In my previous blog, we had discussed the scavenge fire, reasons, indications, remedies, and safety devices fitted in that space. In the series of fire safety onboard we are going to discuss another dangerous situation which may lead to a huge fire on ship i.e. crankcase explosion.
What is a crankcase explosion?
In case of ignition in enclose type crankcase of the engine leads to the fire and explosion known as crankcase explosion. It may happen in all enclose type crankcase engines, including a steam engine. Crankcase explosion occurs in both trunk piston types and in types with a scraper gland seal on the piston rod.
Fire is a continuous chemical reaction of fuel and oxygen in the presence of heat. For this chemical reaction that means to catch fire, there should be a proper ratio of the element (heat, fuel, oxygen).
Mechanism of crankcase explosion –
Crankcase explosion is a sequential event, to understand in a better way we can summarize in following steps –
- A mechanical fault, overload or rubbing of moving parts creates a hot spot somewhere in the crankcase.
- When lube oil comes in contact with hot spot having a temperature above 200°C, starts vaporizing.
- Movement of the engine component allows the vapors to moves inside the engine when vapors pass to a cooler area; they re-condense and forms a white mist of oil droplets of size 0.005-.0.01mm.
- The concentration of oil mist increases steadily and reached the lower explosive limit of around 50 mg/l.
- When the hot spot reached its temperature 500 °C, ignite the oil mist and fire take place.
Here the burning fuel is the Lube Oil droplets. The lube oil is very stable and can withstand a higher temperature(for example boundary lubrication in the combustion chamber) but because of the hot spot, the formation of combustible lube oil droplets will result in crankcase explosion. The crankcase explosion takes place in two steps i.e. primary and secondary explosion. Actually, the other is the result of the first/primary explosion.
When oil mist concentration reached a certain limit and comes in contact with a hot spot, it ignites and fire takes place as primary fire. The extent of fire will depend upon the amount of oil mist formed inside. This primary explosion might be sufficient to lift the crankcase relief valve.
After the primary explosion, the flame front and pressure waves accelerate through the crankcase, vaporized more oil droplets in the path resulting pressure waves build up sufficient to rupture the crankcase doors, if not relieved. If relief valve does not reset after lifting, it will cause fresh air to enter into the crankcase space, because the pressure inside the crankcase will be lower than atmospheric pressure, leading to another flammable mixture and big explosion as secondary fire.
Possible Causes of ‘Hot Spot’
- The failure of the lubrication between rubbing surfaces(a primary cause of Hot Spot formation).
- Improper lube oil causes a rough surface of bearing journal
- Blow by through cracks of piston crowns leads to crankcase causes increase in temperature
- In the case of scavenge fire, heat may transfer to the crankcase and increases the temperature
Indication of a Hot Spot formed inside crankcase will be:
- We can feel hotness by hand touch to crankcase door while in round
- Irregular running of the engine
- Lube oil temperature of the bearing will increase
- Abnormal noise in crankcase space
- Appearance and smell of the dense oil mist when open breather pipe or drain cock.
Safety devices in crankcase space
- OMD (Oil Mist Detector)
- Crankcase door
- Crankcase relief valve
- Breather pipe with flam trap arrangement
- Bearing temperature sensor
- Return lobe oil temperature sensor(piston cooling oil)
OMD (Oil mist detector)-
A safety device, OMD is fitted in the crankcase to continuously monitor the oil mist inside the crankcase by taking sample of air from the space(one at a time from each unit and chain case). Whenever the oil mist level reached set value it will give the alarm to control room.
Oil mist detector consists mainly of 4 components –
- Extraction fan
- Reference tube
- Measuring tube
- Rotary valve
Extraction fan – It draws the sample from the sampling point of the space through reference and measuring tubes via non-return valves.
Reference tube– It is filled with clean air and used as a reference for measuring the oil mist level in measuring tube.
Measuring tube– It has a connection for a sample taken from crankcase space, in which it measures the concentration of oil mist.
Rotary valve– It is connected to each cylinder in sequence to the ODM. When the level of mist reached a set point it will automatically lock at that unit and give a clear indication of the faulty unit.
The continuous sample is taken from space with help of extractor fan pass through measuring tube having a photoelectric cell and electrical connection. This is light When the oil mist level reached a concentration, the intensity of light reaching photocell will reduce and due to different intensity in reference tube and measuring tube deviation will generate in photocells which will give the electrical signal. This signal will activate the OMD alarm.
Maintenance of OMD
It is very important to carry out the routine maintenance of the OMD to prevent failure and false alarms. Routine maintenance are –
- On regular basis, the sensitivity of the OMD should be checked.
- Lenses and mirror should be cleaned periodically as all samples contain a small concentration of the mist, which make them dirty.
