 2000

1001-1500 2 0 150

1501-2000 1 0 100

* This value will limit the silica content of the steam to 0.25 ppm as a function of selective vaporization of silica. * Specific conductance is unneutralized

Selective vaporization occurs due to the solvent properties of steam for certain impurities that are normally present. All sodium-based salts found in boiler water are soluble in water and, to varying degrees, in the steam phase as well. Generally, the solubility of these salts is negligible below pressures of 2400 psig.

Selective vaporization of silica can occur at pressures as low as 400 psig. Usually, the problem of silica vaporization is not a concern at pressures below 900 psig.

Steam Condensing and Receiving Systems :

Condensate system equipment is defined as condensers, receivers, regulators, traps and piping. Normally considered a potential maintenance nightmare, a

properly treated and operated condensate system can be a moneymaker for an industrial plant.

Potential Corrosion Problems

The principal problems associated with return condensate systems deal with corrosion by oxygen, carbon dioxide and copper complexing agents. With significant corrosion, fouling of the return piping can also occur. Each of these sources of corrosive attack is discussed below.

Oxygen Corrosion. The corrosion of a condensate system by oxygen is in the form of severe pitting. As in the feed water train, oxygen corrosion is easily recognized by large pits produced at the point of attack.

Active oxygen pitting is easily recognized by the black oxide present in the pit. The surrounding area may be red ferric oxide, but this is a secondary product of the corrosion mechanism. If the pit itself contains red iron oxide, that simply indicates a past corrosion problem.

Oxygen has many opportunities to enter a condensate system. Leaks due to expansion and contraction of the return system are possible in receiving tanks, pumps, etc. Poor mechanical and chemical deaeration of the feed water can also introduce oxygen in the condensate system.

Carbon Dioxide Corrosion. Carbon dioxide can enter a condensate system as a dissolved gas or it can be chemically combined in the bicarbonate or carbonate alkalinity of the feed water. Generally. dissolved carbon dioxide is removed in the deaerating heater. The following reactions show the breakdown of naturally occurring bicarbonate and carbonate alkalinity to carbon dioxide.

Reaction 1:

NaHCO2 + heat ± NaOH + CO2

Reaction 2:

Na2CO3 + H2O + heat ± 2 NaOH + CO2

Reaction #1 proceeds to completion. Reaction #2 is only about 80% complete.

The manifestation of carbon dioxide corrosion is generalized loss of metal, typified by grooving of the pipe walls at the bottom of the pipe: attack occurs at the threaded or stressed areas. This is the most common form of condensate system attack.

Copper Complexing Corrosion. The most commonly found copper complexing agent in condensate systems is ammonia. This can be present in low concentrations due to the decomposition of organic contaminants, hydrazine or amine type treatment chemicals. A good understanding of the levels of ammonia is necessary in the event the condensate system contains copper-bearing alloys.

Corrosion Control Programs

The basic approach to chemical treatment of condensate systems is through the use of neutralizing amines, filming amines, combinations of both amines and hydrazine.

Neutralising Amines. Simply stated, neutralizing amines hydrolyze in water to generate the necessary hydroxide ions required for neutralization of the carbon dioxide. The normal approach to treating systems with these amines is to feed sufficient quantity to neutralize the carbon dioxide and then provide small additional amounts to buffer the pH to 8.5 or 9.0. At this pH, continued preservation of the magnetite film is also achieved. It is also implied that corrosion will not exist at a pH>8.0-8.5.

A variety of amines are available for condensate neutralization and pH elevation. Based on the complexity of a condensate system, several methods are employed for determining which neutralizing amine is best for the application. Frequently, a blend of amines may appear to be optimum. One method of selection deals with the distribution ratio of the amine. This is defined as the amount of amine in the vapor phase divided by the amount of amine in the condensate. The distribution ratios for commonly utilized neutralized amines are shown in Table 12.5.

TABLE 12.5

Distribution Ratios for Neutralized Amines

Amine Distribution Ratio

Morpholine 0.4

Diethylaminoethanol 1.7

Dimethylisopropanolamine 1.7

Cyclohexylamine 4.0

Ammonia 10.0

For example, because of its distribution ratio, morpholine is an excellent choice for plants that have operating turbines. It begins to leave the gaseous phase as the first condensation occurs in the wet end of the turbine. Conversely, cyclohexylamine will preferentially stay with the steam for a longer period of time. Blends of these amines or others are most desirable for plants with extensive condensate systems.

Filming Amines. Filming amines function by forming a protective barrier against both oxygen and carbon dioxide attack. These amines form films directly with the condensate line metal and develop a barrier to prevent contact of the corrosive condensate with the return piping. By design, film formers have been developed to function best at a pH of 5.5-7.5. In addition, these amines are highly surface-active and will slough loosely adherent iron oxide and other corrosion products back to receiving points or to the boiler. Care must be exercised with the feed of filming amines.

Combination Amines. Over the past several years, combinations of filming and neutralizing amines have been shown to be extremely effective, particularly in complex systems. While the combination amine is still functionally a filmer, the neutralizing amine portions provide for reduction in fouling potential and more uniform coverage of the filmer.

Filming amines and combination amines are generally fed to steam headers. Dosages are based on steam production.

Control of oxygen attack can be accomplished through the proper selection of filming amine programs. In addition, hydrazine has been injected into condensate systems to protect against oxygen attack.

Silica Removal by R.O.:

The study of the chemistry of silica (silicon dioxide, SiO2) and its relationship to reverse osmosis (RO) is of interest on two fronts: One is the challenge silica presents as one of the most difficult-to-handle inorganics for the RO process and the other concerns efforts to better understand the basics of RO.

Both have to do with silica's chemistry, which is unique - especially when dissolved in water. Much of this may be related to silica's relatively moderate polarity; some say it has to do with its similarity to the water molecule itself. What is known is that silicon is one of the most common elements. It has a rather low solubility and tends to interact with water molecules when dissolved. Silica is generally found in water supplies in three different forms, as reactive, colloidal and suspended particles (e.g., sand). The reactive portion of the total dissolved silica readily reacts in the standard molybdate colorimetric test, and the colloidal does not. The reactive form is silicon dioxide dissolved in water, creating the compound monosilicic acid (H4SiO4). In this form, silica is generally un-ionized at most natural pH levels. At a pH of 8.5, only 10 percent of the monosilicic acid is ionized; as the pH reaches 9 to 10, it still is only 50 percent ionized. The colloidal species is generally thought to be either silicon that has polymerized with multiple units of silicon dioxide, or silicon that has formed loose bonds with organic compounds or with complex inorganic compounds, usually aluminum and calcium oxide structures. Monosilicic acid attracts four additional water molecules beyond the two that make up part of its molecular structure in the hydrated state. The overall hydrated structure contains a total of six water molecules that probably play a significant role in its behavior in the RO process. Silicate also plays a role in alkalinity measurements since it is titratable with acid. For example, at pH 9.65, for every 100 milliequivalents (meq) of silicate, 58 meq of alkalinity are contributed to the total alkalinity. For high pH water supplies that contain appreciable amounts of silica, water treatment professionals need to include this equation in the alkalinity calculations in order to create a properly balanced water analysis.

Troublemaker: colloidal silica Colloidal silica creates problems for water treatment because of its stability as an un-ionized compound, which makes it difficult to remove

using ion-exchange processes. It can even cause some resin fouling where colloidal silica levels are exceedingly high. Silica is also found at the lower end of selectivity for anion resins, creating a scenario in which silica is one of the first contaminants to breakthrough. As a result, silica can be effectively removed only if the ion-exchange resins are completely and properly regenerated. With the introduction of RO as a pretreatment process, colloidal silica can be removed effectively using a typical RO membrane. The RO also helps the ion-exchange process by alleviating the overall silica loading on the resin. The downside of silica pretreatment by RO is the effect that the relative insolubility of silica can have on RO membranes as the feedwater becomes more concentrated. For example, at a pH of 7, the solubility of silica is only 120 milligrams per liter (mg/L) at 77 degrees Fahrenheit. This presents a problem for an RO operating on feedwaters having a silica content of more than 30 mg/L when the recovery rate needs to be in the neighborhood of 75 percent. At the resulting four times greater concentration, the RO concentrate would contain roughly 120 mg/L, which at neutral pH might begin to show signs of precipitation. At pH levels that are more alkaline, however, this solubility becomes much greater. It is conceivable to raise the pH in high-silica feedwaters. However, operating most RO feed streams with a higher pH invites other problems, most notably calcium carbonate precipitation. As a result, these circumstances usually call for presoftening of the feedwater supply using ion-exchange softening. Softening offers two advantages: The pH can be elevated, and (it is believed) that the potential for silica precipitation in the concentrate stream is minimized because of the absence of calcium, magnesium and other divalent ions that could serve as precipitation nuclei. lt might also be worth noting that it is generally believed that at neutral or acidic pH levels, it is not a great risk to approach silica concentrations - on softened feedwater - as high as 150 mg/L in the RO concentrate. This is mostly due to the perception that silica precipitation occurs at a relatively slow rate and that the concentrate will be discharged before silica scale is actually formed. Also, some silica inhibitors can be added as a means of pushing past the solubility limits and avoiding precipitation. Silica and the RO process Studies have shown that the colloidal species are removed to >99 percent, even with cellulose acetate (CA) RO nanofiltration (NF) and ultrafiltration (UF) membranes. Reactive silica is rejected well by thin-film polyamide (PA) membranes, yet is not reject as well by CA membranes.

Taking into consideration that reactive silica is essentially un-ionized at neutral pH levels, some of the fundamental rejection theories of the RO process may need to be reconsidered. That is, why does an un-ionized compound such as silica reject to such a high degree if the rejection of salt is typically related to valence, as is so often accepted as the mechanism for ion rejection? It is known that other completely un-ionized compounds at neutral pH levels, such as hydrogen cyanide, pass readily through an RO membrane. So, why does silica reject so well with PA membranes, even at a neutral pH? At a time when CA membranes were the only option and silica rejection was quite poor, all seemed to make sense. Silica always showed greater passage rates than inorganic species that carried a charge, even the relatively high-passage monovalent chloride ion. However, as PA membranes became available, it was found that the rejection of silica was very high, with little variation up and down the pH scale. Could it be that the rejection from RO membranes has little to do directly with the valence, but more to do with the physical size of the ion? This might begin to explain why the rather large silicon dioxide molecule rejects better than a chloride ion, even though the latter carries a much more significant charge. If the fact that most compounds may have an attraction for nearby water molecules (to differing degrees, depending on the nature of the compound) were taken into account, the notion that rejection of ionic species may be more size-dependent ("size" being the hydrated diameter of the compound or element) might become accepted.

Lee Comb is vice president for business development for Osmonics Inc., Minnetonka, MN.

Silica can be removed from raw water easily with standard demineralization techniques. Normally silica is present in a weak acidic form. Ion exchange will remove this as long as the anion resin is the strong base type. Silica in deionized water can easily be reduced to 20-50 ppb. Reverse osmosis will also remove silica by 90-98%. Both of these techniques are non-specific for silica, meaning they also remove all other ionic components along with the silica. Normally these methods work well for most well or surface waters where silica is below 15-20 ppm. In some cases for very large flow rates you can remove silica with lime softening techniques. In lime softening the silica is co-precipitated along with magnesium present in the water (or added if necessary). High temperature softening greatly improves silica removal. For high silica waters as seen in Mexico, Hawaii or other sandy areas where levels are 50-60 ppm or higher, the ion exchange and RO options are more troublesome because you may exceed solubility limits for one, and also you are more likely to have significant levels or non-ionic colloidal silica. This will not ion exchange and may foul an RO membrane. Typically this must be removed with ultrafiltration.

SODIUM HYDROXIDE(LYE) DISSOLVES SILICA SO Ph ashould be under control.

Silica, Hardness, SDI and Colloids Process Removal TechnologyBy: Horace R. Corbin, P.E. - January 7, 1994email to: horace@uswca.comwww.uswca.comBefore a raw water supply can be stripped of dissolved salts such as by reverseosmosis, pretreatment work is essential to avoid process and equipment failure. The pretreatment process must be reliable and flexible. It often must perform in a remote environment under varying and unknown conditions. It must not impose operating complexities on the site personnel.The main pretreatment process needs are:1. Silica, Metals and Hardness Reduction.2. Suspended Solids and Colloid Control, Fouling Factors Minimization.3. Scaling Reduction, Organic Removal and Biological Stabilization.The Pretreatment Process must be flexible to respond to variations in the water supply characteristics, climatic conditions, temperature and flow. The system design often must proceed based on limited information at hand at the project start. Startup adjustment and operation must be responsive to actual site conditions encountered.Communication, Training, Maintenance, Service and Technical Support must beaccessible for the site. For practical reasons, complex issues must be kept to aminimum. However, when needed, service must be available, effective and responsive. Thorough Operating and Maintenance Documents must be prepared in the native language of the operating staff. Often, dual language and dual dimensioned documents are required, such as Spanish/English and with Metric/Imperial dimension standards.

TECHNOLOGY FOR SIO2 REMOVALSilica exists in all waters in multiple chemical and physical/chemical forms. Generally, quasi-soluble silica is analyzed and reported as HSiO2- weak anion. Other forms of silica go undetected. HSiO2- is found in most natural waters at levels generally from 5-30 mg/l.HSiO2- becomes troublesome for piping and other equipment at levels of 150 mg/l and above due to formation of hard, glass-like plating that resists removal. These factors are well understood in evaporative cooling tower applications in power generation and in similar other fields.HSiO2- is a serious problem for reverse osmosis operation as concentration occurs at the liquid/membrane interface. Left uncontrolled, the membranes become destroyed in a short time. The

problem is avoided by removing the silica prior to the Reverse Osmosis unit.

Silica Removal :

A magnified look at silica.

In a water purification system, the treatment objective may include

reducing silica concentration to permit increased cycles of

concentration without scale. Although conventional coagulation and

filtration is effective to remove most (if not all) colloidal silica, this

usually is a small fraction of the total silica in natural water supplies.

Conventional precipitation technologies are reasonably cost-efficient to

achieve partial removal of silica. Drawbacks to precipitation are

numerous. Precipitation of silica is a messy, time-consuming process

and can be difficult to accomplish without upsets.

Where complete removal of silica is required, various combinations of

reverse osmosis (RO) and/or ion exchange processes are used. The

current state-of-the-art technology includes both multiple membrane

and multiple ion exchange steps, and can produce reactive silica

concentrations in the neighborhood of 0.1 ppb.

Chemical Precipitation

Silica almost never precipitates as SiO2. Silica scale almost always

contains a divalent (or trivalent) cation. Although calcium silicate is

quite insoluble, this compound does not form rapidly except at very

high temperatures. Although aluminum salts can be used to precipitate

silica, the consequences of leaving a substantial aluminum residual in

the product water makes this process undesirable. The conventional

method of precipitating silica has always been co-precipitation with

magnesium.

Since silica becomes part of the magnesium precipitant, some means

of adding already precipitated magnesium (magnesium oxide) or of

precipitating magnesium in situ is used. In situ precipitation works

much better than already precipitated magnesium, probably due to

surface area of the precipitant and proximity to a silica molecule.

The addition of soluble magnesium salts (such as MgCl2) often is

desirable due to an increase of total dissolved solids. Even though it is

less effective, MgO is more often used. The advantage of MgO is that it

adds little or no dissolved solid to the water.

Temperature and pH also have important effects on silica removal by

precipitation. The precipita-

tion mechanism occurs faster and more completely at high

temperatures. The pH must be high enough to cause magnesium to

precipitate but not so high as to make the precipitant resoluble.

Reverse Osmosis Systems

RO systems also can be used to reduce silica concentration. Although

cellulose acetate and early thin-film composite materials only provided

moderate silica rejection, newer materials reject silica quite well. The

mechanism of removal probably is by hyper-filtration, but also is

related to degree of ionization since silica is more completely removed

at a high pH. Since silica is concentrated by the membrane in the

reject stream, silica solubility can be an important consideration. RO

systems currently are unable to achieve as complete removal of

reactive silica as ion exchange, but are far better at removing various

forms of non-reactive silica.

E Cells

E Cells are electrodialysis stacks where the water-flow channels are

filled with ion exchange resin. The mechanism of removal probably is

that the resin first exchanges for various ions (including silica), slowing

them down, and then allowing them to be pulled through the

membranes. Since silica is weakly ionized, a higher current density is

needed for a high percentage of silica removal. The E Cell process

currently is more expensive than RO for bulk removal of ions and

suffers the same inability as ion exchange to remove non-reactive

silica. However, it is competitive with RO and ion exchange technology

for polishing and offers a perceived advantage over ion exchange, in

that its use does not involve handling of strong mineral acids and

bases. Still, the lowest silica residuals currently achievable are

produced by ion exchange resins.

Ion Exchange Technology

All strongly basic ion exchange resins have the ability to split salts.

This means that they can remove weakly ionized species such as

carbon dioxide and silica. Although hydroxides form strong base anion

resins, preference for silica is much lower than for sulfates and

chlorides; it is significantly greater than for hydroxide. There is pretty

good evidence that only when in the hydroxide form, does strong base

anion resin exhibit any preference for silica. When in the hydroxide

form, preference for silica and for alkalinity are similar (at least we

know that silica break generally occurs at about the same percentage

exhaustion as alkalinity break). In the chloride form, or any other salt

form, strong base anion resin has zero preference for silica (probably

because silica is nonionized at typical raw water pH), although it still

can be used for alkalinity removal.

