CHAPTER 2: PROPERTIES OF STEAM AND STEAM POWER

Steam produced from water forms one of the most important working fluids in

engineering energy conversion systems. The advantages of using steam (water) as

working fluid are many. Water is abundantly and readily available in our earth to be

used as a working fluid and also as a coolant in condensers. Water could be readily

treated to make it suitable for steam production boilers at reasonably low cost. In

addition, the heat transfers during phase changes essentially take place isothermally

which is the ideal (reversible) heat transfer to be considered in any power cycle. Also

the pumping power needed to raise the pressure from low values to very high values is

very less compound to the power produced in the turbine or the heat added in the

boiler. Thus, water forms the most important working fluid in thermal power plant cycles

and is most of the process industries.

Steam-Water Separation

Steam-water separation is one of critical process in the steam boiler system especially

in steam drum. Steam is usually installed in steam generators and high pressure

boilers. The principal purpose is to allow for separation of saturated steam out of the

mixture of steam-water and then goes to superheater for next process. The saturated

steam is released via several outlet nozzles for special utilization or even further more

heating.

In addition for place steam-water separation, steam drum also assists for following

below:

Offer restricted water storage to support quick adjustments in boiler load.

Eliminate element of water (solids content) via blowdown to manage the

chemistry of boiler water.

Cleanse the steam to eliminate impurities and left over moisture.

Combine the control of chemicals water treatment and also corrosion.

Combine the saturated water staying right after steam-water separation with

feedwater.

Even so, the principal purpose of the steam drum is to allow the efficient performance of

steam-water separation. This particular can be obtained by supplying a huge steam-

water surface area to perform separation through natural gravity or also by way of

owning adequate area for mechanical equipment to perform steam-water separation.

Steam-water separation should be performed efficiently because it is important in

almost all steam boiler uses so that they can:

Avoid the solids broken down in the steam are carried over directly into

superheater in which destroying deposits may possibly be made.

Reduce steam is carried under inside water which departing the steam drum in

which left over steam could decrease the performance of hydraulic head pump.

Avoid water droplet is carried over straight into reheater, superheater and steam

turbine in which thermal destruction could happen.

Boiler water may perhaps consist of impurities, especially in solution. Impurities occur

from inside make-up water, leakage in the condensate process and chemicals

treatment. Moreover small quantities of the solids inside steam could harm reheater,

superheater and steam turbine generator.

Due to the fact the solids solubility is commonly some requirements of specifications

inside steam not as much as inside water, little quantity of water droplet which is carried

over (in excess of 0.26% simply by weight) might effect in substantially enhanced

unwanted deposition and also the carry-over of solids into reheater, superheater and

steam turbine generator. The deposits can lead to steam turbine deterioration in

addition to increases temperature of superheater tube, so it can be over heat.

The methods of steam-water separation process can be performed by natural Gravity

Driven Separation, Baffle Assisted Separation, and Mechanical Primary Separation.

There are some factors of steam-water separation that influence efficiency of its

process both design factors and operating factors.

LOW PRESSURE BOILERS.

A boiler is a closed vessel in which water or other fluid is heated. The heated or

vaporized fluid exits the boiler for use in various processes or heating applications.

Boiler properties:

(i)Safety: The boiler should be safe under operating conditions.

(ii) Accessibility: The various parts of the boiler should be accessible for repair and

maintenance.

(iii) Capacity: Should be capable of supplying steam according to the requirements

(iv) Efficiency: Should be able to absorb a maximum amount of heat produced due to

burning of fuel in the furnace.

(v) It should be simple in construction.

(vi) Its initial cost and maintenance cost should be low.

(vii) The boiler should have no joints exposed to flames.

(viii) Should be capable of quick starting and loading.

Boilers are classified mainly into two categories:

1. Fire tube or Low Pressure boilers.

2. Water tube or High Pressure boilers.

Low pressure boilers are Vertical, Cochran, Lancashire, Cornish, Locomotives, etc.

In fire tube boiler, hot gases pass through the tubes and boiler feed water in the shell

side is converted into steam. Fire tube boilers are generally used for relatively small

steam capacities and low to medium steam pressures. As a guideline, fire tube boilers

are competitive for steam rates up to 12,000 kg/hour and pressures up to 18 kg/cm2.

Fire tube boilers are available for operation with oil, gas or solid fuels.

Vertical Boilers: The vertical boiler is a simple type which consists of a firebox at the

bottom and a copper barrel with a smoke tube. It typically is used to drive stationary

engines and boats. Firing is accomplished by alcohol or solid fuel pellets. More

sophisticated versions of the vertical boiler contain many small tubes and are

sometimes fired by coal or charcoal.

