INTRODUCTION:
For any fire to begin, the fire tringle needs to be completed. To complete a fire tringle there must be present of a combustible material, oxygen or air to support combustion and a source of heat in proportional ratio and within the flammable limits, the reaction which causes fire or explosion becomes cyclic.
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Crankcase explosion normally occurs in trunk engine in which the lubricating oil used in the bearings is splashed around the crankcase and broken down into moderate size particles.
The main cause of crankcase explosions are the development of hot spots at various places in the crankcase. Due to the reciprocating motion of the piston the lubricating oil in the crankcase is splashed in the air.
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Possible causes of “hot spots” in diesel engines:
Failure of oil supply
Bearing journal surfaces becoming too rough due to L.O. becoming corrosive or being polluted by abrasive particles
Atomization of circulating oil, caused by a jet of air/gas 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 outlet pipe)
– As a result of heat from a scavenge fire being transmitted down the piston rod via the stuffing box
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When there is a hot spot formed example a bearing, guide, connecting rod, piston trunk or skirt become overheated, the falling oil particles to the heated area easily vaporize and form a white vapour (in addition some is broken down to flammable gasses such as Hydrogen and acetylene) which spreads around the crankcase.
Some of the vapour condenses to form very small particles (up to 200 micro meters in diameter) which may eventually permeate or travel the away from hotspot to whole of the crankcase space. The condensed droplets, form a dense white mist If the mixture of air, very small particles and vapour reaches a certain proportion and the temperature of the hot spot is high enough to initiate combustion an explosion can occur (850°C to ignite oil mist).
Although the most common cause of of a localized hotspot is due to friction, it is not the only cause of a crankcase explosion. A cracked piston crown, blow-by or an external fire have caused crankcase explosions in the past.
If the mixture of oil vapour, particles and air too rich (due to some reason or possibility air can be drawn in so creating the environment for a second and possible larger explosion.
EXPLOSIONS – PRIMARY AND SECONDARY
PRIMARY EXPLOSION:
Over a period of time the formation of mist starts increasing and when sufficient air/fuel ratio is reached i.e. high enough to exceed the lower explosion limit, the mist comes in contact with the hot spot again and in the presence of sufficient temperature results into an explosion.
The extent of explosion will depend upon the amount of mist produced inside. The primary explosion might be mild and sufficient enough to lift the crankcase relief valves but there could be more severe and dangerous effects during secondary explosion.
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SECONDARY EXPLOSION:
When an explosion occurs a flame front travels down the crankcase with a pressure wave in front of it.
The turbulence caused by moving engine components causing churning and mixing of vapours increase the speed of the flame front and its area, which contribute to the increase in pressure.
Turbulence caused by venting of the pressure through relief valves can also influence the explosion.
Following the venting of the explosion through the relief valves, there is a drop in crankcase pressure to below atmospheric pressure. This can cause air to enter the crankcase resulting in another flammable mixture to be developed resulting in a secondary explosion to occur.
The secondary explosion is more violent and can result in crankcase doors being blown off the engine, and fires starting in the engine room. If the relief valves do not reseal after lifting, or if they do not lift at all in the primary explosion ( due to lack of maintenance etc), then door(s) may be blown off in the primary explosion, giving a ready path for the ingress of air, which will make a secondary explosion more likely.
Air can also be sucked in via the crankcase vent, although rules state that this must be as small as practicable and new installations must have a non return valve fitted.
INDICATION OF CRANKCASE EXPLOSION:
It is a bit difficult to read the early signs of a crankcase explosions. This is because the indications are similar to many other emergency situations. But there are few pre-explosion signs that can be read. Crankcase explosion will lead:
Sudden increase in the exhaust temperature
Sudden increase in the load on the engine
Irregular running of the engine
incongruous noise of the engine
smell of the white mist.
In case of these indications, engine speed should be brought down immediately and the supply of fuel and air should be stopped. The system should then be allowed to cool down by opening the indicator cocks and turning on the internal cooling system.
MEASURES TO BE TAKEN WHEN OIL MIST HAS OCCURRED
Do not stand near crankcase doors or relief valves – nor in corridors near doors to the engine room casing!
Reduce speed/pitch to slow-down level, if not already carried out automatically
Ask the bridge for permission to stop
– Measures to be taken when Oil Mist has occurred
When the stop order is received:
– Stop the engine
– Close the fuel oil supply
Switch-off the auxiliary blowers
Open the skylight and/or “stores hatch”
Leave the engine room
– Measures to be taken when Oil Mist has occurred
Lock the casing doors and keep away from them
Prepare the fire-fighting equipment
Do not open the crankcase until at least 20 minutes after stopping the engine. When opening up, keep clear of possible spurts of flame. Do not use naked light and do not smoke.
– Measures to be taken when Oil Mist has occurred
Stop the circulating oil pump. Take off/open all the lowermost doors on the side of the crankcase. Cut-off the starting air and engage the turning gear.
Locate the “hot spot”. Use powerful lamps from the start.
