Mountings and Accessories of a Boiler | Thermodynamics

Here is a list of twelve major mountings and accessories of a boiler.

1. Dead Weight Safety Valve:

As the name implies, this valve is used to assure safe working of the boiler. When the pressure of steam inside the boiler drum increases beyond the safety valve, it may cause damage to the pressure part, or even a severe accident. It can be prevented by allowing the surplus steam to escape to the atmosphere, by means of the safety valve.

This valve was first introduced by M/s. J.J. Hopkinson and Co. Ltd. It consists of a vertical pipe A, bottom end of which is flanged and through this flanged end it is mounted on the mounting blocks of the boiler. B is the Gun metal valve, resting over the valve seat C. Valve seat C is fixed on the top of the pipe A by means of a securing ring and screw. F is the feather which guides the valve B. Valve B is also guided by a ring in steam discharge casing. The weight carrier is suspended from the top of the valve B. Dead weights are placed on the weight carrier and these are covered by the Cast Iron cover.

Weight of the valve B, spindle, carrier, dead weights and cover will be acting downwards. During normal working conditions, this downward acting load will be balanced by the steam pressure from below. When the steam pressure exceeds pre specified or safe valve, then the valve will be lifted up and the surplus steam will be exhausted to the atmosphere.

Example- Dead weight safety valve used on pressure cookers.

2. High Steam Low Water Alarm:

This valve was also introduced by M/s Hopkinson and Co. Ltd. This is a two-in-one valve, i.e., it consists of 2 valves.

(i) High steam valve – During high steam conditions this valve opens and allows surplus steam to escape to the atmosphere.

(ii) Low water valve – When the water level in the boiler dram falls below the safe level, then this valve opens and allows the steam to escape. When the steam is allowed to escape, it makes whistling sound i.e., it gives warning to the boiler attendant. The whistling sound for high steam conditions will be louder than low water conditions, so that one can distinguish between the two. Since it makes whistling sound it is known as alarm.

As shown in Fig. 11.13, A is the high steam valve, resting over the valve seat B. Valve seat B in secured in position by means of a ring and feather S is cost on it. This feather guides the valve A. C is the low water valve, which is resting over the inner edge of the valve A as shown. R is the rib which guides the valve C. Low water valve. C will be held in position by means of the Dead weight, D which is connected to the rod E. which position by means of the weight D and partly by means of weight and counter weight outside the steam discharge casing.

In the steam space lever FK is provided. A large fire brick float L is suspended from the end K. During normal working conditions, the float will be submerged in water and the weight of this floats in partly balanced by means of counter weight M and partly by means of buoyant force due to water, which is acting in the upward direction. But when the water level goes below the safe level, the float gets uncovered by means of water, then there won’t be buoyant force and counter, weight. M alone will not be sufficient to balance the float. So, the lever FK tilts in the clockwise direction. Then the knife edge projection of the lever FK comes in contact with the collar N and in turn lifts the hemispherical valve C. When the valve C is lifted, steam escapes and makes whistling sound.

During high steam conditions, both the valves are lifted since the low water valve is resting over inner edge of the high steam valve C is lifted, steam escapes and makes whistling sound.

During high steam conditions, both the valves are lifted since the low water valve is resting over inner edge of the high steam valve, and surplus steam will be exhausted.

3. Water Level Indicator:

It indicates the water level inside the boiler drum. This water level can be made visible from the boiler room floor by using mirrors. This unit was also introduced by M/s Hopkinson and Co. Ltd.

The unit consists of two hollow gun-metal stuffing boxes. One end of each of the stuffing box will be flanged and through this flanged end it is connected to the front end plate of the boiler. Upper stuffing box is connected to the steam space and lower stuffing box is connected to the water space. In between the two stuffing boxes a strong glass tube is fitted. (In between the glass and metal, rubber packing is provided in order to have gas tight joint).

Thus two valves A and B which control the passage of steam and water between the boiler and the glass tube. When these valves are open, the handles are vertical and the water level in the glass tube will be the water level in the boiler drum. A third valve C called as Blow-off valve is generally closed. When this valve is closed, its handle is vertical and to blow out sediments and impurities handle is to be made horizontal.

