Steam Generating System - Operator Manual

8/19/2019 Steam Generating System - Operator Manual

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Steam Boiler - Training Workshop for Operators & Technicians

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Steam Generating SystemIn industrial heating, steam is being used most extensively, from process heating topower generation. When it comes to heat transfer, nothing can match steam.The steam is generated by evaporating water with the heat available,

either from combustion of fuel or some other source like waste heatfrom an exothermic chemical process, etc. Heat generated by NuclearFission is also used these days. The equipment used for generating

steam is called a Boiler.Does that mean that a kitchen bowl, or a tea kettle, which are usedto boil water to steam is also a boiler?

Boiler?

By definition, Boiler is a closed vessel, which generates steam under pressure. We

know that being a closed vessel, generating steam under pressure is somethingcritical, which makes a boiler different than a tea kettle above. In industry, for variousprocesses, heating is done at various temperatures and not just at 100oC. We know

from the property of steam that the temperature of steam varies with the pressureunder which the steam is being generated. Again for power generation, one needspressurized steam to expand in the turbine.A boiler’s primary job is to produce steam at required pressure and temperature. Since itis generating steam under pressure, it is also potentially dangerous equipment. As mostof the boiler use the heat produced by combustion of fuel to generate steam, we can say

that In order to achieve optimized Boiler performance, one need to ensure maximumextraction of heat from the fuel and then transfer maximum extracted heat to waterand steam without causing any accident.

The primary requirement of an industrial boiler is to generate steam in large quantities.To produce large quantity of steam, one should transfer large quantity of heat intowater. To achieve that one would also need more surface area through which heat is to

be transferred from heat source (usually hot flue gas from combustion of fuel) to water.It is primarily this need of accommodating more and more heat transfer area in a limitedspace that has initiated the design advancement of a boiler. Based on the heat transfermode, boilers are classified as two types. One is the Fire tube Boiler and the other is

the Water tube Boiler.

Fire Tube Boiler

In a Fire Tube Boiler, the hot flue gaspasses through the tubes and water

remain outside the tube. Heat istransferred from inside the tube to wateroutside the tube. Fire tube boiler isusually preferred where the steampressure required is less than about 30

kg/cm2(g) and steaming rate is lesser

than 30 t/hr. Above this limits, water tubeboilers are more economical.


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In a Water Tube Boiler, water circulates

through tubes and heat source is outside.Heat is transferred from outside the tubeto water inside the tube. Where large

quantity of steam is required, at highpressure – Water Tube type Boiler ispreferred. In power plants, normally, high

pressure water-tube type boilers are used,where capacity rages from 30 to 650 t/hr,having pressure & temperature up to 160kg/ cm2(g) and 540 0C respectively.

Flexibility in design is more but requiresstringent water quality control since waterside cleaning is a complicated and time

taking process.

Water Tube Boiler

If we take a complete steam generating system and break it into various sub systems,

we can have a better understanding of the overall system. For this purpose let us take asteam generating system of a Power Plant, which is comprised of most of the sub-systems.

Let us follow the water route while it is being converted into steam while passingthrough various components of a steam generating system and thereby go thoughvarious sub-systems.

Feed Water System: Properly treated Feed Water first comes to equipment known asDeaerator, where dissolved Oxygen gets removed. Feed water is added as make up

water to the condensate, which is being circulated back to the system. Deaeration isdone by heating up the water by auxiliary steam. This is part of the sensible heatreceived in the cycle, which we can compare with a temperature enthalpy curve.


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From the Deaerator, the feed watergoes to the Boiler Feed Pump,

which pumps the feed water at highpressure into the Evaporator(Circulating water & steam

system). But at this point thetemperature of the water is muchless than saturation temperature.

Therefore a lot of sensible energy isstill required by the water before itreaches the saturation temperatureand evaporation would start.

So it is first taken inside the flue gas path where the gas is about to be released in theatmosphere through chimney at a high temperature (and therefore having a lot of heat

energy) to increase the feed water temperature in an Economizer. Now the heated feedwater enters the Evaporator through steam drum, where water will be circulated whilegetting evaporated. The water circulates usually due to thermo-siphon action and known

as natural circulation. This can be understood from the figure below.

Heat from the hot flue gas is received by the riser,where steaming takes place and therefore riser

contains mixture of water and steam, whereas thedown comer contains denser water. Due to thedensity difference in the two columns, circulationtakes place in the evaporator. In some evaporatorsystem, where the geometry does not allow the

natural circulation, or where the density differenceis less due to high pressure (at 175 bar pressureand above, steam and water density difference isnot sufficient to induce natural circulation), the

circulation is maintained with a pump, which iscalled forced or assisted circulation.

In the Evaporator section, the water receives theremaining amount of sensible heat and total latent

heat of evaporation and finally steam leaves theEvaporator from the steam drum and enters the(Steam system) Superheater section.

It will not be incorrect if we say that maximum heat input is taking place in theEvaporator section since it is getting the latent heat, unless, of course the pressure is

very high.

The steam should be completely dry and saturated before it enters superheater,otherwise, instead of raising the temperature of steam beyond saturation temperature inthe superheater, remaining water would first get converted into steam and then only thetemperature would rise. A number of water separators are used in the steam drum to

ensure that.

In the superheater, the temperature of steam is raised by further heating. The finaltemperature is usually controlled by water spraying through attemperator, which is

placed before the final superheater.The superheated steam then goes to the turbine, which expands in the turbine sectionand rotates the turbine blade. In some power plants, where multiple pressure turbinesection is available, the expanded team, which is at lower pressure and temperature, isfurther brought back to the boiler section. This part is known as Reheater and steam

gets further superheated in this section before it enters the MP or LP stage of theturbine. From the final turbine stage, the exhaust steam (which is partially wet now)goes to the Condenser, where the latent heat is removed by cooling water circulationand the condensed water (condensate) is circulated back (Condensate System) to the

Deaerator with the help of condensate extraction pump.