- To avoid chocking, extractor fan and rotating valve should be checked.
- Length of the sampling tube should not be more than 12.5 meters and should not have any loop.
Action in Case of OMD alarm
- Keep a safe distance from the crankcase door and engine. It may cause personal injury.
- Reduce the rpm of the engine, if not engine will slow down automatically after OMD alarm.
- Inform the bridge and ask them to stop the engine.
- After getting confirmation from bridge stop the engine
- Stop the fuel oil supply to the engine and maintain the cooling system.
- Open the skylight and stores hatch to release pressure from the engine room
- Leave the engine room to avoid human injury.
- Lock the crankcase door and leave the space immediately
- Get prepared for the firefighting because secondary explosion may lead to a large fire in engine room.
- Do not open the crankcase until at least 20 minutes after stopping the engine, fire may come out, which may lead to another fire.
- Open the door carefully, do not use any naked light, lighter may cause re-ignition and fire. All these should be done under the supervision of a competent engineer.
- Circulating oil pump to be stopped. Take off all the lowermost doors on the side of the crankcase. Cut off the starting air supply and engage the turning gear.
- Find out the hot spot. Use the powerful lamps to locate that.
- Keep possible bearing metal found at the bottom of the oil tray for later analyzing of the incident.
- Prevent further hot spot by preferably making a permanent repair of the component.
- Make sure that the respective sliding surfaces are in good condition.
- Circulation of the oil supply to be checked and should b in order.
- Start the circulating oil pump and turn the engine by means of the turning gear to avoid seizing of the piston.
- Ensure the oil is flowing from all bearing, spray pipes and spray nozzles in the crankcase and thrust bearing.
- Check if there are any leakages from piston or piston rods.
Crankcase Door –
Another safety against the crankcase explosion is crankcase door is designed of about 3mm thick steel with a dished aspect(made with aluminum light and strong), which is capable of withstanding 12 bar pressure. For gas-tight, it is secured with rubber seal arrangement. Earlier the door was straight in design, so it gives away very easily in case of crankcase explosion.
Crankcase relief valve –
A non-return valve is fitted on the crankcase door, which only allows the inside excess pressure to escape. It will not allow air to go inside the crankcase space.
The relive valve consists of a light spring that holds the valve closely against its seat. In the case of excess pressure, the spring gets compressed due to the pressure force and allow to the opening of the valve, which passes the excess pressure. Outside of the valve deflector(disc) is fitted which deflect the pressurized gases and provide the safeguards to the personal escaping. An oil wetted gaze(mesh) is also fitted which act as a flame trap and not allow the flame to come out. When pressure goes down to the set pressure spring expands and closes the valve, which will prevent the air to go inside.
The valve plate is made of aluminum, which makes it light, strong and ductile. It doesn’t resist the outgoing gas, because of its low inertia. The opening pressure of the valve disc should not be greater than 0.2 bar. The following defines the rules regarding crankcase explosion doors:
- Relief valves
- Crankcases are to be provided with lightweight spring-loaded valves or other quick-acting and self-closing devices, of an approved type, to relieve the crankcases of pressure in the event of an internal explosion and to prevent any inrush of air thereafter. The valves are to be designed to open at a pressure not greater than 0,2 bar.
- The valve lids are to be made of a ductile material capable of withstanding the shock of contact with stoppers at the fully open position.
- The discharge from the valves is to be shielded by a 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 center 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 chaincases 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.
Indication of Crankcase Explosion
- Exhaust gas temperature will increase suddenly
- Load on the engine will increase suddenly
- Irregular running of the engine, load, and rpm may fluctuate, abnormal noise and vibration in the engine.
- A smell of the white mist from breather pipe.
In the case of the above indications, engine rpm should be brought down immediately and the supply of fuel and air should be stopped. The system should then be cool down by opening the indicator cocks of the engine and turning on the internal cooling system(if provided). Mentioned above under Action in Case of OMD alarm.
Preventive action in case of crankcase fire
For the prevention of the crankcase explosion, we have to prevent the formation of the ‘Hot Spot’ which can we achieve by following methods-
- To avoid the high temperature, providing the proper and sufficient lubrication to the reciprocating part of the engine.
- The white metal bearing can we used, which prevent an increase in the temperature
- By using oil mist detector (OMD) in the crankcase with proper visual and audible alarm.
- Pressure relief valve should be fixed on the crankcase door for the instant release of pressure. They should be periodically pressure tested and maintained.
- Crankcase door should be made of strong and durable material. Vent pipes shouldn’t be too large and should be checked for any choke up at regular interval.
- Pressure relief valves should be provided with wire mesh to prevent the release of flames inside the engine room which may cause human injury.
- Safe distance must be kept from the crankcase and the relief valves in case the indications are sighted.