Cochran Boilers: It is one of the best types of vertical multi-tubular boiler, and has a

number of horizontal fire tubes it also a Low Pressure Boiler.

Construction: Cochran boiler consists of a cylindrical shell with a dome shaped top

where the space is provided for steam. The furnace is one piece construction and is

seamless. Its crown has a hemispherical shape and thus provides maximum volume of

space.

Working: The fuel is burnt on the grate and ash is collected and disposed of from ash

pit. The gases of combustion produced by burning of fuel enter the combustion chamber

through the flue tube and strike against fire brick lining which directs them to pass

through number of horizontal tubes, being surrounded by water. After which the gases

escape to the atmosphere through smoke box and chimney.

Specifications:

Shell diameter 2.75 m

Height 5.79 m

Working pressure 6.5 bar (max. pressure = 15 bar)

Steam capacity 3500 kg/hr (max. capacity = 4000 kg/hr)

Heating surface 120 m2

Efficiency 70 to 75% (depending on the fuel used)

Locomotives boilers: The Locomotive type boiler, shown in Figure 8, consists of a

copper barrel and an outer and inner firebox attached to the rear of the barrel. Perfectly

round pressure vessels carry pressure loads by developing hoop tension stresses in the

wall of the boiler. In this case, there is no tendency for the walls to bend. If a pressure

vessel is not perfectly round, the walls will bend because the walls will bend attempting

to form a perfect circle as it pressurizes. The results in a combined stress field with high

stress levels. Since the firebox is not circular, it is necessary to provide additional

structural members to strengthen non circular surfaces. This is the reason for stay bolts

and structural beams in the firebox assembly.

Thermal stresses also occur as a result of thermal expansion, therefore a boiler must be

designed to carry both thermal and pressure loads. In a Belpaire type firebox, the outer

wrapper and crown sheet may be stiffened by beams or girder stayed to each other.

Both side plates of the outer firebox are cross stayed since they are flat. Flat surfaces

on the back head and smoke box tube plate can be stiffened or stayed. Sometimes a

combination of both is used resulting in a very strong boiler. If two or more stays are

used, once can be hollow and used to route the blower line to the smoke box. Tubes

extend from the front tube plate in the smoke box to the rear tube plate, which is located

at the front of the fire box.

This type of boiler can be fired by coal if grates are installed. Liquid fuel burners can

also be employed. The locomotive type boiler is the most difficult to build but is very

efficient and allows the realism of a coal fire. It requires a suction fan and a blower,

since it is a forced draft system. Its water capacity may be less than a comparable type

C boiler. The water level must be carefully maintained so that the crown sheet is

always covered, otherwise the boiler will be damaged.

HIGH PRESSURE BOILERS

High pressure boilers are Babcock and Wilcox, Sterling, etc.

In water tube boiler, boiler feed water flows through the tubes and enters the boiler

drum. The circulated water is heated by the combustion gases and converted into

steam at the vapour space in the drum. These boilers are selected when the steam

demand as well as steam pressure requirements are high as in the case of process cum

power boiler / power boilers.

Most modern water boiler tube designs are within the capacity range 4,500 – 120,000

kg/hour of steam, at very high pressures.

Babcock and Wilcox boiler: Babcock and Wilcox is a water-tube boiler is an example

of horizontal inclined tube boiler it also a High Pressure Boiler.

Construction: Babcock and Wilcox boiler with longitudinal drum. It consists of a drum

connected to a series of front end and rear end header by short riser tubes. To these

headers are connected a series of inclined water tubes of solid drawn mild steel. The

angle of inclination of the water tubes to the horizontal is about 15° or more.

Working: The fire door the fuel is supplied to grate where it is burnt. The hot gases are

forced to move upwards between the tubes by baffle plates provided. The water from

the drum flows through the inclined tubes via down take header and goes back into the

shell in the form of water and steam via uptake header. The steam gets collected in the

steam space of the drum. The steam then enters through the antipriming pipe and flows

in the superheater tubes where it is further heated and is finally taken out through the

main stop valve and supplied to the Steam turbine or Steam engine when needed.

.

MOUNTING OF BOILER:

There are different fittings and device which are necessary for the operation and safety

of a boiler. The various mountings used on the boiler

1. Water level indicators:

Thefunctionofawaterlevelindicatoristoindicatethelevelofwaterinthelevel constantly. It is

also called water gauge.

2. Fusible plug: The function of a fusible plug is to prevent the boiler against damage

due to overheating for low water level.