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It would be interesting to know that the steam enters the condenser usually attemperature of 40 to 45oC (cold steam). This is because the condenser operates under

vacuum to extract maximum work output from the turbine. If we refer the steam table,we can confirm this fact.

When we are discussing fired boilers, the heat source is combustion of fuel. The fuel canbe of solid, liquid or gaseous type and the combustion takes place in an enclosed sectionknown as the furnace. The first thing the furnace would need is the fuel, which has to

be brought from the place it is stored (Fuel system, which is not shown here). Before itenters the furnace, the fuel might require some preparation. For example, in Pulverizedcoal fired boiler (most commonly used for large thermal power plant), coal has to beground into fine talcum powder consistency and therefore crusher and pulverizing mills

are used. For liquid fuel system, the fuel should be atomized (broken into fine particlesto increase surface area), which is done by steam or air atomizer. Some liquid fuelsrequire certain temperature to retain proper viscosity and therefore suitable heaterwould be there. For gaseous fuel, however, such preparations are not required and onlythe pressure is to be maintained.In the furnace, fuel requires air and ignition temperature for combustion. The air is one

thing, which is available in the nature for free. However, it is still required to be broughtinside the furnace. Once combustion takes place, the furnace gets filled with the productof combustion, i.e. flue gas and this flue gas again is required to be taken out of the

system to allow entry of further air. The system which handles the air and flue gas iscalled Draught System.The draught system is generally of three types:Forced Draught: Where the air is being pushed into the furnace with the help of a fan

known as FD fan, which is located before the furnace. It pushed the air, which in turnpushes the flue gas inside the furnace till it is taken out of the system through thechimney.Induced Draught: Where the flue gas from the furnace is sucked and taken out of the

system through the chimney. The fan used in this system is called ID fan and it islocated just before the chimney, after the Economizer and Air pre-heater in the flue gassystem. Vacuum created by the ID fan sucks in Atmospheric air into the furnace for the

combustion.Balanced Draught: Both FD and ID fans are used in this system. In this system, thefurnace pressure is maintained a little on the negative side.There is of course the Natural Draught system, which uses the chimney height and

higher temperature of the flue gas getting released from the chimney, to create therequired draught pressure. But due to the high temperature of flue gas exiting from thechimney, heat loss is staggering.In the system schematic shown here, we are using a Balanced Draught, where the air ispushed inside the furnace by the FD fan. But before it goes to the furnace, the air is first

taken inside the flue gas path through an Air Pre-heater, where the air is heated byusing the residual heat of the flue gas remaining after it passes the economizer. Theheated combustion air increases the combustion efficiency and also further reduces the

exit gas temperature. After Air Pre-heater, heated air is taken inside the furnace.Inside the furnace, after combustion takes place, tremendous amount of heat is beingreleased. The schematic diagram may not represent it properly, but some of the

superheater and reheater sections are kept in the furnace to take advantage of the

radiation heat transfer. The hot flue gas leaves the furnace and passes through varioussections of Superheater, Reheater, Economizer and Air Pre-heater, while gradually

getting cooled.After the preheater, the exhaust flue gas is taken out of the system by the ID fan anddischarged into the atmosphere through a chimney.

Boilers and IBR:

As per the Indian Boiler Act & Regulations (IBR), a boiler may be defined as a

closed vessel with a capacity of 22.75 liters or more in which water is converted intosteam for external use, under pressurized condition. The term ‘boiler’ includes all such


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mountings fitted to the vessel, which remain wholly or partly under pressure when steam isshut off.

IBR is an Indian regulatory code governing the construction and operation of Boilers to

ensure safety for prevention of loss of lives and properties. However this code is no waybears any guidelines for efficiency, MCR and other operational parameters. Central BoilerBoard is a central authority, which formulates and amends the regulations governing

boilers to keep pace with technological advancement and oversees that the stipulated rulesframed by the different State Governments and regulations under IBR are strictly followedby all concerned authorities. The Indian Boilers Act, 1923 is enacted by the parliament andadministered by the all State Governments through Chief Inspector of Boilers, as the subject

"Boiler" comes under the concurrent list of the constitution of India. The StateLevel Regulatory Bodies under the Chief Inspector of Boilers, in every state, are solelyresponsible for implementation of various regulations of the Indian Boiler Regulation(IBR) concerning all types of boilers for their manufacture, erection, operation and

maintenance. During manufacture and erection the CIB inspects various stages and finallyissues manufacturing certificates in statutory formats and if any owner of a boiler intends torun it, he has to apply to the CIB with manufacturing certificates for registration of the

same. For boiler under use, CIB will inspect it once in a year in normal condition. However,in emergency the frequency of inspections may increase.

The State Directorate of Chief Inspector of Boilers also governs the quality andcapability of personnel handling jobs of manufacturing, maintenance and operating.

Welders deployed in manufacture and maintenance and repairs of pressure vessels arealso covered under such rules. The state Chief Inspector of Boilers is authorized toconduct regular examinations for testing and certifying candidates working on boiler or apressure vessel.

Terminology used in Boilers:

A boiler consists of several components and various parts. Some of them are termed asmountings and accessories and the others are called auxiliary equipment, depending on

the method of mounting or the fittings or the associated functional equipment essentiallyrequired for the boiler operation. Each of the components of a boiler is designated byspecified terminology. A few of such terminology, which are commonly used, are broughtout hereunder.

