What is crankcase explosion ?

• It is an explosion that occurs in the crankcase because of the mixture of oil mist and air in the ratio that is within the range of flammability.
• In addition there must be a source of high temperature energy sufficient to initiate combustion. This source is called hot spot.

What are the causes of crankcase explosion ?

• The normal content of crankcase is air.
• In this air there are oil globules (droplets) formed by the mechanical atomization of the oil as it s sprayed from the edge of the bearing and other places and as it is thrown about and churned by the quickly moving parts.
• If a hot spot occurs the oil particles in the neighborhood will evaporate.
• This evaporation may cause the formation of a quantity of white condense oil mist in the cooler regions.
• The continual generation of heat at hot spot vaporization may proceed a space until the ratio of vapour to air lies within the range of flammability.
• If hot spot can provide the necessary heat for ignition of vapour, a primary explosion may occur.

What is hot spot and why it occur ?

• It is an overheated part, sufficient to initiate combustion.
• A hot spot occur due to the following reasons.
• Failure of lube oil to bearing, sprockets and similar parts.
• Hot gas blowing past the pistons may provide spark sufficient to cause an explosion in the trunk type pistons engine.
• Hot spot temperature: 280 °C  to 400 °C above the L.O flash point

How will you know whether hot spot is there or not ?

1. By hand feel to crankcase door
2. Abnormal noise in crankcase.
3. Irregular running of engine
4. Bearing L.O temperature increase
5. Alarm will operate when it reach at oil mist concentration 2.5 – 5% LEL
6. Smelling & appearance of the dense oil mist when open breather pipe, drain cock.

How to prevent the crankcase explosion ?

• To minimize the formation of explosive mixture breather pipe or exhaust fan fitted on crankcase.
• To prevent the formation of hot spot in the crankcase, the bearing should be in correct running clearance, lubricating oil should be adequately supplied to bearing , chain with sprocket wheels and other running parts.
• The piston with rings and cylinders should be in safe working limits.
• Good fitting and efficient locking of working parts.

 Preventive measure of crankcase explosion

1. There should be adequate cooling of the engine.
2. Proper purification and analysis of lube oil.
3. Lube oil filter to be changed over & cleaned as per schedule.
4. Ensure proper cylinder lubrication by checking the condition of piston, piston rings and liner through scavenge ports.
5. Clean scavenge space as per schedule & drain scavenges space regularly.
6. Maintain the stuffing box gland sealing in good condition.
7. All running gears maintenance & checks to be carried out as per PMS.
8. Be alert & rectify for any abnormal noise in crankcase.
9. All safety trips & alarms fitted on M/E to be tried out for satisfactory.
10. Proper watch on all running gears temperature & pressure to be maintained.
11. Blow through all sampling tubes of Oil Mist Detector (OMD) regularly.
12. Zero adjustment & sensitivity of OMD to be checked regularly.
13. Check for oil leakage at crankcase explosion relief doors & check for the operation by hand.
14. Check flame trap for cleanliness and intact condition.

What will be your action after detecting a hot spot ?

1. Inform chief engineer & bridge
2. Reduce the engine speed to reduce the heat generation
3. Increase lubrication to the running surfaces
4. Keep clear off crankcase relief door to prevent personnel injury, keep fire extinguisher ready and open skylight.
5. Inform bridge, stop engine, open indicator cocks, engage turning gear and turn engine with lube oil circulation continuing to prevent engine seizure.
6. After about 30 minutes when the engine is sufficiently cooled down, stop lube oil pumps and open crankcase door.
7. Try to locate the hot spot by hand feeling over and by observation like change in colour.
8. If the hot spot is located, prevent re-occurrence by making permanent repair.
9. Make thorough inspection of crankcase.
10. Pay particular attention to hot bearings, piston, bottom end bolts, guides and piston rod around stuffing box.
11. Start lube oil pumps and check for proper flow of oil from every lubrication point.
12. If everything is found normal, inform bridge, start the engine and gradually increase speed.
13. Outside feel over sequence to be carried out for satisfactory operation.

What are crankcase safety devices ?

1. Breather pipe with flame trap
2. Crankcase exhaust fan
3. Oil mist detector
4. Crankcase relief doors
5. Bearing temperature sensor
6. L.O return temperature sensor
7. Routine test on used L.O for viscosity, flash point and contamination.

Why is breather pipe fitted in the crankcase ?

• It is fitted to prevent crankcase explosion and to reduce pressure build up in it.
• It maintains the pressure level in the crankcase about 25mm of water below the atmospheric pressure.

Why crank case relief valve is fitted ?

• It is fitted to release any sudden rise of internal pressure with large free escape area thus to prevent secondary explosion.

What are the requirements of crankcase relief valves ?

1. The internal combustion engine of cylinder diameter of 200 mm or a crankcase volume of 0.6 m3 and above shall be provide with crankcase relief valve of a suitable type with sufficient relief area.
2. In small engine, cylinder diameter not more than 300mm, crankcase door of which are usually very strong. It may have relief valve or valves at its end.
3. In large engine, cylinder diameter more than 300mm, require one relief valve to be placed on each crankcase door.
4. Its free area should not be smaller than 45cm2 and there shall be minimum of 115cm2 /m3 of the gross crankcase volume.
5. Spring setting for opening pressure is 0.07 bar at an internal pressure and will close when the pressure has been relieved.
6. The valves open smartly and close positively and rapidly.

How would you test crankcase relief valve ?

• When engine is stopping, after removing the flame trap and press up the valve disc and check the action of opening and closing.
• The valve must be opened smartly and closed positively and rapidly.
• The valve must be oil and gas tight.
• When the engine is running we must only check the oil leakage that shows the v/v is seal or not.

What are the causes of bearing high temperature ?

1. Improper viscosity of oil
2. Insufficient lubrication
3. Misalignment of shaft or bearings
4. Foreign matters in oil
5. Score in journal
6. Poorly fitted bearing
7. Improper oil clearance
8. Whenever the bearing begins to heat up, check the lubrication whether it is clear, correct temperature and flow sufficient or not.

What is the blow past / blow by ?

• Blow past and blow by are the same. Hot gas or flame passed through the piston rings and cylinder liner from the combustion space.

What are the source of oil coming into air bottle ?

• Excessive oil level of compressor
• Excessive cylinder lubrication
• Defective oil scraper rings
• Oil vapours inhaled from the engine room atmosphere.

What point to be checked during crankcase inspection ?

• Appearance of all parts inside the crankcase
• Hot spot
• Corrosion
• Wear and tear
• Locking arrangement
• Bolts tightness
• Chain inspection
• Lube oil system
• Metal chip in the crankcase
• Bearing clearances
• Crankcase relief valves, flame trap
• Crankcase door joints
• Crankshaft deflection

Why oil mist detector is fitted ?

• It is fitted to detect the oil mist concentration in the crankcase and to give early warning.
• To prevent the primary explosion.
• Alarm setting is 2.5% of the lower flammable limit.

What are oil mist detector maintenance ?

• Daily check alarm system.
• Check for any abnormal alarms in the panel.
• Detector head cleaning.
• Make sure fans is working.

How will you take action when oil mist detector alarm is on ?

• Inform to chief engineer
• Inform bridge, take the permission to stop the engine
• Gradually reduce the engine and then stop.
• Cool down the engine.
• Check the inside parts of the crankcase inspection.
• Make necessary repair
• Reset OMD and start engine

Reference:

Oral Guide by – MIN ZAR TAR