Boiler mountings are the devices which are necessary for the safe and satisfactory operation of the boiler. These devices are mounted over the shell or drum of the boiler.
In accordance with IBR, the following mountings must be fitted. There are eight important boiler mountings:
1. Pressure gauge (one)
2. Water level indicator or water gauge (two)
3. Safety valve (two)
4. Steam stop valve or Junction valve (one)
5. Blowdown valve or blow-off cock (one)
6. Feed check valve (one)
7. Fusible plug (one)
1. Pressure Gauge (Bourdon Pressure Gauge):
The function of the pressure gauge is to measure the pressure of steam inside the boiler. The gauge is usually mounted on the front top of the shell or drum such that it should be clearly visible to the operator. It consists of a Bourdon spring tube (B) of bronze metal. One end of this tube is connected with a hollow block (H) and the other end is closed and attached to a connecting rod (C).
The other end of the connecting rod is attached to a toothed sector (T) which gears with a pinion (P). At the center, there is a pointer (PO). The high- pressure steam enters the Bourdon spring tube through hollow block. The interior mechanism of the pressure gauge is shown in Fig. 3.8.
Due to the radial pressure acting on the internal surface of the tube, the tube becomes elliptical in shape and a force is exerted on the connecting rod which is transferred to the toothed sector and hence to the pinion and motion is transmitted to pointer which is fitted at the center and pointer gets moved.
Bourdon pressure gauge is one of the common forms of the gauge. The pressure measured by the gauge will be the gauge pressure. In practice, a U-shape tube filled with water is fitted in between the hollow block and the drum (steam stored space). It is to safe guard the bronze metal tubes with excessive temperature of steam.
2. Water Level Indicator:
The function of a water level indicator is to indicate the level of water in the boiler constantly. It is also called water gauge. It helps the operator in keeping the level constant from outside through a glass tube. In case of need, if the level falls down, water will be pumped by the operator. According to IBR, each boiler should be essentially fitted with two water gauges.
Figure 3.9 shows the water level indicator. It consists of a thick hard glass tube. The ends are connected with two hollow blocks that are connected with boiler end plate such that one end of the blocks is connected with steam space and other end with water storage. Parallel to the glass tube, there is another metallic tube.
In case, the gauge glass breaks, the rush of steam and water will push the steel balls thereby blocking the passage to the position as shown by black color stop and prevents water and steam to come out of the boiler shell or drum. The broken glass tube is replaced immediately to function it as usual.
3. Safety Valves:
The function of safely valve is to safeguard the boiler from the danger of pressure higher than the safe working pressure. Every boiler has a safe working pressure designed by the manufacturer and can be operated safely if it is operated within this pressure limit. The designed pressure may be more than the safe working pressure.
When the boiler is in operation, due to the increased rate of combustion, the rate of steam generation exceeds. This causes a sudden increase of pressure higher than the working pressure. At this instant, the excess steam produced must be instantaneously released from the boiler to reduce the pressure. A safety valve automatically discharges excess steam produced from the boiler by opening the valves provided in it.
The upward force exerted by the steam exceeds the downward force and hence valves lift up. This releases some quantity of steam till upward and downward forces become same, and hence the valve takes the normal position. According to IBR, boilers are fitted with two safety valves.
There are three types of safety valves:
(a) Dead weight safety valve
(b) Lever safety valve
(c) Spring-loaded safety valve
The spring-loaded safety valve is shown in Fig. 3.10.
4. Steam Stop Valve:
It is also known as junction valve. Steam stop valve is used to regulate the flow of steam from one steam pipe to the other or from the boiler to the steam pipe. The valve which is to regulate the steam from boiler to steam pipe is known as junction valve. The valve is mounted on boiler drum or shell. The valve fitted in pipeline to regulate the steam or the valve fitted in pipeline near steam prime mover to get this operated by regulating the steam is known as steam stop valve.
5. Blowdown Valve/Blow-Off Cock:
Blowdown valve is also known as blow-off cock. The function of the blowdown valve is to empty the boiler or may discharge a portion of water or steam periodically during the operation to blow out sediments or mud collected in the lower part of the boiler. It is also used to empty the boiler when necessary for internal inspection and cleaning.
6. Feed Check Valve:
It is a one-way valve and is used to control the supply of water to the boiler and also to prevent the escaping of water from boiler due to high pressure inside. When the level of water falls down, additional water is to be supplied by means of a centrifugal pump. The pressure inside boiler is high and therefore the pressure of the feed water is raised by the pump.
The feed water is supplied to the boiler at a high pressure through a feed check valve by lifting the valve upward. If pump is not in operation, the high-pressure steam inside the boiler keeps the valve on its position seated on the valve seat and does not permit the steam to escape (Fig. 3.11).
7. Fusible Plug:
The main purpose of a fusible plug is to extinguish the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level. Thus, explosion is avoided which may take place due to overheating of the tubes and shell. It is generally fitted over the crown of the furnace or over the combustion chamber. This is shown in Fig. 3.12. Plugs A and B are separated by fusible metal.
In normal case, when the boiler contains sufficient water, the plug also remains covered by water. But when the water level falls, the plug is exposed to steam. Because the steam temperature is larger than the water temperature, the fusible metal melts. Plug B drops down and the steam starts blowing to the combustion chamber to extinguish fire.