Classification of Boilers:

Boilers can be classified on the basis of:

i. Direction of heat transfer in the tubes (Firetube & Watertube boilers),

ii. Mode of circulation of working fluid: i.e., Natural Circulation or Forced Circulation,

iii.Type of fuel: (Solid Fuel e.g., Coal, Lignite and Peat; Gaseous Fuel e.g., Natural Gas,Blast Furnace Gas, Water Gas etc.; Liquid Fuel e.g., Diesel Oil, Furnace Oil, LSHS,Crude, Black Liquor etc.),

iv. Mode of Firing: Stoker, Pulverized, Fluidized Bed etc. in case of Solid Fuel FiredBoilers, Steam Atomized or Air Atomized in case of Liquid Fuel Fired.

v. Fired and Non-fired,

vi. Position of the furnace (Externally fired furnace & Internally fired furnace),

vii.Manufacturer’s trade name,

In Natural circulation type of boilers, circulation of water in the boiler takes place dueto natural convection currents produced by the application of heat. Saturated water flowsdown the unheated down comer and receives heat in the riser whereupon a part of it

gets converted into steam. The difference in densities of saturated water in the downcomer and the steam-water mixer in the riser brings about natural circulation. It isapplicable to all those boilers, which are operating at a pressure less than criticalpressure.


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Natural circulation steam generator

In a Forced circulation, the working fluid is forced through the boiler circuits by an

external pump (Re-circulating Pump).

Forced Circulation Steam Generator

In a natural circulation steam generator, the density difference between the water-steammixture in the riser tubes and the water in the unheated down comers is ever decreasingwith rising pressures. The limit for the maximum possible pressure stage for a natural

circulation steam generator is 175 bar. Moreover, the arrangement of the down comers

and risers in a natural circulation steam generator must be clearly structured in theirgeometry in order to guarantee circulation. This condition cannot always be met inpractice

The re-circulating pump works at a pressure of about 2 - 4 bar in order to overcome thepressure losses in the boiler system and forces the water circulation in the steamgenerator independently of the tube geometry of the down comers and the risers.

The once-through forced flow steam generator design does not include a drum andconsists of tube runs which form the economizer, evaporator and super heater. Thewater is forced through the steam generator in one flow once through by feed pumppressure and, in the course of this, is transformed into hot steam.


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steam generator

HP-turbine

feed-

water

pump economizer evaporator superheater

live steam

reheater

to the IP-

turbine

Once-through forced flow steam generator

Various Types of Boilers:

Fire Tube Boilers:

A fire tube boilers, is the boiler wherein the products of combustion pass through insideof the tubes (either one or several) and water which is to be converted in to steam is

made to surround outside these tubes. Fire Tube Boilers are used where the steampressure is normally low and the steam is not generally required to be the superheated.Fire tube boilers are compact and can be easily manufactured in a factory and assembled

as a packaged boiler. Fire tube boilers cannot be manufactured in large sizes beyond

certain limit due to large size of shell involved.Fire tube boilers have the advantage of low manufacturing and operating cost.

A few of the well-known Boilers, which come under this class of fire tube construction,are:

a) Cornish Boiler,

b) Lancashire Boiler,

c) Simple vertical boiler,

d) Cochran boiler,

e) Locomotive Boiler and

f) Scotch Marine Boiler.

The advantages of fire-tube boilers are:

a) Investment Cost of fire-tube boilers is comparatively lower than water tube Boilers

for the similar capacity,

b) The design of fire-tube boiler is simple and construction is rugged,

c) The water holding capacity of a fire-tube boiler is larger than that of water tube

boilers and fire tube boiler is versatile to meet the steam load changes.

The disadvantages of fire-tube boilers are:

a) Since fire tube boilers have huge capacity for water storage, with big wateraccumulators, its steaming capacity is reduced,

b) In order to resist the internal pressure, non-cylindrical sections and flat surfacesare required to be stayed and

c) High pressure and large diameter fire-tube boilers would need thick shell plates,

which will be uneconomical and impractical.

Water Tube Boilers:

In this type of construction of Boilers the fuel is fired in a confined chamber and thewater is circulated through divided flow path inside a number of small-bore tubes, whichare exposed to the heat generated inside the combustion chamber.

A few of the well known Boilers, which come under this class of water tube construction,are:

a) Babcock and Wilcox Boiler,

b) Stirling Boiler,

c) La-Mont Boiler,

d) Benson Boiler,

e) Yarrow Boiler, and


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f) Loeffler Boiler

Water Tube Packaged Boiler:

Package water tube boilers are as popular as packaged fire tube boilers. Such boilers arecompact and standardized for pressure and capacity. They are shop assembled having afurnace accommodated with water walls. Generating tubes, superheater and economiserready for transportation by road or sea. They are equipped with firing equipments, feed

pumps, auxiliaries and ancillaries, oil pumps, oil heaters, draught fans, feed waterregulator, soot blowers and automatic control for efficient performance.

In such boilers, coal, oil or gas could be burnt efficiently. Such boilers are composed ofone top and one bottom drum. Feed water enters the top drum from where it flows tothe bank of tubes into the lower drum then through the circulating tubes to the water

walls header & up to the water wall tubes into the top drum. Therefore, a continuous & positive circulation of water is established.