This suggests that silica is not exchanged in the same way as other

anions.

Desilicizers

An anion desilicizer is a “poor man’s” demineralizer, consisting of a

strong cation exchanger in the sodium form (a water softener),

followed by a strong base anion exchanger in the hydroxide form.

Thank goodness there are not too many of these critters around

anymore. They have most of the disadvantages of a demineralizer and

few of the advantages. However, they do remove silica along with the

other anions.

Over the years, more than one engineer has wondered if it might be

possible to operate a desilicizer at a real small caustic dose (or perhaps

brine plus caustic) and use the anion resin as a dealkalizer while still

removing some silica. This would avoid the excess causticity created

by the complete anion exchange for hydroxides. This was tried and the

results were quite interesting. As long as the resin had enough

hydroxide exchange sites to remove all the anions, silica was well

removed. As soon as the hydroxide sites were depleted, the resin

dumped the silica. By the time the hydroxide concentration was

substantially reduced in the product water, all the silica had dumped.

There was no net removal.

Basic Measurement/ Analysis TheoryAll silica monitors use the colorimetric principle based on the Molybdenum BlueMethod. The monitor is an automatic chemist and emulates the actions of a laboratory chemist by adding various reagents to the sample and allowing them to react. At the end of these reactions, the final solution develops a blue coloration. The intensity of the coloration is proportional to the concentration of silica in the original sample.Concentration is measured by passing the reacted sample through a cuvette ormeasuring cell. The cuvette has a light source at one side and a photoelectric cell at the other side. The greater the intensity of blue coloration, the greater the amount of light absorbed and hence the smaller the signal from the photoelectric cell. The system is calibrated by the use of a zero silica solution and a solution ofknown silica concentration to set the span.Anion Exchange BedAnions are exchanged for hydroxyl ions in this bed until all the hydroxyl ions in the resin are removed. At this stage (known as resin or bed exhaustion) the levels of dissolved solids in the water from the bed outlet will increase. This is known as breakthrough.

Silica in the form of the bisilicate ions is held loosely by the anion resin and therefore becomes the first impurity to breakthrough when the resin bed is near exhaustion.Mixed BedThe mixed bed contains a mixture of both cation and anion exchange resins tofurther reduce the dissolved solids content, and thus produce the quality of makeup water required.Condensate Polishing Plant (CPP)This type of plant is used mainly on once-through type boilers. It takes the condensate from the steam turbine/condenser and purifies or polishes it to reduce the concentrations of any impurities such as sodium, silica, chloride, sulphate etc. which may have entered the system via condenser tube leaks. Condensate polishing plants (CPP's) only normally remove small levels of contaminants, therefore only have a mixed bed stage (see illustration on previous page).Anion Bed OutletMonitoring the silica content of the anion bed outlet gives an early indication of bed exhaustion. Because silica is very weakly ionised it has a very low conductivity and cannot be detected by a conductivity monitor.A colorimetric type silica monitor is the only way of detecting silica breakthrough.If silica is allowed to breakthrough without taking the anion bed off-line, the anionresin in the mixed bed will quickly become exhausted with the possibility ofunacceptable silica levels in the make-up water.Mixed Bed OutletHere again, silica monitors are used to check the quality of the final water and detect exhaustion of the anion exchange resin. It is essential that the silica concentration in the final water is kept as low as possible to minimize thermal losses due to blowdown.Condensate Polishing Plant (CPP) OutletAs with the complete demineralisation plant, the CPP has a silica monitor on the outlet from the mixed bed unit to detect exhaustion of the anion resin. This is particularly important on once-through boilers as any silica in the feed water after the CPP is deposited in the boiler, causing loss of thermal efficiency and premature shutdown of the system.

Silica in Water

Silica (SiO2) is an oxide of silicon, and is present in

almost all minerals. It is found in surface and well water in the range of

1 - 100 mg/l. Silica in water is considered to be colloidal in nature

because of the way it reacts with adsorbents. A colloid is a gelatinous

substance made up of non-diffusible particles that remain suspended

in a fluid medium. Silica is objectionable in cooling tower makeup and

boiler feed water. Silica evaporates in a boiler at high temperatures

and then re-deposits on the turbine blades. These deposits must be

periodically removed or damage to the turbine will occur. Silica is not

listed in the Primary or the Secondary Drinking Water Standards issued

by the US EPA. Furthermore A study which followed subjects for 15

years found the effects of silica in water appeared to decrease the risk

of dementia. The study found that for every 10 milligram-per-day

intake of silica in drinking water, the risk of dementia dropped by 11%.

 

Silica Water Treatment - There are several effective silica water

treatment and filtration products. Silica in water can be removed by

the anion exchange portion of the demineralization process. Also

Reverse Osmosis Systems will reject 85 - 90% of the silica content in

the water. If you need more information about silica water filtration

please give us a call.

TECHNIFAX®

SILICA CARRYOVERCAUSES TURBINE

DEPOSITSMost boilers making steam forturbines rarely have excessivecarryover of boiler water in thesteam. Good operating practices,improved steam separators, andproper chemical control minimize

this. Silica deposits inturbines, however, can occureven when boiler watercarryover is negligible. Thereason: steam selectively “picksup” silica from the boiler water,dissolves it, and carries it to theturbines, where it redeposits.Investigations show that the keyto minimizing silica carryover isin keeping the boiler water silicacontent below certain levels, theconcentration depending onoperating pressures.SILICA SOLUBILITYResearch on the solubility ofsilica in water and steam showsseveral important points:• Steam is a solvent for silica• For any particular steamdensity and temperature,there is a definite saturationsolubility of silica

Selective silica carryoverFigure 1 — Silica distribution ratioFigure 2 — Maximum boiler water silicapermitted at various pressures to keepsteam silica at or below 0.02 ppm• The maximum solubility ofsilica in steam is a direct functionof both the steam densityand temperature. As steamtemperature or density decreases,the silica solubility alsodecreases.Since pressure affects steamdensity and has a bearing onsteam temperature, it has animportant effect on the solubility

of silica in steam.SILICA CARRYOVERInvestigation of the problem ofsilica carryover in a laboratoryexperimental boiler revealed twoimportant facts:• With constant pressure andboiler water pH, silicacarryover is directly proportionalto the amount of silica inthe boiler water. This was trueover a wide range of boilerwater silica concentrations.• The ratio of silica in the steamto silica in the boiler waterincreases rapidly as boilerpressures increase.These two points are illustratedby the graph in Figure 1.SM

Turbine experts and utilityoperators determined empiricallythat 0.02 ppm of silica in thesteam is a practical maximumlimit for boiler water pH >10.With steam silica contents of0.02 ppm or less, appreciableturbine deposits would notnormally occur. A survey ofoperating experience with utilitytype boilers confirmed that the0.02 ppm figure is a valid andrealistic maximum limit. Figure 2shows boiler water silica limitsneeded to keep steam silica at orbelow 0.02 ppm.An extensive study of silicacarryover in a laboratory boilerconfirmed the results shown inFigures 1 and 2. In addition, thestudy demonstrated that:1. The presence of calcium andmagnesium sludge, antifoam,organic treatments, and combinationsof these do not significantlyaffect silica carryover.2. High ratios of hydroxidealkalinity to silica in the boilerwater reduce the ratio of silicain the steam to silica in theboiler water, thereby reducingpotential silica deposits inturbines.

3. These two curves apply to highalkalinity (pH > 10.5) boilerwater systems.Many new boiler installationshave high purity makeup systemsthat allow acceptable boilerwater pH control at pH 9–10.Since vaporous silica carryoverincreases as boiler water pHdecreases, Figures 3 and 4 shouldbe used for these high puritysystems.In addition, some new turbineinstallations require a silica insteam limitation of 10 ppb asopposed to the traditional 20 ppblimit. This control curve (10 ppb)is also shown in Figure 4.CORRELATING SILICASOLUBILITY ANDCARRYOVERThe conditions under whichsilica carryover occurs had beenthoroughly investigated anddocumented. What was neededat this point was a means ofdetermining the conditions underwhich silica is redeposited in theturbines. To determine thisexperimentally would be difficult.Reasonably valid calculationscould be made, however,based on the prior investigationsof silica solubility and silicacarryover.Researchers have calculated theapproximate relationship betweenboiler pressure, boilerwater silica, silica carryover,and silica solubility in saturatedand superheated steam. Theserelationships are combined inFigure 5.INTERPRETING BOILERWATER AND STEAMSILICA VALUESIn Figure 5, silica in the steam isplotted against boiler pressure.Each diagonal line represents aparticular silica concentration inthe boiler water. The amount ofsilica expected in the steam from

a particular boiler is shown by theintersection of the boiler pressureline and boiler water silica line.For example, a boiler operating at1400 psi with 10 ppm silica in theboiler water would show about0.04 ppm in the steam.Figure 3 — Ratio of silica in steam tosilica in water vs boiler pressureFigure 4 — Boiler water silicaconcentration vs boiler pressureThe solid curved line in Figure 5represents the calculated maximumsolubility of silica insaturated steam. If the steamcontaining 0.04 ppm silica in theabove example is allowed toexpand without being superheated,the graph shows thatsilica will start to precipitate outwhen the pressure drops below300 psi.This point is found by followingthe 0.04 ppm line from the boilerpressure and boiler water silicaintersection across until it intersectsthe silica solubility line.The dotted lines in Figure 5indicate calculated maximumsolubility of silica in superheatedsteam. To illustrate how thesesuperheat lines are used, supposethe steam containing 0.04 ppmsilica has 100F superheat. Followingthe 0.04 line across until itintersects the 100F superheat lineindicates that silica will start toprecipitate out as the pressure isreduced to about 200 psi.

Water Treatment Boilers Requirements are :

Water treatment is required to provide the physical plant with properly treated water in sufficient quantities to meet plant needs. All system require water treatment by using speciality chemicals such as corrosion and scale inhibitors , however open systems require constant water treatment to deal with the constant need for treated water to make-up for system losses of often up to 100%. Closed systems also require water treatment, but due to minimal system losses, that treatment commonly occurs only at the system fill source.

Industrial water treatment for cooling water systems , water treatment boiler requires advanced water technologies ,keeping in mind chemistry of water and use of advanced formulations of corrosion and scale inhibitors .

Three tools can be used to improve water quality:

Internal Treatment – conditioning the boiler water to pre-determined levels by using a variety of chemicals.

Demineralization/ Reverse Osmosis / Electrodialysis - the replacement of specific inorganic salts by ion exchange.

Deaeration – the removal of dissolved oxygen and carbon dioxide by heating and atomizing the water with steam.

While demineralization and deaeration can be accomplished easily by investment in the appropriate support equipment, internal treatment calls for a more concerted effort. However, most organizations large enough to have in-house maintenance will find that the combination of these three tools will more than pay for themselves in defrayed operating costs. A well-implemented program will ensure -

Increased heat transfer Lower fuel expenditures Lower chemical consumption

Except for steam trap maintenance, water treatment has the most potential for reducing annual operating costs in a power plant.

Boiler Losses due to scale

Thickness of Scale Increase in fuel consumption due to scale

   

0.5 mm 2 %

1 mm 4 %

2 mm 6 %

4 mm ( 0.125” ) 10 %

8 mm ( 0.25” ) 20 %

16 mm ( 0.5” ) 40 %

30 mm (1” ) 80 %

Objectives of Water Treatment boiler

Prevention of scaling in boiler.( use speciality chemicals such as scale inhibitors to counter it )

Prevention of corrosion in boiler. (use speciality chemicals such as corrosion inhibitors to counter it )

Prevention of stress corrosion cracking.

Prevention of steam contamination.

What is an External Treatment?

Treatment of water that are done outside of the boiler is called pre-boiler or external treatment. The main physical methods for improving quality of water for boiler include flocculation, clarification, deaeration, oil removal, colour removal, suspended solids removal and blow down. When preparing water for boilers operated at less than 150 psi, all necessary chemical treatments can be accomplished in a clarifier, but as pressure increases; the quality of feed water must improve. The purpose of external treatment is to reduce suspended solids, demineralize the feed water and remove silica. This purpose can be achieved by

Coagulation with chemicals Demineralization/ Reverse osmosis/ Electrodialysis (cold lime, soda process,

hotlime-soda process, mixed bed exchange) Silica removal (coagulation with chemical, Demineralization, Reverse Osmosis,

Electrodialysis)

What is an Internal Treatment?

There are number of treatments that are made within the boiler to minimize the adverse effects of small concentration of components that remain in the feed water after the external treatment. In spite of various external treatments, it is not possible to attain an absolute perfect quality of boiler feed water. Chemical treatment or internal treatment of water inside the boiler is essential to take care of various impurities entering into the boiler such as hardness, dissolved solids, oxygen, and silica.

In many cases, external treatment of water supply is not necessary specifically in low or moderate pressure boilers or where large amount of condensed streams are used or when raw water available is of very good quality

Silicon (Si) and waterSilicon is the most abundant element on earth after oxygen. Large amounts of silicon can be found in various minerals and it is abundant in oceans and nearly all other waters as silicic acid. In the surface layers of oceans silicon concentrations are 30 ppb, whereas deeper water layers may contain 2 ppm silicon. Rivers generally contain 4 ppm silicon. Silicon is usually not ionized when dissolved; it is present as ortho silicic acid (H4SiO4 or Si(OH)4). These compounds are the result of slow dissolution of silica in water. Rivers transport large amounts of silicon to sea. Most likely, less than 20% of dissolved silicon is removed from rivers by means of biological or chemical transformation processes.

In what way and in what form does silicon react with water?

Silicon is never found in nature in free form. In crystallized form it is only reactive under conditions of extremely high temperatures. Water and water vapor probably have little influence upon silicon solubility, because a protective surface layer of silicon dioxide is rapidly formed.There are many examples of silicon compounds reacting with water. Silicon tetra fluoride reacts with water to hydrogen fluoride. Silicon tetra chloride reacts with water quite violently. Silicides of the first and second group are generally more reactive than transitory metals. Typical reaction products include hydrogen and/ or silanes (SiH4), for example Na2Si + 3H2O -> Na2SiO3 + 3H2.

Solubility of silicon and silicon compounds

Silicon compounds differ in water solubility.Silicon oxide is relatively water insoluble compared to other minerals. Upon dissolution the following equilibrium is formulated:

SiO2(s) + 2 H2O(l) <-> H4SiO4(s)

This balance contains silicic acid, a weak acid that also forms during silicon mineral hydrolysis:

H4SiO4(s) + H2O(l) <-> H3O+(aq) + H3SiO4-(aq)

Silicon dioxide has a water solubility of 0.12 g/L, whereas for example silicon carbide is water insoluble.

Why is silicon present in water?

As was explained earlier, silicon is part of various minerals, from which it may be released during weathering processes. It is also released under water during volcanic activity. Water in interspaces of marine sediments contains more silicon than the sea surface. The present current causes silicon to flow from sediments to seawater. Antarctic weathering also releases silicon. Silicon is removed from waters naturally, through plankton fixation, sediment settling, or reactions of dissolved silicon with clay minerals (reverse weathering).Sand is the primary substance for commercially produced silicon. Minerals such as talc, mica, feldspar, nepheline, olivine, vermiculite, perlite and kaolinite also contain silicon. Gemstones such as opal and amethyst also contain silicon.Construction processes silicon compounds in sand and cement, a calcium silicate. Glass and porcelain production is based on sand.Silicon is applied as an aid in steel, chemical and electron industries, where it is processed under high temperatures. Steel and other alloys are eventually processed to for example engine blocks or cylinder heads.Industrially significant silicon compounds are rubber- or resin-like compounds, which are generally water resistant and also withstand oxidation processes and chemical weathering. These are applied as lubricants under high temperatures, as a sealing kit for windows, roofs and pipes, in rubber hoses and in plastic parts for car engines. Silicon oils are applied in cosmetics, and for textile impregnation. In microchips this element is a semi conductor, as it is in transistors and other electronic parts.Solar panels consist of n-semi conductors of silicon and arsenic and p-semi conductors of silicon and boron. It occurs in

elementary form in optic lenses and prisms for infrared light.Silicon carbide is nearly as hard as diamond and is applied as an abrasive. Quartz crystals that exist naturally and are produced chemically have the characteristic of vibrating in very exact frequencies, when they come in contact with electricity. This may be applied in watches, radios and televisions. Alkali silicones are added to cleansing agents, glue and bleaching agents for textiles.Zeolites are silicones that are applied as foam regulators in detergents. These directly influence water quality. Other silicon compounds may be applied as absorbents.

BOILER FEED-WATER TREATMENT

The importance of correct feed-water treatment for economic operation and for extending life of boiler and equipment cannot be over emphasized. Feed-water treatment is essential in boilers, feed-systems, etc., more particularly in modern boilers of a high evaporative rate. (The faster a steam boiler or generator will convert water to steam, the more rapidly will the solids in the water concentrate up.) So, large and small water-tube boilers, the typical fire-tube packaged boiler, and steam generators are all examples of this in varying degrees. As all untreated waters carry natural salts, they have to be treated to prevent scale forming.The three main reasons for water treatment are :

Prevention of Corrosion in feed boiler, steam and condensate systems.