3. Steam stop valve: A junction valve is a valve which is placed directly over a boiler

and connected to a steam pipe which carries steam to the engine. If a valve is placed in

the steam pipe leading steam to the engine and placed near the engine. It usually

termed as stop valve. The larger sizes are called Junction valve and smaller sizes Stop

valve.

4. Feed check valve: The function of a feed check valve is to control the supply of

water to the boiler and to prevent the exception of water from the boiler when the pump

pressure is less as pump is stopped.

5. Blow off cock: A blow off cock or valve performs the two functions. They are it may

discharge a portion of water when the boiler is in operation to blow out mud scale or

sediments periodically and It may empty the boiler when necessary for cleaning,

inspection and repair.

6. Safety valve: The function of a safety valve is to release the excess

steam when the pressure of steam inside the boiler exceeds the rated pressure. The

various type of safety valve are Dead weight safety valve, Spring loaded safety valve

and High steam& low water safety valve.

7. Pressure gauge: The function of a pressure gauge is to measure the pressure

exerted inside the vessels. It is usually constructed to indicate upto double the

maximum working pressure. Its dial is graduated to read pressure in kgf/cm2 gauge.

There are two type of pressure gauges, they are Bourdon tube type pressure gauge and

Diaphragm tube type pressure gauge

8. Man and mud holes

ACCESSORIES OF BOILER:

There are auxiliary plants required for steam boiler for their proper operation & for

increase of their efficiency. The various accessories are

1. Feed pump:

Thefeedpumpisapumpwhichisusedtodeliverfeedwatertotheboiler.Itisdesirablethat

thequantityofwatersuppliedshouldbeatleastequaltothatevaporatedandsuppliedtotheengin

e.Two type of pumps which are commonly used as feed pump are Reciprocating pump

and Rotary pump.

2. Injector: The function of an injector is to feed water in to the boiler. It is commonly

employed for vertical and locomotive boiler and does not find its applications in large

capacity high pressure boiler.

3. Evaporator:

Evaporatorisusedinhighpressureboilerwhichisplacedaftertheairinthewayoffluegases

water are tube.

Henceevaporatorisaunitwhichconsumestheenergyoffluegasesinboiler.Itsmain function is

to convert the water to steam add much to the boiler efficiency.

4. Economiser: An economizer is a device in which the waste heat of the flue gases is

utilized for heating the feedwater. Economizer is very important part of the boiler, with

the help the economizer the efficiency of the boiler increased and the evaporative

capacity of the boiler is increased. Economizers are of two type Independent type and

Integral type.

5. Super heater: The function of a super heater is to increase the temperature of the

steam above its saturation point.

6. Air-pre heater:

Thefunctionofairpreheatereistoincreasethetemperatureofairbeforeisentersthe

furnace.Itisgenerallyplacedaftertheeconomiser.Sothatfluegasespassthroughtheeconomi

serandthen to air preheat. Usually, there are three types of pre-heater are Tubular type,

Plate type and Regenerative type.

Description:

Water level indicators:

Thefunctionofawaterlevelindicatoristoindicatethelevelofwaterinthelevel constantly. It is

also called water gauge.

Water gauge indicates the water level inside the boiler and is hence called as water

level indicator. Usually two water gauges are fitted in front of boiler.

Description of water gauge:

It consists of a glass tube, two gun metal tubes and three cocks. The steam cock C1 is

provided on the gun metal tube M1 which connects the glass tube with the steam space

in the boiler. The water cock C2 is provided on the gun metal tube M2 which connects

the glass tube with the water space. The gun metal tubes M1 and M2 are bolted to the

boiler shell.

The drain cock C3 is used to drain the water from the glass tube at intervals to ascertain

whether the gauge is in proper order or not. The glass tube is protected by means of a

cover, made of specially toughened glass which will prevent any accident that may

happen due to the breaking of glass tube.

Working of Water gauge:

The water gauge shows the level of water in the boiler drum. It warns the operator if the

water level goes below a fixed mark, so that corrective action may be taken in time to

avoid any accident.

For the observation of the water level in the boiler, the water and steam cocks are

opened and drain cock is closed. The steam enters from the upper metal tube M1 into

the glass tube and water enters from the lower metal tube M2 into the glass tube.

Hence, water stands in the glass tube at the same level as in the boiler.

The junctions of the metal tubes and the glass tube are provided with two balls. In case

the glass tube is broken, the balls are pushed to the top nd bottom ends of the glass

tube. Thus the flow of both water and steam out of the boiler is prevented.

When the boiler is not working, the water gauge can be taken out from the boiler for

cleaning purposes by removing the bolts.

