Fouling and Fires in the Scavenge Air Spaces
The principle cause of fouling is blow-by of combustion products,
unburnt fuel and cylinder lubricant between piston and cylinder into the scavenge air
spaces. The fouling will be greater if there is incomplete combustion of the fuel injected (exhaust smoke).
Causes of Poor Combustion:
The fuel injectors are not working correctly; incorrect fuel atomisation.
The fuel is at too low a temperature; resulting in high fuel viscosity and poor fuel atomisation.
Poorly adjusted injection pump timing; late injection results in after burning of fuel.
Operation with a temporary shortage of air during extreme variations in engine loading and with the charge air pressure dependent fuel limiter in the governor set too high.
Engine overloading; too much fuel for the available air.
Insufficient supply of air due to restricted engine room ventilation.
Fouling of the air intake filters and diffuser on the air side of the turbocharger.
Fouling of the air cooler, the air flaps in the charge air receiver and of the scavenge ports; these restrict air flow to the cylinders.
Fouling of the exhaust gas boiler; this increases the back pressure on the turbocharger turbines causing reduction in performance
and reduced air delivery.
Causes of Blow-by of Combustion Products:
Worn, sticking or broken piston rings.
Excessive liner wear or abnormal wear such as 'clover-leafing' which can also result in ring collapse and loss of piston ring to liner seal.
Individual cylinder lubricating quills are not working.
Damage to the running surface of the cylinder liners.
If one or more of these operating conditions prevails, residues, mainly consisting of incompletely burnt fuel and cylinder lubricating oil will
accumulate at the following points:
Between piston rings and piston ring grooves; this can result in the jamming of piston rings causing breakage or blow-past.
On the piston skirts. In the scavenge ports; this can affect the performance of the scavenging process resulting in incomplete
removal of combustion products from the cylinder and subsequent defective combustion.
On the bottom of the cylinder jacket (piston underside).
Causes of the Fires
The blow-by of hot combustion gases and sparks which have bypassed the piston rings between piston and cylinder liner running surface, enter the space
on the piston underside. Any residues present can ignite.
If there is after-burning of fuel in the cylinder due to late injection or poor fuel atomisation, the cylinder pressure, when the scavenge ports are uncovered, may be higher than the scavenge air pressure and hot combustion gases may enter the scavenge space.
A defective piston rod gland may allow oil from the crankcase to enter the scavenge space. The piston rod gland drains should be checked frequently for
signs of crankcase system oil as this indicates defective gland sealing rings.
Indications of a Fire
Sounding of the respective temperature alarms if the engine has the necessary monitoring instrumentation installed.
A considerable rise in the exhaust gas temperatures of the cylinder concerned and a general rise in charge air temperature.
The turbocharger may start surging.
Scavenge Space Fire Fighting Measures
The safety of shipboard personnel should be paramount whenever dealing with fires anywhere aboard ship.
Inform the bridge of the situation
Reduce engine power
Cut out the fuel injection pump of the cylinder concerned
Increase lubrication to the respective cylinder
(Note! If a serious fire occurs, shut down the engine after obtaining permission from the bridge and operate the fixed fire extinguishing system.)
A fire should have died down after 5 to 15 minutes. This can be verified by checking the exhaust gas temperatures and the temperatures of the doors to the piston bottoms.
Caution should be exercised whilst the fire is burning to ensure that it does not cause a fire in the engine room. Extreme care must be taken to ensure that
leakage of oil onto the hot scavenge space sides does not happen.
After it has been confirmed that the fire has been extinguished the engine must be stopped as soon as possible and the cause of the fire established. The
scavenge space must be allowed to cool completely before access doors are opened to allow inspection.
Checks should be made on the cylinder running surfaces, piston rings, fuel injectors, valve groups in the scavenge space, piston rod gland and liner seals.
Tie rod tension should be checked if the fire has been severe.
After a careful check, or if necessary repair, the engine can be put back on load with cut-in fuel injection pump and automatic cylinder lubrication.
Should a stoppage of the engine not be feasible and the fire has died down, the fuel injection pump can again be cut in, the load increased slowly and the
cylinder lubrication brought back again to the normal output. Avoid prolonged running with the considerably increased cylinder lubrication.
Preventive Measures
As can be seen from the causes, good engine maintenance goes a long way to safeguarding against fires in the scavenge air spaces. The following measures have a particularly favourable influence:
Use of correctly spraying fuel injectors and keeping the air and gas passages clean.
Optimum
adjustment of the fuel cams and of the fuel injection pump timing.
When running continuously at reduced load, check the cylinder lubricating oil feed rate and readjust if necessary. Ensure that fuel atomisation and combustion is correct for the reduced load condition.
The permanent drain of residue from the piston underside must always be checked. To prevent accumulation of dirt, the drain cock on the collector main must be opened for a short time each day.
