3_blowdown_alkalinity_af.pdf

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BLOW DOWN

PURPOSE: To control the concentration of one or more natural

components of boiler water.

* * * * * * * * * * * * *

Boiler feed water usually contains dissolved or suspended materials introduced with raw or treated

make-up water or picked up by condensate returns. These solids concentrate as evaporation proceeds

in the boiler. Unless something is done to keep the boiler water solids from concentrating excessively,

serious difficulties may result.

The boiler water may take on a foaming character which causes water to be entrained in the out going

steam; this is the most immediate effect of excessive solids concentrations. Sludge may build up

abnormally to such an extent that it interferes with the heat transfer, hampers circulation, or necessitates

an undue amount of boiler cleaning. Boiler water silica, if allowed to concentrate without restraint, may

participate in formation of siliceous scale, or it may leave the boiler as an entrained or volatilized

contaminant in the steam, especially in high pressure boilers.

Blow down represents the most direct - - often the only-- means for effectively controlling the

concentration of solids in boiler water. When properly controlled, blow down removes solids in the

concentrated boiler water solution at exactly the same rate with which they are introduced. Blow

down is regulated to avoid exceeding some specified maximum concentration of either

total dissolved solids, suspended solids,

iron silica or

alkalinity.

Although the permissible maximum concentrations are lower with increase in boiler pressures, there

actually is no simple rule for judging what the limiting concentrations should be in any given boiler. Itdepends upon steam pressure, steam temperature (whether saturated or super heated), the type of

boiler involved, steam purification facilities in the steam drum, method of blow down (whether manual

or continuous), water level control, the nature of fluctuations in boiler load, the purposes for which

steam is used, and the kind of turbines involved, if any -- plus, of course, the kinds and the amounts of

impurities in the boiler feed water.


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As a guide for blow down control in your particular boilers, maximum and minimum concentrations

have been recommended for one or more components of the boiler water. You may be controlling

blow down directly on the basis of the test for boiler water alkalinity, silica, or solids (dissolved,

suspended, or total). Or you may be testing for some reference constituent such as chloride, sulfate, or

total electrolytes (conductivity); by regulating blow down so as to keep the reference constituent within

some specified concentration range, you automatically maintain proper control over other constituentsin the boiler water (dissolved solids, suspended solids, total solids, or silica, whichever the case may

be.

In any case, the blow down rate must be increased whenever the maximum limiting concentration is

approached or exceeded. This may sometimes call for emergency manual blow downs, for operating

with concentrations that are to high can be seriously detrimental to boiler and plant performance.

Minimum limiting concentrations are specified to avoid undue wastage of water, heat and treating

chemicals through blow down. In general, the range of permissible concentrations is fixed at the highest

level considered to be safe for the particular boilers involved.

Antifoam materials are sometimes used to allow an increase towards the tolerance for higher solids

concentrations. This allows a reduction in blow down losses. Antifoam treatment is economically

justified for this purpose if the cost of such treatment does not off set the resulting savings in blow

down. The risk of serious consequences, however, can not too great if there is a sudden and

unavoidable interruption in antifoam feed.

Blow down losses can be reduced, also, by the use of pre-treating equipment to remove raw water

constituents which otherwise would have to be removed through boiler blow down. This is a very

important economic consideration in the selection of pre-treatment equipment.

Good blow down control is one of the most important requirements

in boiler water conditioning.

If blow down is not regulated properly, it will be almost impossible to maintain satisfactory control over

the chemical treatment program. Blow down has a direct influence over the concentrations of all

constituents in the boiler water. Take particular pains to stay within the limiting concentrations specified

for blow down.

A continuous blow down valve is the preferred method for the control of boiler water component build

up. This will gradually remove boiler water on a continuous basis, thus preventing the radical change in

boiler water chemistry as seen during excessive manual bottom blow downs. Manual bottom (sludge)

blow downs are still required to remove the heavy sludge, even with continuous blow down systems.

Boiler makers usually provide instructions covering manual blow down from drums and headers.


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Heavy manual blow downs should be avoided where practical. Mud drum connections should be

"tried" at least every 24 hours. It is generally poor practice to blow down water wall headers while the

boiler is in full load service.

Continuous blow down represents the most effective means for controlling boiler water solids

concentrations. It:

Provides ideal boiler water samples for testing and control purposes.

Permits close regulation of blow down without wide fluctuations in

concentration.

Makes possible the use of blow down heat recovery equipment--an

important economic factor in many plants.


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ALKALINITY CONTROL

PURPOSE: To aid in formation of desirable sludge and to maintain a protective

coating of iron oxide on steel surfaces.

* * * * * * * * * * * * *

The total alkalinity of boiler water includes all dissolved components that are capable of reacting with

acid: These are most commonly hydroxide (OH),carbonate (CO), bicarbonate (HCO), phosphate

(PO), and silicate (Sio). Among these alkaline components, the hydroxide is most important from a

chemical control standpoint; it is measured directly by pH determination or by titration.

Boiler water must contain a certain amount of caustic (i.e. Hydroxide alkalinity in order to accomplish

three things:

1. To maintain a protective coating of iron oxide over the

metal and thereby prevent certain types of corrosion.

2. To provide a proper environment for precipitation of

desirable sludge materials.

3. To keep silica in solution so as to avoid deposition of siliceous

scales.