Packaged Horizontal Smoke Tubes Boilers:

2 pass Dry Back Packaged Boiler

3 pass Wet Back Packaged Boiler

These boilers are these days installed in almost every industry. There is a horizontalcylindrical shell which is fitted with flat plates supported by means of gussets, roundsectioned longitudinal stay bars or tee stiffeners. There are dry back types as well as wet

back types of such boilers. In dry back the radiant heat is lost through the dry back ofthe combustion chamber ends. These boilers are designed as three passes. There are

one or two horizontal either plain furnaces supported by stiffening rings at intervals orcorrugated furnaces or partly plain and corrugated furnaces; laid in between end plates

in dry back and in between front end plate and front tubes plate of the combustionchamber, in case of wet back boiler. The combustion chamber is often called reversingchamber and is fabricated of a cylindrical shell horizontally fitted with end plates. The

front plate is used as tube plate for second pass and the rear flat plate is supported byscrew stays fitted in between rear end plates of the shell. The set of oil or gas burnerassembly is fitted to the front part of the furnace and fire/gases run through the furnace

and enters to the combustion or reversing chamber and passes through the second passtubes and then onwards through the front part of the shell through third pass tubes insmoke box from where they pass out through the chimney. These boilers are verycompact and can be installed in industries conveniently.

The shell at sides and bottom are fitted with hand holes/sight holes or mud holes

with covers. The shell at the bottom is fitted with an oval shaped man hole with cover.There is a set of stand pipes or pads fitted for set of water gauges, one for pressuregauge and for feed inlet pipe on the shell side. A stand pipe at the shell bottom is fittedfor blow off and at the top of the shell one for main steam outlet, one or two for safety

valves and one for the auxiliary valves. The shell is installed on M.S. fabricated chairs.

Efficiency of such boiler is about 82% on gross calorific value of fuel oil. Packaged firetube boilers are designed for coal, oil and gaseous fuels.

Waste heat boiler:

It is a special purpose boiler designed to generate steam by removing the generated


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heat, as called for, by the

a) Chemical processes involvingexothermic reactions viz.,

i. the partial gasification of fuel oil;

ii. synthesis of ammonia;

iii. oxidation of Sulphur dioxide totrioxide;

iv. conversion of ammonia to nitricoxide

b) Recovering heat that is:

i. evolved as an integral part of theprocess and would otherwise gowaste, such as from an open-hearthfurnace

ii. a by-product of chemical process,e.g. black liquor recovery

iii. made available by burning waste,e.g. wood scraps.

It reduces air and water pollution andlowers the flue gas temperature, reducingthe maintenance of flues, fans and stacks.

Fluidized Bed Combustion:

When air or gas is passed through an inert bed of solid particles such as sand or crushedrefractories supported on a fine mesh or grid the air will initially seek a path of least

resistance, and pass upwards through the bed.

With further increase in the velocity, the air starts bubbling through the bed and theparticle attains a state of high turbulence. Under such conditions, the bed assumes theappearance of a fluid and exhibits the properties associated with a fluid and hence the

name ‘FLUIDISED BED’.

If the bed material in fluidized state isheated to the ignition temperature ofthe fuel and fuel is injectedcontinuously into the bed, the fuel will

turn rapidly and the bed attains auniform temperature due to effectivemixing. This, in short, is fluidized bedcombustion (or FBC)

While it is essential that the

temperature of the bed should be atleast equal to ignition temperature ofcoal, it must not be allowed toapproach the adiabatic combustion

temperature (1600-1700ºC) to avoidmelting of the ash. The combustion iscarried out essentially at a

temperature below ash fusion temp.This is achieved by extracting heatfrom the bed through heat transfertubes immersed in the bed, as well as

through walls of the bed.


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If the air velocity is too high the bed particles are entrained in the air stream and arelost. Hence air velocity is maintained between minimum fluidizing velocity and particle

entrainment velocity. The integration of the fluidized bed combustion process with aboiler results in a system that has several advantages.

Coil type Steam Generator:Historically, coil type steam generators have not been commonly used in typical steam

heating plants and therefore not many people are familiar with the technology. A coiledtype steam generator is a forced circulation, water tube boiler in which water underpressure circulates at high velocity. The water circulates through sets of coils in eitherseries, or, parallel flow, while forced draft combustion gases travels across the coils. The

hot gases envelop the entire tube surface making maximum use of both radiant andconvective heat to achieve very high heat transfer rates.Coil-type steam generators are used primarily for industrial process application. Theseare particularly useful, when there is

• A need for additional steam capacity,• A restriction of floor space• An application in which there are significant cyclical or seasonal load fluctuation• An operation which experiences short duration load swing

There are two general types of coil type steam generators: re-circulating and oncethough.

Re-circulating type Steam Generator

In the case of the re-circulating coil type steam generator, water at saturationtemperature is drawn from a steam drum and pumped through a set of nested, parallelconnected coils at several times maximum desired steaming rate. The water is thencarried back to a steam lance, in the top of the same drum, where steam is released and

effectively separated. Dry steam, which has a dryness fraction more than 0.99 is drawnfrom the drum.

Once through Coil type boilersThese are a 'once through' type of water tube boiler, and referred to in some regulations

as, 'boilers with no discernible water level'.


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Water supply to the boiler will usually be at 10 to 15% above the steaming rate to:

• Ensure that all the water is not evaporated, thus ensuring that superheated

steam is not produced.• Provide a vehicle for the feed water TDS to be carried through. If this vehicle was

not available, the salts in the feed water would be deposited on the insides of the

tubes and impair heat transfer, leading to over heating and eventually to tubefailure. Clearly, a separator is an essential component of this type of boiler toremove this contaminated water.

Being of the water tube type, they can produce steam at very high pressures.

Flow diagram of a once through coil type steam generator

The once through coil type steam generator differs significantly from the recirculationdesign in that the water is pumped through multiple coils connected in series and then

into an integral cyclonic separator. This results in approximately 90% of the water beingconverted to steam with a dryness fraction of something like 0.99. The remainingapproximately 10% un-evaporated water can then be blown down or re-used.


