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In this article, we will discuss the Crankcase explosion. Crankcase explosion is the serious threat to the Ships safety and may be fatal to human beings. Crankcase explosion takes place in two stages known as primary and secondary occurring in the Engine Crankcase.
Crankcase explosion is the greatest and severe threat to the safety of Ship and fatal to the human beings working in the close vicinity. This phenomenon occurs due to the oil mist in the crankcase getting ignited after coming into contact with any hot spot. A hot spot can be due to various reasons like poor maintenance of the Engine, neglected maintenance schedule of the Engine. If the primary explosion is not taken then, the secondary explosion takes place
The most hazardous fires causing disastrous accidents onboard ships is known as the Scavenge fire.There is significant threat to the safe operation of marine diesel Engines due to the Scavenge fire. Let us understand the scavenge fire and the reasons for its occurrence. We shall also discuss the preventive major to combat such fires.
What is Scavenge Fire?
During the engine in operation, Lubricating oil with heavy carbon contents, partially burnt fuel, and carbon from residual products get accumulated in the scavenge box. Over a period, the density of these products increases. Due to some abnormal conditions in the engine, these stocked products that catch fire in the
scavenge box are known as scavenging fire.
Causes of Scavenge Fire
- Sticky or Broken Piston Rings
- Excessive wear in Cylinder Liners
- Excessive or insufficient Cylinder Lubrication
- Excess vice Piston ring axial and radial clearances.
- Overheated Piston
- Faulty fuel Injection Equipment leading to poor combustion.
- Low compression pressure
- Overloaded Engine
- Exhaust back pressure duet the fouled exhaust gas pathways.
- Fouled Scavenged box.
Indication of scavenge fire
Following indication will indicate the sign of Scavenge fire to prevent Major Disastrous accidents in Marine Engines
- Sudden Loss in power with high exhaust temperatures
- Drop-in Engine Speed
- Excessive gases in the exhaust
- Increase in Scavenge Temperature
- Noticeable surging in Turbocharger
- Noticeable smoke from scavenging drains
- Burnt paint on scavenging doors in case of major scavenge fire.
- Abnormal exhaust smoke.
- Visible smoke in a transparent window type installation.
- Increased cooling water temperature.
Three essential elements of Fire
Fire triangle consists of three essential elements.These three elements are Oxygen, Heat, and fuel. Due to the presence of these three elements in proportionate, will cause the fire and subsequent result in crankcase explosion.
In the Engine’s crankcase, the oil travels in the vapor form of tiny particles of 200 microns. These small particles can never ignite in the presence of even naked flame. Accidently due to any hot spot in the crankcase, these particles will be reduced and form the mist. This mist will get easily ignited once in contact with the hotspot.
If there are multiple hot spots, then the density of the mist will increase in Geometrical progression. This movement, once the mist is ignited, will result in the primary explosion. If the created pressure is not relieved during this period, this will rapture crankcase doors and relief valves. The low pressure will suck in the fresh air from the damaged parts and leaky crankcase relief valves. Another phenomenon is serious, which will cause a significant explosion.
Action to be Taken
For Small Fire
- Reduce the Engine Speed.
- Cut off the fuel supply of the particular unit.
- Don’t increase the cylinder lubrication, In case the fire is more
- Maintain the cooling flow through the Jacket and piston
- Shut the scavenge drain cock.
- Cut off the Fuel injection pump of the affected unit.
- Monitor the exhaust and scavenge temperatures
For Major Fire
- Stop the engine immediately
- Engage the turning gear and start turning the engine to prevent the seizure of significant components.
- Attempt to extinguish the fire with the fixed fire fighting system or the
- Open and inspect the scavenge space after proper cooling down once it is confirmed that the fire is extinguished.
Prevention of Scavenge Fire
- Clean the scavenge space, scavenge box, and drain regularly.
- Avoid excessive cylinder lubrication.
- Regularly inspect the Piston rings, cylinder liners, and piston skirt.
- Carry out the proper honing of cylinder liners.
