Answer EKM Question 29

Q29. With respect to scavenge fires in Large two stroke Marine engines:

A. State the common causes of scavenge fires.

B. List the indication that a scavenge fire is in progress.

C. State the immediate action to be taken in the event of a scavenge fire.

D. List with reasons the checks and precautions necessary before an engine is put back in to service following a scavenge fire.

Answer: A. State the common causes of scavenges fires;
-A prolonged blow-by owing to ineffective sealing of combustion space
- Leaky piston ring, sticky ring, broken ring, badly worn out Liner, scoring and scuffing at the liner surface, faulty lubrication (quality, quantity or timing), insufficient ring axial clearance are some of the probable causes Listed which will adversely affect proper sealing of the combustion space.
- An overheated piston arising out of continued slow burning in the cylinder. Faulty atomization, faulty fuel timing, low compression, ineffective sealing, failure of coolant circulation, generation of frictional heat are some of the reasons for which the piston may accumulate higher temperature level in its body. The heat will be dissipated to the underside causing heating of the air space.
- A blow back of the exhaust gases through the scavenge parts. Such a blow back is possible due to a rise in exhaust back pressure. Carbonizing of pads, fouling of grid before turbine fouling of turbine blades, chocking of silencer etc. are some of the reasons.
B. List the Indications that a scavenge fires is in progress;
Detection:
- A scavenge fire is likely to be initiated first in the space at the immediate vicinity of the piston. It should be detected at that stage. A scavenge fire Will Manifest Itself by the following indications:
(1) An Increase in the exhaust temperature of the affected cylinder as the cylinder is not receiving fresh air.
(2) A drop in revolutions of the engine as the power generation in the affected cylinder is less.
(3) Black smoke  with exhaust.
(4) Discharge of spark, flame or smoke through drain cocks from scavenge air box.
(5) Unstable operation of Turbocharger.
(6) Evidenice of local overheating of scavenge air box.
(7) Visible evidence of fire if a transparent window is provided.
(8) Cooling outlet temperatures of the affected cylinder will indicate a rise.
(9) Rise of pressure and temperature of air in air box below the piston. A temperature rise will be sense and signaled by a transducer if provided which is amplified to energize an alarm circuit
(10) As the fire spreads, there will be more smoke in exhaust and further drop in revolutions or sign of overloading.
C. State the Immediate action to be taken in the event of a scavenge fire;
- If the fire is localized, the affected cylinder will have to be isolated. Prompt action should extinguish the fire.
- The fuel pump plunger of the affected cylinder is lifted up and the fuel inlet valve to the pump should be shut.
- The coolant flow rate through the piston and jacket can be increased.
- The rate of lubrication in the affected cylinder may be increased.
- Drains are to be shut to prevent a blow of sparks in the engine room 
- Nobody should be standing near the scavenge space relief doors after the tire is extinguished an investigation should follow to ascertain the cause of fire. Crankcase or scavenge doors must not be vented until the engine is sufficiently cooled.
If the fire spreads:
- Inform the bridge and stop the engine
- The scavenge air duct before the engine is flapped and the smothering gas charged
- Cooling and bearing circulation is maintained till the engine is cooled.
- Alter the engine is sufficiently cooled down, It is necessary to open the  boxes of all cylinders for Cleaning of deposits and burn products. The cause of fire should be ascertained and rectified. Some of the probable places where damage may occur as a result of fire are:
(a) The piston rod stuffing box gland,
(b) The piston rod and cylinder liner surfaces
(c) The alignment of piston and Straightness of piston rod.
(d) scoring or crack on liner.
(e) The diaphragm below the piston for detection of crack
(f) The tie-rods near the fire should be re-tensioned.
D. List with reasons the chocks and precautions necessary before an engine is put back into service following a scavenge fire.
- After extinguishing the fire and oiling down, the scavenge trunking and scavenge ports
should be cleaned and the trunking together with cylinder liner and Water Seals, piston, piston rings, piston skirt, piston rod and gland must be inspected. 
- Tightness of tie bolts should be checked before restarting the engine. Fire extinguishers should be recharged at the first opportunity and faults diagnosed as having caused the fire must be rectified.
- To prevent scavenge fires good maintenance and correct adjustment must be carried out.
- Scavenge trunking must be periodically inspected and cleaned and any build-up of contamination noted and remedied. Scavenge drains should be blown regularly and any passage of oil noted.