Boiler Mountings and Accessories
These are the fittings, which are mounted on the boiler for its proper and safe functioning. Though there are many types of boiler mountings, yet the following are important from the subject's point of view:
1. Water level indicator
2. Pressure gauge
3. Safety valves
4. Stop valve
5. Blow off cock
6. Feed check valve
7. Fusible plug.
Water Level Indicator: It is an important fitting, which indicates the water level inside the boiler to an observer. It is a safety device, upon which the correct working of the boiler depends. This fitting may be seen in front of the boiler, and is generally two in number.
|Fig: Water level indicator.|
Pressure Gauge: A pressure gauge is used to measure the pressure of the steam inside the steam boiler. It is fixed in front of the steam boiler. The pressure gauges generally used are of Bourden type.
|Fig: Pressure gauge.|
Safety Valves: These are the devices attached to the steam chest for preventing explosions due to excessive internal pressure of steam. A steam boiler is, usually, provided with two safety valves.These are
directly placed on the boiler. In brief, the function of a safety valve is to blow off the steam when the pressure of steam inside the boiler exceeds the working pressure.
The following are the four types of safety valves:
1. Lever safety valve.
2. Dead weight safety valve.
3. High steam and low water safety valve.
4. Spring-loaded safety valve.
Blow-off Cock: The principal functions of a blow-off cock are to empty the boiler whenever required and to discharge the mud, scale, or sediments that are accumulated at the bottom of the boiler.
It is fitted to the bottom of a boiler drum and consists of a conical plug fitted to the body or casing.
|Fig: Blow off Cock.|
Schematic Diagram of a Boiler plant:
|Fig: Schematic diagram of a boiler plant.|
These are the devices that are used as integral parts of a boiler, and help in running efficiently.
Though there are many types of boiler accessories, yet the following are important from the subject point of view:
1. Feed pump
4. Air preheater.
Feed Pump: We know that water, in a boiler, is continuously converted into steam, which is used by the engine. Thus, we need a feed pump to deliver water to the boiler. The pressure of steam inside a boiler is high. So, the pressure of feed water has to be increased proportionately before it is made to enter the boiler.
Generally, the pressure of feed water is 20% more than that in the boiler.
Superheater: A superheater is an important device of a steam-generating unit. Its purpose is to increase the temperature of saturated steam without raising its pressure. It is generally an integral part of a boiler, and is placed in the path of hot flue gases from the furnace. The heat, given up by these flue gases, is used in superheating the steam.
Economizer: An economizer is a device used to heat feed water by utilizing the heat in the exhaust flue gases before leaving through the chimney. As the name indicates, the economizer improves the economy of the steam boiler.
A well-known type of economizer is the Greens economizer. It is extensively used for stationary boilers, especially those of Lancashire type.
Following are the advantages of using an economizer:
1. There is about 15 to 20% of coal savings.
2. It increases the steam-raising capacity of a boiler because it shortens the time required to convert water into steam.
3. It prevents the formation of scale in boiler water tubes, because the scale formed in the economizer tubes, can be cleaned easily.
4. Since the feed water entering the boiler is hot, therefore strains due to unequal expansion are minimized.
Air Preheater: An air preheater is used to recover heat from the exhaust flue gases. It is installed between the economizer and the chimney. The air required for combustion is drawn through the air preheater where its temperature is raised. It is then passed through ducts to the furnace.
The air is passed through the tubes of the heater internally while the hot flue gases are passed over the outside of the tubes.
The following advantages are obtained by using an air preheater:
1. The preheated air gives a higher furnace temperature which results in more heat transfer to the water and thus increases the evaporative capacity per kg of fuel.
2. There is an increase of about 2% in the boiler efficiency for each 35-40° C rise in the temperature of the air.
3. It results in better combustion with less soot, smoke, and ash.
4. It enables low-grade fuel to be burnt with less excess air.