Boiler Mountings – Functions, Parts, Diagram, Working

Boiler Mountings – Introduction

These are different fittings and devices which are necessary for the operation and safety of a boiler. Normally these devices are mounted on boiler shell.

Functions Of Boiler Mounting:

These are the fittings that are mounted on the boiler for its proper functioning. They include water level indicator, pressure, gauge, safety valve, etc. It may be noted a boiler cannot function safely without the mounting.

According to IBR the following are the list of mountings should be fitted to the boilers.

1. Water level indicators/Water Gauge
2. A pressure gauge
3. Safety valves
4. A Steam stop valve or Junction Valve
5. A feed check valve
6. A blow-off cock
7. Fusible plug
8. A manhole, Mud holes or sight holes
9. Steam Scrubbers/Anti-priming pipe
10.Air vents and vacuum breakers
11.Soot blowers

Main Boiler Mountings and their Functions : 

Boiler mountings:Function
1) Feed check valveTo allow water flow into boiler
2) water level indicatorTo check water level in boiler drum
3) Blow off cockTo pass water away from boiler for maintenance purpose
4) Pressure GaugeTo measure steam pressure generated in boiler drum
5) Steam Stop ValveTo control flow of steam to turbine
6) Safety ValveTo relieve excess steam from boiler drum generated at high pressure
7) Fusible PlugTo extinguish fire in furnace by allowing spray of water by melting molten material

Water level indicators


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 are generally two in number.

Number and Location: Normally two water level indicators/water gauge are fitted in front of the boiler.

Construction of Water level indicator : 

It consists of three cocks and a glass tube. Stem Cock 1 keeps the glass tube in connection with the steam space. Water cock 2 puts the glass tube in connection with the water in the boiler. Drain cock 3 is used at frequent intervals to ascertain that the steam and water cocks are clear.

Diagram of Water level Indicator :

water level indicator diagram
water level indicator diagram

Working of Water level indicator : 

In the working of a steam boiler and for the proper functioning of the water level indicator, the steam and water cocks are opened and the drain cock is closed. In this case handles are placed in a vertical position. The rectangular passage at the ends of the glass tube contains two balls. In case the glass tube is broken, the two balls are carried along its passages to the ends of the glass tube. It is thus obvious, that water and steam will not escape out. The glass tube can be easily replaced by closing the steam and water cocks and opening the drain cock.

Pressure Gauge


It is used to measure the pressure of the steam inside the steam boiler. The pressure gauges generally used are of Bourdon tube type.

Number and Location: All boilers must be fitted with at least one pressure gauge and it is usually mounted at the front top of the boiler shell or drum so that attendant can easily read the pressure reading.

There are two types of pressure gauges:

  • Bourdon tube pressure gauge and
  • Diaphragm type pressure gauge.

The common type of pressure gauge for steam boilers is Borden’s pressure gauge, construction, and operation of which is described as follows:

Construction of Borden’s pressure gauge

It consists of an elliptical elastic tube XYZ bent into an arc of a circle, as shown in fig.  This bent up tube is called Bourdon’s tube. One end of the gauge is fixed and connected to the steam space in the boiler. The other end is attached by links and pins to a toothed quadrant. This quadrant meshes with a small pinion on the central spindle.

Diagram Of Borden Pressure gauge :

Bourdon pressure Gauge diagram
Bourdon pressure Gauge diagram

Working of Borden’s pressure gauge

The steam under pressure flows into the tube. As a result of this increased pressure, the tube tends to straight itself. Since the tube is encased in a circular curve, therefore it tends to become circular instead of straight. With the help of simple pinion and sector arrangement, the elastic deformation of the Bourdon tube rotates the pointer. This pointer moves over a calibrated scale, which directly gives the gauge pressure.

Safety Valves

Function: The function of a safety valve is to prevent excessive pressure from building up in a steam boiler. If the steam pressure in the boiler drum exceeds the working pressure, it permits the steam in the boiler to escape to the atmosphere until the safe working pressure in the boiler is reached again. The safety valve also warns the boiler attendant as the steam escape through the safety valve.

Principle: The principle of operation of a safety valve depends on the fact that a valve is pressed against a seat through some external force. When the steam force corresponding to boiler pressure acting under the valve exceeds the external force, the valve gets lifted off its seat allowing some of the steam to escape until the working pressure is restored again.