Keep possible bearing metal found at the bottom of oil tray for later analyzing.
– Measures to be taken when Oil Mist has occurred
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 the circulating oil supply is in order.
– Measures to be taken when Oil Mist has occurred
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, chaincase and thrust bearing.
Check for possible leakages from piston or piston rods.
PREVENTION
Prevention of crankcase explosion can be done by preventing the generation of hot spots. It can also be prevented by the following ways:
By providing proper lubrication to the reciprocating parts,thus avoiding high temperatures.
Avoiding overloading of the engine
Using bearings with white metal material which prevents rise in temperature.
Using oil mist detector in the crankcase with proper visual and audible alarm.Oil mist detectors raise an alarm if the concentration of oil mist rises above the permissible limit.
Pressure relief valves should be fixed on the crankcase for the instant release of pressure. They should be periodically pressure tested.
Crankcase doors should be made of strong and durable material. Vent pipes shouldn’t be too large and should be checked for any choke up
Pressure relief valves should be provided with wire mesh to prevent the release of flames inside the engine room.
Safe distance should be kept from the crankcase and the relief valves in case the indications are sighted.
In case of indication, the crankcase doors should never be opened till the time the system has totally cooled down. Once the system has cooled down, proper inspection and maintenance should be carried out.
Fire extinguishing medium should be kept standby. In many systems, inert gas flooding system is directly connected to the crankcase.
Crankcase safety fittings
• Crankcase Relief valves
• Vent pipes
• Alarms
• Warning notice
• Fire-extinguishing system for scavenge manifolds
Relief Valves
The valve lids are to be made of ductile material capable of withstanding the shock of contact with stoppers at the full open position.
The discharge from the valves is to be shielded by flame guard or flame trap to minimize the possibility of danger and damage arising from the emission of flame.
Number of Relief Valves
In engines having cylinders not exceeding 200 mm bore and having a crankcase gross volume not exceeding 0,6 m3, relief valves may be omitted.
In engines having cylinders exceeding 200 mm but not exceeding 250 mm bore, at least two relief valves are to be fitted; each valve is to be located at or near the ends of the crankcase. Where the engine has more than eight crank throws an additional valve is to be fitted near the centre of the engine.
In engines having cylinders exceeding 250 mm but not exceeding 300 mm bore, at least one relief valve is to be fitted in way of each alternate crank throw with a minimum of two valves. For engines having 3, 5, 7, 9, etc., crank throws, the number of relief valves is not to be less than 2, 3, 4, 5, etc., respectively.
In engines having cylinders exceeding 300 mm bore at least one valve is to be fitted in way of each main crank throw.
Additional relief valves are to be fitted for separate spaces on the crankcase, such as gear or chain cases for camshaft or similar drives, when the gross volume of such spaces exceeds 0.6 m3.
Size of Relief Valves
The combined free area of the crankcase relief valves fitted on an engine is to be not less than 115 cm2/m3 based on the volume of the crankcase.
The free area of each relief valve is to be not less than 45 cm2.
The free area of the relief valve is the minimum flow area at any section through the valve when the valve is fully open.
In determining the volume of the crankcase for the purpose of calculating the combined free area of the crankcase relief valves, the volume of the stationary parts within the crankcase may be deducted from the total internal volume of the crankcase.
Vent Pipes
Where crankcase vent pipes are fitted, they are to be made as small as practicable to minimize the inrush of air after an explosion. Vents from crankcases of main engines are to be led to a safe position on deck or other approved position.
If provision is made for the extraction of gases from within the crankcase, e.g. for oil mist detection purposes, the vacuum within the crankcase is not to exceed 25 mm of water.
Lubricating oil drain pipes from engine sump to drain tank are to be submerged at their outlet ends. Where two or more engines are installed, vent pipes, if fitted, and lubrication oil drain pipes are to be independent to avoid intercommunication between crankcases.
Alarms
Alarms giving warning of the overheating of engine running parts, indicators of excessive wear of thrusts and other parts, and crankcase oil mist detectors are recommended as means for reducing the explosion hazard. These devices should be arranged to give an indication of failure of the equipment or of the instrument being switched off when the engine is running.
Warning Notice
A warning notice is to be fitted in a prominent position, preferably on a crankcase door on each side of the engine, or alternatively at the engine room control station. This warning notice is to specify that whenever overheating is suspected in the crankcase, the crankcase doors or sight holes are not to be opened until a reasonable time has elapsed after stopping the engine, sufficient to permit adequate cooling within the crankcase.
Crankcase Access and Lighting
• Where access to crankcase spaces is necessary for inspection purposes, suitably positioned rungs or equivalent arrangements are to be provided as considered appropriate.
• When interior lighting is provided it is to be flameproof in relation to the interior and details are to be submitted for approval. No wiring is to be fitted inside the crankcase.
Fire-Extinguishing System for Scavenge Manifolds
• Crosshead type engine scavenge spaces in open connection with cylinders are to be provided with approved fixed or portable fire-extinguishing arrangements which are to be independent of the fire-extinguishing system of the engine room.
