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APPENDIX: STEAM BOILERS AND REFRIGERATION SYSTEMS
Boiler Mountings
These are the fittings and devices necessary for the safety of the boiler and complete control of the process of steam generation. According to Indian Boiler Act Regulation, the following mountings should be always fitted to a boiler.
- Two safety valves
- Two water level indicators
- A pressure gauge
- A feed check valve
- A fusible plug
- A steam stop valve
- A blow-off cock
- A man hole
- An attachment for inspector’s test gauge
1. Safety Valves
As per Boiler Act Regulation every boiler must be fitted with two safety valves for precautionary measures.
Function
The function of the safety valve is to blow off steam from the boiler to the atmosphere when the pressure of the steam inside the boiler exceeds the working pressure. As soon as the steam pressure inside the boiler exceeds the external force equivalent to rated pressure, the safety valve opens automatically and the excess steam rushes out to the atmosphere till the internal pressure drops down back to its normal value.
Location These are the devices attached to the steam chest generally on the top of the boiler shell.
Types There are four types of safety valves
a) Dead weight safety valve b) Lever safety valve c) Spring-loaded safety valve d) High-steam low water safety valve
Three of these four safety valves are described below:
A Dead Weight Safety Valve
Construction : Figure 1 shows a dead weight safety valve. It consists of a large vertical steel pipe with a flange at the bottom for fitting it on the boiler shell. At the top of the pipe, valve seat is fixed. The valve rests on this valve seat. The valve supports a large weight carrier in which dead weight rings are placed and enclosed by cover. Thus the total load on the valve is made up of the weight of the carrier, the dead weight rings, the cover and the valve itself.
Figure 1: Dead Weight Safety Valve
Working : When the force due to steam pressure is less than the total load of the valve, the steam will not escape. When the force due to the steam pressure is equal to the total load of the valve, it will be in the position of just blowing. When the force due to steam pressure exceeds the total load of the valve, it lifts and the steam escapes through the discharge pipe. The valve returns automatically when pressure drops to its normal value. Any steam condensed while passing through the valve is drained through the drain pipe line. The uplift of the valve is guided by the feather.
Advantages
- Simple in construction.
- Reliable with regard to Satisfactory performance during operation
- Suitable for low-pressure stationery boilers. 4 it cannot be readily tempered with because any change in weight made by any unauthorized personnel can be easily detected.
Disadvantages
- Large amount of weight required.
- The effective weight changes when the boiler is on the slope
- Unsuitable for marine and locomotive boilers 4 Unsuitable for high-pressure boilers because a large amount of weights are required to balance th steam pressure.
C. Spring-Loaded Safety Valve
Construction: Figure 3 shows Rams bottom spring-loaded safety valve, usually employed for all the boilers. It
consists of a casing having two valve chess for flow of the steam. Valve seat is fixed at the upper end of both valve chests. Valves are held down on their seats by helical spring and lever arrangement. The lever is provided with two conical pivots. One pivot is integrally forced with the lever while the other is pin connected to its one end. The helical spring has one end fixed to the lever and the other end to a shackle fixed on the valve chest. Thus spring pulls the lever downward and applies load on the valve. A link is also provided to prevent the valves being blown away in case of spring break.
Figure 3: Spring-loaded safety valve
Working : When the boiler pressure is less than or equal to the working pressure equivalent to the thrust of spring over the valve, it remains on its seat firmly. But when the boiler pressure exceeds the working pressure, the valve automatically lifts, opening the passage for steam to escape to the atmosphere until boiler pressure falls back to its working pressure. So the spring is subjected to a tensile load during release of excess pressure. The valve spring pressure can be altered by tightening or loosening the screw nut. The lever projected on one side has an extension projected into the driver’s cabin. This is for manual lifting of the valve and blowing off the steam in order to ensure satisfactory working of the device and the valves are not stuck to their seats.
Advantages
- Elimination of heavy weights.
- Easy maintenance and examination
- Not affected by jerks, pitching and rolling.
- Suitable for stationery and mobile boiler.
- Suitable for both high- and low-pressure boilers.
Disadvantages
- The load on the valve increases with the lift of the valve as stiffness of the spring is not constant. This means that the pressure required to open it is less than that required to open it fully.
2. Water-Level Indicator
Function The function of the water-level indicator is to show the level of water in the boiler.
Location It is fixed at the front end of the boiler to make it easy visible to the attendant. To carry out the function, its upper end opens to the steam space while the lower end opens in the water space.
Construction Figure 4 shows a Hopkinson’s water-level indicator. It consists of glass tube which is very tough and hard to withstand boiler pressure. The stuffing boxes connect the ends of the glass tube to the metal pipe flanged to the front plate of the boiler. A steam cock is fixed in the upper pipe controlling passage between steam space and the glass tube. A water cock is fixed in the lower pipe controlling passage between water space and the glass tube. A third cock known as the drain cock is fixed to drain out the sediments and foreign matter deposits in water-level indicator at a regular interval. To make the drainage comfortable, a hollow metal column with their ends joining both upper and lower pipe is provided just above the drain cock. A red mark on the glass tube indicates the safe water level.