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Because the water flowing through the tube as well as the combustion gases outside thetubes are traveling at a high velocity, turbulent flow is created. This reduces the film

resistance to heat transfer and results in a high overall heat transfer coefficient.The biggest advantage of this type of coil type once through steam generator is thesmall physical size and weight of the boiler. Due to this relatively small water content,

this type of boilers enables to go from cold start to full steam output in approximately 5mins. Hence the term steam generator instead of boiler, reflecting its ability to producedry steam very quickly. In many of such boiler construction, the furnace is designed for

Reverse Flame Technology.

Reverse Flame Technology:

The flame reverses within the furnace and thereafter passes along with the membraneswall provided. Due to this, the flame has a longer residence time for total combustion.

The reverse flame technology ensures a very high radiation heat transfer and completecombustion.

Thermal Shock: Coil type steam generators are immune to thermal shock because they

employ a wound coil that, when differing rates of expansion and contraction occur, tendsto unwind like a clock spring without the stress associated with rigid tubes. These typesof steam generators also have modulating feed water control that further reduces thepossibility of thermal shock.

When do you select a coil type steam generator?Full modulation is required: In case of rapidly fluctuating loads, where full modulation –of steam out put and not just fuel – is desired. And, full range response time of the coiltype steam generator, measured at the header is typically about 15 secs.

High output turndown ratio is needed: This type of steam generators generallyoffers turndown ratio of 8:1 to as high as 13:1. So if wide load swing is expected, thismay be an important feature.

Rapid startup is desired: If a boiler is to be base loaded and left at constant fire on acontinuous basis, then quick steaming capability is not a tangible benefit. But for thoseapplications, where it is advantageous to go from cold start to full steam output inshortest possible time, this type of steam generator is the best option.

Small size, low weight: Perhaps the biggest advantage the coil type steam generatorhas is its compact size. That, along with its modular construction, offers substantialsavings in construction and installation cost. Since the total water holding capacity areusually less than 22.75 litres, this steam generators are not falling under IBR purview

and sometimes termed as Baby Boilers.

Boiler Mountings, Accessories and Auxiliaries

Boiler Mountings and Accessories:

In order to facilitate trouble free operation of a boiler, certain devices are attached or


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mounted on the boiler as a regular feature and as integral components. These mountingsare to ensure that a boiler is operated in safe and proper manner. Without these

mountings it will be difficult to run the boiler safely. Let us now learn about such devices,which are termed as Boiler Mountings. A boiler cannot function safely without mountings.

Boiler Mountings: For the safety of the boiler and for better control over the steam

generation process, various fitting are provided on the boiler, which are called boilermountings. A boiler cannot function safely without mountings.

They are mainly,

(1) Safety valves

(2) Water level indicator or gauge glass

(3) Steam Pressure Gauge

(4) Main Steam stop valve

(5) Feed water check valve

(6) Drum Vent valve

(7) Blow-down valve, Drain, etc.

(8) Fusible Plug

Boiler Accessories: These are the items that form an integral part of the boiler but are

not mounted on the boiler. They are mainly for increasing the plant load and overallboiler efficiency and they help in smooth running of the plant under desired operating

conditions.

They are mainly,

(1) Superheater

(2) Economizer

(3) Air Pre-heater

(4) Deaerator

(5) Feed water pumps

(6) Feed water heaters etc.

Description of Mountings, Accessories and Auxiliaries:

Safety Valves:

Safety Valve is a mounting, which protects a boiler or any pressure vessel against overpressure. A pressure vessel is always designed to withstand a safe working pressure at aparticular temperature and it is supposed to work within the safe limits of pressure andtemperature. Safety against over pressure is very essential due to the fact that thepressure vessel may burst causing extensive damages, if the safe working pressure limit

is exceeded. Therefore, if the working pressure exceeds limit at any time, the pressure isto be instantaneously released and the vessel has to be brought back within the safelevel of pressure limits. Safety Valve is the safety device for any pressure vessel and isan essential and important mounting for a boiler. IBR stipulates stringent conditions for

periodic inspection and testing of safety valves. Every year before renewal of boilerlicense, it is mandatory to conduct steam test for a boiler by successfully demonstratingthe working of the Safety Valves. There are several types and designs of safety valves in

use since earlier days, such as Lever and weight safety valve; Dead Weight SafetyValve; Hopkinson combined low water level and high steam pressure safety valves;Ramsbottom spring loaded safety valves:

Pop spring-loaded safety valves:

This type of safety valves is most commonly seen these days. This consists of mainly a

valve seat, which is exposed to system pressure in the vessel and the spindle whichcarries the valve seat is kept tight in position by the force of a coil spring, exerting equaland opposing force over the valve seat through the valve spindle, to counter the upwardforce acting over the valve seat. The valve disc is loosely fitted to the lower end of the


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vertical valve spindle and exposed to force of the pressure in the drum. There is coilspring positioned inside the yoke in between the valve body and a setscrew, fixed at the

top of the yoke with brass washer holding it compressed. The set screw has a female

threaded bush fixed on to top of the yoke and exerts direct pressure on the spindlewhich further transfers the pressure on to the valve seat, keeping the valve in tight shutposition against force of the steam pressure.

The top set screw is tightened to set the valve to open only at preset pressure, which is

normally at safe limit over the normal working pressure. When the steam pressure indrum tends to exceed the set pressure of the screw, the valve seat opens upcompressing the springs against the preset pressure, relieving the over pressure in the

drum to atmosphere.

These types of valves are mainly used in high-pressure boilers. These valves are verycompact and require very less space and can be locked. The illustration is detailed in thefigure above.