- Prevent the Engine vibrations
- The trunk should be cleared of improperly burnt oil and carbon particles.
- Periodically drain the scavenge drains and keep the drains cleared.
- Install the high-temperature alarm to provide the high-temperature indications.
- Test the pressure regulating valves regularly.
- Test the fire alarm regularly
- Avoid prolonging engine overloading.
- Open the drain cocks during the low load operation to clear the oil accumulation in the manifold.
Post Scavenge Fire Inspection
- Check the piston alignment.
- Check the moment of Pistons in the cylinder while turning the engine. Look for any binding of piston with liner. Also, look for any dry patched across the cylinder liner working surface.
- Inspect the spring of the safety relief device.
- Check the tension of bolts.
What is a Hot Spot?
The hot spot is the heat source produced due to the rubbing of metal surfaces such as Piston rod and glands, crosshead guides, Bearings getting overheated due to oil starvation, or excessive blow-by in the crankcase. The hot spot will be termed only if the spot’s temperature is more than the ignition point of the fuel. The hotspot temperature in the crankcase is in the range of 200 to 400 Degree C.
Hotspots are generally resulted due to poor maintenance and maintaining the improper clearances between the moving parts.
When the oil comes in, the contact of hot spots will get converted into vapors. These vapours travelling towards the colder surface will form a white mist. Over a period, the density of this mist will keep on increasing due to repeated contact with the hot spot. The extent of an explosion will depend upon the amount of mist produced.
Causes of Hot Spots
- High temperature is created due to the reciprocating moment of pistons.
- The high temperature of the bearings.
- Heavy blow by in the crankcase from the combustion chamber.
- Fire in the scavenge trunk.
Methods to detect the presence of hot spots
- Abnormal Engine operation with obnoxious noise.
- The alarm on Oil mist detector.
- High Lubricating oil temperature.
- Dense smoke from crankcase breather.
- High exhaust temperature with sudden surging.
What is Blow-past/ Blow-by
Blowy is the exhaust gases leaking from the combustion chamber to Crankcase through poor sealing between Piston rings and cylinder liner
Affect of the blowy
- In a two-stroke engine, blow-by will enter into the scavenge post and cause heavy fouling. Accumulation of these particles of carbon will lead in to scavenge fire.
- The blowy will enter into the Crankcase in the four-stroke Engine, causing the hot spot and serious crankcase explosion.
Causes of blow-by/blow-past
- Excessive worn-out piston rings or damaged piston rings
- Lubricating oil starvation
- Inferior quality of oil or oil not meeting the OEM specifications.
- Piston ring grooves fouled with carbon deposits.
- Poorly maintained piston rings butt and axial clearances.
- The cylinder liner is worn out more than the specified limits.
- Overloading of the Engine.
Prevention of Blow by or Blow past
- During maintenance, replace all the piston rings with excessive clearances.
- Carry out the gauging of cylinder liner bore. Carry out honing, and after honing if the dimensions are more than the specified limits, replace the liners.
- Use the recommended grade of lubricating oil and maintain the proper lubrication.
- Prevent overloading of Engine.
Measures for prevention of Crankcase explosion
- Avoiding the high temperature of the parts by providing the appropriate lubrication.
- Engine not to be overloaded.
- Using proper bearing with proper material composition.
- Use of oil mist detector
- Crankcase doors with pressure relief valves.
- Periodical testing of crankcase pressure relief valves.
- Check the vent pipe for any choking etc.
- Proper standby fire extinguishing system
Crankcase Safety Devices
- Crankcase relief door.
- Crankcase exhaust fan
- Bearing temperature sensor
- Breather pipe
- Oil mist detector
- Sensors for the Lube oil temperature.
- Alarm for the Lube oil temperature.
All the above-listed devices play an essential role in the safety protection of crankcase explosions. Out of the above devices Mist detector and crankcase relief door play a significant role in preventing the crankcase explosion.
Oil Mist Detector
Marine Diesel is provided with an essential instrument toMarine Diesel Engine is provided with an essential instrument to detect the level of mist in the crankcase. Function of this device is to check and monitor the mist in the crankcase.