Elimination of Scale. Economic boiler operation without carryover.

Corrosion will reduce metal thickness of tubes or shell. Result : pressure must be reduced and finally boiler condemned.

Scale reduces the heat flow from fire side to water. Result : high fire temperatures are needed to maintain down is insufficient.

Basic Chemistry of the Effect of Impurities in the Boiler. If we could use water completely free from all impurities, there would be no need for water treatment.

  IMPURITY EFFECT ON A BOILER

1.  Dissolved gases  Corrosion

2.  Calcium salts and magnesium salts

 These salts are the 'hardness in the boiler. Some salts can also cause corrosion

3.  Silica  Can form a very hard scale.

4.  Suspended solids and dissolved solids

 Contribute to, or cause, carryover (*)

(*) Carryover is a collective term to describe the entrainment of a relatively small quantity of boiler water solids with the steam. Carryover occurs as a result of either

foaming or priming, or by a combination of both. Foaming is the formation of bubbles on the surface of the boiler resulting in the throwing over of slugs of boiler water with

the steam. This is similar to the 'bumping' experienced when water is boiled in an open vessel.

Even on ships and in powerhouses, however, where evaporated water is used, the small quantities of impurities are sufficient to cause corrosion, scale and carry-over,

and must therefore be treated. A table of the impurities is as follows :

1. Dissolved Gases : the two gases which cause corrosion are oxygen and carbon dioxide. The

carbon dioxide does so simply by dissolving in the water and forming a weak carbonic acid which attacks the metal in feed systems, boiler or condensate

system. Oxygen is present in all waters, so that red iron oxide forms on a mild steel surface immersed in water. This rusting or, as we call it, corrosion

triunes until the metal is corroded away. If the amount of oxygen in the water is restricted, the oxide film does not form so readily; but instead, the surface of the steel tarnishes. This tarnish is usually the development of a thin film of iron oxide on the metal surface which is not so fully oxidized as the red iron oxide, and is more dense, thus tending to resist further corrosive attack. In water of increasing alkalinity, the oxide film becomes more stable and gives more protection to the steel, but until a definite alkalinity is reached, it still

tends to break down in selective areas, where pits will develop. 2. Calcium and magnesium salts :

There are two forms of hardness; temporary and permanent. Temporary hardness is due bicarbonates of calcium and magnesium which break down to carbonates when the water is boiled. In the boiler the following chemical

reaction takes place : Calcium Bicarbonate + heat. Calcium Carbonate+carbon dioxide+water. Calcium and magnesium bicarbonate are soluble in water but the carbonates are insoluble and therefore precipitate as a fine white powder. This precipitate will bake unto the heating surface of a

boiler and form a scale. Permanent hardness is due to calcium and magnesium sulphates, chlorides and nitrates, and these salts cannot be

removed by boiling. However, under boiler conditions (resulting in successive concentrations of these hardness salts) the solubility of these salts is soon exceeded and they deposit on the hottest part of the heating surface. The

salts of magnesium that form permanent hardness sometimes tend to cause corrosion instead of hard scale formation, e.g. magnesium chloride in an

untreated boiler hydrolyses to form corrosive hydrochloric acid. 3. Silica :

Silica forms scale in a similar way to the permanent hardness salts. When the scale formed is a mixture of silica, calcium and magnesium salts, it is very

hard and therefore presents a difficult problem at inspection time. 4. The suspended and dissolved solids :

The suspended and dissolved solids cause foaming by becoming absorbed unto the walls of individual bubbles so that small bubbles, instead of

coalescing to form large ones and bursting early, repel one another and build up a large volume of small bubbles. If these bubbles burst near the steam

outlet, the spray is taken over with the steam. If the bubbles do not burst high in the steam space, the foam can be drawn over with the steam.

Water, the raw material for making steam : Water is the only common substance that exists in three forms (ice, water, steam) at normal earth

temperatures. It absorbs more heat for a given temperature rise than any other common inorganic substance. Water expands 1600 times as it evaporates to form

steam at atmospheric pressure. The steam is capable of carrying large quantities of heat. These unique properties of water make it an ideal raw material for heating and

power generating processes.All natural waters contain varying amounts of dissolved and suspended matter and

dissolved gases the amount of minerals dissolved in water varies from 30kg. per 10001 in sea water to anything from 5 g to 1 kg per 10001 in fresh water supplies.

The source (lake, river, well, etc.) and also with the area of the country. The impurities in water are important considerations when it is to used for steam

generation.The composition of boiler feed water must be such that the impurities in it can be

concentrated a reasonable number of times inside the boiler, without exceeding the tolerance limits of the particular boiler design. If the feed water does not meet these requirements it must be pretreated to remove impurities. The impurities need not be completely removed in all cases, however, since chemical treatment inside the boiler

can effectively and economically counteract them.

Let us thoroughly examine water the raw material for making steam through a series of questions and answers.

"94" QUESTIONS & ANSWERS

Hereunder we would like to list up many questions that may be asked by the users of boilers and answers are given in details with as much information available.

Questions :

1. What are the Physical Properties of Water? 2. What is the Chemical Composition of Water?

3. At Temperatures Does Water Boil? 4. Why is Steam Ideal for "Carrying" Heat Energy?

5. Why is Water Not Always Satisfactory for Boiler Use? 6. What General Type of Impurities Does Water Contain? 7. What Dissolved Minerals Do Natural Waters Contain?

8. What is Water Hardness? 9. What Gases are Dissolved in Natural Waters?

10. What Types of Other Impurities May Water Contain? 11. What is the Difference between Sea Water and Fresh Water?

12. What are the Sources of Fresh Water and How do they Vary in Composition? 13. How Does Water Composition Vary from Geographical Standpoint?

14. What is Boiler Feed Water? 15. Is there any Relationship between Good water for drinking use and for Boiler

Feed? 16. How Pure Must Feed Water be?

17. How does Operating Pressure Influence Boiler Water Composition requirements?

18. What is Meant by 'External' and 'Internal' Feed water Treatment? 19. What Causes Boiler Deposits?

20. Which are some Common Types of Boiler Deposits? 21. What are the Characteristics of a carbonate deposit? 22. What are the Characteristics of a Sulphate Deposit?

23. What are the Characteristics of a Silica Deposit? 24. What are the Characteristics of an Iron Deposit?

25. What Problems do Deposits Cause? 26. What is Corrosion?

27. Where is Corrosion Usually Experienced? 28. What is Corrosion Fatigue? 29. What is Caustic Cracking?

30. What Problems does Corrosion Cause? 31. What Measures are taken to Prevent Boiler System Corrosion?

32. What is Boiler Water Carry-over? 33. What Causes Foaming? 34. What Causes Priming?

35. How Does Oil Affect Carry-over? 36. How Do Suspended Solids Affect Carry-over?

37. What is Selective Silica Carry-over? 38. What Problems are Caused by Carry-over?

39. What Measures are Usually Taken to Prevent Carry-over? 40. What is Clarification? 41. What is Coagulation?

42. What Various Types of Coagulants are Used? 43. What is Chemical Precipitation?

44. How Does Lime React in the softening Process ? 45. How Does soda Ash React in the Softening Process?

46. What are the Various Methods of Lime Soda Softening? 47. Why are Coagulants Used in the Lime Soda Process?

48. Under What Conditions Are Phosphate Softeners Use? 49. What are the Disadvantages of Lime Soda Softening?

50. What are the Advantages of Lime Soda Softening? 51. What is Ion Exchange?

52. What are the Various Types of Ion Exchange Materials? 53. What is Boiler Water Carry-over?

54. What are the Disadvantages of Ion Exchange? 55. What are the Advantages of Ion Exchange?

56. How Does Oil Affect Carry-over? 57. How Do Suspended Solids Affect Carry-over?

58. What is Selective Silica Carry-over? 59. What Problems are Caused by Carry-over?

60. What Measures are Usually Taken to Prevent Carry-over? 61. What is Clarification? 62. What is Coagulation?

63. What Various Types of Coagulants are Used? 64. What is Chemical Precipitation?

65. What is the Purpose of Deaeration? 66. How are Evaporators Employed?

67. What Combinations of External Treatment Methods are Generally Used? 68. When is Internal Treatment of Boiler Feed water Necessary?

69. What Should a Good Internal Water Treatment Programme Accomplish? 70. What chemicals are Used in Internal Treatment?

71. How are Carbonates Reacted on by Internal Treatment? 72. How are Sulphates Reacted on by Internal Treatment?

73. How is Silica Reacted upon by Internal Treatment? 74. How is Sludge Conditioned in Internal Treatment?

75. What Difficulties are Encountered in Internal Treatment? 76. What are the Advantages of Internal Treatment?

77. How are Internal Treatment Chemicals Fed? 78. How are Chemical Dosages Controlled?

79. What Boiler Water Tests are Used for Treatment Control?

80. What Tests are Usually Made as a Check for Contaminants? 81. What Units are Used in Expressing Water Analysis Results?

82. Why are some Analysis Results Express ‘As CaCO2E 83. What is Blow-down?

84. How much Blow-down is Needed? 85. What Tests are Made in Regulating Blow-down?

86. What is the Difference between Continuous and ‘Puff Blow-down? 87. What Causes Corrosion in Steam Condensate Systems?

88. How is Steam Condensate Corrosion Prevented? 89. How do Chemical Oxygen Scavengers Help Control Condensate System

Corrosion? 90. What is the Basis for Choice between Neutralizing and Filming Inhibitors?

91. What Characteristics Should a Good Condensate Corrosion Inhibitor Have? 92. How are Deposits and Corrosion Prevented in Feed water Systems?

93. What is the Wet Method of Boiler Lay-Up? 94. What is the Dry Method of Boiler Lay-Up?

 

Answer to : back

What are the Physical Properties of Water?Water is a tasteless, odorless, colourless liquid in its pure state. It is the only

inorganic material which occurs in three forms (ice, water, steam) within the natural temperature range on earth. Because water can be converted to steam at a

convenient temperature.

Answer to : backWhat is the Chemical Composition of Water?

Pure water is a simple combination of hydrogen and oxygen. The common formula is H2O. As a matter of general interest, however, there are several 'hybrid' forms of

water which are present in all supplies. Water contains about 300 ppm of deuterium oxide (D.,O) or "heavy water". This doesn't quench thirst or make plants grow but in a pure form it has found use in moderating nuclear reactors. Another form of water,

tritium oxide (T2O), is made radioactive by cosmic rays. Although only a minute fraction of water exists in this form, its radioactivity serves as a means of measuring

the age of a water supply. For all practical purposes, though, only ordinary water (H2O) is considered for use in boilers.

Answer to : backAt Temperature Does Water Boil?

The boiling point of water depends on the pressure to which the water is subjected. At atmospheric pressure water boils at about 100°C. As pressure increases the

boiling point also increases. At the critical pressure near 22.000kPa (where water can be converted to steam without a change in volume) the boiling point is lowered.

Under extreme vacuum conditions, water will boil at temperatures as low as 2°C.

Answer to : backWhy is Steam Ideal for "Carrying" Heat Energy?

The Traditional formula was : 1 Btu raises 1 pound of water 1°F. This can be converted to 4.2 kJ to raise 1 kg. of water 1°C. It takes an additional 2256 kJ/kg. to

change 1 kg. of water (at the boiling point) to steam. This amount of heat (the heat of vaporization) is then "stored" in the steam. When steam condenses, this heat energy is given off. Consider, for example, the amount of heat which can be "carried" by 1

kg. of water. If the water temperature is originally 38°C (100°F) it takes 260 kJ/kg. to heat it to 100°C and 2256 kJ/kg. to convert it to steam. A total of 2516 kJ/kg. has

been added and will be released as the water is condensed and cooled back to 38°C. Since this transfer is never 100 percent efficient, some of the heat energy will be

dissipated in the process. But much of the heat from burning fuel can be absorbed by boiler water, transported with the steam, and released at the desired points.

Answer to : backWhy is Water Not Always Satisfactory for Boiler use?

Completely pure water is nonexistent. All natural waters contain various types and amounts of impurities. Since water impurities cause boiler problems, careful

consideration must be given to the quality of the water used for generating steam.

Answer to : backWhat General Types of Impurities Does Water Contain?

Impurities picked up by natural waters may be classed as : (a) dissolved solids, (b) dissolved gases, and (c) suspended matter. Water is a good solvent; it dissolves the

rocks and soil it contacts. It dissolves gases from the air and gases given off from organics is the soil. It picks up suspended matter from the earth. It is also subject to contamination with trade wastes, oils and process materials. In general, the type of

impurities water contains depends on what it contacts; the amount of impurities deepens on the contact times.

Answer to : backWhat Dissolved Minerals Do Natural Waters Contain?

The minerals which water picks up from rocks consist chiefly of calcium carbonate (limestone); magnesium carbonate (dolomite); calcium Sulphate (gypsum),

magnesium Sulphate (Epsom salts); silica (sand); sodium chloride (common salt); sodium Sulphate (Glaubers salts); and small quantities of iron, manganese, fluorides, aluminum, and other substances. Wastes from mines and certain industrial processes make some surface waters very acin, while minerals in the earth make some ground waters very alkaline. Sometimes Nitrates are found in water and in many cases, this

is associate with sewage contamination.

Answer to : backWhat is Hardness?

Waters which contain large amounts of calcium and magnesium minerals are "hard to wash with". The calcium and magnesium compounds react with soap to form a

curb in the water. These compounds are therefore referred to as water hardness. The amount of hardness in a natural water may vary from several parts per million to

over 500 parts per million. Since calcium and magnesium compounds are relatively insoluble in water, they tend to precipitate out to cause scale and deposit problems. Water hardness, therefore, is an important consideration in determining suitability of

a water or use in generating steam.

Answer to : backWhat Gases are Dissolved in Natural Waters?

Water dissolves varying amounts of air which is composed of 21% oxygen, 78% nitrogen, 1% other gases (including 0.03 - 0.06% carbon dioxide). Oxygen is soluble

in water at room temperature and atmospheric pressure to the extent of about 9ppm. The solubility of oxygen decreases as the temperature of the water goes up, but water under pressure can hold larger quantities of dissolved oxygen. Although

nitrogen is dissolved in natural water it is inert gas and has little effect on the character of water used in boilers. Water doesn't usually pick up more than 10 ppm of carbon dioxide from the air. Carbon dioxide may be dissolved in water to a much

greater extent, though, through the decay of vegetation and organics in the soil. Hydrogen Sulphate and methane may be dissolved in some waters, but this is not a general occurrence. These gases, however, can be important when they occur as

contaminants.

Answer to : backWhat Types of Other Impurities May Water Contain?

Natural waters may contain turbidity, colour, soil, and precipitated minerals, as well as oil and other trade wastes. Colour comes from decaying vegetable matter.

Turbidity may consist of very finely divided organic material and microorganisms, as well as suspended clay and silt. Oils, fats, greases, sewage and other wastes may

contaminate water supplies.

Answer to : backWhat is the Difference between Sea Water and Fresh Water?

The main difference is in the amount of dissolved minerals. Sea water contains about 30 kg. of minerals per 1000 liters (mostly salt). The mineral content of fresh water

supplies is much lower and generally ranges form 5 g. to 1 kg. per 10001. While work is now in progress on methods of purifying sea and brackish water at the present

time only fresh water supplies are generally used for boiler feed purposes.

Answer to : backWhat are the Sources of Fresh Water and How do they Vary in Composition?

Fresh water supplies may be either surface water (rivers, streams, reservoirs, etc.) or ground water (shallow or deep well waters). In general, ground water supplies are

more consistent in composition and contain less suspended matter and turbidity than surface supplies which are affected directly by rainfall, soil erosion, and trade wastes.

On the other hand, ground waters are usually harder than surface waters. For example, an average surface supply will contain about 95 ppm total hardness as

opposed to an average of about 200 ppm total hardness for ground supplies. In some instances where the ground water is normally encountered in surface supplies.

Answer to : backHow Does Water Composition Vary from Geographical Standpoint?

Water composition varies throughout the states depending upon the type and strata of the earth formations. In the limestone areas the waters contain large quantities of calcium carbonate. In parts of the country where there is more of the granite type of rock formation, much less mineral matter is dissolved and the water will not contain much hardness. Throughout some state, there are deposits of alkali which the water will pick up. These are the general geographic variations, but local conditions in any

area may greatly influence the composition of the water.

Answer to : backWhat is Boiler Feed Water?

The water added to a boiler to repave evaporation and blow-down losses is termed 'feed water'. In may cases, steam is condensed and returned to the boiler as part of the feed water. Whatever water is needed to supplement the condensate returned is termed 'make-up water'. The make-up water is usually natural water either in its raw

state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned

to the boiler.

Answer to : backIs there any Relationship between Good water for drinking use and for

Boiler Feed?

Except that sewage pollution is harmful to both, there is not a great deal of similarity between the requirements for drinkable water and the requirements for boiler feed-

water. The minerals in drinking water, while they might affect the taste, are absorbed by the body and many so called 'health waters' are high in minerals. On the other hand, water impurities cannot be handled as well by boilers. Although a boiler is a

mass of steel, it is generally more sensitive about what it consumes than is the

human stomach. For this reason, much care is needed in selecting water treatment.

Answer to : backHow Pure Must Feed Water be?

