Crankcase Explosion

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Crankcase Explosion

Crankcase explosion, starting air line explosion and explosion in scavenge spaces is more dangerous and devastating situation on a ship. We know that to carry out such a long voyages the ships are made to carry a huge amount of lube oils, fuel oils and diesel. Though they help in propulsion and maintenance of the ships but they are also the source of major fire hazards which can lead to explosion also.

What is Crankcase Explosion ?

It is Explosion or fire takes place in crankcase of marine diesel engine when mixture of oil mist and air is in the proper proportion is within range of flammability.

Must Read :- Scavenge fire

Causes of Crankcase Explosion

  • Air is the normal content of the crankcase.
  • In this air there are oil globules (droplets) created by the mechanical atomization of the oil as it is sprayed from the edge of the bearing and other areas, and as it is thrown around and churned by fast moving parts.
  • If there is a hot spot, the oil particles in the neighborhood will evaporate.
  • This evaporated oil go on cooler area and get condense cause formation of white mist.
  • Continuous generation of heat during hot spot vaporization may take place until the ratio of vapor to air is within the range of flammability.
  • If the hot spot can provide the necessary heat for vapour ignition , a primary explosion may occur.

Basic Elements Required for Crankcase Explosion

As we know that ,In the presence of heat, fire is a continuous chemical reaction of fuel and oxygen. There should be a proper ratio of the element (heat, fuel, oxygen) for this chemical reaction that means to catch fire.

Crankcase Explosion in I.C Engine usually occur as when Requirements of Fire triangle completed.

  • Fuel:-Mist is burning fuel material in explosion.Indirectly we can say that lubricating oil.

When the lube oil comes into contact with a hot spot with a temperature above 200 ° C, it begins to vaporize.

Movement of engine components cause the vapors move inside the crankcase of engine .When vapors come in contact with cooler area ,they started re-condensing and it forms a white mist of oil droplets (Size range 0.005- 0.01 mm ).

  • Heat Source :- Heat source is Hot Spot.

Due to mechanical fault , overloading of engine or Rubbing of moving parts of engine creates a hot spot in crankcase.

For Explosion take place in crankcase ,there three basic elements I.e fire triangle must be completed.

When three elements (heat,fuel and oxygen ) in proportional ratio & within flammable limits,then fire takes place.

What is Hot Spot ?

Hot spot is nothing but a heat source produced as a result of friction between two metal surfaces or friction between two metal parts, such as piston rod and gland, cross-head guides, chain and drive, etc. In general, the hot spot is caused by improper maintenance and insufficient or less clearance.

Possible causes of hot spot

  • Lubrication failure between two rubbing surface of engine parts (It is primary cause of formation of hot spot )
  • Lube oil failure to bearing, sprockets and similar parts.
  • Blow past of of hot gases to crankcase of engine causes increase in temperature.
  • Scavenge fire cause increase in temperature of crankcase due to heat transfer in it.

Indication of Hot Spot

  1. By hand feel by touching door of crankcase.
  2. Running diesel engine irregularly.
  3. Abnormal Noise in Crankcase of diesel engine.
  4. Lube oil temperature of bearing increases.
  5. Alarm start ringing when concentration of mist reach to 2.5 – 5_ of LEL.
  6. Smelling and appearance of White oil mist when breather pipe or drain cock open.

 

How Explosion Takes place in Crankcase ?

When the oil comes in contact with the hot spot, these oil particles vaporize and smaller particles are formed.

These particles move towards the colder region inside the crankcase space and when in contact with the cold region, form a white mist.

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 effect of explosion will depend upon the formation of mist produced inside.

Primary Explosion

When the concentration of oil mist reaches a certain limit and comes into contact with a hot spot, it ignites and fire takes place as a primary fire.

The level of fire depends on the amount of oil mist formed inside. This primary explosion may be sufficient to lift the relief valve of the crankcase.

Crankcase Explosion

Secondary Explosion

After the primary explosion,the flame front and pressure wave travels down the crankcase with increasing speed.

The moving engine components cause turbulence and mixing of vapors ,which increase the speed of flame front and it’s area which contribute to increase in pressure.

Turbulence cause venting of pressure through relief valve .

Owing the venting of the explosion through relief valve cause drop in pressure of crankcase to below atmospheric pressure.

This can cause air to enter the crankcase resulting in another flammable mix to be developed.It cause another explosion called secondary explosion.

Note :- It is more violent and result in crankcase v door being blown off the and fires starting in engine room

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Indication of Crankcase Explosion

  • Temparature of exhaust gas increase suddenly
  • Sudden increase in the load of engine.
  • White mist smell through breather pipe of drain cock.
  • Noise and vibration in engine.
  • Irregular running of engine i.e load ,r.p.m rapidaly fluctuate.

What should do after indication of fire

If this indication seems ,

  • The speed of diesel engine should be decrease immediately.
  • Fuel supply and air in system should be stopped.
  • The system should then be cooled by opening the engine’s indicator cocks and Turning  ON the internal cooling system (if provided).

