Safety interlocks for boiler control

  • Many of the interlocks related to the start-up, shutdown, and operation of a boiler are implemented for the purposes of protecting personnel and equipment.
  • Most of the interlock and safety features directly related to the boiler can be classified as either burner management or combustion control.
  • This delineation is made because boiler safety standards define very specific functions for burner management and require it to be implemented in a dedicated system, separate and apart from other control functions.
  • A burner management system (BMS) is primarily concerned with the interlock, sequence, and timing functions required to safely put burners into service and to stop fuel and trip the boiler on detection of potentially unsafe conditions (master fuel trip).
  • Other combustion control interlocks and protection functions, not necessarily a part of BMS, include furnace draft (implosion protection) control, fuel/air cross-limiting, and “runbacks.”
  • An overview of some of the most common boiler safety interlocks is as follows:

PURGE INTERLOCK Prevents fuel from being admitted to an unfired furnace until the furnace has been thoroughly air purged.

LOW AIR FLOW INTERLOCK OR FAN INTERLOCK Fuel is shut off upon loss of air flow or combustion air fan or blower.

LOW FUEL SUPPLY INTERLOCK Fuel is shut off upon loss of fuel supply that would otherwise result in unstable flame conditions.

LOSS FLAME INTERLOCK All fuel is shut off upon loss of flame in the furnace, or fuel to an individual burner is shut off upon loss of flame to that burner.

FAN INTERLOCK Stops forced draft upon loss of induced-draft fan.

LOW WATER INTERLOCK (OPTIONAL) Shuts off fuel on low water level in boiler drum.

HIGH COMBUSTIBLES INTERLOCK (OPTIONAL) Shuts off fuel on highly combustible content in the flue gases.