Number and Location: As per boiler regulations, each boiler must be fitted with at least two safety valves. Safety valves are located above the steam space in the boiler.

Types: There are four types of commonly used safety valves as given below:

(i) Deadweight safety valve,

(ii) Spring-loaded safety valve,

(iii) Lever safety valve and

(iv) High steam and low water safety valve.

The choice of a safety valve for a particular boiler depends on the type of the boiler and its safe working pressure.

Lever Safety Valve

A lever safety valve used on steam boilers is shown in fig. A lever safety valve consists of a valve body with a flange fixed to the steam boiler. The bronze valve seat is screwed to the body, and the valve is also made of bronze. The thrust on the valve is transmitted by the strut. The guide keeps the lever in a vertical plane.
When the pressure of steam exceeds the safe limit, the upward thrust of steam raises the valve from its seat. This allows the steam to escape until the pressure falls back to its normal value. The valve then returns back to its original position.

lever safety valve Diagram
lever safety valve Diagram

Dead Weight Safety Valve

When the steam pressure exceeds the normal limits, this high-pressure steam creates upward force on the valve, thus valve V lift with its weights and the excess steam escapes through the pipe to the outside.

dead weight safety valve Diagram
dead weight safety valve Diagram

High Steam Low Water Safety Valve

This valve has a combined safety arrangement against

a) high steam pressure in the boiler and

b) low water level in the boiler.

It allows the steam to escape out of the boiler when steam pressure exceeds the normal value or water lever in the boiler falls below the normal level. It consists of lever A which is hung inside the boiler shell and it is hinged at point C. One end of the lever carries a balance weight and the other end carries an earthen float immersed in water. The balance weights are kept in such a way that the knife-edge of the lever just touches the projection when the float just dips into the water. It also consists of two valves. One is the main valve V1 which rests on its seat. The edge of the central opening in the valve V1 forms the seat for the hemispherical valve V2 and the end of the valve rod carries a weight.

High stem low ware safety valve diagram
High stem low ware safety valve diagram

When the water level falls and floats are sufficiently uncovered from water, the weight of the float increases and no longer. It is balanced by the balance weights. Consequently, the float end of the lever will descend and causes a swing in the lever A. When the lever swings, the valve rod is pushed up. It also pushes up the hemispherical valve V2 and the steam leaks through the gaps provided with a loud noise. This acts as a warning to the boiler attendant. When the hemispherical valve is closed, the main valve V1 acts as an ordinary lever safety valve and it guards against the high pressure in the boiler. The valve V1 is held in position partly by the weight on the rod of valve V2 and partly by the loaded lever above the valve casing.

When the steam pressure exceeds the limiting working pressure, the main valve V1 along with valve V2 lifts up and the steam leaks out through the discharge duct.

Spring Loaded Safety Valve

A Ramsbottom spring-loaded safety valve is shown in Fig.  It is usually, fitted to locomotives. This valve consists of a cast iron body having two branch pipes. Two valves sit on corresponding valve seats at the end of the pipes. The lever is placed over the valves utilizing two pivots. The lever is held tight at its position utilizing a compression spring. One end of this spring is connected with the lever while the other end with the body of the valve.
Under normal conditions, the spring pulls the lever down. This applies downward force on valves which is greater than the upward force applied by steam. When steam pressure exceeds normal value, upward force becomes, larger than the downward force on the valve due to spring. Thus the valves are lifted from their seats, opening the passage for steam to release out. The valve closes due to spring force when the pressure in the boiler becomes normal.

spring loaded safety valve diagram
spring loaded safety valve diagram


When the steam pressure in the boiler is equal to the working pressure:

The upward force exerted by boiler steam on the valve is balanced by the downward force of spring. Under this condition, the valve lies on its seat tightly and the steam will not escape from the boiler as shown in Fig. (a)

When the steam pressure in the boiler is more than the working pressure:

The upward force exerted by boiler steam on the valve is overpowered by the downward force of spring. Under this condition, the valve automatically opens and the steam escapes to the atmosphere(Fig. (b)) until the pressure falls back to the working pressure.

working of spring loaded safety valve
working of spring-loaded safety valve


These valves are suitable for both low and high-pressure boilers stationary and non-stationary boilers


  • Elimination of heavyweight.
  • Easy maintenance and examination.
  • Not affected by jerks and vibrations.