Drum Water Level Indicator or Gauge:

The most satisfactory water-level indicator, and the one now almost exclusively used, isthe glass tube water gauge, by means of which the exact level of the water in the boileris constantly shown.

A good form of glass tube water gauge, is shown in the figure below.


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The level gauge is one of the most important mountings in a boiler as it gives thephysical representation of the drum water level. As per IBR norms, there should be at

least two numbers of level indicators on-line in a running boiler at all times. Operatorsshould periodically clean the level gauge by systematically blowing both the watersideand steamside valves.

Fusible Plug:

This is a safety device commonly used for fire tube boilers against high metaltemperature resulting from low water level. The crown of the furnace of some earlierdays boilers is fitted with a plug held in position by fusible metal or alloy. This plug undernormal conditions is covered with water in the boiler which keeps the temperature of the

fusible metal below its melting point. But when the water level in the boiler falls lowenough to uncover the top of the plug, the fusible metal quickly melts, the plug dropsout, and the opening so made allows the steam to rush into the furnace. The steamthus, puts out the fire or gives warning that the crown of furnace is in danger of being

overheated.

Above Figure illustrates a common form of fusible plug. Hollow gunmetal plug screwedinto the crown plate. A second hollow gunmetal plug screwed into the plug. A thirdhollow gunmetal plug is separated from second by fusible metal. The inner surface of


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second hollow gunmetal and the otter surface of third gunmetal are grooved as shown sothat when the fusible metal is poured in, the second and the third plugs are locked

together. Hexagonal flange is provided in the base of plug so that it can be removed byusing a spanner. There is another hexagonal flange on second plug for finding orremoving it.

Pressure Gauge:

Dial Pressure gauge internal view

The shape of the pressure gauge is like a clock having a dial and pointer withgraduations of the pressure gauge. If a pressure gauge is opened, by removing the rearcover plate, it can be seen that there is a pivot at the center, with a small pinion gearattached to the external indicator needle. The pinion gear is in turn connected with a

worm gear sector, whose one end is connected with a Borden tube through a set oflevers. The Bourdon tube is a metallic, thin walled, flexible construction with semicircular or oval cross section. The other end of the Borden tube is attached to the siphonpipe with a cock, which is either fitted on steam/water pipe or to the boiler or pressure

vessel. One helical hairspring is fitted to the central pivot to keep the pointer at fixedposition when the pressure is released. One dial with pressure range is fitted at the backof this mechanism concentrically. The pivot center is extended through the dial and onearm/pointer with sharp edge is fitted on this extension, which actuates with the pivot.When the steam/water, under pressure, is introduced inside the Borden tube through the

siphon pipe the Bourdon pipe expands actuating the central gear through the sector

worm, thus actuating the arm/pointer on the dial bringing the helical spring undertension (the spring is wound). The pointer indicates correct pressure in theboiler/vessel/pipe. The mechanism can be calibrated with standard gauges, so that the

error in indicating can be minimized. When the siphon cock is closed the tension on thehelical spring is withdrawn and it releases the spring tension to bringing the pointer backto its original position.

Siphon cock:

This is a U shaped pipe-having socket and female threadsat one end and other end is fitted with one cock havingfemale threads. The cock has got a three wayarrangement, with the facility to disconnect the Borden

pipe and to turn on the impulse line to atmosphere, sothat the entrapped air can be cleared, before connectingthe Borden gauge to read pipe line pressure. The Bordenshould always be filled with cold leg of condensate and

under no circumstance hot oil or steam can come incontact and fill it up to avoid any damage. Since thewater in the siphon prevents the hot steam from entering

the Bourdon tube, the gauge is kept comparatively cool.

Continuous overheating of the Bourdon tube would injure it and may permanently affectthe accuracy of the gauge. When in use, the siphon should be cool enough to be graspedby hand without discomfort


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Main Steam Stop valves/Auxiliary steam valves:

There are different types of such valves viz. right angled stop valves, Globe type valves,Wedged type parallel seat valves and piston-typed valves.

Feed-check valves:

It is essentially a stop cum non-return type of valve

used earlier. A stop valve is provided on the uppersection and the non-return valve is provided at thebottom so that when the valve is opened due tobackpressure the non-return valve automaticallycloses due to backpressure of the steam in theboiler. There is a plug provided at the bottom most

portion of the valve body as shown so that in casethe non-return valve is found leaking the stop valvecan be closed and plug can be opened to grind thenon-return valve with the help of screw driver

through the slot provided on the non-return valveextension spindle. The valve otherwise is just as asteam stop valve. These days a simple non-

return valve is used. This non-return valve isan essential mounting as required by IBR. It isplaced in the feed water line before it enters theboiler to prevent back flow of feed water from

Boiler.

Blow off valve; blow off cock or mud valve:

The blow off cock is required to empty the boiler periodically for cleaning and inspection.It will let out any sediment along with a portion of BFW when it is opened. It is fitted

directly to the boiler shell or to a pipe connected with the boiler.

Steam Traps:

Steam traps are devices used to collect and automatically discharge the water resultingfrom partial condensation of steam without allowing any steam to escape. The trap is solocated that water from the condensation of the steam in the steam pipe flows by gravity

to it.

In order to receive the greatest return from the cost of generating and using steam as aheating and process medium a good working knowledge of steam traps, and applicationis essential.

Why A Steam Trap?Steam generated by a boiler contains heat units, which are released in heating, processapplications and pipe radiation loss and in doing so the steam returns to its condensatestate. If condensate is not drained immediately or "trapped” from the system, itreduces operating efficiency by slowing the heat transfer process and can causephysical damage through the phenomenon known as "Water Hammer".

