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BASICS OF MECHANICAL ENGINEERING (ME-107)

Lectures onSTEAM BOILERS

ByJAYASHISH PANDEYNational Institute of technology KarnatakaSurathkal

IntroductionBoiler is a device which is used to produce steam at high pressure.Steam is being used in thermal power plant, textile industries and for domestic uses during winter to heat the room.

Types of boilersHorizontal, Vertical and Inclined boilersFire tube and water tubeExternally fired and internally firedForced circulation and natural circulationHigh pressure and low pressureStationary and portable(moving)Single tube and multi tube

Horizontal, vertical or inclinedIf the axis of the boiler is horizontal, vertical or inclined then it is called horizontal, vertical or inclined boiler respectively.

Fire tube and water tubeIf hot gases are inside the tube and water is outside the tube, it is called fire-tube boiler.Examples: Cochran, Lancashire and locomotive boilers.If water is inside the tube and hot gases are outside the tube, it is called fire-tube boiler.Examples: Babcock and Wilcox, Sterling, Yarrow boiler etc.

Externally fired and internally firedThe boiler is known as externally fired if the fire is outside the shell.Examples: Babcock and Wilcox, SterlingThe boiler is known as internally fired if the furnace is located inside the boiler shell.Examples: Cochran, Lancashire

Forced circulation and natural circulationIn forced circulation type of boilers, the circulation of water is done by a forced pumpExamples: Velox, Lamont, Benson boiler

In natural circulation type of boilers, circulation of water in the boiler takes place due to natural convection currents produced by the application of heat.Examples: Lancashire, Babcock and Wilcox

High pressure and low pressureThe boilers which produce steam at pressures of 80 bar and above are called high pressure boilers.Examples: Babcock and Wilcox, Velox, Lamont, Benson boilers.

The boilers which produce steam at pressure below 80 bar are called low pressure boilers.Examples: Cochran, Cornish, Lancashire and locomotive boilers.

Stationary and portableStationary boilers are used for power plant-steam, for central station utility power plants, for plant process steam etc.

Mobile or portable boilers include locomotive type, and other small unit for temporary use at sites.

Single tube and multi tubeThe fire tube boilers are classified as single tube or multi-tube boilers, depending upon whether the fire tube is one or more than one.Examples of single tube boilers are Cornish and simple vertical boiler

Parts and terms in BoilerShellConsists of one or more steel plates bent into a cylindrical form and riveted or welded together. The shell ends are closed with end platesGrateIt is a platform in the furnace upon which fuel is burntFurnaceIt is the chamber formed by the space above the grate and below the boiler shell, in which combustion takes place. RefractoryInsulation material used for lining combustion chamberCombustion chamberPart of furnace where combustion of fuel takes place.SettingThe primary function of setting is to confine heat to the boiler and form a passage for gases. It is made of brick work and may form the wall of the furnace and combustion chamberWater space and steam spaceThe volume of the shell that is occupied by the water is termed as water space while the entire shell volume less the water and tubes is called steam space.Water levelThe level at which water stands in the boiler is called water level.RefractoryInsulation material used for lining combustion chamber.

Boilers and PartsFoamingFormation of steam bubbles on the surface of boiler water due to high surface tension of water.ScaleA deposit of medium due to extreme hardness occurring on the water heating surfaces of boiler because of an undesirable condition in the boiler water.Blowing offThe removal of mud and other impurities of water from the lowest part of the boiler. Accomplished with the help of blow off cock or valve.LaggingInsulation wrapped on the outside of the boiler shell or steam piping.PrimingItis 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 totalsalinityor TDS. The degree of priming also depends on the design of the boiler and its steaming rate.MountingsEquipment and devices, which are used for safety of boiler are called mountings, these are required parts for the successful operation of a boiler.Ex. Feed-check valve, safety Valve etc.

AccessoriesThe items which are used for increasing the boiler efficiency are called accessories.Ex. Super heaters, Steam separators etc.BOILER MOUNTINGSImportant boiler mountings are as follows,

Pressure gauge

Safety valves

Fusible plug

Steam stop valve

Feed check valve

Blow off cock

Mud and man holes

Water level Indicator

PRESSURE GAUGE

Fitted in front of the boiler to record the steam pressure at which steam is generated in the boiler.

