Fire in incinerator exhaust gas manifold

What happened?

During burning of sludge in the vessel incinerator, there was a high exhaust gas temperature alarm.  Smoke appeared from the incinerator exhaust gas manifold, followed by a fire inside the exhaust pipe.

19:34: the alarm sounded, and the crew mustered.  The vessel course was altered to prevent the spread of smoke.  Boundary cooling of the funnel was started; the fire team entered the incinerator room with extinguishers to fight the fire but were unable to extinguish it.

19:41: the fire team left the incinerator room and sealed the door.  Minutes later, the sprinkler system for the incinerator room was activated.

20:01: the fire extinguished and the sprinkler system was stopped.

Smoke and high temperatures remained for over an hour; boundary cooling continued until the situation was stabilized at around 21:00.  A fire watch was maintained until midnight.  There were no injuries but there was some damage to the equipment in the incinerator room.

Exhaust sensor was tampered with
Exhaust sensor
Setpoint on sludge resistor is set to low

The incinerator was only used for burning sludge, at a temperature somewhere between 810-850°C.  For some reason the flame extinguished.  

Detailed Sequence of Cause of Fire

  • A set point value on a control resistor was set too low, such that the equipment was not able to detect that the flame had failed, and so kept pumping sludge into the combustion chamber;
  • The temperature dropped slowly to below 810°C; at that point the automatic diesel burner triggered and started its cycle – the burner ignited and started burning fuel;
  • This re-ignited the hot sludge already in the combustion chamber and caused a ‘fireball’ inside the exhaust pipe;
  • This in turn caused the exhaust temperature to rise, which triggered a high exhaust gas temperature alarm;
  • This stopped the sludge system from pumping more sludge into the incinerator, and the system should then have started to cool down;
  • However, since there was an accumulation of sludge left inside the combustion chamber, the sludge just kept on burning and the temperature continued to rise;
  • After five minutes, the emergency stop button was activated and this cut the power to the incinerator, and all fans stopped;
  • There was still sludge left inside the burning chamber floor, which continued to burn unassisted;
  • Since the flue gas fan was not running, the exhaust started going out through ventilation holes on the incinerator and this triggered the smoke detectors inside the incinerator room.

What went wrong?

  • It was discovered afterwards that a set point value on a control resistor was set too low, such that the equipment was not able to detect a flame failure;
  • The exhaust sensor had been tampered with.

What lessons were learned?

Have a full, detailed and comprehensive understanding of what happens to incinerators and other similar equipment when the EMERGENCY STOP button is pressed, and/or when power fails and restarts.  This applies not only to incinerators but to all complex machines.  Ensure that incinerator flame failure devices are tested as part of the vessels’ planned maintenance system.

Members may wish to refer to the following three incidents, none of which are fires.  Two have as a cause, lack of understanding of the technicalities of how certain equipment works.  In the third, a serious incident arose from poor understanding of the consequences of a power outage.