Figure 4: Water level Indicator
Working At normal working condition, the steam cock and the water cock is in open position and the drain cock is in close position. During this, the handles of all the cock are in vertical position. The glass tube indicator shows the level of water inside the boiler. As the indicator tube is made up of glass, there is a possibility of its breakage. So a provision is made for automatically shutting off the steam and water supply to the glass tube. For this purpose, two balls are provided whose position under normal working condition is shown in the figure. When the glass tube breaks, the steam and water at a high pressure try to rush out with a high velocity from upper pipe end and the lower pipe end, respectively. This rushing out steam carries the bails along with it at a position, shown by dotted ball in the figure, which block the water and steam escaping from the boiler. The operator can now easily turn off the cocks and replace a new gauge glass tube. To protect the operator from the flying pieces of the broken glass, the glass tube is covered with a guard made up of tough glass.
4. Feed Check Valve
Function The function of feed check valve is to regulate the supply of the water under pressure into the boiler and simultaneously to prevent the back flow of water from the boiler in case of the pump failure or stopped.
Location It is fitted in the water space of the boiler, slightly below the working level of the water in the boiler. Construction Figure 6 shows the schematic diagram of a feed check valve. It consists of a body with two flanges at right angle to each other. Flange F1 is for bolting it to the boiler shell and flange F2 is for bolting it to make connection with the feed pump supply line. The body contains two valves; one is called feed valve while other is called check valve. The function of feed valve is to control rate of water supply to the boiler. Its position can be adjusted by turning the hand wheel of the feed valve. The raising and lowering of the feed valve also limits the maximum lift of the check valve. The function of the check valve is to allow the feed water to flow unidirectional and that too from the feed pump to the boiler. It moves automatically up and down within its limit due to pressure difference of water acting on its top and bottom side.
Working When it is desired to feed water to the boiler, the feed valve is raised up from its seat. Under normal working conditions, the pressure on the feed pump side is more than the boiler side. This pressure difference lifts the check valve and water flows from the side of the check valve into the boiler. In the event of failure of the feed pump, the pressure reduces on the feed pump side. The pressure exerting from the boiler side provides greater force on the top side of the check valve. This force closes the check valve and prevents the return flow of water from the boiler to the feed pump. In this manner the unidirectional flow is maintained.
Figure 6: Feed Check Valve
5. Fusible Plug
Function The function of fusible plug is to extinguish the fire in the furnace of the boiler when the water level in the boiler shell falls to an unsafe extent. Thus, it protects the boiler against damage due to overheating.
Location It is fitted at the crown plate of the furnace or the combustion chamber at the lowest permissible water level.
Construction Figure 7 shows the schematic diagram of a fusible plug. It consists of a hollow gun metal body having hexagonal flange at the bottom for screwing to the crown plate. On the upper portion of the body a hollow gun metal plug is screwed. There is another solid plug made of copper with a conical top and round bottom kept in the hole of gun metal plug and held firmly by fusible metal likes tin or lead as shown in the figure.
Working Under normal operating condition, the fusible plug is covered with water. The heat received by the fusible plug is conducted to the water in the boiler. This keeps the fusible plug metal at an almost saturation temperature of water, which is below the melting point of the fusible metal. When the water level falls below the safe level, the fusible plug gets exposed to steam. The heat received by the fusible plug is now conducted to the steam. As steam has poor conductivity compared to water, lesser heat is being conducted. So the plug gets overheated resulting into the melting of fusible metal. Therefore, the inner solid plug falls down thereby opening a passage between the steam space and the furnace. The steam and water under pressure immediately rush into the fire box through the opened passage and extinguish the fire. Before restarting the boiler, a new fusible plug should be screwed.
Figure 7: Fusible Plug
7. Blow-Off Cock
Function The function of blow' off cock is to (i) discharge periodically the sentiments and other impurities settled at the bottom of the boiler, (ii) empty the boiler for internal cleaning, inspection and repair and (iii) lower the water level rapidly, in case of excess feed water input to the boiler
Location The blow-off cock is fitted directly through an elbow- pipe at the lowest portion of the boiler shell.
Construction Figure 9 shows the schematic diagram of a blow-off cock. It consists of a casing secured with two flanges. One of the flanges is connected to the boiler and the other (outer) flange is connected to a discharge pipe which rakes the blow off water out of the boiler house. For regulating the discharge, casing consists of a plug valve. The plug valve is of conical shape having a hole/slot made into it and is fitted into a corresponding hole of the casing. The shank of the plug valve passes through the stuffing box to prevent leakage when the boiler is steaming. At the top of the plug valve a yoke is provided for fixing the spanner when the cock is in open position.
Working The figure shows the stop/shut-off position of the valve. When it is desired to discharge the water or sediments, the plug is rotated through a quarter so that the hole of the plug valve comes in line with the passage of the casing. The water will flow out of the boiler and will get discharged out of the boiler house. To stop the discharge, a quarter revolution of the plug is made either in clockwise or in anticlockwise direction. This brings the solid part of the plug valve in line with the passage of the casing and the flow of water stops. This indicates that during the shut-off position the axis of the hole of the plug remains perpendicular to the axis of the passage of the casing.
