BASICS OF MECHANICAL ENGINEERING (ME-107)

Transcription

1 BASICS OF MECHANICAL ENGINEERING (ME-107) Lectures on STEAM BOILERS Introduction Boiler 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 boilers Horizontal, Vertical and Inclined boilers Fire tube and water tube Externally fired and internally fired Forced circulation and natural circulation High pressure and low pressure Stationary and portable(moving) Single tube and multi tube

2 Horizontal, vertical or inclined If the axis of the boiler is horizontal, vertical or inclined then it is called horizontal, vertical or inclined boiler respectively. Fire tube and water tube If 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 fired The boiler is known as externally fired if the fire is outside the shell. Examples: Babcock and Wilcox, Sterling The boiler is known as internally fired if the furnace is located inside the boiler shell. Examples: Cochran, Lancashire Forced circulation and natural circulation In forced circulation type of boilers, the circulation of water is done by a forced pump Examples: 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

3 High pressure and low pressure The 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 portable Stationary 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 tube The 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

4 Parts and terms in Boiler Shell Consists of one or more steel plates bent into a cylindrical form and riveted or welded together. The shell ends are closed with end plates Grate It is a platform in the furnace upon which fuel is burnt Furnace It is the chamber formed by the space above the grate and below the boiler shell, in which combustion takes place. Refractory Insulation material used for lining combustion chamber Combustion chamber Part of furnace where combustion of fuel takes place. Setting The 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 chamber Water space and steam space The 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 level The level at which water stands in the boiler is called water level. Refractory Insulation material used for lining combustion chamber.

5 Boilers and Parts

6 Foaming Formation of steam bubbles on the surface of boiler water due to high surface tension of water. Scale A 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 off The 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. Lagging Insulation wrapped on the outside of the boiler shell or steam piping. Priming It is the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the super heaters and in the turbines. Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes aggravated by impurities in the boiler-water. Some mechanical entertainment of minute drops of boiler water in the steam always occurs. When this boiler water carryover is excessive, steam-carried solids produce turbine blade deposits. The accumulations have a composition similar to that of the dissolved solids in the boiler water. Priming is common cause of high levels of boiler water carryover. These conditions often lead to super heater tube failures as well. Priming is related to the viscosity of the water and its tendency to foam. These properties are governed by alkalinity, the presence of certain organic substances and by total salinity or TDS. The degree of priming also depends on the design of the boiler and its steaming rate.

7 Mountings Equipment 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. Accessories The items which are used for increasing the boiler efficiency are called accessories. Ex. Super heaters, Steam separators etc.

8 BOILER MOUNTINGS Important 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

9 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 type Bourdon 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.

10 Diaphragm Type

11 SAFETY 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 valve Lever safety valve Spring loaded safety valve High steam and low water safety valve Dead weight safety valve Lever safety valve

12 Spring loaded safety valve High steam and low water safety valve

13 FUSIBLE PLUG To extinguish fire in the event of water level in the boiler shell falling below a certain specified limit. It is installed below boiler s water level on the crown plate Fusible Plug

14 STEAM 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

15 FEED CHECK VALVE To 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

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

17 WATER LEVEL INDICATOR The function of water level indicator is to show level of water present in the boiler. A B&C D&E F G : End plate of boiler : Hollow gun metal casting : Cocks : Gauge glass : Hollow metal column H&J : Two balls K : Drain cock L : Guard glass M,N,P& R: Screwed caps X,Y : Flanges

18 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 heater Economizer Air preheater Steam separator Feed Pump Injector

19 STEAM 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 blades Increase in efficiency. There are two types of super heaters. 1. Convective Super heater 2. Radiant Super heater

20 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 economizer 1. Independent type (not a part of boiler) 2. Integrated type (a part of boiler)

21 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 upon 1. Type of fuel 2. Type of fuel burning equipment 3. Rating of the boiler and furnace Two types of preheaters commonly used, 1. Recuperative type (Both the fluids pass simultaneously) 2. Regenerative type (Fluids pass alternatively)

22 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 separator 1. Impact or baffle type 2. Reverse current type 3. Centrifugal type Baffle type steam separator

23 FEED 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, 1. Reciprocating feed pump (Piston cylinder arrangement) Single acting Double acting 2. Rotary or centrifugal feed pump Duplex reciprocating pump

24 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. Advantages 1. Low initial cost 2. Simplicity 3. Compactness 4. No dynamic parts 5. High thermal efficiency Disadvantages 1. Low pumping efficiency 2. Can t work for very hot steam 3. Irregularity in the operation when steam pressure varies considerably Injector

25 STUDY OF BOILER Fire tube boiler (Cochran boiler) Water tube boiler (Babcock and Wilcox boiler) FIRE TUBE BOILER (COCHRAN BOILER) Introduction It s a multi tube internally fired, fire tube boiler, as an improvement of traditional vertical boiler, providing more heating space relatively. Construction It 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.

26 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. Construction It 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.

27 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.

28 COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS Particulars Fire-tube boilers Water-tube boilers Position of water and hot gases Hot gases inside the tubes and water outside the tubes Water inside the tubes and hot gases outside the tubes Mode of firing Generally internally fired Externally fired Operation pressure Limited to 16 bar Can go up to 100 bar Rate of steam production Lower Higher Suitability Not suitable for large power plants Suitable for large power plants Risk on bursting Involves lesser risk of explosion due to lower pressure More risk on bursting due to high pressure Floor area For a given power it occupies more floor area For a given power it occupies less floor area Construction Difficult Simple

29 COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS Particulars Fire-tube boilers Water-tube boilers Transportation Difficult Simple Shell diameter Large for same power Small for same power Chances of explosion Less More Treatment of water Not so necessary More necessary Accessibility of various parts Various parts not so easily accessible for More accessible cleaning, repair and inspection Requirement of skill Require less skill for efficient and economic working Require more skill and careful attention