INTRODUCTION:
For any fire to begin, the fire tringle needs to be completed. To complete a fire tringle there must be present a combustible material, oxygen or air to support combustion and a source of heat at a temperature high enough to start combustion.
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In the case of scavenge fires:
the combustible material is oil. The oil can be cylinder oil which has drained down from the cylinder spaces, or crankcase oil carried upwards on the piston rod because of a faulty stuffing box. In some cases the cylinder oil residues may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinders and other similar causes.
The oxygen necessary for combustion comes from the scavenge air which is in plentiful supply for the operation of the engines.
The source of heat for ignition comes from piston blow-by, slow ignition and afterburning, or excessive exhaust back pressure, which causes a blowback through the scavenge ports.
• A scavenge fire can cause serious damage to the piston rod diaphragm gland as well as leading to possible distortion of the air box and cracking of the liner. Tie rod tension will almost certainly be affected.
• The worst case scenario for a scavenge fire is it leading to a crankcase explosion
• The fire may also spread outside the scavenge box due to relief doors leaking or oil deposits on the hot casing igniting. For these reasons a scavenge fire should be dealt with as quickly as possible.
INDICATION
Loss in power and irregular running of the engine,
High exhaust temperatures of corresponding units,
High local temperature in scavenge trunk,
Surging of turbocharger,
Sparks and smoke emitted from scavenge drains.
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 the fire started and will show itself as very hot spots or areas of the scavenge trunk surfaces.
In ships where the engine room is designed as UMS, temperature sensors are fitted at critical points within the scavenge spaces. So, activation would cause automatic slow down of the engine.
ACTION TO BE TAKEN WHEN SCAVENGE FIRE OCCURRED
In the event of scavenge fire the engine must be put to dead slow ahead as soon as possible and the fuel must be taken off the cylinders affected by the fire or preferably stopped.
The turning gear should be put in and the engine continuously turned with increased cylinder oil to prevent seizure (jam).
All scavenge drains must be shut to prevent the discharge of sparks and burning oil from the drains into the engine room.
Air supply should be cut off by enclosing the turbocharger inlets, for mechanically operated exhaust valves the gas side should also be operated, (hydraulically operated exhaust valves will self close after a few minutes).
For a minor scavenge fire:
– A minor fire may shortly burn out without damage, and conditions will gradually return to normal. The affected units should be run on reduced power until inspection of the scavenge trunking and overhaul of the cylinder and piston can be carried out at the earliest safe opportunity.
– Once navigational circumstances allow it, the engine should be stopped and the whole of the scavenge trunk examined and any oil residues found round other cylinders removed.
– The actual cause of the initiation of the fire should be investigated
For a major scavenge fire:
– If the scavenge fire is of a more major nature, if there is a risk of the fire extending or if the scavenge trunk is adjacent to the crankcase with risk of a hot spot developing it sometimes becomes necessary to stop the engine.
– Normal cooling is maintained, and the turning gear engaged and operated. Fire extinguishing medium should be applied through fittings in the scavenge trunk: these may inject carbon dioxide, dry powder or smothering steam.
The fire is then extinguished before it can spread to surfaces of the scavenge trunk where it may cause the paint to start burning if special non inflammable paint has not been used.
Boundary cooling of the scavenge trunk may be necessary. Keep clear of scavenge relief valves, and do not open up for inspection until the engine has cooled down.
After extinguishing scavenge fire:
After extinguishing the fire and cooling 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.
Heat causes distortion and therefore checks for binding of piston rod in stuffing box and piston in liner must be carried out.
Tightness of tie bolts should be checked before restarting the engine.
Inspect reed valves if fitted, and scavenge relief valve springs.
Fire extinguishers should be recharged at the first opportunity and faults diagnosed as having caused the fire must be rectified.
SAFETY FITTING
- Scavenge belt relief door
- Fire Fighting Media
1. SCAVENGE BELT RELIEF DOOR:
Scavenge belt relief door Fitted to both ends of the scavenge belt and set to lift slightly above the maximum normal working scavenge air pressure.
2. FIRE FIGHTING MEDIA
Carbon dioxide- will put out a fire but supply is limited. Susceptible to loss if dampers do not effective prevent air flow
Water spray- perhaps the ideal solution giving quick effective cooling effect to the fire.
Dry powder- will cover the burning carbon and oil but is messy. As the fire may still smoulder below the powder care must be taken when the scavenge doors are removed as the powder layer may be blown away.
Steam smothering-plentiful and effective
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PREVENTION
Good maintenance and correct adjustment must be carried out
Scavenge trunking must be periodically inspected and cleaned and any buildup of contamination noted and remedied.
The drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludges which do not drain down so easily and which are a common cause of choked drain pipes
Scavenge drains should be blown regularly and any passage of oil from them noted.
The piston rings must be properly maintained and lubricated adequately so that ring blow-by is prevented.
At the same time one must guard against excess cylinder oil usage.
With timed cylinder oil injection the timing should be periodically checked.
Scavenge ports must be kept cleared
The piston-rod packing rings and scraper rings should also be regularly adjusted so that oil is prevented from entering the scavenge space because of butted ring segments.
This may and does occur irrespective of the positive pressure difference between the scavenge trunk and the crankcase space.
Fuel injection equipment must be kept in good condition, timed correctly, and the mean indicated pressure in each cylinder must also be carefully balanced so that individual cylinders are not overloaded.
If cylinder liner wear is up to maximum limits the possibility of scavenge fires will not be materially reduced until the liners are renewed
REFERENCES:
1. www.marineengineering.co.uk
2. The Running and Maintenance of Marine Machinery – Cowley
3. Reeds Marine Engineering Series, Vol. 12 – Motor Engineering Knowledge for Marine Engineers
4. Lamb’s Question and Answers on Marine Diesel Engines – S. Christensen
5. Diesel Engines – A J Wharton
6. www.marinediesels.info