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Boiler Mountings and their functions

Fittings on a Boiler which are required for its safe and efficient operation are called boiler mountings.  Mountings directly fitted with boiler and designer also provide specific space/area for specific mounting. A boiler cannot be operated without mountings. There are many types of boiler mountings, following are some important boiler mountings:

  • Water level sight glass (gauge glass)
  • Pressure gauge
  • Safety valve
  • Main steam stop valve
  • Blow off cock
  • Fusible Plug
  • Feed water check valve (NRV)
  • Blowdown valve
  • Manhole
  • Air Vent
  • Start-up vent

Water Level Indicator:

Water level indicator shows the water level in the boiler. This is located in front of the boiler and easily visible to the boiler attendant. Generally, two water level indicators used for safe operation in a vertical position.

Pressure Gauge:

Pressure gauge used to measure the steam pressure inside the boiler. It is fitted in front of the boiler and connected with steam space with the help of a siphon tube. Bourdon tube pressure gauge used for measuring the boiler pressure.

Safety Valve:

Safety valves attached to the steam chest/ top of the boiler. It prevents explosions due to overpressurization of steam inside the boiler. When the steam pressure inside the boiler exceeds normal working pressure then safety valves operate. Safety valves blow off excessive steam into the atmosphere and maintain the pressure within the safe limit. During the opening of the safety valve, heavy noise creates that alerts the boiler attendant. Two safety valves provided for safe operation. Following are the types of safety valves:

  • Lever safety valve
  • Dead weight safety valve
  • Spring loaded safety valve
  • High steam low water safety valve

Main Stop Valve:

It is the largest valve on the steam boiler.  Stop valve usually fitted on the highest part of the shell with the help of flange. The stop valve can be operated manually or automatically. The main functions of the stop valve are:

  • To control the steam flow from the boiler to the main steam pipe
  • To shut off the steam completely on demand

Sometimes a non-return valve fitted near to stop valve to prevent accidental admission of steam from other the boilers. This will happen if the number of boilers connected with the same steam pipe. In these conditions, when one boiler is empty and under maintenance, non-return valve prevents accidental admission of steam in under maintenance boiler.

Feed Check Valve:

It is a non-return valve fitted to the shell slightly below the normal water level of the boiler. Feed Check valve regulates feed water pump supply to the boiler. It also prevents backflow from the boiler to the pump when the feed pump fails. Perforated pipe used to distribute the feed water uniformly in the boiler.

Blow off Cock:

Blow off cock fitted on the bottom of the boiler drum. The principal functions of blow off cock are:

  • To empty the boiler for maintenance or other purposes
  • To discharge the scale, mud or sediments from the bottom of the boiler

Fusible Plug:

Fusible plug fitted to the crown plate of the furnace. It consists of gunmetal plug with fusible molten metal. Its object is to put off the fire in the furnace of the boiler when the water level falls to an unsafe limit. The fusible plug prevents explosion due to overheating of the furnace plate.

During normal operation, fusible plug dipped in water and the molten metal temperature remains below melting temperature. When the water level falls too low then fusible plug uncovered or above the water. Furnace gases heat up the fusible plug and plug metal. The inner molten metal falls down. The water and steam then rush to the hole and extinguish the fire. Thus fusible plug prevents major damage to the boiler due to overheating.

Blowdown Valve:

Blowdown valve used to remove scale, dirt, and sediment from boilers or associated piping to maintain boiler efficiency and integrity. Period blowdown and continuous blowdown performed in the boiler. Periodic blow down reduces the amount of total dissolved solids (TDS) and performs once per shift.  Continuous blowdown reduces suspended solids/ light particles and forth removal. Two types of blow down valves used, one is quick to open and second is the slow open valve. Quick open valve first opens and then the slow open valve will open. If the sequence is reversed, hammering will occur in the piping.


Manhole allows access to the boiler for repairs and cleaning. Usually, it is an elliptical shaped hole that has enough area for workers to enter in the boiler shell.


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