Two types of pressure gauges are being used in boiler operations.Diaphragm typeBourdon Tube

A bourdon pressure gauge in its simplest form consists of a simple elastic tube, one end of the tube is fixed and connected to the steam space in the boiler, other end is connected to a sector through a link.

Diaphragm TypeSAFETY VALVE

Safety valves are needed to blow off the steam when the pressure of the steam in the boiler exceeds the working pressure. It is placed on the top of the boiler.

Dead weight safety valveLever safety valveSpring loaded safety valveHigh steam and low water safety valve

Dead weight safety valve

Lever safety valve

Spring loaded safety valve

High steam and low water safety valveFUSIBLE PLUG

To extinguish fire in the event of water level in the boiler shell falling below a certain specified limit.It is installed below boilers water level on the crown plate

Fusible PlugSTEAM STOP VALVE

A valve is a device that regulates the flow of a fluid (gases , fluidized solids slurries or liquids) by opening or closing or partially obstructing various passageways

Function : to shut off or regulate the flow of steam from the boiler to the steam pipe or steam from the steam pipe to the engine

FEED CHECK VALVETo allow the feed water to pass in to the boiler

To prevent the back flow of water from the boiler in the event of the failure of the feed pump

BLOW OFF COCK To drain out water from the boiler for internal cleaning inspection or other purposes

WATER LEVEL INDICATORA: End plate of boilerH & J: Two ballsB & C: Hollow gun metal castingK: Drain cockD & E: CocksL: Guard glassF: Gauge glassM,N,P& R: Screwed capsG: Hollow metal columnX,Y: FlangesThe function of water level indicator is to show level of water present in the boiler.BOILER ACCESSORIES

Accessories are the devices being used to increase the efficiency of the boiler. A large amount of heat is being carried out by the flue gases, this is wastage of useful energy, which can be recovered. Accessories are those equipment which recovers the wastage along with smoothing the operation to increase the utilization of energy as well as reducing the cost of operation. The waste recovery takes place by the help of flue gases, which has a large amount of heat.Accessories are not the mandatory parts or devices but being used for efficient operation.

Commonly used accessories are as follows,Steam Super heaterEconomizerAir preheaterSteam separatorFeed PumpInjectorSTEAM SUPER HEATER

The function of a super heater is to increase the temperature of steam above its saturation point. That means it gives assurance of the quality of steam. During superheating pressure of steam remains same but the volume increases with its temperature, increasing the internal energy which in turns prove to increase in kinetic energy, resulting in

Reduction of steam consumption of turbine.Reduction in losses due to condensation in steam pipes.Elimination of erosion of turbine bladesIncrease in efficiency.

There are two types of super heaters.Convective Super heaterRadiant Super heater

ECONOMIZER

In best way it is known as feed water heater, that refers heating of feed water, which is supplied to the boiler shell to get vaporized. It utilizes heat carried out but the waste furnace gases to heat the water before it enters boiler. By increasing the temperature of water, chilling of the boiler surface is prevented and then a less amount of sensible heat is required to achieve saturation temperature, it reduces then the input heat to the boiler and increasing efficiency.

There are two types of economizerIndependent type (not a part of boiler)Integrated type (a part of boiler)

AIR PREHEATER

The function of an air preheater is to heat the inlet air before it is sent to the furnace. It is placed after economizer, flue gases coming from economizer is being utilized to heat air. Preheated air accelerates combustion and increasing the amount of heat produced.

Degree of preheating depends uponType of fuelType of fuel burning equipmentRating of the boiler and furnace

Two types of preheaters commonly used,Recuperative type (Both the fluids pass simultaneously)Regenerative type (Fluids pass alternatively)

STEAM SEPARATOR

The basic work of steam separator is to ensure the quality of steam, steam from the boiler may be in the form of wet steam, or incase of regenerative cycle, where condensate from turbine is supplied back or being used by smaller capacity turbines to recover heat, steam must be in the wet format. Steam separator removes water particles.

