Tuesday, January 7, 2020

Scavenge fire

What is scavenge fire ?

Carbonized lube oil, unburnt fuel oil and carbon from the residual products of the combustion spaces are accumulated in the scavenge spaces with the running of the engine. Under certain faulty running condition of the engine, these may ignite causing a fire in the enclosed scavenge space, known as scavenge fire.

Causes of Scavenge Fire

  1. Blow past of combustion products caused by leaky, sticky or broken piston rings, worn out liner, faulty cylinder lubrication, or insufficient axial clearance of the piston rings.
  2. Overheated piston dissipates heat to the under piston area caused by faulty atomization and injection pressure, faulty fuel pump timing, loss of compression, engine overload, failure of coolant circulation or insufficient cooling due to formation of scale.
  3. Blow back of exhaust gases caused by exhaust back pressure or deposits on exhaust ports, fouling of grid before turbine inlet, fouling of turbine blades, choking of EGB or economiser gas outlet.
  4. Presence of fuel oil in the scavenge spaces due to defective fuel injectors, incorrect pressure setting of injectors or fuel particles landing on the cylinder liner due to excessive penetration.
  5. Excessive cylinder lubrication which is drained down to scavenge spaces.
  6. Oxygen is plenty during engine operation.
  7. Fouled scavenge manifold.

Indication of Scavenge Fire

  1. Engine revolution drops as the power generation in the affected cylinder is reduced.
  2. Unstable operation of Turbo charger and may cause surging.
  3. Exhaust temperature of the affected cylinder increases, as the cylinder is not receiving fresh air.
  4. Black smoke with exhaust due to incomplete combustion in that unit.
  5. Discharging of spark, flame or smokes through drain cocks from scavenge air boxes.
  6. Rise of pressure and temperature of air in the air box below the piston.
  7. Visible evidence of fire if a transparent window is fitted.
  8. Cooling water outlet temperature of the affected unit is increased.

Action to be Taken

  1. Inform bridge and reduce the speed.
  2. Cut of fuel supply to the affected unit and shut off valves.
  3. Cylinder lubrication is increased.
  4. Coolant flow through jacket and piston is maintained.
  5. Drains to be shut to prevent flow of spark in engine room.
  6. Keep clear of scavenge space relief valve to prevent human injury.
Fire should be extinguished after some time. If the fire spread in the other scavenge spaces along with the scavenge manifold, then –
  1. Inform bridge and stop the engine
  2. Stop fuel oil booster pump
  3. Open indicator cocks, engage turning gear and turn engine to prevent engine seizure
  4. Normal engine cooling and lubrication are maintained
  5. Scavenge air duct flap valve before engine is to be shut
  6. Release the smothering gases (Steam or CO2) to extinguish fire
  7. Before opening scavenge door ventilate the space thoroughly if CO2 is released.

Inspection after Scavenge Fire

  1. Intense fire can cause distortion and may upset piston alignment
  2. Check by turning the engine and watch movement of piston in the liner, check for any occurrence of binding  at part of stroke (Binding indicates misalignment of piston)
  3. Check spring on scavenge space relief device, if the device was near the set of fire
  4. Piston rod packing spring also should be checked, which may have become weakened by overheating
  5. Check piston rings and liner for any distortion or reddish burning mark
  6. Check diaphragm and frame near affected part
  7. Check guides and guide shoes
  8. Check tension of tie bolts

Prevention of Scavenge Fire

  1. Clean scavenge space and drain at regular intervals
  2. Keep scavenge space drain open at regular intervals
  3. Excess cylinder lubrication must be avoided
  4. In case of timed lubrication, the time has to be checked as per PMS
  5. Piston rings must be properly maintained and lubricated adequately
  6. Piston rod stuffing box must be maintained to prevent oil ingress in the scavenge space.
  7. Prolong engine or any cylinder over loading to be avoided
  8. Cylinder liner wear must be within admissible limits

Scavenge Space Protection Devices

  • Electrical temperature sensing device fitted within the trunking, which will automatically sound an alarm in the event of excessive rise in local temperature.
  • Pressure relief valves consisting of self closing spring loaded valves are fitted and should be examined and tested periodically.
  • Fixed fire extinguishing system may be CO2, Dry Powder or Steam.

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