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Boiler Mountings and Accessories: Types, and Their Working.

Boiler mountings and accessories are required for the proper and satisfactory functioning of the steam boilers.

Boiler Mountings and Accessories

The boiler mountings are fittings which are mounted on the boiler for its proper functioning. Mountings are water level indicator, safety valve, pressure gauge, etc. It may be noted that a boiler cannot function safely without the mountings.

The boiler accessories are the devices, which form an integral part of a boiler but are not mounted on it. They include superheater, economiser, feed pump etc. It may be noted the accessories help in controlling and running the boiler efficiently.

Read also: Steam Boilers: Parts, Types, Classification, Advantages, Application, and More

Now let’s discuss different types of boiler mountings and accessories.

Boiler Mountings

Following are the important boiler mountings:

  1. Water level indicator
  2. Pressure gauge
  3. Safety valve
  4. Stop valve
  5. Blow off
  6. Feed check valve
  7. Fusible plug

Water Level Indicator

It is an main fitting in the boiler, Water level indicator indicates the water level inside the boiler. It is a safety device upon which safe working of the boiler depends.

Water level indicator diagram

Working Principle

Water-tube indicator the water consists of a vertical hard glass tube G which is fitted with two gunmetal tubes A and B. The tubes A connects the steam space of the boiler with the glass tube and the tube B connects the water space of the boiler with the glass tube.

The tube A is provided with a valve ‘S’, called a steam valve, and tube B is fitted with another valve ‘W’, called a water valve. In addition to these valves, a third valveD, called drain valve, is fitted to the water level indicator through which water together with condensed steam from the gunmetal tube A is drained from time to time.

Pressure Gauge

Pressure gauges are used to measure the pressure of steam inside a steam boiler. The pressure gauge is fixed in front of a steam boiler.

Pressure Gauge diagram

Working Principle

The pressure gauge shown in fig is bourdon pressure gauge. It consists of a circular spring tube A. One end of the bourdon tube is closed and connected to a link L and the other end is sequared to a Hollow block B. The link L connects the closed end of the tube to the toothed sector C which is hinged at O. The toothed sector gears with pinion D which carries a pointer P. The pointer moves on a dial graduated in pressure units.

Safety valves

These are the devices attached in the steam boiler for preventing explosions due to excessive internal pressure of steam.

Safety Valve diagram

The safety valve is an instrument which prevents the boiler pressure from rising above its normal working pressure by automatically opening when the boiler pressure exceeds the normal working pressure, Thus allowing excess steam to escape into the atmosphere until the pressure comes down to its normal valve. Thus, a safety valve ensures safety to a boiler from being damaged due to excessive steam pressure.

The safety valves commonly used are:

  1. Deadweight safety valve,
  2. Lever safety valve,
  3. Spring-loaded safety valve,

Steam stop valve

The function of a stop valve is to control the flow of the steam from within the boiler and to stop it completely when required. A stop valve or junction valve is used to regulate the flow of steam from the boiler.

Steam Stop Valve diagram

The valves mounted on the boilers, which change the direction of flow of steam by 90° are called junction valves, while valves fitted in pipelines which allowing the steam in the same direction are called stop valve.

Blow-off Valve

The function of a blow-off valve is to remove periodically the sediments deposited at the bottom of the boiler while the boiler is in operation and to empty the boiler while it is being cleaned or inspected.

Blow off Valve or Cock - Boiler Mountings and Accessories

When the blow-off valve is opened the water which is under the pressure of steam, rushes out with tremendous velocity thus carrying out the sediments along with it.

Feed Check Valve

When the level of water in the boiler falls, it is brought back to the specified level by supplying the additional water called feed water. The pressure inside the boiler will be high therefore the pressure of the feed water has to be raised by a pump before it is fed into the boiler. The feed water under high pressure is fed into the boiler through the feed check valve.

The function of a feed check valve is to control the flow of water from the feed pump to the boiler and to prevent the backflow of water from the boiler to the pump when the pump pressure is less than the pressure or when the feed pump ceases to work. Evidently feed check valve is placed at the boiler end of the delivery pipe of the feed pump.

Fusible Plug

Fusible Plug - Boiler Mounting and Accessories

The plug is made up of tin or leads alloy, which has a low melting point. The function of the fusible plug is to put-off the fire in the furnace of the boiler when the water level falls below an unsafe level and thus avoids the explosion, which may take place due to overheating of the tubes and the shell. It is fitted over the crown of the furnace or the combustion chamber.

Boiler Accessories

The boiler accessories are required to improve the efficiency of the steam power plant and to enable for the proper working of the boiler. The boiler accessories aren’t mounted directly on the boiler.

The essential boiler accessories are:

  1. Economiser
  2. Air pre-heater
  3. Superheater
  4. Feed pump
  5. Steam Separator
  6. Steam trap


The combustion gases coming out of the boiler contain a large quantity of heat. Therefore the maximum amount of heat from the gases should be recovered before it escapes to the chimney.

Economiser - Boiler Mountings and Accessories

In the economiser, heating the feed water does the recovery of heat in the flue gases. The economiser is placed in the path of the gases. They improve the overall efficiency of the boiler by reducing fuel consumption.

Air Pre-heater

The air preheater is an accessory that recovers the heat in the exhaust gas by heating the air supplied to the furnace of the boiler. Supplying preheated air into the furnace produces a high furnace temperature and accelerates the combustion of the fuel. Thus the thermal efficiency of the plant will be increased.

Air Pre-heater - Boiler Mountings and Accessories

The advantages of air pre-heater are,

  1. Increase in the steam generation rate.
  2. Better combustion with less soot, smoke and ash, and
  3. Low-grade fuels can be used.


The superheater is used in boilers to increase the temperature of the steam above the saturation temperature.

Superheater - Boiler Mountings and Accessories

The dry saturated steam generated in the boiler is passed through a set of tubes placed in the path of the flue gases, in which it will be heated further by the hot gas to increase its temperature about the saturation temperature.

Feed Pump

A feed pump is a boiler accessory required to force the feed water at high pressure into the boiler. Commonly used pumps are,

  1. Reciprocating pumps
  2. Rotary pumps

The reciprocating pumps are driven directly by coupling them to the steam engine. The rotary pumps are driven by the steam turbines or by electric motors.

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About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

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