Excessively high concentrations of hydroxide alkalinity may result in foaming difficulties or caustic

corrosion of the boiler steel. Caustic alkalinity may also cause caustic embrittlement of boiler steel

under some conditions. Alkalinity control is correct when the boiler water contains enough hydroxide

to secure the desired benefits without inviting trouble associated with aggressively high concentrationsof caustic. The P alkalinity test provides a sound basis for control of the alkalinity in boiler water. pH

determination is also widely used for this purpose where a low level alkalinity is to be maintained.

Hydroxide alkalinity in boiler water may come from a variety of sources. It may be added directly in

the form of caustic soda (sodium hydroxide). When soda ash is fed as a water conditioning chemical, it


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decomposes under boiler steaming conditions, producing sodium hydroxide (plus carbon dioxide which

goes off with the steam).

Sodium hydroxide and/or sodium carbonate may be present as excess treating chemicals in the effluent

from a lime-soda softener; the use of such pre-treating equipment automatically provides the alkali

required for alkalinity control in the boiler water. Most raw waters contain bicarbonate (HCO). So do

effluent waters from sodium cycle and from most sodium-acid cycle zeolite softeners. Under boilerwater conditions, bicarbonate decomposes first to carbonate and then to hydroxide. In all cases it is

the boiler water alkalinity, as measured by the p alkalinity test or pH value, that concerns us, regardless

of whether this hydroxide is added as such or represents the decomposition product of soda ash or

alkaline components in raw water or treated makeup water.

Maximum and minimum P alkalinity values (and/or pH) limits are specified for proper control of boiler

water alkalinity in your plant. Alkalinity concentrations below the specified minimum may result in

corrosion difficulties or the formation of objectionable sludge or scale. Whenever boiler water

alkalinities fall below the recommended minimum. do not hesitate to increase the feed of alkali (caustic

soda or soda ash) or to increase the total alkalinity of the treated water if a softener is involved.Alkalinities above the specified maximum may result in foam-type carry over or caustic corrosion, and

they may complicate protective measures employed to prevent caustic embrittlement. When boiler

water alkalinities rise above the recommended maximum, reduce the feed of alkali (caustic soda or

soda ash) or reduce the total alkalinity of the treated water if a softener is used. In an emergency,

increase the blow down rate if necessary to bring boiler water alkalinity under control.

Sodium hydroxide is the preferred approach, as the break down components do not form carbon

dioxide. Carbon dioxide will carry off the steam and form carbonic acid in the condensate return lines.

For this reason, sodium carbonate is not generally used in todays boilers. It is never recommended for

use by good water treatment consultants.


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ANTIFOAM MATERIALS

Purpose: To minimize the amount of solids carried by the steam

* * * * * * * * * * * * *

Many different factors - some chemical, some mechanical -- entrained boiler water in the steam leavingthe boiler. "Foaming" is a general term applied to carryover, which is caused by objectionable

concentrations of certain components of the boiler water. Foam aggravating substances such as

excessive concentration of mineral solids, high alkalinity, or some types of organics) steam bubbles do

not allow steam bubbles to collapse or coalesce as rapidly as they should. They rather collect as a

relatively stable layer of foam on the surface of the water. When this foam layer becomes thick enough

to invade the active steam separation section of the boiler, masses of foam are sucked off with the

steam and carryover results.

Antifoam agents (polyalkylene glycol) are used specifically to speed up the coalescence of steam

bubbles and thereby minimize the tendency for foam to accumulate in any part of the boiler.Antifoamtreatment is often effective in preventing carryover caused by uncontrollable feed water contamination

or by high concentrations of boiler water solids, which for physical or economic reasons, can not be

adequately controlled by normal blowdown.

Even when no foaming problem exists, the cost of antifoam treatment can be justified because of

savings resulting from reduced blowdown and the maintenance of higher boiler water solids. Under

certain operating conditions, some types of boilers may have a tendency to allow steam bubbles to

collect in downcomer tubes and thereby cancel part of the hydrostatic head required for proper

circulation of water through the boiler. Antifoam treatment may improve circulation in such cases.

An antifoam may be beneficial by producing a higher quality boiler water that is ideal for uniform

steaming and by encouraging steam bubbles to coalesce with the greatest possible speed. It also can

minimize the carryover effects of mechanical or operating factors for which there is no immediately

practical or economic remedy. Of course, the best approach to problems of this nature is through

physical changes, which correct the difficulty at its source.


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Whenever possible, antifoam should be fed continuously, although shot feeding is permissible if

additions are made at frequent intervals. Antifoam can be fed in solution along with other chemicals

used for internal treatment of boiler water. Avoid over-feed of antifoam in that gross over-treatment

may actually cause boiler water to foam.

Another consideration is that starting with a full boiler antifoam treatment program to boilers that

contain foam may result in low water because of to the sudden coalescence of steam bubbles in certain

banks of tubes. When the application is to be intermittent, feed about one-fourth of the required

amount durng the first hour. Follow this with one-third the second hour, one-half the third hour,

three-fourths the fourth hour and the full treatment requirement for each hour there after. Unlike most

other water conditioning chemicals, antifoam loses its effectiveness rather rapidly under boiler steaming

conditions, therefore, feed antifoam continuously or in shots at frequent intervals. Take special

precautions to assure that the antifoam treatment is notinterrupted once it gets under way.

3_blowdown_alkalinity_af.pdf

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