This device can identify the mist within its ignitable territory. An oil mist detector consists of flowing parts
- A sampling Point
- Reference tube
- Extraction fan
- Light source
- Measuring tube
- Rotary vane
Reference and meMist detector consists of reference and measuring tube as the elements. The light source is detected through mirrors to the cell placed on the opposite side of the tubes. Fresh air is filled in the reference tube, and the measuring tube is connected to the sampling point for the sample to be tested. The sample air is induced into these tubes through the sampling points with the help of a suction fan.
Rotary vane then shifts between the sampling point at fixed intervals. This process allows the detection of mist in other cylinders using a mono oil mist detector.
Crankcase Relief Valves
The crankcase relief valve is the spring-loaded non-return valve fitted on the crankcase doors. It discharges any sudden increase in the crankcase pressure during the primary explosion state. Being a relief valve, its operation is only one way that is from inside to outside. The spring fitted in the valve ensures instant closer of the valve after the release of gas.
The no. of valves per engine is decided by the classification societies to prevent Explosion And Fire In Marine Engines
Starting air line explosion on ships
Causes of starting airline Explosion
The most unfortunate explosion is that of starting an airline. The reason attributing to this disaster is listed below.
- Accumulation of compressor oil carried along in the starting airline. Leakage of Starting valve or failure of the valve not getting closed.
- Malfunction of starting air relief valve.
- Poor maintenance of flame arrester.
Preventions and precautions (Crankcase Explosion and Scavenge Fire)
- Carry out the leakage test on starting valves.
- Drain the condensate from the air bottle regularly.
- Proper maintenance of Starting valve
- Maintenance of Airline relief valve
- Care of bursting disc.
- Maintenance of flame arrester.
Safety devices fitted in starting an airline. (Crankcase Explosion)
- Relief valve
- Bursting disc
- Flame arrester
- No return valve
A relief valve is installed in the common air manifold. It releases the excess pressure in the event of a rise in pressure. Being the relief valve, it will sit back to its original position after the release of excess pressure for the system,
Bursting Disc (Crankcase Explosion)
Bursting disc is the safety device installed in the starting airline. This device consists of a porous disc made of steel sheets. This sheet is having the properties of bursting in the event of excess pressure during the explosion. A protective cap provided in the disc will cover the whole after bursting and will enable the supply of air as and when needed.
NRV (Non-return valve)
NRV is installed between starting airline and air bottle. NRV plays a crucial role in protecting the air bottle from exploding by not allowing the backflow of air to the bottle.
A flame arrester is a slight safety device that will arrest any flame coming from the cylinder. This device fitted on each cylinder before the air starting valve. Flame arrestor is the best and simple device to avoid explosion and fire in the marine Diesel Engines.
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Frequently asked Questions
The condition inside a crankcase is steady, and combustion will not be permitted since there is no combustion source.
Thus the first occasion is the creation of a proper air-fuel mixture. The right air-fuel mixture will be formed when the lubricating oil in the crankcase will contact the heat caused by the hotspot. Oil gets evaporated in this process. As a result, vaporized oil will rise and condense in the cold part of the crankcase. Condensate will create a mist formation within the explosive parameters and will ignite.
The second occasion will be of the ignited mist coming further in contact with the hot spots and resulting in the crankcase explosion raising the pressure in the crankcase.
High bearing temperature is a positive indication of overheating in the crankcase.
Use of Crankcase door relief door
Take the following actions.
- Evacuate the personals from the vicinity.
- Reduce the speed of the engine and stop the machine with information to bridge.
- Close the fuel supply after the engine has stopped.
- Stop the blowers.
- Evacuate the engine room.
- Put the lock on the engine room entry door.
- Prepare the fire fighting system ready.
- Don’t attempt to open the crankcase doors before 20 to 25 minutes of stoppage of the engine.
- Ensure that the mist detector alarm has been reset.
- Stop the lube oil priming pump.
- Close the starting air system
- Engage the shaft barring arrangements.
- Identify the hot spot location.
- Repair the defect permanently.
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