An arrangement is also provided, in order to shut off automatically the supply of steam and water to the glass tube, if the glass tube breaks due to any reason. This arrangement consists of a hollow gun-metal column D and two steel balls as shown.

During normal working conditions these balls will be in position shown by full circles if the glass tube breaks the rush of water in the bottom passage carries the ball into the position shown by dotted circle and shuts off the supply of water. At the same time the steam rushing through the upper passage aided by the water rushing upwards through the column D carries the ball into the position shown by dotted circle and shuts off the steam supply. Then the boiler attendant can safely close the valves A and B and can replace the tube.

Screw plugs provided at the end of stuffing boxes, give access for cleaning of passages. The glass tube is generally covered from the front and from sides by Guard glass in order to protect the boiler attendant from the flying fragments of glass tube, if it breaks.

4. Fusible Plug:

It actuates itself as the last resort, when the water level in the boiler drum falls below the minimum permissible water level. As a result of which, fusible metal melts by the heat of steam, the plug drops out and the steam rushes into the combustion chamber and extinguishes the fire. Thus it safeguards the combustion chamber crown from burning and also warns the boiler attendant about the fall of water level in the drum below minimum permissible level.

Function:

Fusible plug is fitted on the combustion chamber crown, but in the drum. As shown in Fig. 11.16, A is the hollow gun metal plug screwed into the crown plate. B is the second hollow gun metal screwed into the plug A and C is the third solid copper conical plug. The inner surface of B and C for removal and refitting purpose by using spanners.

Under normal working conditions, fusible plug will be covered by means of water in the drum. This water keeps the temperature of the fusible metal below its melting point. But when due to any reason, water level in the drum falls below the minimum permissible water level, the plug gets uncovered from water and is exposed to steam.

Due to heat of steam, fusible metal melts and copper plug drops and the steam rushes through the access into the combustion chamber. Before referring the boiler, the attendant removes and refits the plug after interposing the fusible metal between the plugs B and C.

5. Bourdon Pressure Gauge:

This is the most common type of pressure gauge. This is used to measure pressure inside the containers and pipe lines etc.

The basic element of this gauge is the tube A. It is elliptical in cross section and is bent into and arc of circle as shown in Fig. 11.17.

End B of this tube is sealed, while open end C is connected to the connecting union D. Through the connecting union the gauge will be mounted over the containers or pipes of whose pressures are to be measured.

When it is mounted on the systems and if the pressure of the system is more than atmosphere (i.e., + ve pressure) then the tube will tend to curl out. Conversely if the pressure of the system is less than the atmosphere, (i.e., – ve pressure) the tube will tend to curl in. Thus the change of pressure will therefore appear as a movement of the end B. This end B is connected by means of a link E to a quadrant gear F. The quadrant gear engages with a small gear G on to which a point H is attached.

Thus then change of pressure will appear as the movement of the end B, which will be transmitted to the quadrant gear, which in turn rotate the pinion and hence the pointer. The pointer rotates over the calibrated scale, which directly gives the pressure readings.

6. Steam Stop Valve:

Through the steam stop valve, steam will be taken out for power generation purpose.

As shown in Fig. 11.18 valve body is made up of C.I. Through its bottom flange A it is mounted in the boiler drum, and its side flange F is connected to the pipe through which steam goes to the steam engine or steam turbine.

It is clear from full figure that when the hand wheel is rotated in clockwise or anticlockwise direction, the valve will move up and down. And the required quantity of steam can be taken out for power generation.

7. Feed Check Valve:

Through the feed check valve, water enters the boiler drum. It allows the water to flow into the drum and it will not allow the water to flow out of the drum, so it is called as check valve.

8. Blow off Valve:

Mud or any other settled impurities are blown out through this valve.

9. Air Preheater:

The air is heated, by using the heat of flue gases in a device called preheater, which otherwise would have been lost to the atmosphere. By using air preheater the overall efficiency will increase by about 10%.

10. Feed Pump:

It is an appliance used to pump the feed water into the boiler drum. The feed pump may be

11. Steam Injector:

When the feed pump fails due to any reasons, then steam injector, injects or pumps or delivers feed water to the boiler drum, by the use of steam.

12. Steam Trap:

Steam trap automatically collects and returns to the boiler, the water resulting from partial condensation of steam, without allowing any steam to escape.