Feed-water purity is a matter both of quantity of impurities and nature of impurities... some impurities such as hardness, iron and silica are of more concern, for example, than sodium salts, The purity requirements for any feed-water depend on how much

feed water is used as well as what the particular boiler design (pressure, heat transfer rate, etc.) can tolerate. Feed-water purity requirements therefore can vary widely. A low pressure fire-tube boiler can usually tolerate high feed-water hardness with proper treatment while virtually all impurities must be removed from water used

in some modern, high pressure boilers.

Answer to : backHow does Operating Pressure Influence Boiler Water Composition

requirements?

The Boiler and affiliated Industries Manufacturers' Association has established limits for boiler water composition with respect to operating pressure to assure good quality steam. Pending the evolving of new, round figure ratios in S.I. units, we

literally convert the p.s.i.g. column below :

  BOILER PRESSURE TOTAL SOLIDS

ALKALINITY SUSPENDED

(kPa) (psig) (ppm) (ppm) solids silica*

0-2070 0-300 3500 700 300 125

2070-3100 301-450 3000 600 250 90

3100-4135 451-600 2500 500 150 50

4135-5170 601-750 2000 400 100 35

5170-6200 751-900 150 300 60 20

6200-6890 901-1000 1250 25 40 8

6890-10335 1001-1500 1000 20 20 2.5

10335-13780 1501-2000 750 50 10 1.5

over 13780 over 2000 500 100 5 0.5

(*) Silica limits based on limiting silica in steam to 0.02-0.03 ppm.

Answer to : backWhat is Meant by 'External' and 'Internal' Feed water Treatment?

External treatment is the reduction or removal of impurities from water outside the boiler. In general, external treatment is used when the amount of one or more of the

feed=water impurities is too high to be tolerated by the boiler system in question. There are many types of external treatment (softening, evaporation, deaerarion, etc.)

which can be used to tailor make feed-water for a particular system. Internal treatment is the conditioning of impurities within the boiler system. The reactions

occur either in the feed lines or in the boiler proper. Internal treatment may be used alone or in conjunction with external treatment. Its purpose is to properly react with feed water hardness, condition sludge, scavenge oxygen and prevent boiler water

foaming.

Answer to : backWhat Causes Boiler Deposits?

Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and

becoming hard and adherent. Evaporation in a boiler causes impurities to concentrate. The high temperatures break down some minerals, cause others to

become less soluble. In general, water in contact with hot metal will tend to deposit out impurities as it evaporates.

Answer to : backWhich are some Common Types of Boiler Deposits?

In untreated boiler water, the formation of deposits is like a back to nature movement. That is as minerals are deposited out from water they form many types of

crystalline and rock like structures such as are encountered in the earth's strata. Deposits are seldom composed of one constituent alone but are generally a mixture of various types of minerals, corrosion products and other water contaminants. The most common types of boiler deposits may contain : Calcium carbonate, Sulphate or silicate; magnesium hydroxide or silicate, iron oxide, and silica, sludge deposits form

boiler water which has been treated may also contain calcium and magnesium phosphates.

Answer to : backWhat are the Characteristics of a carbonate deposit?

A carbonate deposit is usually granular and sometimes of a very porous nature. The crystals of calcium carbonate are large but usually are matted together with finely divided particles of other materials so that the scale looks dense and uniform. A

carbonate deposit can be easily identified by dropping it in a solution of acid. Bubbles of carbon dioxide will effervesce from the scale.

Answer to : backWhat are the Characteristics of a Sulphate Deposit?

A Sulphate deposit is much harder and more dense than a carbonate deposit because the crystals are smaller and cement together tighter. A Sulphate deposit is brittle,

does not pulverize easily, and does not effervesce when dropped into acid.

Answer to : backWhat are the Characteristics of a Silica Deposit?

A high silica deposit is very hard, resembling porcelain. The crystal of silica are extremely small, forming a very dense and impervious scale. This scale is extremely

brittle and very difficult to pulverize. It is not soluble in hydrochloric acid and is usually very light coloured.

Answer to : backWhat are the Characteristics of an Iron Deposit?

Iron deposits, due either to corrosion or iron contamination in the water, are very dark coloured. Iron deposits in boilers are most often magnetic. They are soluble in

hot acid giving a dark brown coloured solution.

Answer to : backWhat Problems do Deposits Cause?

The biggest problem that deposits cause is overheating and failure of boiler tubes. A deposit acts as an insulator and excessive deposits prevent an efficient transfer of

heat through the tubes to the circulating water. This causes the metal itself to become over heated. When the overheating is severe enough the metal fails. Boiler

deposits can also cause plugging or partial obstruction of corrosive attack underneath the deposits may occur. In general, boiler deposits can cut operating

efficiency, produce boiler damage, cause unscheduled boiler outages, and increase cleaning expense.

Answer to : backWhat is Corrosion?

Stated simply, general corrosion is the reversion of a metal to it 's form. Iron, for example, reverts to iron oxide as the result of corrosion. The process of corrosion,

however is a complex electro chemical reaction and it takes many forms. Corrosion may produce general attach over a large metal surface or it may result in pinpoint

penetration of metal. While basic corrosion in boilers may be primarily due to reaction of the metal with oxygen, other factors such as stresses, acid conditions, and specific chemical corrodents may have an important influence and produce

different forms of attack.

Answer to : backWhere is Corrosion Usually Experienced?

Corrosion may occur in the feed-water system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. Corrosion in the boiler proper

generally occurs when the boiler water alkalinity is low or when the metal is exposed to oxygen bearing water either during operation or idle periods. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the

boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system corrosion is generally the result of c contamination with carbon

dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system.

Answer to : backWhat is Corrosion Fatigue?

This type of cracking in boiler metal may occur by two different mechanisms. In the first mechanism, cyclic stresses such as are created by rapid heating and cooling are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually associated with improper corrosion prevention. The second type of corrosion fatigue cracking occurs in boilers with properly treated water. In these

cases corrosion fatigue is probably a misnomer. These cracks often originate where the metal surfaces are covered by a dense protective oxide film and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are usually thick, blunt and cross the metal grains. They usually start at internal tube surfaces and are

most often circumferential on the tube.

Answer to : backWhat is Caustic Cracking?

Caustic cracking (caustic embrittlement) is a serious type of boiler metal failure characterized by continuous, mostly inter granular cracks. The following conditions

appear to be necessary for this type of cracking to occur :

1. The metal must be stressed, 2. the boiler-water must contain caustic,

3. at least a trace of silica must be present in the boiler-water, and 4. some mechanisms, such as a slight leak, must be present to allow the boiler

water to concentrate on the stressed metal.

Answer to : backWhat Problems does Corrosion Cause?

Corrosion, in general, causes difficulty from two standpoints. The first is deterioration of the metal itself, and the second is deposition of the corrosion products to form deposits. Generally, uniform corrosion of boiler surfaces is seldom of real concern. Corrosion, however, takes many insidious forms and deep pits resulting in only a

minimum of iron loss may cause penetration and leaking of boiler tubes. Corrosion underneath certain types of boiler deposits can so weaken the metal that failure of

tubes occurs. In steam condensate system, replacement of lines and equipment due to corrosion can be a costly problem.

Answer to : backWhat Measures are taken to Prevent Boiler System Corrosion?

Corrosion, in general, causes difficulty from two standpoints. The first is deterioration of the metal itself, and the second is deposition of the corrosion products to form deposits. Generally, uniform corrosion of boiler surfaces is seldom of real concern. Corrosion, however, takes many insidious forms and deep pits resulting in only a

minimum of iron loss may cause penetration and leaking of boiler tubes. Corrosion underneath certain types of boiler deposits can so weaken the metal that failure of

tubes occurs. In steam condensate system, replacement of lines and equipment due to corrosion can be a costly problem.

Why Water Treatment is Needed :As feed-water enters a boiler the heat causes hardness (calcium and magnesium salts) to come out of solution. Untreated the hardness deposits on the hot boiler metal to from scale. As water evaporates in the boiler the feed-water impurities

concentrate. Even small amounts to iron, copper, and silica can accumulate in the boiler-water and cause serious deposit problems in higher pressure boilers. Since

scale can cause overheating and failure of boiler metal, preventive water treatment is needed. The corrosion of boiler system metal is a complex process and takes many

forms: general attack, localized pitting, and various types of cracking in stressed metal. In general, the main factors causing corrosion are dissolved gases in the water

(primarily oxygen) and acid conditions. High temperatures speed up the corrosion process. Corrosion is damaging from several standpoints: it causes weakening and failure of metal and produces corrosion products which can cause boiler deposits. High concentrations of dissolved and suspended matter in boiler-water can cause foaming of the water at the steam release surface. This produces carry-over of the

water and its impurities into the steam. Carry-over results in deposits and other problems in turbines, engines and other processes using steam. While mechanical and operational factors also cause carry-over, proper control of water conditions is

important in producing pure steam.

Answer to : backWhat is Boiler Water Carryover?

Boiler water carry-over is the contamination of the steam with boiler-water solids. There are four common types of boiler-water carry-over. In one bubbles or froth

actually build up on the surface of the boiler-water and pass out with the steam. This

is called foaming and can be compared to the stable foam found on beer. In the second type small droplets of water in the form of spray or mist are thrown up into the steam space by the bursting of the rising steam bubbles at the steam release

surface. This is sometimes called ‘aquaglobejectionEand is like ginger ale or champagne where no stable foam is formed but droplets of liquid are ejected from

the liquid surface. The third condition of carry-over, called priming, is a sudden surge of boiler-water that carries over with the steam, similar to the effects produced in

uncapping a bottle of charged water. stem contamination may also occur from leakage of water through improperly designed or installed steam separating

equipment in a boiler drum.

Answer to : backWhat Causes Foaming?

Very high concentrations of any solids in boiler-water cause foaming. It is generally believed, however, that specific substances such as alkalis, oils, fats, greases, certain types of organic matter and suspended solids are particularly conducive to foaming.

Answer to : backWhat Causes Priming?

Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. priming is sometimes aggravated by

impurities in the boiler-water

Answer to : backHow Does Oil Affect Carryover?

Oil contamination in boiler feed-water, usually form reciprocating engines, pumps, etc., can cause serious foaming. This is generally attributed to the formation of soaps

in the boiler-water due to specification of the oil by boiler-water alkalis.

Answer to : backHow Do Suspended Solids Affect Carryover?

The theory advanced is that suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking

and builds up a foam. It is believed that the finer the suspended particles the greater their collection in the bubble. Experience indicates, however, that many boilers

operate with exceedingly high suspended solids without carry-over while others have carry-over with only a trace of suspended solids. This would seem to indicate that the

type as well as the quantity of suspended solids has much to do with carry-over.

Answer to : backWhat is Selective Silica Carryover?

Silica can carry over into the steam in two ways. It can be present in the steam as the result of general boiler-water carry-over or it can go into steam in a volatile form.

In the latter case silica acts much like a gas and is considered to be selectively carried over. As Pressures increase above 2760 kPa (400 p.s.i), there is an increased tendency for silica to be selectively carried into the steam in amounts proportionate

to the amount of silica in the boiler-water.

Answer to : backWhat Problems are Caused by Carry-over?

The disadvantages of wet steam include a general decrease in operating efficiency and erosion of turbines and engines. In addition any dissolved or suspended solids in the boiler-water tend to deposit out in the steam and condensate system,. when the solids deposit in super heaters and turbine, overheating and failure of superheated tubes and reduction in turbine efficiency can result. Impurities carried over with the

can cause difficulties in many processes for which the steam is used.

Answer to : backWhat Measures are Usually Taken to Prevent Carryover?

The most common measure is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads,

and sudden load changes also helps. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases the condensate

should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foam agents can be very effective in preventing

carry-over due to high concentrations of impurities in the boiler-water.Removing Impurities from Water :

Coagulants are chemicals to enmesh fine particles of suspended matter in a water supply to form a flock which settles or can be filtered out. Adding softening chemicals (lime, soda, ash, etc.) to a water causes some dissolved hardness salts to precipitate

and the suspended matter can then be coagulated and filtered out. Precipitation processes such as lime soda softening can effectively remove suspended matter, hardness and alkalinity and in some cases reduce the silica content of the water. When a salt dissolves in water it forms positive ions (cations) and negative ions

(anions). For example, calcium carbonate (CaCO3) forms a calcium cation (Ca++) and a carbonate anion (CO3=). The most common form of ion exchange involves

passing water through material which substitutes sodium for calcium and magnesium cations. This is a typical softening treatment. Anions can also be removed from water by the use of special ion exchange resins. Demineralization or complete removal of dissolved minerals involves the use of both cation and anion exchange materials. In

removing impurities from water there are many possible combinations of coagulation, precipitation and ion exchange methods. Other methods of treatment

include: deaerarion (heating the water and venting the gases) for reduction of oxygen and carbon dioxide; and evaporation to produce distilled water.

Answer to : backWhat is Clarification?

Clarification is the removal of suspended matter and/or colour from water supplies. The suspended matter may consist of large particles which settle out readily. In these cases clarification equipment merely involves the use of settling basins and/or filters.

Most often, however, suspended matter in water consists of particles so small that they do not settle out and even pass through filters. The removal of these finely

divided or colloidal substances therefore requires the use of coagulants.

Answer to : backWhat is Coagulation?

Coagulation is the clumping together of finely divided and colloidal impurities in water into masses which will settle rapidly and/or can be filtered out of the water. Colloidal particles have large surface areas which keep them in suspension and in addition the particles have negative electrical charges which cause them to repel

each other and resist adhering together. Coagulation, therefore, involves neutralizing the negative charges and providing a nucleus for the suspended particles to adhere

to.

Answer to : backWhat Various Types of Coagulants are Used?

The most common coagulants are iron and aluminum salts such as ferric Sulphate, ferric chloride, aluminum Sulphate (alum) and sodium acuminate. Ferric and alumina

ions each have three positive charges and therefore their effectiveness is related their ability to react with the negatively charged colloidal particles. With proper use

these coagulants form a flock in the water which serves as a kind of net for collecting suspended matter. In recent years synthetic materials called polyelectrolyte have

been developed for coagulation purposes. these consist of long chain like molecules with positive charges. In some cases organic polymers and special types of clay are

used in the coagulation in making the flock heavier, causing it to settle out more rapidly.

Answer to : backWhat is Chemical Precipitation?

In precipitating processes the chemicals added react with dissolved minerals in the water to produce a relatively insoluble reaction product. Precipitation methods are

used in reducing dissolved hardness, alkalinity and in some silica. The most common example of chemical precipitation in water treatment is lime soda softening.

Answer to : backHow Does Lime React in the softening Process ?

Hydrated lime (calcium hydroxide) reacts with soluble calcium and magnesium bicarbonates to form insoluble precipitates. This is shown by the following equations:

 

  Ca(OH)2+

  Ca(HCO3)2 ======>   2CaCO3+

  2H2O

 Lime 

  Calcium     Calcium 

  Water

      Bicarbonate     Carbonate 

  

  Ca(OH)2+

  Mg(HCO3)2======>   Mg(OH)2+

  2CaCO3+

  2H2O

 Lime 

  Magnesium    Magnesium  

  Calcium 

  Water

  

  Bicarbonate     Hydroxide 

  Carbonate  

  

Most of the calcium carbonate and magnesium hydroxide come out of solution as a sludge and can be removed by settling and filtration. Lime, therefore, can be used to

reduce hardness present in the bicarbonate form (temporary hardness) as well as decrease the amount of bicarbonate alkalinity in a water. Lime reacts with

magnesium Sulphate and chloride and precipitates magnesium hydroxides but in this process soluble calcium Sulphate and chloride are formed. Lime is not effective in

removing calcium Sulphate and chlorides.

Answer to : backHow Does soda Ash React in the Softening Process?

Soda ash is used primarily to reduce non-bicarbonate hardness (also called Sulphate hardness or permanent hardness). It reacts as follows:

 

  Na2CO3+

  CaSO4 ======>   CaCO3+

  Na2SO4

 Soda Ash 

  Calcium     Calcium 

  Sodium

  

  Sulphate     Carbonate 

  Sulphate

 

  Na2CO3+

  CaC12 ======>   CaCO2+

  2NaCl

  

  Calcium     Calcium 

  Sodium

  

  Chloride     Carbonate 

  Chloride

The calcium carbonate formed by the reaction tends to come out of solution as a sludge. The sodium Sulphate and chloride formed are highly soluble and non-scale

forming.

Answer to : backWhat are the Various Methods of Lime Soda Softening?

The two general types are intermittent (batch type) and continuous. The older method of intermittent softening consists of mixing the chemicals with the water in a tank, allowing time for reaction and settling of the sludge, and drawing off the clear water. The more modern method of continuous lime soda softening involves the use

of specially compartmented tanks with provisions for

1. proportioning chemicals continuously to the incoming water 2. retention time for chemical reactions and settling of sludge, and

3. continuous draw-off of softened water. Lime soda softening may also be classified as hot or cold, depending on the temperature of the water. Hot process softeners increase the rate of chemical reactions and give better

quality water.

Answer to : backWhy are Coagulants Used in the Lime-Soda Process?

Just as coagulants are used for removing suspended matter in clarification processes, they serve to clump together precipitates in the softening process. Coagulants can speed up settling of sludge as much as 25 - 50 per cent. Sodium acuminate has a special advantage as a coagulant in lime soda softening since unlike most other

coagulants it is alkaline and also contributes to the softening redactions, particularly in reducing magnesium. Effective use of coagulants helps remove silica in the

softening process. Silica tends to be absorbed in the flock produced by coagulation of sludge.