PRESSURE GAUGE:

A pressure gauge is used to indicate the steam pressure of the boiler. It is also called as

steam gauge. It is usually mounted in the front top of the steam drum.

Figure shows a commonly used pressure gauge known as bourdon type. It consists of

an elastic metallic bourdon spring tube S of elliptical cross section and bent in the form

of circular arc. One end of the tube is fixed at the block B. it is connected to the steam

space of the boiler by menas of cock C. The other end is connected to a toothed sector

T through a link L hinged at the point H. The sector is in mesh with a pinion P fixed on a

spindle. An indicating pointer N is attached to the spindle to read the pressure on a dial

gauge D.

Working:

When steam enters the elliptical tube, the tube section tries to become circular, which

causes the other end of the tube to move outward. The movement of the closed end of

the tube is transmitted and magnified by the link and the tooth sector. The magnitude of

the movement of the sector is indicated by the pointer on the dial.

The Dead Weight safety valve:

The Dead Weight safety valve consists of a valve V which is made of gun metal to

prevent rusting. It rests on the gun metal seat S and is fixed to the top of a vertical

steam pipe P. The pipe has a flange F at the bottom for fixing at the top of the boiler

shell.

A weight carrier C is suspended from the top of the boiler. It carries cast iron rings (i.e.,

weight W). the total weight must be sufficient to the keep the valve on its seat against

the normal working pressure.

Working of Dead Weight safety valve:

When the steam pressure in the boiler exceeds the normal working pressure, it lifts the

valve with its weight. The excess steam therefore escapes through the pipe to the

atmosphere, until the pressure reaches its normal value.

It is the simplest type of safety valve; it is suitable for stationary boilers only, because it

cannot withstand the jerks and vibration of mobile (marine) boilers. Another

disadvantage of this valve is the heavy weight required to balance the steam pressure.

Hence, it is not suitable for high pressure boilers.

Lever safety valve

The advantage of level safety valve over the dead weight safety valve is that the heavy

dead weight is replaced by a lever with a smaller weight.

Description of Lever safety valve:

It

consists of a valve resting over a gun metal seat. The valve seat is fixed on a mounting

block, fitted over the boiler shell. One end of the level is hinged to a rod of the mounting

block, while the other end carries a weight. A short strut is placed over the valve.

Working of Lever safety valve:

The thrust of the lever with its weight is transmitted to the valve by the strut. When the

steam pressure exceeds the safe limit, the upward thrust of steam lifts the valve from its

seat and the lever with its weight. The excess steam escapes till the pressure falls back

to the normal value. The valve then returns back to its original closed position.

The required weight W at the end of the lever for maintaining the pressure P in the

boiler is obtained by taking moments about the hinged point.

SPRING LOADED SAFETY VALVE:

It is loaded with a spring instead of weights. Hence it is called spring loaded safety

valve. It consists of a cast iron body having two branch pipes P1 and P2. Two separate

valves are placed over the valve seatings, which are fixed to the top of the branch

pipes. A lever is placed over the valves by means of two conical pivots.

The lever is attached to a spring at its middle. The spring pulls the lever in downward

direction. The lower end of the spring is attached to the valve body by means of a

shackle. Thus the valves are held tight to their seats by the spring force.

Working of spring loaded safety valve:

When the steam pressure exceeds the normal working pressure , the valves rise up

against the action of the spring and allow the steam to escape from the boiler till the

pressure in the boiler reaches its working pressure.

The spring loaded safety valve is much lighter and compact compared with other safety

valves. For locomotive or marine service, the safety valve should be such that it is

unaffected by jerks and vibration likely to occur in such device. Hence spring loaded

safety valve is preferred for locomotive and marine services, in addition to stationary

boilers.

FUSIBLE PLUG:

The function of the fusible plug is to put-off the fire in the furnace of the boiler when the

water levels falls below an unsafe level and thus avoid the explosion which may take

place due to overhearing of the tubes and the shell.

Description of fusible plug:

It consists of a hollow gun metal body screwed into the fire box crown plate. A hollow

gun metal plug is screwed into the gun metal body by tightening the hexagonal flange in

it. There is another copper plug locked with the gun metal plug by pouring a low melting

point metal (lead) into the groove provided for the same.

Working of fusible plug:

During the normal operation, the fusible plug is submerged in water which keeps the

temperature of the fusible metal below its melting point.

But when the water level falls below the top of the fusible plug, it is uncovered by the

water. The fusible plug therefore melts by the heat of the furnace. Thus the copper plug

drops down and is held within the gun metal body by the ribs. The opening so made

allows the steam rush into the furnace and extinguish the fire. The damage to the fire

box which could burn up, is avoided.