Prevention of Crankcase Explosions
The oil mist in the crankcase is inflammable over a very narrow range of mixture. Weaker or richer mixtures do not ignite. There must always be an
extraneous cause to set off ignition, such as hot engine components. Only under these circumstances and the presence of a critical mixture ratio of oil
mist and air can an explosion occur. A 'hot spot' is the common feature of all crankcase explosions and this can be due to metal-to-metal contact at a wiped bearing, rubbing guide, defective piston rod gland, damaged thrust, un-lubricated gear wheel, etc. or even due to a prolonged scavenge fire. The 'hot spot' provides the heat source to evaporate oil, which condenses to form mist-like droplets which will ignite readily, and ignite the mist. If the mist concentration in the crankcase reaches a critical level an explosion can occur.
Engines are equipped with an oil mist detector, which constantly monitors intensity of oil mist in the crankcase and triggers an alarm if the mist exceeds
the density limit.
Measures to be Taken When Oil Mist Has Occurred
a) Do not stand near crankcase doors or relief valves or in corridors near doors to the engine room casing.
b) Reduce speed to slowdown level immediately, if not already carried out automatically. Explain the situation and ask the bridge
for permission to stop.
c) When the engine STOP order is received, stop the engine. Close the fuel oil supply. Maintain engine cooling and lubrication as the supply of lubricant will assist the cooling of the hot spot.
d) Switch-off the auxiliary blowers.
e) Open the skylight(s) and/or stores hatch.
f) Leave the engine room as a fire can still occur even with the engine stopped because the mist will circulate in the crankcase and can come into contact with the hot spot.
g) Lock the casing doors and keep away from them,
h) Prepare the fire-fighting equipment.
i) Do not open the crankcase until at least 20 minutes after stopping the engine. Ideally leave the engine for as long as possible before opening the crankcase doors as this will ensure that the hot spot has cooled below the ignition temperature and so any mist which persists will not ignite from this source. It is important that no naked lights exist in the vicinity of the crankcase when the doors are opened in order to prevent ignition of any residual mist from that source.
j) Stop the lubricating oil pump.
Take off-open all the doors on one side of the crankcase. Cut off the starting air, and engage the turning gear.
Main Engine Graviner Oil Mist Detector
k) Locate the hot spot. Feel over, by hand, all the sliding surfaces (bearings, thrust bearing, piston rods, stuffing boxes, crossheads, lubricant supply toggle lever pipes, gears, vibration dampers, moment compensators, etc.)- Look for squeezed-out bearing metal and discolouration caused by heat (blistered paint, burnt oil, oxidised steel). Keep possible bearing metal found at the bottom of the oil tray for later analysing. Prevent further hot spots by
preferably making a permanent repair. Ensure that the respective sliding surfaces are in good condition. Take special care to check that the circulating oil supply is in order. The engine should not be restarted until the cause of the hot spot has been located and rectified.
1) Start the circulating oil pump and turn the engine by means of the turning gear. Check the oil flow from all bearings, spray pipes and spray nozzles in the crankcase, camshaft drive gear wheel case and thrust bearing. Check for possible leakages from pistons or piston rods.
m) Start the engine. After running for about 30 minutes stop and feel over surfaces for signs of abnormal temperature rise. Especially feel over the sliding surfaces which caused the overheating. There is a possibility that the oil mist is due to atomisation of the circulating oil, caused by a jet of air/gas, e.g. by combination of the following: Stuffing box leakages (not air tight). Blow-by through a cracked piston crown or piston rod (with direct connection to crankcase via the cooling oil outlet pipe). An oil mist could also develop as a result of heat from a scavenge fire being transmitted down the piston rod or via the stuffing box. Hot air jets or flames could also have passed through the stuffing box
into the crankcase.
WARNING
Special Engine Room Dangers
Keep clear of spaces below loaded cranes.
The opening of cocks may cause discharge of hot liquids or gases.
The dismantling of parts may cause the release of springs.
The removal of fuel valves or other valves in the cylinder cover may cause oil to run onto the piston crown. If the piston is hot an explosion may blow out the valve.
When testing fuel valves do not touch the spray holes as the jets may pierce the skin.
Beware of high-pressure oil leaks when using hydraulic equipment, wear protective clothing.
Arrange indicator cocks with pressure relief holes directed away from personnel, wear goggles when using indicator equipment.
Do not weld in the engine room if the crankcase is opened before fully cooled.
Turning gear must be engaged before working on or inside the engine as the wake from other ships in port or waves at sea may cause the propeller to turn. Also, isolate the starting air supply.
Use warning notices at the turning gear starter and other control stations to warn personnel that people are working on the engine.
Use gloves when removing O-rings and other rubber/plastic based sealing materials, which have been subjected to abnormally high
working temperatures as they may have a caustic effect.
Do not allow oil patches to remain on floors as personnel can easily slip resulting in injury.
Oil spills, and particularly oily rags, anywhere present a fire hazard.
Do not remove fire extinguishers from designated positions and ensure that any fire extinguishers which have been used are replenished immediately.
Only use lifting equipment which has current certification.