Answer to : backUnder What Conditions Are Phosphate Softeners Use?

Sodium phosphates react readily with calcium and magnesium salts. Phosphate softeners are generally used only on naturally soft or pre softened waters, however,

because relatively high amounts of magnesium in the water cause a very sticky

precipitate in reacting with phosphate. Properly used, phosphate softeners can effectively reduce hardness to very low levels. Improved ion exchange softening

methods have largely supplanted phosphate softeners in new installations.

Answer to : backWhat are the Disadvantages of Lime Soda Softening?

The main disadvantage is that while hardness is reduced it is not completely removed. Wide variations in raw water composition and flow rate also make control

of this method difficult since this involves adjusting the amounts of lime and soda ash being fed.

Answer to : backWhat are the Advantages of Lime Soda Softening?

The main advantage is that in reducing hardness, alkalinity and silica can also be reduced. In addition, prior clarification of the water is not usually necessary since

suspended matter and turbidity are also removed in the process. Another advantage is that with continuous hot process softening some removal of oxygen and carbon

dioxide can be achieved.

Answer to : backWhat is Ion Exchange?

When minerals dissolve in water they form electrically charge particles called ions. Calcium carbonate, for example, forms a calcium ion with plus charges (a cation) and

a carbonate ion with negative charges (an anion). Certain natural and synthetic materials have the ability to remove mineral ions from water in exchange for others.

For example, in passing water through a simple cation exchange softener all of calcium and magnesium ions are removed and replaced with sodium ions. Ion

exchange materials usually are provided in the form of small beads or crystals which compose a bed several feet deep through which the water is passed.

Answer to : backWhat are the Various Types of Ion Exchange Materials?

Ion exchange materials are basically of two types: cation and anion exchangers. Cation exchange materials react only with positively charged ions such as Ca++ and Mh++. Anion exchanger materials react only with the negatively charged ions such

as carbonate (CO3) and Sulphate (SO4). Zeolite materials are cation exchangers composed chiefly of sodium, aluminum and silica. There are several other types of

cation exchange materials of an organic or resinous nature. The anion materials are usually organic in nature and are of two basic types: weak base and strong base

types. Weak base exchangers don’t take out carbon dioxide or silica (actually carbonic acid and silica acid) but remove strong acid anions by a process that is more like adsorption than ion exchange. Strong base anion exchangers on the other hand

can reduce silica and carbon dioxide to very low values. Cation exchangers usually opals which settle out readily. In these cases clarification equipment merely involves the use of settling basins and/irate on either a sodium or hydrogen ‘cycleE That is,

they may be designed to replace all cations in the water with either sodium or hydrogen. Strong base anion exchangers are generally operated on a hydroxide weak base on a carbonate cycle. Chloride anion exchange is also used in some processes.

Why Water Treatment is Needed :As feed-water enters a boiler the heat causes hardness (calcium and magnesium salts) to come out of solution. Untreated the hardness deposits on the hot boiler metal to from scale. As water evaporates in the boiler the feed-water impurities

concentrate. Even small amounts to iron, copper, and silica can accumulate in the boiler-water and cause serious deposit problems in higher pressure boilers. Since

scale can cause overheating and failure of boiler metal, preventive water treatment is needed. The corrosion of boiler system metal is a complex process and takes many

forms: general attack, localized pitting, and various types of cracking in stressed metal. In general, the main factors causing corrosion are dissolved gases in the water

(primarily oxygen) and acid conditions. High temperatures speed up the corrosion process. Corrosion is damaging from several standpoints: it causes weakening and failure of metal and produces corrosion products which can cause boiler deposits. High concentrations of dissolved and suspended matter in boiler-water can cause foaming of the water at the steam release surface. This produces carry-over of the

water and its impurities into the steam. Carry-over results in deposits and other problems in turbines, engines and other processes using steam. While mechanical and operational factors also cause carry-over, proper control of water conditions is

important in producing pure steam.

Answer to : backWhat is Boiler Water Carryover?

Boiler water carry-over is the contamination of the steam with boiler-water solids. There are four common types of boiler-water carry-over. In one bubbles or froth

actually build up on the surface of the boiler-water and pass out with the steam. This is called foaming and can be compared to the stable foam found on beer. In the

second type small droplets of water in the form of spray or mist are thrown up into the steam space by the bursting of the rising steam bubbles at the steam release

surface. This is sometimes called ‘aquaglobejectionEand is like ginger ale or champagne where no stable foam is formed but droplets of liquid are ejected from

the liquid surface. The third condition of carry-over, called priming, is a sudden surge of boiler-water that carries over with the steam, similar to the effects produced in

uncapping a bottle of charged water. stem contamination may also occur from leakage of water through improperly designed or installed steam separating

equipment in a boiler drum.

Answer to : backWhat are the Disadvantages of Ion Exchange?

The main disadvantage with sodium cycle ion exchange softening is that the total solids, alkalinity and silica contents of the raw water are not reduce. A problem

encountered with cation exchange on the hydrogen cycle is corrosion from acidity of

the effluent. With demineralization the chief difficulties are with cost particularly on high solids raw waters, and the corrosive nature of the effluent water. In general, fouling of the ion exchange material with suspended or colloidal matter in the raw water can produce difficulties and some water impurities cause degradation of the

material. In many cases, therefore, ion exchange processes require pretreatment of the water supply.

Answer to : backWhat are the Advantages of Ion Exchange?

The main advantage of zeolite softening is ease of control. Ordinary variations of hardness in the raw water or in flow rate do not affect completeness of softening.

Also the system generally takes up less space than the lime-soda system and in most cases gives a softer water. The use of acid exchangers has advantages when a low

alkalinity soft water is required. The main advantage of ion exchange demineralization is its ability to produce better quality water than can be obtained by

any other method.

Answer to : backHow Does Oil Affect Carry-over?

Oil contamination in boiler feed-water, usually form reciprocating engines, pumps, etc., can cause serious foaming. This is generally attributed to the formation of soaps

in the boiler-water due to specification of the oil by boiler-water alkalis.

Answer to : backHow Do Suspended Solids Affect Carry-over?

The theory advanced is that suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking

and builds up a foam. It is believed that the finer the suspended particles the greater their collection in the bubble. Experience indicates, however, that many boilers

operate with exceedingly high suspended solids without carry-over while others have carry-over with only a trace of suspended solids. This would seem to indicate that the

type as well as the quantity of suspended solids has much to do with carry-over.

Answer to : backWhat is Selective Silica Carry-over?

Silica can carry over into the steam in two ways. It can be present in the steam as the result of general boiler-water carry-over or it can go into steam in a volatile form.

In the latter case silica acts much like a gas and is considered to be selectively carried over. As Pressures increase above 2760 kPa (400 p.s.i), there is an increased tendency for silica to be selectively carried into the steam in amounts proportionate

to the amount of silica in the boiler-water.

Answer to : backWhat Problems are Caused by Carry-over?

The disadvantages of wet steam include a general decrease in operating efficiency and erosion of turbines and engines. In addition any dissolved or suspended solids in the boiler-water tend to deposit out in the steam and condensate system,. when the solids deposit in super heaters and turbine, overheating and failure of superheated tubes and reduction in turbine efficiency can result. Impurities carried over with the

can cause difficulties in many processes for which the steam is used.

Answer to : backWhat Measures are Usually Taken to Prevent Carry-over?

The most common measure is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads,

and sudden load changes also helps. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases the condensate

should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foam agents can be very effective in preventing

carry-over due to high concentrations of impurities in the boiler-water.

Removing Impurities from Water :Coagulants are chemicals to enmesh fine particles of suspended matter in a water

supply to form a flock which settles or can be filtered out. Adding softening chemicals (lime, soda, ash, etc.) to a water causes some dissolved hardness salts to precipitate

and the suspended matter can then be coagulated and filtered out. Precipitation processes such as lime soda softening can effectively remove suspended matter, hardness and alkalinity and in some cases reduce the silica content of the water. When a salt dissolves in water it forms positive ions (cations) and negative ions

(anions). For example, calcium carbonate (CaCO3) forms a calcium cation (Ca++) and a carbonate anion (CO3). The most common form of ion exchange involves

passing water through material which substitutes sodium for calcium and magnesium cations. This is a typical softening treatment. Anions can also be removed from water by the use of special ion exchange resins. Demineralization or complete removal of dissolved minerals involves the use of both cation and anion exchange materials. In

removing impurities from water there are many possible combinations of coagulation, precipitation and ion exchange methods. Other methods of treatment

include: deaerarion (heating the water and venting the gases) for reduction of oxygen and carbon dioxide; and evaporation to produce distilled water.

Answer to : back

Answer to : backWhat is Coagulation?

Coagulation is the clumping together of finely divided and colloidal impurities in water into masses which will settle rapidly and/or can be filtered out of the water. Colloidal particles have large surface areas which keep them in suspension and in addition the particles have negative electrical charges which cause them to repel

each other and resist adhering together. Coagulation, therefore, involves neutralizing the negative charges and providing a nucleus for the suspended particles to adhere

to.

Answer to : backWhat Various Types of Coagulants are Used?

The most common coagulants are iron and aluminum salts such as ferric Sulphate, ferric chloride, aluminum Sulphate (alum) and sodium acuminate. Ferric and alumina

ions each have three positive charges and therefore their effectiveness is related their ability to react with the negatively charged colloidal particles. With proper use

these coagulants form a flock in the water which serves as a kind of net for collecting suspended matter. In recent years synthetic materials called polyelectrolyte have

been developed for coagulation purposes. these consist of long chain like molecules with positive charges. In some cases organic polymers and special types of clay are

used in the coagulation in making the flock heavier, causing it to settle out more rapidly.

Answer to : backWhat is Chemical Precipitation?

In precipitating processes the chemicals added react with dissolved minerals in the water to produce a relatively insoluble reaction product. Precipitation methods are

used in reducing dissolved hardness, alkalinity and in some silica. The most common example of chemical precipitation in water treatment is lime-soda softening.

Answer to : backWhat is the Purpose of Deaeration?

Since dissolved oxygen in water is a big factor in corrosion in boiler systems it is desirable that this be removed before the water is put into a boiler. Feed-water

deaerarion is accomplished by intimately mixing the water and steam in a deaerating heater. Part of the steam is vented, arraying with it the bulk of the dissolved oxygen from the water. There are two basic types of steam deaerators: the spray type and the tray type. In the spray deaerator a jet of steam mixes intimately with the feed water being sprayed into the unit. In the tray type the incoming water is allowed to fall over a series of trays causing the water to be broken up into small droplets to

permit intimate contact with incoming steam.

Answer to : backHow are Evaporators Employed?

Water is sometimes pretreated by evaporation to produce relatively pure vapor which is then condensed and used for boiler feed purposes. Evaporators are of

several different types, the simplest being a tank of water through which steam coils are passed to heat the water to the boiling point. Sometimes to increase the

efficiency the vapor from the first tank is passed through coils in a second tank of water to produce additional heating and evaporation. Other types of evaporation

include a ‘flash type which operates under a partial vacuum causing a lowering of the boiling point of water and evaporation at lower temperatures. Evaporators have

advantages where steam as a sources of heat is readily available. They also have particular advantages over demineralization, for example, when the dissolved solids

in the raw water are very high.

Answer to : backWhat Combinations of External Treatment Methods are Generally Used?

As mentioned previously, water containing suspended solids, organics, and/or turbidity usually requires clarifications prior to ion exchange methods. Also, since simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation type softening. One of the most common and efficient combination treatments is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity and in

some cases silica, and subsequent treatment with a cation exchange softener. This system of treatment accomplishes several functions: softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.

Answer to : backWhen is Internal Treatment of Boiler Feed-water Necessary?

Chemical treatment of water inside the boiler is usually essential whether or not the water has been pretreated. Internal treatment, therefore, complements external treatment by taking care of any impurities entering the boiler with the feed water

(hardness, oxygen, silica, etc.) regardless of whether the quantity is large or small. In many cases external treatment of the water supply is not necessary and the water

can be treated by internal methods alone. Internal treatment can constitute the sole treatment when boilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when the raw water available is of good

quality.

Answer to : backWhat Should a Good Internal Water Treatment Programme Accomplish?

The purpose of an internal treatment programme is fourfold: (1) react with any feed-water hardness and prevent it from precipitating on the boiler metal as scale, (2)

condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non-adherent to the boiler metal, (3) provide anti-foam protection to permit a reasonable concentration of dissolved and suspended solids in the boiler

water without foam carry-over, and (4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion. In addition, as supplementary measures

an internal treatment should prevent corrosion and scaling of the feed-water system and protect against corrosion in the steam condensate systems.

Answer to : backWhat chemicals are Used in Internal Treatment?

The softening chemicals used include soda ash, caustic and various types of sodium phosphates. These chemicals react with calcium and magnesium compounds in the feed water. At times sodium silicate is used to contributed alkalinity as well as react

selectively with magnesium hardness. The materials used for conditioning sludge include various organic materials of the tannin, lignin or alginate classes. It is

important that these organics are so selected and processed that they are both effective and stand stable at the boiler operating pressure. Certain synthetic organic

materials are used as anti-foam agents. The chemicals used to scavenge oxygen include sodium sulphite and hydrazine. Various combinations of polyphosphates and organics are used for preventing scale and corrosion in feed-water systems. Volatile

neutralizing amines and filming inhibitors are used for preventing condensate corrosion.

Answer to : backHow are Carbonates Reacted on by Internal Treatment?

Calcium bicarbonate entering with the feed water is broken down at boiler temperatures or reacts with caustic soda to form calcium carbonate. Since calcium carbonate is relatively insoluble it tends to come out of solution. Sodium carbonate

partially breaks down at high temperature to sodium hydroxide (caustic) and carbon dioxide. When phosphates are used in internal treatment they react with calcium

carbonate to form calcium phosphate and sodium carbonate (soda ash). In the presence of sufficient hydroxides (caustic) alkalinity, magnesium bicarbonate will precipitate as magnesium hydroxide or will react with any silica present to form magnesium silicate. The minerals precipitated from solution (calcium carbonate,

calcium phosphate, magnesium hydroxide, magnesium silicate, etc..) form sludge in the water which must be conditioned to prevent its sticking to the metal. The

conditioned sludge is removed from the boiler by blow-down.

Answer to : backHow are Sulphates Reacted on by Internal Treatment?

High temperatures in the boiler water reduce the solubility of calcium Sulphate and tend to make it precipitate out directly on the boiler metal as scale. Consequently

calcium Sulphate must be reacted upon chemically to cause a precipitate to form in the water where it can be conditioned and removed by blow-down. Calcium Sulphate

is reacted on either by sodium carbonate, sodium phosphate or sodium silicate to form insoluble calcium carbonate, phosphate or silicate. Magnesium Sulphate is

reacted upon by caustic soda to form a precipitate of magnesium hydroxide. some magnesium may react with silica to form magnesium silicate. Sodium Sulphate is highly soluble and remains in solution unless the water is evaporated almost to

dryness.

Answer to : backHow is Silica Reacted upon by Internal Treatment?

In untreated waters silica tends to precipitate out directly as scale at hot spots on the boiler metal or it may combine with calcium to produce a hard calcium silicate scale. Treatment for silica involves keeping the boiler-water alkalinity high enough to hold

silica in solution. Usually there is enough magnesium in the water to precipitate some of the silica as sludge. At times proper treatment with magnesium can tie up silica

when it is a special problem. Some organic materials such as starches tend to prevent the adherence of silica to the boiler metal probably by a physical action.

Answer to : backHow is Sludge Conditioned in Internal Treatment?

There are two general approaches to conditioning sludge inside a boiler: by coagulation or dispersion. When the total amount of sludge is great (as the result of

high feed-water hardness) it is practical to coagulate the sludge to form large flocculent particles. This flow readily with the boiler water and can be removed by

blow-down. This can be accomplished by careful adjustment of the amounts of alkalis, phosphates and organics used for treatment, based on the fee-water analysis.

When the amount of sludge is not great (low hardness feed-waters) it is more practical to use a higher percentage of phosphates in the treatment. Phosphates

form finely divided sludge particles. A higher percentage of organic sludge dispersants is used in the treatment to keep the sludge particles dispersed

throughout the boiler water.

Answer to : backWhat Difficulties are Encountered in Internal Treatment?

The main difficulty is the presence of a large amount of sludge formed when feed-water hardness is high. This may increase the amount of blow-down required. When

internal treatment is used alone (without pretreatment of the water by external means) there is more possibility for scale in the pre-boiler system and fee-water

lines. it is important that someone experienced in the technology helps to set up an internal treatment programme which will minimize these difficulties.

Answer to : backWhat are the Advantages of Internal Treatment?

The prime advantage is that in many instances internal treatment can eliminate the need for extensive external treatment equipment. This gives a definite economic advantage. In addition, the simplicity of an internal treatment programme offers a

decided savings in manpower for feeding and control. A qualified consultant can help decide what water quality is required for a specific boiler system, and choose the

most economical means of obtaining the required quality.

Answer to : backHow are Internal Treatment Chemicals Fed?