There are three types of steam separatorImpact or baffle typeReverse current typeCentrifugal type

Baffle type steam separatorFEED PUMP

Feed pumps is the device required to supply water to the boiler. The quantity of feed water should be at least equal to the amount of steam delivered to the turbine or required space. For open cycle boiler in case of large plants, where there is no condenser or the amount of feedback water is less, pumps are inevitable.

There are two types of feed pumps,Reciprocating feed pump (Piston cylinder arrangement)Single actingDouble actingRotary or centrifugal feed pump

Duplex reciprocating pump

INJECTOR

The basic work of an injector is to feed water to the boiler on high pressure, it finds its application in such places where there is no space to install feed pumps. It works by the help of steam pressure in a way that the pressure of steam is being utilised to increase the kinetic energy of feed water.

AdvantagesLow initial costSimplicityCompactnessNo dynamic partsHigh thermal efficiency

DisadvantagesLow pumping efficiencyCant work for very hot steamIrregularity in the operation when steam pressure varies considerablyInjectorSTUDY OF BOILER

Fire tube boiler (Cochran boiler)Water tube boiler (Babcock and Wilcox boiler)

FIRE TUBE BOILER(COCHRAN BOILER)IntroductionIts a multi tube internally fired, fire tube boiler, as an improvement of traditional vertical boiler, providing more heating space relatively.

ConstructionIt consists of a vertical cylindrical shell with a hemispherical top and hemispherical shaped furnace. Furnace has ash pit at the bottom end above which fire grate lies. Combustion chamber of the furnace widens to connect the flue passage pipes, lined with fir bricks and insulated with refractory materials on the shell side. Smoke box is fitted in the hinged door for easy access to tubes, and allow easy cleaning. A number of horizontal fire tubes of equal lengths and diameter connects the combustion chamber with smoke box. Furnace is surrounded by water on all the side except the opening for fire door and combustion chamber.

WATER TUBE BOILER(BABCOK & WILCOX BOILER)

This is a type of water tube boiler used when pressure exceeds 10 bar and capacity 7000 kg per hour.

ConstructionIt consists of a horizontal high pressure drum, from each end of it connections are made with an uptake and a down take header. Headers are joined to each other by large number of water tubes inclined at an angle of 15 degree to provide water circulation. Hand holes are provided for the maintenance of tubes. The entire assembly of water tubes is hung in a room made of masonry work, lined with fire bricks to resist thermal expansion. As super heater additional U-shaped tubes are arranged between drums and water tubes.Furnace is arranged below the uptake header. Baffles are provided across the water tubes to guide flue gases. A Chimney is provided for exit of gases and a damper at the opening of chimney to provide draught. Cleaning doors are provided to access the tubes for cleaning and removal of soot and various mountings for successful operation.Advantage of Babcock and Wilcox boiler

Suitable for all types of fuels and hand stokers for firing.

Draught loss is small.

All components are accessible for inspection during operation.

Expansion and contraction has no harm on masonry work(construction).

Replacement of defective tubes are easy.

Applications of Cochran boiler and Babcock and Wilcox boiler

Cochran boiler finds its application in cranes, excavators and in small factories for general contract work where temporary supply of steam is required.

Babcock and Wilcox boiler finds its application in sugar mills and textile industries for power generations and processing works.COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS

Particulars Fire-tube boilersWater-tube boilersPosition of water and hot gasesHot gases inside the tubes and water outside the tubesWater inside the tubes and hot gases outside the tubesMode of firingGenerally internally firedExternally firedOperation pressureLimited to 16 barCan go up to 100 barRate of steam productionLowerHigherSuitabilityNot suitable for large power plantsSuitable for large power plantsRisk on bursting Involves lesser risk of explosion due to lower pressureMore risk on bursting due to high pressureFloor areaFor a given power it occupies more floor areaFor a given power it occupies less floor areaConstructionDifficultSimple COMPARISON OF FIRE TUBE AND WATER TUBE BOILERSParticularsFire-tube boilersWater-tube boilersTransportationDifficult SimpleShell diameterLarge for same powerSmall for same powerChances of explosionLess MoreTreatment of waterNot so necessaryMore necessaryAccessibility of various partsVarious parts not so easily accessible for cleaning, repair and inspectionMore accessibleRequirement of skillRequire less skill for efficient and economic workingRequire more skill and careful attention