Wednesday, June 22, 2011

Scavenge Fire

  In the name of ALLAH who is the most beneficient and merciful

List the various factors which must be present for a scavenge fire to start?

 For any fire to begin there must be present a combustible material, oxygen or air to support combustion and a source of heat of a temperature high enough to start combustion. In case of scavenge fires the combustible material is oil. The oil is usually cylinder oil which has drained down from the cylinder material is oil. The oil is usually cylinder oil which has drained down from the cylinder spaces, in some cases the cylinder oil residue may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinder, and other similar causes. The oxygen necessary for combustion comes from the scavenge air which is plentiful supply for the operation of the engines. The heat in the scavenge space, around the cylinder, brings the oil to a condition where it is easily ignited. The high temperature required to start combustion may arise from piston-ring blow past.


How would you become aware of a scavenge fire? How would you deal with a scavenge fire?

The first indication of a scavenge fire may be a slight reduction in the engine speed due to the reduction in power which comes about when a fire starts. Other indications are a higher exhaust temperature at the cylinders where the scavenge fire has started and irregular speed of turbo-blowers. External indications will be given by a smoky exhaust and the discharge of sooty smuts or carbon particles. If  the scavenge trunk is oily the fire may spread back from the space around or adjacent to the cylinders where fire started and will show itself as very hot spots on areas of the scavenge trunk surfaces. In ships where the engine room is periodically unmanned, temperature sensors are fitted at critical points within the scavenge spaces. On uniflow-scavenged engines the sensors are fitted round the cylinder liner just above the scavenge ports. A temperature higher than reference or normal then activates the alarm system.

If a scavenge fire starts, two immediate objectives arise; they are to contain the fire within the scavenge space of the engine and to prevent or minimize damage to the engine. The engine must be put to dead slow ahead and the fuel must be taken off the cylinders affected by fire (see note). The lubrication to these cylinders must be increased to prevent seizure and all scavenge drains must be shut to prevent the discharge of sparks and burning oil from the drains into engine rooms. In allows the fire to burn itself out without damage. Once the fire is out and navigational circumstances allow it, the engine should be stopped and the whole of the scavenging port examined and any oil residues found round other cylinders removed. The actual cause of initiation of the fire should be investigated. If the scavenging fire is more major nature. It sometimes become necessary to stop the engine and use the steam or extinguishing arrangement fitted to the scavenging trunk. The fire is then extinguished before it can be spread to surfaces of the scavenging trunk. Where it may cause the paint to start burning if special non-flammable paint has not been used.

 How can the incidence of scavenge fires be prevented or reduced?
One of the first things that must receive attention is maintaining the scavenge space in as clean a condition as possible. This can be done by keeping scavenge drain pipes clear and using them regularly to drain off any oil which comes down into scavenge space drain pockets. The scavenge space and drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludge which don’t drain down so easily and which are a common cause of choke drain pipes. The piston rings must be properly maintained and lubricated adequately so that ring blow-by (blow-past) is prevented. At the same time one must guard against exceed cylinder oil usage. With timed cylinder oil injection the timing should be periodically checked. Scavenge ports must be kept clear.

WRITTEN BY S/C ZEESHAN AHMED (A/C 2988)
INSTRUCTOR: CHIEF ENGINEER YASEEN SAHAB
DATE: 25/10/2009
TIME: 10:05:00 AM