Figure 9: Blow-off cock
It may be noted that it the sediments will not be removed tor a long time, it will lead to accumulation of the sediments on the side wall of the furnace. This reduces the heat transfer from the crown plate to the water. The crown plate getting overheated will get fired, which will destroy the whole boiler. Thus periodic removal of the sediments is essential.
Two other mountings are not described and students are encouraged to prepare descriptive materials on them for completeness.
ACCESSORIES
These are the fittings and devices installed to increase the efficiency of the boiler and for smooth working of the plant. The following accessories are commonly employed in boilers:
- Economizer
- Air pre-heater
- Superheater
- Feed pump or injector
- Steam separator or steam driver
- Steam trap The economizer, the air pre-heater and the super-heater are discussed below and students are encouraged to add on the description of the feed pump, the steam separator and the steam trap.
1. Economizer
Function The function of economizer is to extract some of the waste heat carried by flue gases going to the chimney and utilize it for pre-heating the feed water supplied to the boiler. Location It is installed in the path of the flue gases between the boiler and the chimney. Construction Figure 10 shows Greens vertical tube economizer commonly employed in medium pressure range boilers. It consists of a large group of vertical tubes which are fitted in between two heaters. The groups of vertical tubes are built in section of 4, 6, 8, 10 or 12 tubes. The whole arrangement is placed in the passage of pipes of the against flue gases entering into the chimney. A safety valve is fitted on the top heater for the safety of the pipes against any high pressure of water that may be developed. A blow-off valve is also fitted at the lowest point of the economizer to discharge the sediments collected from the feed water. A bypass arrangement is provided to the economizer so that it may be put out of the action for necessary maintenance and inspection work. Working The feed water from the feed pump enters the bottom heater, passes through the vertical tubes, reaches the upper header and is fed to the boiler through feed check valve. The flue gases flow over the external surfaces of the vertical tubes. The water, while flowing upwards inside the vertical tubes, gains heat from the hot flue gases. The flow of flue gases over the vertical tubes is likely to deposit soot which will retard the heat transfer from hot flue gases to the water in the tube. To prevent the deposition of soot, a set of scrapper hung by a chain passing over the pulley is fitted over the vertical pipes. The mechanically driven pulley moves the scrapper up and down continuously over the vertical tubes and keeps the surfaces of the tubes free from soot. The scrapped away soot is collected in the soot chamber placed below the bottom heater and removed periodically through the soot door.
Figure 11: Air pre-heater
Location It is installed between the economizer and the chimney. Construction The air preheater is generally classified as recuperative-type air preheater and regenerative-type air preheater. The recuperative-type air preheater is either tubular-type air preheater or plate-type air preheater. Figure 11 shows the tubular-type air preheater that is commonly used in small boiler plants. It consists of an insulated tubular casing having a large number of tubes held by horizontal baffles. The baffles are placed in such a way that it gives the passage to fresh air in a defined manner. The upper and lower ends of the tubes are fixed to the flat plate so that flue gases and fresh air do not able to mix with each other. Working The flue gases are passed through the tubes and the air circulates around them. The air on its travel outside the tubes is forced to defect by baffles in a zig-zag path for a number of times. This ensures a better heat gain from the flue gases. Soot and other materials carried with the flue gases are collected in the hopper at the bottom and removed periodically through the soot gate.
3. Super-heater
Function The function of superheater is to remove the moisture from the steam coming out of the boiler and to increase the temperature sufficiently above the saturation temperature. Location It is located in the path of the furnace gases. Construction Superheater is classified on the basis of arrangement as (i) attached super-heaters (here single furnace is used for both boiler and superheater); (ii) separately fired superheaters (here superheater is placed in a separate furnace, other than the furnace used for heating water). Super-heater used for Locomotive and Babcock-Wilcox boilers. Figure 12 shows the attached-type superheater used in the Lancashire boiler. It consists of two headers connected by a number of U-shaped tubes called superheater tubes. The header through which boiler steam enters the superheater tubes is called saturate steam header and the other header through which the superheated steam is obtained is called discharge header. It is fitted with a safety valve so as to avoid overheating of the superheater tubes. Working
The steam generated in the boiler enters the superheater tube through the steam entrance valve and saturated steam header. In the superheater the steam gains heat from the flue gases flowing over the superheater tubes and get superheated. The superheated steam comes out through the discharge header
Figure 12: Super-heater
and is regulated by superheated steam valve control valve.1he wet steam of the boiler can be mixed to this superheated steam, if desired, by regulating the wet steam control valve. The amount of hot gases passed over the superheater tubes should be in proportion to the amount of superheated steam passing through the tubes. Otherwise, the tubes will be overheated. To avoid overheating, a damper is provided. When the damper is horizontal, the hot gases pass over the superheater tubes and when it is vertical, the Hue gases directly pass to the bottom flue.
Other Accessories Accessories other than the economizer, air preheater and superheater include feed pump or injector, steam separator or steam driver and steam trap. These are out of scope of the present text lecture material and students are encouraged to document the description of the three other accessories on their own from other sources of information.