What should be do in case of fire

In case of explosion in crankcase,Reduce engine speed and increase the lubrication.

Do not open the door for 30 minutes when engine is sufficiently cooled down.

We take same measure as we take in case detection of oil mist.

Measures taken when oil mist is detected

  • First,inform the chief engineer and bridge
  • Reduce the speed of engine to reduce generation of heat.
  • Lubrication between rubbing surfaces should be increased.
  • Do not stay near crankcase door.
  • Ask,the bridge for permission to stop engine.If get order, open indicator cock and engage the turning gear and rotate engine with proper circulation of lube to prevent engine seizure.
  • Open the skylight or stores hatch si that engine room is open to sea.
  • Leave the engine room.
  • Prepare fire fighting equipment.
  • Don’t open the door for 30 minutes so that engine cooled down sufficiently.After that stop lube oil pumps.
  • Try to locate the hot spot by hand feeling over and analysing it as a change of color.
  • If the hot spot is located, avoid re – occurrence by making permanent repairs.
  • Do the crankcase inspection.
  • Pay special attention to the piston,guide and guide shoe,piston rod , stuffing box,bottom end bolts and hot bearings.
  • Start the lube oil pumps and test the proper flow of oil from every lubrication stage.
  • If all is found to be fine, notify the bridge, start the engine and gradually increasing the speed.
  • Outside feel over the sequence to be performed for a satisfactory operation.

How to prevent crankcase Explosion ?

It is done by

  • Preventing the generation of hot spots or minimize ( breather pipe or exhaust fan installed on crankcase )
  • Hot spot formation in crankcase prevented by using proper bearing clearance, adequate lube oil provided to bearing,chain with sprocket wheels and other running parts.
  • Provide proper lubrication to reciprocating parts and thus avoiding high temperature.
  • Avoiding overloading of the engine.
  • Use bearings of white metal material which helps in preventing rise in temperature.
  • The piston rings and cylinder liner of engine should be in safe working limits.
  • Use Oil Mist Detector (OMD ) in the crankcase.
  • Good fitting and efficient locking of the working parts.
  • Try to find locate hot spot hand feeling over and by observation like change in colour.

Crankcase explosion video

What are the safety devices fitted in crankcase ?

  1. Crankcase relief valve
  2. Breather pipe with flame trap
  3. Crankcase exhaust fan
  4. Oil Mist Detector
  5. Bearing temperature sensor
  6. Lube oil return temperature sensor
  7. Fire extinguishing system
  8. Warning notice
  9. Routine test of used lube oil for its viscosity,flash point and contamination.

Tha above mentioned are crankcase safety devices

Crankcase Explosion relief valve

Crankcase relief valve is a non return valve fitted on the door of crankcase which only allow to escape inside excess pressure.It will not allow air to come back into crankcase space from atmosphere.

Crankcase Explosion
Crankcase relief valve

How crankcase relief valve work ?

The relive valve consists of a light spring which did the work of holding the valve closely against its seat. 

Due to Excess pressure ,the springs get compressed because of pressure force and it allow to open the valve to transfer excess pressure inside.

Deflector (Disc ) is fitted outside the valve ,which deflect the pressurised gases & providing safeguard to the personal escaping.

An oil wetted gaze (mesh ) is also fitted which act as a flame trap .It does not allow Flame to come out.

When the pressure is reduced, the pressure spring expands and closes the valve, which prevents the air from entering.

The valve plate is made of aluminum, making it light, strong and ductile. Because of its low inertia, it does not resist the outgoing gas.

The valve disk opening pressure should not be greater than 0.2 bar.

Rules related crankcase relief valve

Crankcases shall be equipped with lightweight spring-loaded valves or other quick-acting and self-closing devices of an approved type, for the relief of pressure crankcases in the event of internal explosion and for the prevention of any air intrusion thereafter.

The valves shall be designed to be opened at a pressure not exceeding 0,2 bar.

The valve lids must be made of a ductile material capable of resisting the shock of contact with the stoppers in a fully open position.

The discharge of the valves shall be secured by a flame guard or a flame trap to minimize the possibility of danger and damage resulting from the emission of flames.

Number of relief valve

  • In diesel engine of cylinder bore not more than 290 mm and gross volume of crankcase not exceeding 0.6 m3 ,relief valve may be fitted.
  • In diesel engine of cylinder bore mire than 200 mm and not more than 250 mm ,at least two relief valve fitted.Each valve must be located at or near the ends of the crankcase. Where the engine has more than eight cranks, an additional valve shall be installed near the center of the engine.
  • In diesel engine of cylinder bore more than 250 mm but not more than 300 mm ,at least one relief valve is fitted in way of each alternate crank throw.
  • In diesel engine of cylinder bore more than 300 mm ,at least one valve is to be fitted in way of each main crank throw.
  • Additional relief valve are also fitted fir the separate spaces on the crankcase,for example gear or chain cases for camshaft or similar drives,when the gross volume of such spaces more than 0.6 m3.

