Thursday, October 19, 2017
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Marine Diesel Engine Scavenge Space Fires

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Scavenge fires

These can occur in the scavenge trunking of large bore, 2 stroke engines as a result of oil and carbon that has been blown from the cylinders through the scavenge port and collected in the spaces. The source of heat to ignite the fire is usually sparks and the gases which get blown throught the scavenge ports or past worn out piston rings. The oil is mainly lub-oil or can be unburnt fuel oil.

Causes:

  1. Presence of combustible material in scvange spaces and sparks from piston rings will lead to fire.
  2. Combustion gases blown past worn out piston rings or worn out liner.
  3. Poor combustion leading to accumulation of combustible material in the scavenge space caused by faulty exhaust valves / fuel injectors / fuel pumps etc.
  4. Excessive cylinder lubrication which will cause deposits in the scavenge space.
  5. Under cylinder lubrication which will lead the liner to wear.
  6. Leakage of piston cooling oil findings its way into the scavenge space.
  7. Incomplete combustion due to prolonged slow speed operation of the engine.
  8. Piston rod stuffing box leakage.

Confirmation of a scavenge fire

  1. Scavenge space high temperature alarm (set at 200 deg c).
  2. Exhaust temperatures high.
  3. Jacket cooling water outlet temp high.
  4. Piston cooling water outlet temp high.
  5. Acrid smell.
  6. Irregular running of the engine.
  7. Turbocharger surging.
  8. Possible smoke from turbocharger.
  9. Scavenge manifold relief valve lifting.
  10. Sparks from funnel.

Action in case of scavnege fire

  1. In case of a scavenge fire, slow down engine and then stop it.
  2. Stop scavenge blowers to cut off supply of fresh air.
  3. The turning gear should be engaged and the engine continually turned to avoid seizure.
  4. Fight fire by fixed fire fighting system i.e. Injection of inert gas or steam.
  5. If engine cannot be stopped due to any navigational constraint, then cut the fuel supply to the affected unit, increase cylinder lubrication to the affected unit. Slow down the engine further and stop it when possible.
  6. Cause of the fire to be ascertained and rectified. Also check and rectify any damage due to fire. Clean the scavenge space before restarting the engine.

Precautions to minimize scavenge fires

  1. Regular inspection and cleaning of scavenge spaces, rectify anything abnormal.
  2. Maintain good combustion by regular routine maintenance of combustion system.
  3. Correct rate of cylinder lubrication.
  4. Check temperature sensors regularly.

 

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