Boilers High Furnace Pressure - Specific causes

Boilers High Furnace Pressure - Specific causes
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Combustion in a boiler furnace is normally considered a controlled explosion, as all boilers operate at a regime where explosion pressure is the maximum. This is mainly due to the fact that it is at this regime we get the maximum efficiency of the boiler. Furnace pressure in boilers is one of the most critical and important parameter to be maintained and monitored. Furnace pressure in a balanced draft furnace is always kept negative, and in pressurized furnace it is positive. Present days designers adopt balanced draft furnace for all solid fuel fired boiler, however there are installations of pressurized furnace even in the case of solid fuel fired boilers. In the case of gas and liquid fuel, generally designers adopt pressurized furnaces. It should be understood that the selection of pressurized or balanced draft furnace depends upon many other factors.

Present day boilers have online monitoring of furnace pressure and are controlled on auto. There are also alarms and trips provided for furnace pressure to alert the operator and also to protect the boiler from very high pressure surges. In the case of furnace explosion, if the explosion process is triggered then the boiler cannot be protected by any of these devices. Only the good and safe operating procedures can prevent explosions. There are many reasons why furnace pressure goes high, and the plant responds to these changes in furnace pressure like any other boiler parameter.

Specific causes

  • Tripping of induced draft fan
  • Mal-operation of regulating vanes of the fans
  • Unstable flame
    • Low wind box pressure
    • Improper burner operation
    • Sudden starting of mills with fuel in the mill
    • Sudden fuel input in to the furnace
    • Loss of ignition energy
  • Gradual buildup of fouling in air pre-heaters
  • Tripping of air pre-heater
  • Furnace water seal broken
  • High excess air levels
  • Large amount of air ingress in the second pass of boiler

The causes given are all applicable for balanced draft furnace, but in the case of forced draft furnace some of the causes are not applicable.

Plant response

  • Furnace pressure high alarm appears for corrective action by operator
  • Furnace trip activates if pressure goes above trip limit
  • Boiler furnace pressure surge
  • Can lead to explosion

Immediate operator action

  • Check draft reading for any damper closure
  • Check vane control mechanism of fans for any mal-operation
  • If induced draft fan tripped, reduce load and stabilize boiler parameters
  • Stabilize combustion if combustion is unstable
  • Restart air pre-heater if tripped
  • Check furnace seal and establish if broken
  • Check induced draft fan for any mal-operation

Immediate local operator action

  • Check for any opening of man holes in second pass
  • Check air pre-heater rotation
  • Check dampers in flue gas path for proper position
  • Check induced draft fan fully to ensure it is in proper operating condition
  • Check and ensure furnace bottom seal water flow
  • Check combustion condition and inform control room of any disturbance

It is a good practice for the local operator to get clearance from the control room before making any change (unless it is a standard procedure to take corrective action by the local operator). As furnace pressure increase is one of the indications for boiler explosion boiler operating engineers and all concerned must view increase in furnace pressure very critical and take corrective action and ensure safety.

High Furnace Pressure in Boiler

Specific Causes

Plant Response & Operator Action