When condensate accumulate on the bottom of a horizontal pipe, is swept along by thevelocity of the steam passing over it. As the velocity of condensate increases it forma solid slug of incompressible water, when it is suddenly stored by a pipe bend, fitting

or valve produces shock wave of tremendous momentary force which can cause greatphysical damage. So in low pressure as well as in high pressure systems, the steam


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piping must be designed in such a way that it will drain condensate properly for avoidingwater pockets.

A trap is therefore required at all locations where condensate is formed and collects,

principally

Location of trap in steam distribution system:

a) At elevation changes such as risers and expansion loops.

b) At all low points and at intervals of 200 to 500 ft. on long horizontal runs.

c) Ahead of all possible "dead-end" areas such as shut-off valves, pressure andtemperature control valves and at ends of system.

Location of trap in steam operated equipments:

A) Ahead of humidifiers, pumps, turbines etc.

B) Draining directly below heat exchangers coils, unit heaters and driers etc.

Types of Traps:

Traps are of two broad kinds. Those, which distinguish water from steam owing to thedifference in density of the two-phases, are the mechanical traps, and those which

distinguish by means of temperature these are the thermostatic or expansion traps.

Mechanical Traps:

Plain Float Traps

Bucket Traps

Thermostatic or Expansion Traps: Metallic Expansion Traps;

Liquid Expansion Traps;

Balanced Pressure Expansion Traps;

Relay Traps;

Thermodynamic Trap

Boiler Accessories and Auxiliaries:

Superheater:

It is an important accessory, where the temperature of steam is increased above

saturation temperature by further heating dry saturated steam.

We have seen that in thermal powergeneration using steam, the cycleefficiency improves when steam is

superheated. Mixture of water and watervapour enters drum through waterseparators, where the dry steam isseparated and comes out through steamraiser pipes. The steam coming out from

drum through saturated steam pipes isdry and saturated. The saturated steamflow takes its passage through set of

coils, called Superheater, where furtherheat is added to increase steamtemperature at constant pressure.

Superheater is generally located above furnace, at high radiant and convection heatzone. Superheating is also done in stages through a set of Superheaters e.g., PrimarySuperheater and Secondary Superheater.

Economizer:

Economizer is another important accessory to a boiler plant. It is similar to feed water

heater but it is called Economizer, since, by its function; it helps to improve uponefficiency of the boiler and saves on fuel consumption.


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The temperature of the combustion gases, leaving final Superheater zone is very highand has got good potential of heat energy which can be effectively tapped off. Further

ahead of all Superaheaters, Economizer is located so that the heat potential still left over

can be recovered by heating boiler feed water, before it enters the drum. Economizer isnothing but a heat exchanger, where in the heat available in waste gases can be

effectively utilized to preheat the feed water going to boiler drum.

Location and Arrangements of Economisers:

The economizer is located, along the flue gas path, before the last two stages of Airheaters and following the primary Superheater or Reheater. Hence it will generally be

contained in the same casing along the second pass of the flue gas path, where theprimary superheater or reheater is located. Counterflow arrangement is normallyselected so that heating surface requirement is kept at minimum for the sametemperature drop in the flue gas. Economizer coils are designed for horizontal

placement, which facilitates draining of the coil and favors the constructionarrangements in the second pass of boiler. Water flow is from bottom to top so thatsteam, if any formed during the heat transfer can move along with water and prevent

the lock up of steam, which will cause overheating and failure of economizer tube.Economiser coils are arranged in

horizontal direction across the width. Thecoils are made to take several turns inloops welded with ‘U’ bends at the ends

and finally connecting to headers at inletand outlet. The economizer coils can bearranged one over the other in straightline or with cross pitch arrangements.Some of the economizers are designedwith finned tubes to increase the heat

pick up area.

Economizer tubes are supported in such a manner that sagging, undue deflection andexpansion prevention will not occur at any condition of operation. A recirculation line

with a stop valve and non-return valve may be incorporated to keep circulation throughthe economizer back into steam drum, when there is fire in furnace but feed flow todrum is cut off due to high water level. (e.g. during start up period). Tube elementscomposing the unit are built up into tiers or banks and these are connected to inlet and

outlet headers. Manholes and adequate access and spacing between banks of tubes areprovided for inspection and maintenance work. Normally the tube bank arrangement andsteam soot blowers provision at appropriate location will facilitate efficient on load

cleaning. An ash hopper below the economizer is provided if the flue gas duct is taking aturn from vertical.


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Air-preheater:

Combustion is a process of vigorous chemical reaction of a fuel with Oxygen and therequired Oxygen is received from atmospheric air, which is available in abundance in

nature. We have seen earlier for proper combustion of a fuel three ‘T’s i.e. Time,Temperature and Turbulence are essential. Time and Turbulence can be created bysupply of combustion air at a required pressure, velocity and direction. To sustain the

temperature of burning fuel, it is also essential to supply combustion air preheated tohigher Temperature. Perfect and efficient combustion of fuel can be achieved in a boiler,if the atmospheric air can be preheated by the low temperature flue gas, which isotherwise going to be wasted through the chimney. Hence a device called ‘Air Preheater’

is used in boilers for the purpose of preheating combustion air by the flue gas and thenis supplied for fuel combustion.

Based on the operating principle, the Air Preheaters are classified into two differenttypes, namely, Recuperative (Tubular) and Regenerative (Rotating) Air Preheaters.

Feed Water Pump

The feed pump is used to deliver feed water to the boiler, and it is required to supply a

quantity of water at least equal to that converted into steam and used by the engine.Feed pumps may be either reciprocating or rotary pumps.

The feed pump is sometimes worked from the engine direct or from the shaft by an

eccentric attached to the plunger. Pumps are also worked independently by using steamdirectly from the boiler. Such pumps are called direct-acting pumps.