Common feeding methods include the use of chemical solution tanks and proportioning pumps or special ball briquette chemical feeders. In general, softening chemical (phosphates, soda ash, caustic, etc.) are added directly to the fee-water at

a point near the entrance to the boiler drum. They may also be fed through a separate line discharging in the feed-water drum of the boiler. The chemicals should discharge in the fee-water section of the boiler so that reactions occur in the water

before it enters the steam generating areas. Softening chemicals may be added continuously or intermittently depending on feed-water hardiness and other factors.

Chemicals added to react with dissolved oxygen (Sulphate, hydrazine, etc.) preferably should be fed continuously as far back in the feed-water system as

possible. Similarly, chemicals used to prevent scale and corrosion in the feed-water system (polyphosphates, organics, etc.) should be fed continuously. Chemicals used to prevent condensate system corrosion may be fed directly to the steam or into the feed-water system, depending on the specific chemical used. continuous feeding is

preferred but intermittent application will suffice in some cases.

Answer to : backHow are Chemical Dosages Controlled?

Chemical dosages are based primarily on the amount of impurities in the feed-water. For example, the amount of softening chemicals needed depends on fee-water hardness; the amount of sodium Sulphate needed depends on the amount of

dissolved oxygen in the feed-water. In addition, however, a set amount of extra chemical treatment is added to provide a residual is alkies of insurance and serves as

the basis for treatment control.

Answer to : backWhat Boiler Water Tests are Used for Treatment Control?

Routine control test of the boiler water vary according to the type of chemical treatment used but they may include tests for: alkalinity M phosphate, Sulphate and organic color. Boiler water hardness tests are not often made because it is generally

assumed that if there is enough alkalinity and/or phosphate present in the boiler-water, the hardness has reacted completely. In testing for Sulphate it is assumed that

if an adequate residual is present, the feed-water oxygen has been removed; this may not always be true, especially if the Sulphate feed is not continuous and if

ordinary unanalyzed sodium Sulphate is used. Generally antifoams are incorporated in organic treatments so testing for organic color gives an indication both of sludge

conditioner present as well as level of antifoam treatment.

Answer to : backWhat Tests are Usually Made as a Check for Contaminants?

Here, again, the specific tests made vary with the type of contamination suspected. Some checks made fairly often, however, include test for: iron, oil and silica. Usually

the iron test serves as a check on corrosion products brought back with the condensate but may also be used when appreciable iron is present in the make up water. Oil tests usually require laboratory facilities but visual inspection of samples can show up gross contamination. While silica is usually present to some extent in

boiler waters, periodic checks are sometimes made to detect unusual contamination or to indicate when additional blow-down is needed to keep silica concentrations

below a preset limit.

Answer to : backWhat Units are Used in Expressing Water Analysis Results?

The most common unit is parts per million. One p.p.m. of a substance in a water sample represents one unit mass of the substance in each million unit mass of the water. For example, one p.p.m of salt (NaCl) means one kg of salt per million kg of

water. There is still some diehard use of the classic unit grains per gallon (g.p.g.) but this expected to disappear due to universal S.I. usage as will the unit equivalents per

million (e.p.m). This mention is therefore made merely as a matter of record.

Answer to : backWhy are some Analysis Results Express ‘As CaCO2E

Water treatment reactions are based on the combining mass of the reacting substances. For example, 106 kilograms of soda ash (molecular mass 106) reacts

with 136 kilograms of calcium Sulphate (molecular weight 136). The molecular mass of calcium carbonate (CaCO3) is the round number 100. In order to simplify chemical

dosage calculations all hardness and alkalinity results are usually based on the molecular mass of calcium carbonate and are expressed as ‘CaCO3E For example, using this system, one p.p.m of calcium Sulphate (expressed as CaCO3). This is the same as converting English pounds, German marks, or fresh francs into a 100 cent

dollar.

Answer to : backWhat is Blow-down?

Blow-down is the removal from the boiler of water containing concentrated dissolved and suspended solids. As the blow-down water is replaced with lower solids feed

water the boiler water is essentially being diluted. By regulating the amount of blow-down, therefore, the amount of solids in the boiler-water can be controlled.

Answer to : backHow much Blow-down is Needed?

This depends on how many concentrations of the various feed-water impurities a given boiler can tolerate; the more concentrations possible the less blow-down

needed. For example, with 10 feedwater concentrations in a boiler, blow-down equal to 10 per cent of the feed-water flow rate is needed; with 20 concentrations only 5

per cent blow-down is needed. To illustrate how blow-down requirements are calculated let us assume that the maximum amount of suspended solids (sludge) in

the boiler water that a particular boiler can tolerate is 500 p.p.m. If the fee-water contains 50 p.p.m. of hardness it can be concentrated only about 10 times (since feed water hardness is precipitated as suspended solids in the boiler water). This

means that for every 50 kg of water fed to the boiler about 5 kh of boiler water must be blown down to keep the suspended solids from exceeding 500 p.p.m. Suspended solids, however, may not be the limiting factor in all cases; other factors which may

limit feed-water concentrations include dissolved solids, alkalinity, silica or iron.

Answer to : backWhat Tests are Made in Regulating Blow-down?

Since there are no simple test for routinely checking the amount of suspended solids in boiler-water, blow-down is usually controlled through use of a simple instrument which measures the electrical conductivity of the water. This test gives an estimate of the dissolved solids present in the boiler-water. Chloride tests are also used for blow-down control since chlorides are not reacted on by chemical treatment. By checking both the fee-water and boiler-water chlorides the number of feed-water

concentrations can be calculated. In some higher pressure boilers, silica or iron tests may also be made to control blow-down.

Answer to : backWhat is the Difference between Continuous and ‘PuffEBlow-down?

All boilers have blow-down connections located at low points where sludge is likely to collect. Opening these blow-down valves periodically for shot intervals gives a

‘puffEor intermittent removal of sludge and solids. Many boilers also have blow-down connections consisting of an overtake located just below the water level in the steam

release area. A small amount of water is continuously removed through these connections. The use of continuous blow-down in addition to ‘puffEor bottom blow-

down keys it possible to maintain the solids and chemical residuals at more consistent levels in the boiler water. Continuous blow-down also minimizes the amount of blow-down required with resultant savings in heat and chemicals. Continuous blow-down also causes less upset in boiler water circulation and

operation.

Answer to : backWhat Causes Corrosion in Steam Condensate Systems?

Most condensate system corrosion is caused by carbon dioxide and oxygen, arrived into the system with the steam. Carbon dioxide, dissolved in the pure condensed steam, form corrosive carbonic acid. if oxygen is present with carbon dioxide, the

corrosion rate is much higher, and is likely to produce localized pitting. Ammonia, in

combination with carbon dioxide or oxygen, attacks copper alloys.

Answer to : backHow is Steam Condensate Corrosion Prevented?

The general approach may involve removing oxygen from the feed-water mechanically and chemically, and providing pretreatment of the make-up water to minimize potential carbon dioxide formation in the boiler. In addition, an effective

chemical treatment programme is required. This may consist of using volatile amines to neutralize carbon dioxide and/or a volatile filming inhibitor to form a barrier

between the metal and the corrosive condensate. Mechanical conditions such as poor trapping and draining of lines, and air in-leakage may need to be correct-ed.

Answer to : backHow do Chemical Oxygen Scavengers Help Control Condensate System

Corrosion?

As previously mentioned mechanical equipment (de-aerator) is often used to reduce feed-water oxygen. The best designed and operated de-aerators can reduce oxygen to as low as 0.007 parts per million or less. Most de-aerators or feed-water heaters are less effective. Since very small amounts of oxygen, however, can cause boiler

corrosion and corrosion in steam condensate system, chemical treatment is therefore, needed to assure complete oxygen removal. Sodium Sulphate is the

chemical most commonly used for this purpose. Greatly improved oxygen removal is obtained, however, when the Sulphate is catalyzed. Catalyzed sodium Sulphate can reduce oxygen content of water (at room temperature) from the saturation point to zero in less than 30 seconds. Without a catalyst it takes up to 10 minutes under the

same conditions to reduce the oxygen content by only about 30 per cent. Fast reactions are important since oxygen should be removed before the water enters the boiler. Otherwise some oxygen will escape form the boiling water into the steam lines

and corrosion in the condensate system.

Answer to : backWhat is the Basis for Choice between Neutralizing and Filming Inhibitors?

The proper choice of inhibitor depends on the boiler system, plant lay-out operating conditions and fee-water composition. In general, volatile amines are better with low

make-up, low feed-water alkalinity, and good oxygen control. Filming inhibitors usually give more economical protection with high make-up, air in-leakage high feed-

water alkalinity or where the system is operated internally. In some cases a combination of treatments is needed.

Answer to : backWhat Characteristics Should a Good Condensate Corrosion Inhibitor Have?

A good volatile neutralizing amine should have a favorable distribution ratio in steam and condensate so that it protects the entire steam-condensate system. It should

have no insoluble reaction products and should be stable at high temperatures and pressures. A good filming inhibitor should be easy to disperse in water so that it can

be fed uniformly. It should be stale under usage conditions and form a thin protective film without causing deposits in either the boiler or the steam-condensate system.

Answer to : backHow are Deposits and Corrosion Prevented in Feed-water Systems?

Deposits in feed-water systems are most frequently caused by hardness coming out of solution as the water goes through feed- water heaters or as the feed lines enter the boiler. Deposits also can occur from premature reaction of treatment chemicals with hardness in the feed-water. Prevention involves the use of stabilizing chemicals fed continuously to retard hardens precipitation. Proper design of the chemical feed system can minimize premature chemical reactions. Corrosion of feed-water system generally results from low alkalinity or dissolved oxygen in the water. Raising the pH of the water and the continuous feed of catalyzed sodium Sulphate will minimize this

problem.

Answer to : backWhat is the Wet Method of Boiler Lay-Up?

This is a method of storing boilers full of water so that they can be readily returned to service. it involves adding extra chemicals (usually caustic, organics, and sodium

sulphite to the boiler-water.) The water level is raised in the idle boiler to eliminate air spaces and the boiler is kept completely full of treated water. Special

considerations are needed for protecting super heaters.

Answer to : backWhat is the Dry Method of Boiler Lay-Up?

This method of lay-up is usually for longer boiler outages. It involves draining, cleaning and drying out the boiler. a material which absorbs moisture such as

hydrated lime or silica gel is placed in trays inside the boiler. The boiler is then sealed carefully to prevent in leakage of air. Periodic inspection and replacement of the

drying chemical is required during long storage periods.

DATA USED IN WATER CHEMISTRY

The chemicals listed in this section include those found as impurities in water and also those used as treatments. The chemical formulas, ion forms, and molecular and

equivalent weights are given for each substance. Abbreviations and symbols are used extensively to simplify water analysis reports and calculations. This section

explains the meanings of some common symbols and what they represent in water

analyses. Very often the units used in water chemistry need to be converted back and forth for practical application. For example, parts per million may be converted

to grams per 1000 liters and vice versa. The conversion factors in this section simplify this type of calculation.

 

  CATIONS IonFormula

IonicWeight

EquivalentWeight

   AluminumA1+++ 27.0 9.0

   AmmoniumNH4+ 18.0 18.0

   CalciumCA++ 40.1 20.0

   HydrogenH+ 1.0 1.0

   Ferrous IronFe++ 55.8 27.9

   MagnesiumMg++ 24.3 12.2

   ManganeseMn++ 54.9 27.5

   PotassiumK+ 39.1 39.1

   SodiumNa+ 23.0 23.0

   

   ANIONS

   

   BicarbonateNCO3- 61.0 61.0

   ChlorideCO3- 60.0 60.0

   FluorideF- 19.0 19.0

   NitrateNO3- 62.0 62.0

   HydroxideOH- 17.0 17.0

   Phosphate (PO4-- 95.0 31.7

   Phosphate (dibasic)HPO4-- 96.0 48.0

   Phosphate (monobasic)H2PO4- 97.0 97.0

   SulphateSO4-- 96.1 48.0

   SulphiteSO3-- 80.1 40.0

   COMPOUNDSFormula Molecular

WeightEquivalent

Weight

   Aluminum hydroxideAl(OH)3 78.0 26.0

   Aluminum SulphateAl2(SO4)3 342.0 57.0

   AluminaAl2O3 102.0 17.0

   Calcium bicarbonateCa(HCO3)2 162.1 81.1

   Calcium carbonateCaCO3 100.1 50.1

   Calcium chlorideCaCl2 111.0 55.5

   Calcium hydroxide (pure)Ca(OH)2 74.1 37.1

   Calcium hydroxide (90%)Ca(OH)2 -- 41.1

   Calcium Sulphate (anhydrous) CaSO4 136.2 68.1

   Calcium Sulphate (gypsum) CaSO4.2H2O 172.2 86.1

   Calcium phosphateCa3(PO4)2 310.3 51.7

   Disodium phosphate Na2HPO4.12H2O

358.2 119.4

   Disodium phosphate   (anhydrous) NaHPO4 142.0 47.3

   Ferric oxideFe2O3 159.6 26.6

   Iron oxide (magnetic)Fe3O4 321.4 -

   Ferrous Sulphate (copperas) FeSO4.7H2O 278.0 139.0

   Magnesium oxideMgO 40.3 20.2

   Magnesium bicarbonateMg(HCO3)2 146.3 73.2

   Magnesium carbonateMgCO3 84.3 42.2

   Magnesium chlorideMgCl2 95.2 47.6

   MagnesiumMg(OH)2 58.3 29.2

   Magnesium phosphateMg3(PO4)2 263.0 43.8

   Magnesium SulphateMgSO4 120.4 60.2

   Monosodium phosphateNaH2PO4.H2O 138.1 46.0

   Monosodium phosphate   (anhydrous) NaH2PO4 120.1 40.0

   MetaphosphateNaPO3 102.0 34.0

   Sodium acuminateNa2Al2O4 164.0 27.3

   Sodium bicarbonateNaHCO3 84.0 84.0

   Sodium carbonateNa2CO3 106.0 53.0

   Sodium chlorideNaCl 58.5 58.5

   Sodium hydroxideNaOH 40.0 40.0

   Sodium nitrateNaNO3 85.0 85.0

   Sodium SulphateNa2SO4 142.0 71.0

   Sodium sulphiteNa2SO3 126.1 63.0

   Trisodium phosphateNa3PO4.l2H2O 380.2 126.7

   Trisodium phosphate   (anhydrous) Na3PO4 164.0 54.7

   

   GASES

   

   AmmoniaNH3 17 --

   Carbon DioxideCO2 44 --

   HydrogenH2 2 --

   OxygenO2 32 --

   

   ACIDS

   

   CarbonicH2CO3 62.0 31.0

   HydrochloricHCl 36.5 36.5

   PhosphoricH3PO4 98.0 32.7

   SulphuricH2SO4 98.1 49.1

Deaeration in boilersIn order to meet industrial standards for both oxygen content and the allowable metal oxide levels in feed water, nearly complete oxygen removal is required. This can be accomplished only by efficient mechanical deaeration supplemented by a properly controlled oxygen scavenger.

Deaeration is driven by the following principles: the solubility of any gas in a liquid is directly proportional to the partial pressure of the gas at the liquid surface, decreases with increasing liquid temperature; efficiency of removal is increased when the liquid and gas are thoroughly mixed.

Deaeration can be performed using a physical medium such as deaerating heaters or vacuum deaerators or a chemical medium such as oxygen scavengers (polishing treatment) or catalytic resins. Membrane contractors are increasingly being used. Carbon dioxide is often removed using a physical medium.

The purpose of a deaerator is to reduce dissolved gases, particularly oxygen, to a low level and improve plant thermal efficiency by raising the water temperature. In addition, they provide feed water storage and proper suction conditions for boiler feed water pumps.

Pressure deaerators can be classified under two major categories: tray type and spray type.

The tray type desecrating heaters consist of a shell, spray nozzles to distribute and spray the water, a direct contact vent condenser, tray stacks and protective interchamber walls. The chamber is constructed in low carbon steel, but more corrosion-resistant stainless steels are used for the spray nozzles and the other parts.

Incoming water is sprayed into steam atmosphere, where it is heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases (principally oxygen and free carbon dioxide) are released to the steam as the water is sprayed into the unit. Seals prevent the recontamination of tray stack water by gases from the spray section. Water falls from tray to tray, breaking into fine droplets of film, which intimately contact the incoming steam.

The steam heats the water to the steam saturation temperature and removes the very last traces of oxygen. Deaerated water falls to the storage space below, where a steam blanket protects it from recontamination. It is usually stored in a separate tank.

The steam enters the deaerators through ports in the tray compartment, flows down through the tray stack parallel to the water flow. A very small amount of steam condenses in this section as the water temperature rises to the saturation temperature of the steam. The rest of the steam scrubs the cascading water. Before leaving the tray compartment, the steam flows upward between the shell and the interchamber walls to the spray section. Most of the steam is condensed and becomes part of the deaerated water. A small portion of the steam, which contains the non-condensable gas released from the water, is vented to the atmosphere. It is essential that sufficient venting is provided at all times or deaeration will be incomplete. Steam flow through the tray stack may be cross-flow, counter-current, or co-current to the water.