- In case of indication of crankcase explosion, 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 by a competent engineer.
- Fire extinguishing medium should be kept standby. In many systems, inert gas flooding system is directly connected to the crankcase space.
Once the crankcase fire is extinguished, the foremost work is to find out the cause of the fire. For this crankcase inspection to be carried out. The procedure for crankcase inspection after fire and inspection during normal inspection is almost the same, only a few things get change which I have mentioned below. Permission has to be taken before reaching port to make sure that the port authority is not having any problem with this(for routine inspection). This is called Immobilization permission of the main engine. If the ship is at sea, put navigational status as “Not Under Command NUC”. After the permission is received from port authority or NUC at sea, ensure enclosed space entry procedure checklist has to be followed up with all it’s required permission. Safety issues have to be discussed with the people taking part in the inspection (Toolbox meeting).
Safety Precautions carried Out before Crankcase Inspection:
- Isolate the main engine starting system by shutting-off of main air starting valve and air bottle main valve.
- Open indicator cocks and engage turning gear.
- Ensure engine has been tuned for at least half an hour before stopping lube oil pump and crosshead lubrication pump(nowadays no separate pump is there), due to the shortage of time, you can reduce this interval. But in any case not less than 15 minutes.
- Stop lube oil pumps and crosshead oil pump and place the circuit breakers in off position.
- Post notice “Don’t start– Men at work”.
- Ask duty officer for propeller clearance.
- Open crankcase door and ventilate properly.
- Wear safety gear; e.g. helmet, boots, raincoat, gloves etc.
- Use safety lamps, torches, and tools.
- All pockets should be emptied of contents and all tools taken inside must to be accounted for.
- One responsible person must stand outside.
Procedure to be carried out during Crankcase Inspection:-
- After stopping the engine and the pumps the crankcase doors have to be opened and sufficient time is to be given to cool and ventilate the space as the temperature inside is very hot and deprived of air.
- After the cooling and ventilating the space, the person entering the space should be with proper personal protective equipment (PPE) like boiler suit, safety harness, and anti-slipping pads for shoes.
- Make sure there are no tools, pen etc in your pockets which might drop inside and cause damage to bearing and machinery parts
- Before entering, the person has to be detailed as what needs be checked inside. After fire inspection is to find out the hotspot or any other cause of the fire.
- Turn the engine to BDC and start checking from under stuffing box area for any sign of black oil, an indication of stuffing box leaking.
- Check for any bluish dark patches, this indicates that hot spots are caused by friction of insufficient lubrication.
- Check the piston rod surface for scoring marks and roughness.
- Check piston palm bolts and locking device for slackness and fretting.
- Check the guide and guide shoe bearing general condition and the area around the frame where the guide is attached for any visible cracks.
- Check that the guide shoe end cover bolts are in place and not slack.
- Check crosshead bearing general condition.
- Evaluate top and bottom end of connecting rod bolt, nut and locking device for slackness, signs of fretting, etc.
- Check for sliding of bottom end bearing (axial movement) or floating of connecting rod.
- Check web in the area of stress concentration and check tie bolt (bottom side).
- Check cross girder, the area around the main bearing and bearing keep for a sign of cracks and check around the main bearing.
- All bearings must be checked for silvery color (indicates bearing wiping).
- Check the surroundings of the oil pan area of all units for any sludge deposits, bearing metal pieces, etc.
- Check for the slip of web and journal by checking the reference mark.
- Check crankcase relief door- wire mesh (should be wet), spring tension, sealing ring condition, etc.
- Check the teeth of transmission gear for a sign of wear.
- Check chain drive for tightness.
- Use oil mist detector on sampling pipe to check for clear passage.
- Check the bed plate for any welding cracks etc.
- Check for piping and any loose connections between them.
- Before coming out make sure there is nothing left inside.
- Clear all the foreign materials from the crankcase, and make sure all tools are accounted for.
- Start lube oil pump and crosshead lubricating pump and check for the oil flow and distribution.
- Check crankcase door sealing condition and close the crankcase door.
- Inform chief engineer (for satisfactory checking of the crankcase) and the duty officer.
A table of major crankcase explosion(by MAN B&W) is given below:
|Year||Cause of Explosion||Cause of Failure|
|1995||Bearing in PTO gearbox|
|1996||Inlet pipe for piston cooling oil falling off||Incorrect tightening|
|1997||Incorrect spring mounted in piston rod stuffing box||Unauthorized spare part|
|1997||Piston rod interference with cylinder frame|
|1999||Weight on chain tightener falling off||Incorrect tightening|
|1999||Fire outside the engine|
|2000||Incorrect shaft in camshaft drive||Unauthorized spare part|
|2001||Piston crown failure|
|2002||Inlet pipe for piston cooling oil falling off||Incorrect tightening|