Handle Very Low Primary Air Header Pressure in Boilers

In coal fired boiler, the primary air has two functions: one to dry and carry the pulverized coal from the pulveriser to the furnace and the other is to supply oxygen for primary combustion. There are many layouts that are used to position the primary air fans in boilers. Many boiler designers prefer the cold primary air system, where the primary air fan handles cold air. There will be two or three primary air fans which cater to all mills’ requirement. The air from the fan outlet goes to an air pre-heater and then to a distribution header from where the hot air is distributed to the working mills though a regulating system. The regulating system works for a set point of mill outlet temperature. The primary air header pressure is maintained in such a way that it can carry the required pulverized coal overcoming the pressure drop in the system as well as maintaining the required velocity in coal pipes to avoid settling. Some designers go for a hot primary air system where the primary air fans handle hot air and is as many in number as the number of mills. When primary air header pressure goes below a particular value, then the boiler will be tripped due to flame failure because of high furnace disturbance. The boiler trip can be avoided if the cause for very low primary air header pressure is quickly analysed and corrective action taken both from desk operator and local operator side.

Specific causes

  • One of the primary air fans operating trips
  • Starting more mills without properly loading the primary air fans and stabilizing
  • Closing of primary air fan inlet damper by mistake or malfunction
  • Closing of air pre-heater outlet damper due to any reason
  • Closing of primary air fan discharge damper due to any reason

Plant response

  • Mills trip by control system as per the designed logic
  • There will be a rapid drum level fall needing regulation
  • Boiler pressure and temperature will fall due to fuel input reduction if turbine and generators do not respond to the required level.
  • Boiler furnace pressure becomes unsteady
  • Combustion disturbance are noticed
  • When oil support is not available flame failure can result
  • With flame failure the boiler will trip

Control room operator action

  • Reduce turbine and generator load to the required level
  • Maintain drum level within the trip levels preferably on the higher side
  • Open high pressure and low pressure by pass valves (HP-LP by pass) if required
  • Cut in oil support taking care not to do so under unstable furnace
  • Maximize oil load and reduce HP LP by pass valve opening

Local operator action

  • Check the tripped fan
  • Make ready for starting the tripped fan
  • Confirm to control room for starting the fan

Very Low Primary Air Header Pressure

Very Low Primary Air Header Pressure