The spray type deaerating heaters consist of a shell, spring-loaded inlet spray valves, a direct contact vent condenser section and a steam scrubber for final dearetion; the shell and steam may be low carbon steel, the spray valves and the direct contact vent condenser section are in stainless steel. The incoming water is sprayed into a steam atmosphere and heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases are released to the steam, and the heated water falls to water seals and drains to the lowest section of the steam scrubber. The water is scrubbed by a large volume of steam and heated to the saturation temperature prevailing at that point. As the water-steam mixture rises in the scrubber, the deaerated water is a few degrees above the saturation temperature, due to a slight pressure loss. In this way a small amount of flashing is produced, which aids in the release of dissolved gases. The deaerated water overflows from the steam scrubber to the storage section below.

Steam enters the deaerator through a chest on the side and flows to the steam scrubber. After flowing into the scrubber it passes up into the spray heater section to heat the incoming water. Most of the steam condenses in the spray section to become a part of the deaerated water. A small portion of the gases is vented to the atmosphere to remove the non-condensable gases.

Vacuum deaeration is used at temperatures below the atmospheric boiling point to reduce the corrosion rate in water distribution systems. A vacuum is applied to the system to bring the water to its saturation temperature. Spray nozzles break the water into small particles to facilitate gas removal and vent the exhaust gases. Incoming water enters through spray nozzles and falls through a columns packed with Raschig rings to other synthetic packing. In this way, water is reduced to thin films and droplets, which promote the release of dissolved gases. The released gases and water vapor are removed through the vacuum, which is maintained by steam jet eductors or vacuum pumps, depending on the size of the system. Vacuum deaerators remove oxygen less efficiently that pressure units.

Corrosion fatigue at or near welds is a major problem in deaerators. It is the result of mechanical factors, such as manufacturing procedures, poor welds and lack of stress-relieved welds. Operational problems such as water/steam hammer can also be a factor.

Find extra information about boiler feed water and boiler water treatment.Check also our web page about he main problems occurring in boilers: scaling, foaming and priming, and corrosion. For a description of the characteristics of the perfect boiler water click here.

Foaming and priming in boilersBoiler water carry-over is the contamination of the steam with boiler-water solids. Bubbles or froth actually build up on the surface of the boiler water and pass out with the steam. This is called foaming and it is caused by high concentration of any solids in the boiler water. It is generally believed, however, that specific substances such as alkalis, oils, fats, greases, certain types of organic matter and suspended solids are particularly conducive to foaming. In theory suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking and builds up foam. It is believed that the finer the suspended particles the greater their collection in the bubble.

Priming is the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the super heaters and in the turbines. Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes aggravated by impurities in the boiler-water. Some mechanical entertainment of minute drops of boiler water in the steam always occurs. When this boiler water carryover is excessive, steam-carried solids produce turbine blade deposits. The accumulations have a composition similar to that of the dissolved solids in the boiler water. Priming is common cause of high levels of boiler water carryover. These conditions often lead to super heater tube failures as well. Priming is related to the viscosity of the water and its tendency to foam. These properties are governed by alkalinity, the presence of certain organic substances and by total salinity or TDS. The degree of priming also depends on the design of the boiler and its steaming rate. The most common measure to prevent foaming and priming is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads, and sudden load changes also helps. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases the condensate should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foaming and anti-priming agents, mixtures of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry-

over of fine water particles in the stream, can be very effective in preventing carry-over due to high concentrations of impurities in the boiler-water.

Scaling in boilersBoiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. Evaporation in a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots. Leading to local overheating. Scaling mechanism is the exceeding of the solubility limits of mineral substances due to elevated temperature and solids concentration at the tube/water interface. The deposition of crystalline precipitates on the walls of the boiler interferes with heat transfer and may cause hot spots, leading to local overheating. The less heat they conduct, the more dangerous they are.

Common feed water contaminants that can form boiler deposits include calcium, magnesium, iron, aluminum, and silica. Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the deposit site in a soluble form and precipitate. The values corresponding to their thermal conductivity are:

Steel 15 kcal/m2.h per degree CCaSO4 1-2 kcal/m2.h per degree CCaCO3 0.5-1 kcal/m2.h per degree CSiO2 0.2-0.5 kcal/m2.h per degree C

Scaling is mainly due to the presence of calcium and magnesium salts (carbonates or sulphates), which are less soluble hot than cold, or to the presence of too high concentration of silica in relation to the alkalinity of the water in the boiler. A carbonate deposit is usually granular and sometimes of a very porous nature. The crystals of calcium carbonate are large but usually are matted together with finely divided particles of other materials so that the scale looks dense and uniform. Dropping it in a solution of acid can easily identify a carbonate deposit. Bubbles of carbon dioxide will effervesce from the scale. A sulphate deposit is much harder and more dense than a carbonate deposit because the crystals are smaller and cement together tighter. A Sulphate deposit is brittle, does not pulverize easily, and does not effervesce when dropped into acid. A high silica deposit is very hard, resembling porcelain. The crystal of silica are extremely small, forming a very dense and impervious scale. This scale is extremely brittle and very difficult to pulverize. It is not soluble in hydrochloric acid and is usually very light coloured.Iron deposits, due either to corrosion or iron contamination in the water, are very dark coloured. Iron deposits in boilers are most often magnetic. They are soluble in hot acid giving a dark brown coloured solution.

If unchecked, scaling causes progressive lowering of the boiler efficiency by heat retardation, acting as an insulator. Eventually, scale built-up will cause the tube to overheat and rupture. Boiler deposits can also cause plugging or partial obstruction of corrosive attack underneath the deposits may occur. In general, boiler deposits can cut operating efficiency, produce boiler damage, cause unscheduled boiler outages, and increase cleaning expense.

The first anti-scaling preventative measure is to supply good quality demineralised water as make–up feed water. The purer the feed water is, the weaker the driving mechanism to form scale. Scale-forming minerals that do enter the boiler can be rendered harmless by internal chemical treatment. A long-established technique is to detach the hardness cations, magnesium and calcium, from the scale forming minerals and to replace them with sodium ions.

Images Source: http://www.aalborg-industries.com/ifs/files/AI/eng/Presentation/Website/Downloadablefiles/pdf/Aalborg_Solutions_download/aal_sol_6_mar04.pdf

Presence of Silica

Silica can vaporize into the steam at operating pressures as low as 28 bars. Its solubility in steam increases with increased temperature; therefore, silica becomes more soluble as steam is superheated. The conditions under which vaporous silica carryover occurs have been thoroughly investigated and documented. Researchers have found that for any given set of boiler conditions using demineralized or evaporated quality make-up water, silica is distribute between the boiler water and the steam in a definite ratio. This ratio depends on two factors: boiler pressure and boiler water pH. The value of the ratio increases almost logarithmically with increasing pressure and decreases with increasing pH.If the silica enters the boiler water, the usual corrective action is to increase boiler blowdown, to decrease it to acceptable levels and then to correct the condition that caused the silica contamination.

Corrosion in boilers

Corrosion is the reversion of a metal to its ore form. Iron, for example, reverts to iron oxide as the result of corrosion. The process of corrosion, however is a complex electro chemical reaction and it takes many forms. Corrosion may produce general attach over a large metal surface or it may result in pinpoint penetration of metal. Corrosion is a relevant problem caused by water in boilers. Corrosion can be of widely varying origin and nature due to the action of dissolved oxygen, to corrosion currents set up as a result of heterogeneities on metal surfaces, or to the iron being directly attacked by the water. While basic corrosion in boilers may be primarily due to reaction of the metal with oxygen, other factors such as stresses, acid conditions, and specific chemical corrodents may have an important influence and produce different forms of attack. It is necessary to consider the quantity of the various harmful substances that can be allowed in the boiler water without risk of damage to the boiler. Corrosion may occur in the feed-water system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide.Starting form these figures, and allowing the amount that can be blown down, the permitted concentration in the make-up water is thus defined.

Corrosion is caused principally by complex oxide-slag with low melting points. High temperature corrosion can proceed only if the corroding deposit is in the liquid phase and the liquid is in direct contact with the metal. Deposits also promote the transport of oxygen to the metal surface. Corrosion in the boiler proper generally occurs when the boiler water alkalinity is low or when the metal is exposed to oxygen bearing water either during operation or idle periods. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system corrosion is generally the result of contamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system.Corrosion is caused by the combination of oxide layer fluxing and continuous oxidation by transported oxygen.

Cracking in boiler metal may occur by two different mechanisms. In the first mechanism, cyclic stresses are created by rapid heating and cooling and are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually associated with improper corrosion prevention. The second type of corrosion fatigue cracking occurs in boilers with properly treated water. In these cases corrosion fatigue is probably a misnomer. These cracks often originate where a dense protective oxide film covers the metal surfaces and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are usually thick, blunt and cross the metal grains. They usually start at internal tube surfaces and are most often circumferential on the tube.

Corrosion control techniques vary according to the type of corrosion encountered. Major methods include maintenance of the proper pH, control of oxygen, control of deposits, and reduction of stresses trough design and operational practices.Deaeration and recently the use of membrane contractors are the best and most diffused ways to avoid corrosion removing the dissolved gasses (mainly O2 and CO2).

For further information about the different types of corrosion check the following web pages:

Galvanic corrosion Caustic corrosion Acidic corrosion Hydrogen embrittlement Oxygen attack Carbon dioxide attack

Protection of steel in a boiler system depends on temperature, pH, and oxygen content. Generally, higher temperatures, high or low pH levels and higher oxygen concentrations increase steel corrosion rates. Mechanical and operation factors such as velocities, metal stresses, and severity of service can strongly influence corrosion rates. Systems vary in corrosion tendencies and should be evaluated individually.

Characteristics of boiler feed waterWater absorbs more heat for a given temperature rise than any other common inorganic

substance. It expands 1600 times as it evaporates to form steam at atmospheric pressure. The steam is capable of carrying large quantities of heat. These unique properties of water make it an ideal raw material for heating and power generating processes.All natural waters contain varying amounts of dissolved and suspended matter and dissolved gases the amount of minerals dissolved in water varies from 30 g/l in sea water to anything from 0.005 to 1500 mg/l in fresh water supplies. Since water impurities cause boiler problems, careful consideration must be given to the quality of the water used for generating steam. The composition of boiler feed water must be such that the impurities in it can be concentrated a reasonable number of times inside the boiler, without exceeding the tolerance limits of the particular boiler design. If the feed water does not meet these requirements it must be pretreated to remove impurities. The impurities need not be completely removed in all cases, however, since chemical treatment inside the boiler can effectively and economically counteract them.

Feed-water purity is a matter both of quantity of impurities and nature of impurities: some impurities such as hardness, iron and silica are of more concern, for example, than sodium salts. The purity requirements for any feed-water depend on how much feed water is used as well as what the particular boiler design (pressure, heat transfer rate, etc.) can tolerate. Feed-water purity requirements therefore can vary widely. A low-pressure fire-tube boiler can usually tolerate high feed-water hardness with proper treatment while virtually all impurities must be removed from water used in some modern, high-pressure boilers.Only relatively wide ranges can be given as to maximum levels of alkalis, salt, silica, phosphates etc, in relation to working pressure. The actual maximum levels must be obtained fro the boiler manufacturer, who will base them on the characteristics of the boiler in question.

The following tables are extracts of recommended levels from APAVE (Association of electrical and steam unit owners), up to pressures of 100 bar for medium steaming rates and for volumes of water in the chambers sufficient to properly control the blow down rates, and from ABMA (American Boiler Manufacturers Association) in its standard guarantee of steam purity.

Working Pressure (Bar)

  0 - 20.720.8 - 31.0

31.1 - 41.4

41.5 - 51.7

51.8 - 62.1

62.2 - 68.9

69.0 - 103.4

103.5 - 137.9

Feed water

Dissolved oxygen (measured before oxygen scavenger addition)  

0.04 0.04 0.007 0.007 0.007 0.007 0.007 0.007

Total Iron mg/l 0.1 0.05 0.03 0.025 0.02 0.02 0.01 0.01

Total copper 0.05 0.025 0.02 0.02 0.015 0.015 0.01 0.01

Total hardness (CaCO3)

0.3 0.3 0.2 0.2 0.1 0.05 not detectable

Non volatile 1 1 0.5 0.5 0.5 0.2 0.2 0.2

TOC

Oily matter 1 1 0.5 0.5 0.5 0.2 0.2 0.2

pH at 25  7.5 - 10.0

7.5 - 10.0

7.5 - 10.0

7.5 - 10.0

7.5 - 10.0

8.5 - 9.5 9.0 - 9.6 9.0 - 9.6

Boiler Water                

Silica

mg/l

150 90 40 30 20 8 2 1

Total alkalinity CaCO3

350 300 250 200 150 100 not specified

Free hydroxide alkalinity CaCO3

not specified not detectable

Specific conductance at 25 without neutralization

mS/cm 3500 3000 2500 2000 1500 1000 150 100

Working Pressure (Bar)

  0 - 15 15 - 25 25 - 35 35 - 45 40 - 60 60 - 75 75 - 100

Feed water

Dissolved oxygen (measured before oxygen scavenger addition)

mg/l 0.02 (Physical removal of dissolved oxygen)

Total hardnessFrench degrees

0.5 0.3 0.2 0.1 0.05 0.05 0.05

Oily matter mg/l absence 0.05 0.05 0.05

pH   > 8.5

Total Iron

mg/l

not specified 0.05 0.05 0.03

Total copper not specified 0.03 0.03 0.01

Boiler water

M alkalinityFrench degrees

100 80 60 40 15 10 5

P alkalinity 0.07 M 0.07 M 0.07 M 0.07 M > 0.5 M > 0.5 M > 0.5 M

SiO2

mg/l

200 150 90 40 15 10 5

TDS 4000 3000 2000 1500 500 300 100

Phosphates30 to 100

31 to 100 20 to 80 21 to 80 10 to 60 10 to 40 5 to 20

pH   10.5 to 12 10 to 11

Make up water  Softened or softened and carbonate

freeDemineralized

Boiler feed waterA boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. The steam or hot fluid is then recirculated out of the boiler for use in various processes in heating applications.

The water circuit of a water boiler can be summarized by the following pictures:

The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). The make-up water is usually natural water either in its raw state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial outlay. The tendency is to reduce this percentage to a very small figure.

Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment. For more information check the boiler water treatment web page.

In the following table you can find a list of the common boiler feed water contaminants, their effect and their possible treatment.

Find extra information about the characteristics of boiler feed water.

IMPURITY RESULTING IN GOT RID OF BY COMMENTS

Soluble Gasses

Hydrogen Sulphide (H2S)Water smells like rotten eggs: Tastes bad, and is corrosive to most metals.

Aeration, Filtration, and Chlorination.

Found mainly in groundwater, and polluted streams.

Carbon Dioxide (CO2)Corrosive, forms carbonic acid in condensate.

Deaeration, neutralization with alkalis.

Filming, neutralizing amines used to prevent condensate line corrosion.

Oxygen (O2)Corrosion and pitting of boiler tubes.

Deaeration & chemical treatment with (Sodium Sulphite or Hydrazine)

Pitting of boiler tubes, and turbine blades, failure of steam lines, and fittings etc.

Suspended Solids

Sediment & TurbiditySludge and scale carryover.

Clarification and filtration.

Tolerance of approx. 5ppm max. for most applications, 10ppm for potable water.

Organic MatterCarryover, foaming, deposits can clog piping, and cause corrosion.

Clarification; filtration, and chemical treatment

Found mostly in surface waters, caused by rotting vegetation, and farm run offs. Organics break down to form organic acids. Results in low of boiler feed-water pH, which then attacks boiler tubes. Includes diatoms, molds, bacterial slimes, iron/manganese bacteria. Suspended particles collect on the surface of the water in the boiler and render difficult the liberation of steam bubbles rising to that surface.. Foaming can also be attributed to waters containing carbonates in solution in which a light flocculent precipitate will be formed on the surface of the water. It is usually traced to an excess of sodium carbonate used in treatment for some other difficulty where animal or vegetable oil finds its way into the boiler.

Dissolved Colloidal Solids

Oil & GreaseFoaming, deposits in

boilerCoagulation & filtration

Enters boiler with condensate

Hardness, Calcium (Ca), and Magnesium (Mg)

Scale deposits in boiler, inhibits heat transfer, and thermal efficiency. In severe cases can lead to boiler tube burn thru, and failure.

Softening, plus internal treatment in boiler.

Forms are bicarbonates, sulphates, chlorides, and nitrates, in that order. Some calcium salts are reversibly soluble. Magnesium reacts with carbonates to form compounds of low solubility.

Sodium, alkalinity, NaOH, NaHCO3, Na2CO3

Foaming, carbonates form carbonic acid in steam, causes condensate return line,

Deaeration of make-up water and condensate return. Ion exchange; deionization, acid

Sodium salts are found in most waters. They are very soluble, and cannot be removed by chemical

and steam trap corrosion, can cause embrittlement.

treatment of make-up water.

precipitation.

Sulphates (SO4)Hard scale if calcium is present

DeionizationTolerance limits are about 100-300ppm as CaCO3

Chlorides, (Cl)

Priming, i.e. uneven delivery of steam from the boiler (belching), carryover of water in steam lowering steam efficiency, can deposit as salts on superheaters and turbine blades. Foaming if present in large amounts.

Deionization

Priming, or the passage of steam from a boiler in "belches", is caused by the concentration sodium carbonate, sodium sulphate, or sodium chloride in solution. Sodium sulphate is found in many waters in the USA, and in waters where calcium or magnesium is precipitated with soda ash.