Steam Stop Valve or Junction Valve


  • To control the flow of steam from the boiler to the main steam pipe.
  • To shut off the steam completely when required.

Location: This valve is usually mounted over the boiler drum, in the steam pipe leading to-the steam turbine/engine, and between the steam, pipes to stop or regulate the flow of steam.

Construction of Steam Stop Valve:

The body of the stop valve is made of cast iron or cast steel. The valve, valve seat, and the nut through which the valve spindle works, are made of brass or gunmetal. The spindle passes through a gland and stuffing box. The spindle is rotated utilizing a handwheel. The rotation of the spindle causes the valve to move up and down.

Diagram of Steam Stop Valve

Steam stop valve diagram
Steam stop valve diagram

Working of Steam Stop Valve :

When the valve sits over the valve seat, the passage of steam is completely closed. The passage may be partially or fully opened for the flow of steam by moving the valve up, rotating the handwheel.

Feed Check Valve

Function: The function of the feed valve is

  • To control the supply of feed water to the boiler from the feed pump and
  • To prevent any water escaping back from the boiler in the event of failure of the feed pump or the pump pressure less than the boiler side.

Location: It is fitted over the boiler shell slightly below the normal water level of the boiler.

Construction of Feed check valve : 

It is a non-return valve, fitted to a screwed spindle to regulate the lift. This valve must have its spindle lifted before the pump is started. Pump pressure acts from below the non-return valve and boiler pressure acts from above it. Under normal working conditions, the pump delivery pressure is higher than the boiler pressure. So the valve is lifted from its seat and allows the water to flow to the boiler. The lift of the valve is controlled by moving the spindle up and down with the help of the handwheel. Thus, the flow of water can be controlled.

Diagram of Feed Check Valve : 

Feed check valve diagram
Feed check valve diagram

Working of Feed check valve :

If the boiler pressure is higher than pump pressure or the pump is stopped, the upward force on the non-return valve is higher. So it sits on its seat and closes the passage. Thus water from the boiler is not allowed to flow backward.

Blow Off Cock


The blow-off cock serves three purposes:—

(i) To blow out sediments, precipitated sludge, loose scale, or other impurities periodically when the boiler is in operation.

(ii) To empty the boiler when necessary for cleaning, repair, and inspection.

(iii) To permit a rapid lowering of water level in the boiler if accidentally it becomes too high.

Location: The blow-off cock is fitted at the lowest water space of a boiler shell.

Construction of Blow- off Valve :

A common type of blow-off cock is shown in fig. A conical plug is fitted accurately into a similar casing. The plug has a rectangular opening. The plug slot is perpendicular to the flow passage.

Diagram of Blow- off Valve :

Blow off cock Diagram
Blow off cock Diagram

Working of Blow-off valve : 

When the plug slot is brought in line with the flow passage of the body by rotating the plug, the water from the boiler comes out with a great force. If sediments are to be removed, the blowoff cock is operated when the boiler is on. This forces the sediments quickly out of the boiler.

Fusible Plug


The main function of the fusible plug is to extinguish the fire when the water level in the boiler falls below an unsafe level.

Location: The fusible plug is generally inserted in the crown of the furnace or in the combustion chamber at the lowest permissible water level.

Construction of Fusible Plug : 

The construction of the fusible plug is shown in fig. which consists of three plugs. The hollow plug A having hexagonal flanges is screwed to the fire box crown plate. The plug B gunmetal plug is screwed to the body A. The third plug C is made up of copper is locked with metal like tin or lead which has a low melting point.

Diagram of Fusible Plug

Fusible plug Diagram
Fusible plug Diagram

Working of Fusible Plug :

In normal working conditions, water covers the fusible plug remains cool. In case the water level falls below the danger levels, the fusible plug gets exposed to steam. This overheats the plug and fusible metal having a low melting point melts quickly. Due to this plug S falls. The opening so made allows the steam to rush on to the furnaces and extinguishes the fire or it gives warning to the boiler attendant that the crown of furnace is in danger of being overheated.

Manhole and Handhole

Function: Manholes and hand holes are required for cleaning, inspection, and repairing of the boiler. The manhole is provided for the entrance of a man inside the boiler shell whereas the hand hole is for a hand.

Location: The manhole is provided on the boiler shell at a convenient place.

Construction: Generally they are elliptical in shape and cover is provided. The size of the manhole is usually 400 mm x 300 mm.

Read More about Boiler : 

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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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