Size of relief valve

  • The crankcase relief valves combined free area is to be nit less than 115 cm2/m3 bases on crankcase volume.
  • The free area of each relief valve is not to be less than 45 cm2.
  • The free area of the relief valve is the minimum flow area in any section of the valve when the valve is fully opened.
  • In order to calculate the volume of the crankcase for the purpose of calculating the combined free area of the crankcase relief valve, the volume of the stationary parts within the crankcase may be deducted from the total internal volume of the crankcase.

How would you test crankcase relief valve

When the engine stopping, after removing the flame trap, press the valve disk and check the opening and closing action. The valve must be opened smartly and closed quickly and positively.

The valve must be strong with oil and gas. When the engine is running, we must only check the leakage of oil that the v / v is sealed or not sealed.

2.Breather Pipe

Breather pipe is fitted to prevent explosion in crankcase and it reduces pressure build up inside crankcase.

It maintains pressure level in the crankcase about 25 mm of water below the atmospheric pressure.

Oil Mist Detector

Oily Mist Derector (OMD), is fitted in the crankcase to continuously monitor the oil mist inside the crankcase by taking a sample of the air out of the space (one at a time from each unit and chain case).

Whenever the oil mist amount has exceeded the set value, omd give the alarm to the control room.

Why Oil Mist Detector fitted

  • It is installed to detect the concentration of oil mist in the crankcase and to give early warning.
  • In order to prevent a primary explosion.
  • Alarm is set at 2.5% of the lower flammable limit.

Oil mist detector consist of 4 parts:-

  1. Extraction fan
  2. Reference tube
  3. Measuring tube
  4. Rotary valve
Crankcase Explosion
Oil Mist Detector

1.Extraction Fan :- This Fan did the work of drawing the sample from the space of sampling point measuring tubes and references tubes via non-return valves.

2.Measuring Tubes :- This tube has a connection for a sample taken from crankcase space, in this tube the concentration of oil mist is measured.

3.Reference Tube:- This tube is filled with clean air and used as a reference for measuring the level of oil mist in the measuring tube.

4.Rotary valve:-It is connected to each cylinder in sequence to the (Oil Mist Detector )ODM. When the oil mist level 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 sample space by means of extractor fan pass through measuring tube having a photoelectric cell and electrical connection.

When the level of oil mist reached a certain concentration, the intensity of light reaching photocell will decrease. 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 alarm of OMD.

OMD Maintenance

It is very important that the routine maintenance of the OMD is carried out to prevent failure and false alarms.

Routine Maintenance are –

  • The sensitivity of the OMD should be tested on a regular basis.
  • Tube Lenses and mirrors should be cleaned periodically, as all samples contain a small concentration of mist, which makes them dirty.
  • The extractor fan and the rotating valve should be checked to avoid shock.
  • The length of the sampling tube does not exceed 12.5 meters and should not have a loop.

 

What Action will take you in case of OMD Alarm

Or

OMD Alarm Actions

  • Keep yourself at a safe distance from crankcase door and engine. This may cause personal injury.
  • Reduce the engine rpm if the engine does not slow down automatically after OMD alarm.
  • Please notify the bridge and ask them to stop the engine.
  • After the bridge confirmation stop the engine.
  • Cut the fuel supply of the engine and start circulation of cooling system.
  • Open the skylight and store the hatch to release pressure from the engine room.
  • Leave the engine room to prevent human injury.
  • Get ready for firefighting because a secondary explosion can lead to a large fire in the engine room.
  • Do not open the crankcase until at least 30 minutes after the engine stops, a fire may come out, which could lead to another fire.
  • Carefully open the door, use no naked light, lighter may cause fire and re-ignition. All of these should be done under the supervision of the competent engineer.
  • Try to find out hot spot using the powerful lamps.
  • For later analyzing of the incident,keep possible bearing metal at the bottom of oil tray.
  • To prevent further formation of hot spot,make a permanent repair of the components.
  • Do the crankcase inspection.
  • Pay special attention to the piston,guide and guide shoe,piston rod , stuffing box,bottom end bolts and hot bearings.
  • Start the lube oil pumps and test the proper flow of oil from every lubrication stage.
  • If all is found to be fine, notify the bridge, start the engine and gradually increasing the speed.
  • Outside feel over the sequence to be performed for a satisfactory operation.

Crankcase Door

It is another safety device fitted on crankcase for the safety of person from explosion.

It is made of about 3 mm thick steel with a stainless steel appearance (made of light and strong aluminum) capable of withstanding 12 bar pressure.

For gas-tight, it is secured with a rubber seal.

Earlier the door was straight in design, so it gives away very easily in case of crankcase explosion.

Oil Mist Wikipedia