Rotary feed pumps are generally of the high speed centrifugal type, driven directly by asmall steam turbine or by an electric motor. Rotary pumps may be single-stage or multi-

stage according to whether one or more impellers are used. In the single-stage pump,the full pressure of water is obtained in one chamber in which the impeller revolves. In amulti-stage pump a number of impellers are keyed on the same shaft, each impellerworking in its own chamber or stage. These pumps are also known as turbine pumps.

The discharge pressure of a feed pump should have enough margins over the highest

boiler pressure (Safety valve over pressure) to overcome frictional resistance of the feedline.

Deaerator:

One of the feedwater heaters is a contact-type open heater, known as deaerator, othersbeing closed heaters. It is used for the purpose of deaerating (primarily removal ofdissolved oxygen) the feedwater.

The presence of dissolved gases like oxygen and carbon dioxide in water makes thewater corrosive, as they react with the metal to form iron oxide. The solubility of these

gases in water decreases with increase in temperature and becomes zero at the boilingor saturation temperature. These gases are removed in the deaerator, where feedwateris heated to the saturation temperature by the steam extracted (sometimes called

pegging steam) from the turbine. Feedwater, after passing through a heat exchanger,called vent condenser, is sprayed from the top so as to expose large surface area, andthe bled steam from the turbine is fed from the bottom. By contact the steam condensesand the feedwater is heated to the saturation temperature. Dissolved oxygen and carbon

dioxide gases get released from the water and leave along with some vapour, which is

condensed back in the vent condenser, and the gases are vented out.To neutralize the effect of residual dissolved oxygen and carbon dioxide gases in water,sodium sulphite (Na2SO3) or hydrazine (N2H2) is injected in suitable calculated doses intothe feedwater at the suction of the boiler feed pump (BFP).

During suction of the BFP, some of the saturated feed water may flash into vapour due

to reduction in pressure causing vapour lock and cavitations problems in the pump. Toprevent this from occurring the deaerator is located at a sufficient height (H) from thelevel where the pump is installed so that the pressure before suction remains positive.

This is called NPSH (Net Positive Suction Head). When this water is sucked by the pump,


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the pressure does not fall below Psat and there is no flashing of any water into vapour,which protects the BFP from any damage due to vapour lock and cavitations.

Deaerator with storage tank

The deaerator is usually placed near the middle of the feedwater system so that the total

pressure difference between the condenser and the boiler is shared equitably between

the condensate pump and boiler feed pump. The feedwater heaters before the deaeratorare often termed as low pressure (L.P.) heaters.

Steam Separators:

When steam supplied to an engine is not superheated it is almost certain to contain

water suspended in it. The ultimate use of steam in an industrial plant determines thedegree of steam purity required for operation. While carryover can never be eliminated,even with the best boiler designs and boiler water chemistry, it can be reduced to a

tolerable level.

When a boiler must have optimum steam purity, operating pressure is a deciding factorin the design. At low pressures, the density of water is many times greater than that of

steam. Therefore, adequate disengaging space and water surface area can be provided

to achieve desired steam purity. As pressures increase, the density difference betweensteam and water decreases. To size a steam drum to separate steam and wateradequately at higher pressure is cost prohibitive; therefore, mechanical separators are

employed. The two major types of mechanical separation are primary separation andsecondary separation (or steam scrubbing).

Primary Separation: This form of mechanical carryover prevention utilizes rapid andabrupt changes in the direction of steam flow. Major separation of the steam and wateroccurs in these primary devices. Simple baffles, curtain baffles, belly baffles, and cyclone

separators are used in primary separation. Over 98% of the steam purity is achieved inthe primary separation phase.

Secondary Separation: With the small amounts of moisture that remain after primaryseparation, the scrubbing or drying process has to be accomplished through rapiddirectional changes of the steam flow in conjunction with large surface areas for

collection of the mist. In addition, steam velocity must be low to avoid reintrainment ofthe boiler water. Normally, screens or corrugated plates with close tolerances are usedas steam scrubbers.


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As the steam-water mixture risesabove the tube bundle it enters the

Steam Drum. An arrangement of steelplates as shown here, called “CycloneSeparators” force the steam-water

mixture into a swirling centrifugalmotion, which results in the waterdroplets moving to the outside area of

the separator where they are drainedoff. The steam flows upwards as itbecomes lighter with the removal ofthe moisture content. The final stage

of drying the steam takes place in asecond stage of water removal called

“scrubbers”, which are located abovethe cyclone separators. The steam thatleaves the steam drum had itsmoisture content reduced to about

0.1% from the 90% that entered thesteam drum.

Fans:The prime function of a fan is to move mass of air or gas, constantly through closed and

guided passage, at a moderate pressure and velocity, for distribution at targetedlocation or locations. Though a little amount of compression does occur, fans are notsupposed to compress or expand air/gas during fanning action. Fans can also handlegases carrying dust particles in suspension. In a boiler fans are deployed for different

applications, such as supply of the required quantity of atmospheric air at requiredpressure for enabling fuel combustion, instant removal of products of combustion,supplying air for cooling of equipment working in hot zones etc. Fans are designed and

designated according to the function they are meant for in boiler (e.g.) Induced DraftFan, Forced Draft Fan, Primary Air Fan, Seal Air fan and Motor Cooling Fans etc.

Fans are broadly classified into axial and radial (Centrifugal) types, according to flowdirection of the medium at inlet or / and outlet, with reference to axis of shaft rotation.

The construction and shape of the fan will be according to its function as axial flow orradial flow fan.

Steam Generating System - Operator Manual

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Transcript of Steam Generating System - Operator Manual