Iron (Fe) and Manganese (Mn)

Deposits in boiler, in large amounts can inhibit heat transfer.

Aeration, filtration, ion exchange.

Most common form is ferrous bicarbonate.

Silica (Si)Hard scale in boilers and cooling systems: turbine blade deposits.

Deionization; lime soda process, hot-lime-zeolite treatment.

Silica combines with many elements to produce silicates. Silicates form very tenacious deposits in boiler tubing. Very difficult to remove, often only by flourodic acids. Most critical consideration is volatile carryover to turbine components.

Oxygen attack in boilers

Galvanic corrosion

Caustic corrosion

Acidic corrosion

Hydrogen embrittlement

Oxygen attack

Carbon dioxide attack

Without proper mechanical and chemical deaeration, oxygen in the feed water enters the boiler. Much is flashed off with the steam; the remainder can attack boiler metal. Oxygen in water produces pitting that is very severe because of its localized nature. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper bearing alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability.

Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems: iron oxide generated by the corrosion can produce iron deposits in the boiler. Oxygen corrosion may be highly localized or may cover an extensive area. Oxygen attack is an electrochemical process that can be described by the following reactions: Anode: Fe è Fe2+ + 2e-

Cathode: ½ O2 + H2O + 2e- è 2 OH-

Overall: Fe + ½ O2 + H2O è Fe(OH)2

In this reaction a temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in a rapid and severe corrosion.

The acceptable dissolved oxygen level for any system depends on may factors, such as feed water temperature, pH, flow rate, dissolved solids content, and the metallurgy and physical condition of the system. In general, the limit value of oxygen in make up water can be stared 0.10 mg/kg

For a complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration. Membrane contractors are also a possibility

Boiler Water TreatmentWater is the essential medium for steam generation. Conditioning it properly can increase theefficiency of boiler and as well as extend the boiler’s life. Treating boiler water also insures safe and reliable operation: without proper treatment, severe problems can develop, some so severe that boiler itself can be destroyed. Boiler water problem generally falls into classes: deposit related and corrosion related. Because the two often interact, it is very common to find a boiler experiencing both simultaneously. There are many instances where deposit causes

corrosion and corrosion causes deposits. The other problem is of steam purity.Therefore the aim of the boiler water treatment is1) To prevent the formation of scales and deposits on heating surface2) To prevent corrosion in the boiler and steam system.3) To maintain high level of steam purity.The pressure and design of boiler determines the quality of water it requires for steam generation. The sequence of treatment depends on the type and concentration of the contaminants found in water supply and the desired quality of finished water to avoid three major problems in boiler systems – Deposits, Corrosion and Carryover.ScalingOne of the aims of boiler water treatment is to prevent the formation of scales and deposits in the boiler systems. Scale can be prevented by external method or by conditioning with internal treatment. At times combination of both external and internal treatment is done. Water gets evaporated due to high heat transfer rate. This concentrates the water and scale precipitates. The type of scale will depend upon the chemical composition of the concentrated water.Scales formed in boiler systems can be divided into four groups:-a) Scale due to calcium & magnesiumb) Scale due to iron oxidec) Scale due to copperd) Scale due to silicaThe combination in which they exist will not be same. It will vary from boiler to boiler. In some boiler the scale can be due to Calcium and in some due to Iron.Scale forms as the solubilities of the scale forming salts in water decreases and the temperature and concentration increases. When feed water temperature is elevated to boiler water temperature, the solubility of scale forming salt is decreased, and solid scale begins to form on the boiler system.Thus we can say that Scale formation is a function of two criteriaa) The concentration and solubility limits of the dissolved salts.b) The retrograde solubility (inversely proportional to temperature) characteristics of some salt.Causes of deposit formationBoiler deposits result from the impurities carried in with feed water. Their source is either make up water containing mineral salt, condensate containing process contaminants, corrosion products or in the case of condensers – in leaking cooling water. Deposits can also be formed due to the internal chemicals used.CorrosionCorrosion is the destructive attack of metal by chemical or electrochemical reaction. Corrosion is always because of chemical

reaction. Physical deterioration is termed as erosion, wear or galling. Deterioration can be due to both chemical and physical attack.Water corrosiveness is determined by the impurities present in it. Oxygen, dissolved solids, and dissolved acids in water attack the common construction material. Alkali can also be corrosive, at high temperature, as in boiler.Problems due to corrosion1. Thinning of metal2. Development of crack3. Pitting of metal4. Metal perforation5. Interference with heat transfer6. Contamination of waterCorrosion is a complex problem and many factors influence corrosion. The factors to be considered are physical, chemical and biological.Factors influencing corrosionPhysical Factors1. System construction2. System Pressure3. Temperature4. Flow Velocity5. Water ChemistryChemical factorsChemistry plays a very important role in corrosion. We have already explained earlier that corrosion is electrochemical reaction and is influenced by chemical factors like pH, alkalinity, dissolved salts and others.a) AlkalinityAlkalinity in water is due to presence of Bicarbonates, carbonates and hydroxyl ions. In raw water alkalinity is mainly due to bicarbonates. Some times carbonates ions may also be present.Carbonates and particularly hydroxyl ions are rarely encountered in untreated waters. Hydroxyl ions normally get introduced during treatment of water.Alkalinity is determined by using standard acid solution using methyl or phenolphthalein indicator. Alkalinity determined by using methyl orange indicator is termed as M-Alkalinity or Total Alkalinity. P-Alkalinity is determined by using phenolphthalein as indicator. The different type of alkalinity present in water supplies can be calculated from M and P-Alkalinity value determined by titration. Alkalinity is the ability of natural water to neutralize acid. This happens because of buffering mechanism. Alkalinity in raw water is primarily composed of bicarbonates and carbonates. Acid compounds having free H+ ions react with CO3 and HCO3 ions and conversely OH ions also reacts with CO3 and HCO3 ions.CO3

2- + H+ → HCO3

-

HCO3

- + H+ →H2CO3

HCO3

- + OH- → CO3

2- + H2OIn either case acid or base is neutralized by the carbonate or bicarbonate. Thus it can be seen that when Acid (or caustic ) is added to water having high concentration of bicarbonate or carbonate the pH of water does not change much compared to when the same amount of acid(or caustic) is added to pure water. This is known as buffering capacity.b) pHWhen pure water dissociates, the number of hydrogen ions is equal to number of hydroxyl ions. Such a solution is called neutral solution . pH is defined as negative logarithm of H+ ions.Solution having pH less than 7 are acidic and those greater than 7 are basic. Low pH is Corrosive and high pH is protective to pipe. Very high can cause scaling and deposits.c) Dissolved OxygenOxygen is considered has one of the most corrosive components in water chemistry. Dissolved O2 with traces of chlorides or solids can cause pitting corrosion of metallic surface. The resulting condition may be severe, even at low pressure.d) Dissolved SolidsDissolved solids or salt content of water present as ion increases electrical conductivity of water.Higher the conductivity, greater the potential for corrosion. Some salts like CaCO3 are involved in scale forming and thereby reducing corrosion.e) HardnessHardness is generally associated with scale forming. Hardness is composed primarily of Ca & Mg ions but may also include other metallic ions like iron and manganese. All hardness ions have the property of forming scales. One of the methods for corrosion control is by planned deposition of CaCO3

f) Chloride & SulphateChloride and sulphate ions inhibit the formation of scale by keeping hardness ions in solution. Trace amount of Chlorides even with dissolved oxygen can cause corrosion in boiler.Type Of CorrosionThe type of corrosion classified with respect to outward appearance or altered physical properties areUniform attackPittingCavitation erosionDezincification & parting

Intergrannular corrosionCrackingOxygen CorrosionWater coming out of deaerators has residual oxygen. As explained earlier even a trace amount of oxygen can cause corrosion. This last trace of oxygen is removed chemically. Sodium sulphite and hydrazine or one of its product is used for removal of residualoxygen. Sodium sulphite is used for low pressure boiler. Amine is preferred in high pressure boiler because it does not add to TDS, unlike Sodium sulphite.Effect of pHBoth high and low pH can cause corrosion in boiler. In acidic range the protective layer of magnetite is not able to form and it cause corrosion. In very high pH range the protective layer of magnetite breaks down and this leads to caustic corrosion. For corrosion prevention maintaining proper pH and alkalinity is very important.Acid CorrosionExcess acid cause damage at more rapid rate than excess base. Simply because this happens, it should not be taken as an operating guideline. Magnetite film forms due to corrosion but once formed adhere tightly and acts as a barrier between boiler water and steel. Acids are capable of destroying this film and hence water chemistry must be so maintained that the protective film is not disrupted. This can be done by keeping the water in alkaline range.Caustic Corrosion.Feed water is maintained at alkaline pH. Alkali is added to provide optimum pH in the feed water to prevent corrosion of piping and equipment. Caustic soda (sodium hydroxide) is generally added for this purpose. Sometimes sodium carbonate is also added. Even though caustic soda is added with control, there are occasions when pH increases and cause corrosion as shown by the equation below. The damaged caused by excess alkali is because it dissolves the magnetite film forming sodium hypo ferrite and sodium ferrite both of which are soluble in hot concentrated caustic soda. In addition concentrated reacts directly and more rapidly with iron to form hydrogen and sodiumferrorate.Fe3O4 + 4NaOH → 2 NaFeO2 + Na2FeO2 +H2OFe + 2NaOH → Na2FeO2

Caustic attack on boiler can two forms - Gouging or cracking. Caustic cracking is also known as caustic embrittlement.Caustic gouging causes deep elliptical depression in boiler metal surface. This occur in areas of high heat flux or under heavy porous deposits. Underneath these deposits , boiler water can concentrate to the point where high concentrate of caustic can accumulate causing a localized corrosion. This action can be rapid. Boiler water chemistry if properly maintained will prevent caustic gouging. Caustic

embrittlement or cracking is a form of stress corrosion. Cracks occur rapidly and are often undetectable leading to sudden failure of boiler –at times causing a violent failure. All parts of boiler are subjected to this type of corrosion. The only way to stop this type of corrosion is to prevent high concentration from forming.Caustic corrosion is generally confined to :a) Water cooled tubes in region of high heat flux.b) Slanted and horizontal tubes.c) Location beneath heavy deposits.d) Heat transfer region at or adjacent to packing rings.Caustic corrosion is prevented by coordinate caustic program. Phosphate ions act as a buffer ion. It does not allow pH to increase in water, no matter how concentrated OH ions become. Buffer ions are also useful in avoiding similar high OH concentration which leads to stress corrosion cracking (caustic embrittlement). In low pressure boiler sodium nitrate is added in a definite ratio to caustic alkalinity to prevent caustic embrittlement.Galvanic corrosionWe have already explained what galvanic corrosion is. A metal or alloy if it is electrically coupled, galvanic corrosion occurs. Corrosion by copper is the most common form of galvanic corrosion in boiler system. Copper can be carried from pre boiler section. Water deposits copper as decomposition of bicarbonates or as ammonia complexes. Pitting of boiler tubes has been reported due to copper deposit.Iron Oxide depositsIn boiler the steel reacts with water in absence of oxygen to form a magnetite film. This film than acts as a protective layer for further corrosion. Iron oxide also enters with feed water into boiler as corrosion product. This layer is very porous and can be easily penetrated. This allows boiler water to seep through and flash into steam leaving behind dissolved solids which concentrates in localized areas. This excessive concentration can lead to metal dissolution and metal failure.Condensate corrosionSteam generated in boiler is transported to point of use through pipes. Steam condensate is also returned to boiler feed water. Corrosion of steam lines and condensate return line occurs because of the low pH. The chief source of acid in steam is carbon di oxide( CO2). High temperature and pressure decomposes alkalinity ( Bicarbonate –HCO3

)to carbon di oxide, some of which dissolves in steam making it acidic ( Carbonic Acid H2CO3 ).HCO3

- ↔ HO + CO2

HO + CO2 + H2O ( Steam ) → H2CO3

This lowers the condensate pH and leads to corrosion of return lines. Oxygen can enter a

condensate system from other sources even if the deaerator is functioning properly. Oxygen causes a deep pitting of condensate lines. High Velocity and low pH can result in extremely severe corrosion conditions. The best way to minimize this is by keeping the pH above 9.0 Other gases which can be corrosive in the condensate system are Ammonia, Hydrogen sulphide and sulphur di oxide.Impure steam can create problems of carryover, priming and foaming in boiler. Steam gets contaminated because of the boiler water it carries with it or because of salt and silica which are, soluble in steam at high pressure. Solids carried over with steam can get deposited on super heater and turbine. Carryover can also effect the product quality.CarryoverCarryover is defined as contamination of steam with droplets of boiler water. Carry over can be due to entrainment of water drops in steam or due to property of certain salt like silica in boiler water to get vaporized and get into steam.The factors responsible for carry over area) Amount of dissolved solids in boiler water.b) Chemical nature of dissolve solids.c) Suspended solids in boiler waterd) Boiler designe) Boiler operating conditionMany factors, both mechanical and chemical contribute to carryover.7.2.1 Mechanical CausesBoiler design & operating conditions plays an important role in carryover. Without going into details we can say that major design & operating factors responsible for carryover are :a) Design pressureb) Steam drum sizec) Design generating rated) Circulation rate.e) Arrangement of down comers and risersf) Type of mechanical separating equipment.For example even when the TDS is within the limit, carryover can still occur because of change in operating condition. For example sudden increase in steam demand may lower the steam header pressure. This reduces drum pressure and water in the drum gets mixed with steam bubbles and the level rises. The rise in the drum level can cause carryover.Chemical CausesPriming and foaming are two terms used with carryover. Priming is the surging of water in the steam outlet and is caused by factors like high water level in boiler ,steaming rate, load fluctuations and boiler design. Priming is thus due to mechanical factors.Foaming is formation of stable bubbles. The bubbles don’t break because of high surface tension .

Causes of foaming are : a. High Alkalinity : Caustic soda (NaOH) or sodium carbonate (Na2CO3) havegreater influence on foaming than neutral salts.b. High TDS : High TDS causes carryover. For a given boiler design and a given set of operating condition There is a limiting dissolved solid content above which a serious steam contamination occurs. Reducing blow down by small amounts every few days and measuring steam purity this limiting TDS value can be found out. This value is found by keeping boiler operating conditions and other operating variables such as feed water composition and treatment constant. If a graph is plotted between conductivity of condensed steam & boiler water TDS the limiting figure is that corresponding to slightly less than where steam quality deteriorates.c. .Suspended solids also cause foamingd. Oil is not present n boiler water. It can enter boiler system through leaks in condenser orother heat exchanger. Oil can also system because of lubrication of steam driven reciprocating equipment. Oil is undesirable in boiler for two reasona) It acts as binder to form scale, b) It also causes foaming . Even a very small amount can cause severe foaming and hence immediate action should be taken for complete removal of oil.e. Silica is another chemical which causes carryover. This is dealt separately.

SilicaAll natural water contains silica. Like calcium and magnesium silica also forms scale. Removing silica from water is more difficult than removing Hardness (calcium and magnesium ). At high temperature Silica volatizes and gets carried into steam and forms hard coating on turbine blades. Silica can form various kind of scale such as amorphous silica or magnesium Silicate. Amorphous silica scale forms like a glassy deposit which is very difficult to remove. Hydrofluoric acid is used to remove such scales. Silica scale is generally found in low pressure boiler because only softener is used and softener does not remove silica. Silica is not considered to be a big problem in low pressure boiler.

MANUAL BLOWDOWN

Intermittent manual blowdown is designed to remove suspended solids, including any sludge formed in the boiler water. The manual blowdown take-off is usually located in the bottom of the lowest boiler drum, where any sludge formed would tend to settle.

Properly controlled intermittent manual blowdown removes suspended solids, allowing satisfactory boiler operation. Most industrial boiler systems contain both a manual intermittent blowdown and a continuous blowdown system. In practice, the manual blowdown valves are opened periodically in accordance with an operating schedule. To optimize suspended solids removal and operating economy, frequent short blows are preferred to infrequent lengthy blows. Very little sludge is formed in systems using boiler feedwater of exceptionally high quality. The manual blowdown can be less frequent in these systems than in those using feedwater that is contaminated with hardness or iron. The water treatment consultant can recommend an appropriate manual blowdown schedule.

Blowdown valves on the water wall headers of a boiler should be operated in strict accordance with the manufacturer's recommendations. Usually, due to possible circulation problems, water wall headers are not blown down while the unit is steaming. Blowdown normally takes place when the unit is taken out of service or banked. The water level should be watched closely during periods of manual blowdown.

CONTINUOUS BLOWDOWN

Continuous blowdown, as the term implies, is the continuous removal of water from the boiler. It offers many advantages not provided by the use of bottom blowdown alone. For instance, water may be removed from the location of the highest dissolved solids in the boiler water. As a result, proper boiler water quality can be maintained at all times. Also, a maximum of dissolved solids may be removed with minimal loss of water and heat from the boiler.

Another major benefit of continuous blowdown is the recovery of a large amount of its heat content through the use of blowdown flash tanks and heat exchangers. Control valve settings must be adjusted regularly to increase or decrease the blowdown according to control test results and to maintain close control of boiler water concentrations at all times.

When continuous blowdown is used, manual blowdown is usually limited to approximately one short blow per shift to remove suspended solids which may have settled out near the manual blowdown connection.