SPECIFIC REGULATIONS
1. Non-return doors must be fitted to engines with a bore greater than 300mm, at each cylinder with a total area of 115sq.cm/m3 of gross crankcase volume. The outlets of these must be guard to protect personnel from flame. For engines between 150 to 300mm relief doors need only be fitted at either end. Below this bore there is no requirement. The total clear area through the relief valve should not normally be less than 9.13cm2/m3 of gross crankcase volume
2. Lub Oil drain pipes to the sump must extend below the surface and multi engine installations should have no connections between the sumps
3. Large engines, of more than 6 cylinders are recommended to have a diaphragm at mid-length and consideration should be given to detection of overheating (say by temperature measuring probes or thermal cameras) and the injection of inert gas.
4. Engines with a bore less than 300mm and a crankcase of robust construction may have an explosion door at either end
5. Means of detection of oil mist fitted.
Crankcase Doors
• These when properly designed are made of about 3mm thick steel with a dished aspect and are capable of withstanding 12 bar pressure. They are securely dogged (gripped) with a rubber seal arrangement.
Crankcase Relief Door
Due to the heavy force of momentum the gas shockwave is not easily deflected.
Thus any safety device must allow for a gradual change in direction, and be of the non-return type to prevent air being drawn back into the crankcase.
The original design was of cardboard discs which provided no protection against the ingress of air after the initial explosion, in addition it was known for these discs to fail to rupture in the event of an explosion.
The valve disc is made of aluminium to reduce inertia.
The oil wetted gauze provides a very effective flame trap this reduces the flame temperature from 1500’C to 250’C in 0.5 m.
The ideal location for this trap is within the crankcase where wetness can be ensured.
The gas passing from the trap is not normally ignitable. The gauze is generally 0.3mm with 40% excess clear areas over the valve.
Continuous extraction by exhauster fan may be used but this tends to be costly, flame gauzes must be fitted to all vents. Similarly a continuous supply of air can be used to reduce gas mist levels.
Crankcase oil mist detector
Source: OIL MIST DETECTOR MD-SX (Daihatsu, Japan)
Oil mists can be readily detected at concentrations well below that required for explosions, therefore automated detection of these oil mists can be an effective method of preventing explosions.
The Graviner oil mist detector is in common use in slow speed and high speed engines.
The disadvantage of this type if system is that there is a lag due to the time taken for the sample to be drawn from the unit and for the rotary valve to reach that sample point.
Consists of :-
I. Extraction fan
– draws the sample from the sample points through the reference and measuring tubes via non-return valves.
II. Reference tube
– measures the average density of the mist within the crankcase, as there will always be some mechanically generated mist.
III. Rotary valve
– this valve is externally accessible and is so marked so as to indicate which sample point is on line. In the event on exceeding the set point , the valve automatically locks onto that point so giving a clear indication of the locality of the fault condition.
IV. Measuring tube
– measures the opacity(darkness) of the sample by means of a photoelectric cell as with the measuring cell. To exclude variables in lamps a single unit is used with beams directed down the tube by mirrors.
Oil mist detector function:
- To monitor samples taken continuously from the crankcase of a diesel engine.
- Detect the presence of oil mist at concentrations well below the level at which crankcase explosions may occur. This gives warning in time to allow avoiding to slow down the engine and prevent either serious bearing damage or an explosion.
- The detector consists basically of two parallel tubes of equal size each having a photo-electric cell fitted at one end. Photo-electric cells are light sensitive and generate an electric current directly proportional to the intensity of the light falling on their surface. Lenses are fitted to seal the ends of each tube but allow light to pass.
- Two identical beams of light from a common lamp are reflected by mirrors to pass along the tubes on to the cells which are then in electrical balance.
- One tube is sealed to contain clean air and is termed the reference tube. The other termed the measuring tube, has connections through which samples of the vapor content of the engine crankcase are drawn by means of an electrically extractor fan.
- In the event of a concentration of oil mist being present in the sample, light will be obscured before reaching the cell of the measuring tube. Electrical balance between the two cells will be disturbed and an alarm will be operated.
- Sampling points should be fitted to each cylinder crankcase and their connections are brought to a rotating selector valve which is driven from the motor. This repeatedly connects each sampling point to the measuring tube in sequence.
- In the event of oil mist being detected the rotator stops to indicate which sampling point is concerned. The instrument must be reset below the alarm ceases and sampling will recommence its sequence.
Crankcase oil mist detector maintenance:
- Sampling connections should not exceed 12.5 meters in length and must slope to ensure positive drainage of oil; they must avoid any loops in which could fill with oil.
- The detector should be tested daily and the sensitivity checked. The lenses and mirrors should be cleaned periodically.
REFERENCES:
- www.marineengineering.co.uk
- The Running and Maintenance of Marine Machinery – Cowley
- Lamb’s Question and Answers on Marine Diesel Engines – S. Christensen
- Diesel Engines – A J Wharton
- www.marinediesels.info