Crankcase Explosion and Scavenge Fire-Air starting line explosion-hotspot

In this post, we will discuss the Crankcase Explosion and Scavenge Fire. This post will be useful for the Marine Engineers and Captive Diesel Power pant Engineers. The major type of accidents with their causes and remedies have been discussed in detail.

Scavenge fire is considered one of the most Engine room hazardous fires causing disastrous accidents onboard ships. These fires are a significant threat to the safe operation of marine diesel Engines.

Let us understand the scavenge fire and the reasons for its occurrence. We shall also discuss the preventive major to combat such fires.

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

• 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.

The following indication will indicate the sign of Scavenge fire to prevent Major Disastrous accidents in Marine Engines

1. Sudden Loss in power with high exhaust temperatures
2. Drop-in Engine Speed
3. Excessive gases in the exhaust
4. Increase in Scavenge Temperature
5. Noticeable surging in Turbocharger
6. Noticeable smoke from scavenging drains
7. Burnt paint on scavenging doors in case of major scavenge fire.
8. Abnormal exhaust smoke.
9. Visible smoke in a transparent window type installation.
10. Increased cooling water temperature.

Action to be Taken

For Small Fire

1. Reduce the Engine Speed.
2. Cut off the fuel supply of the particular unit.
3. Increase the cylinder lubrication. In case the fire is more than, don’t increase the cylinder lubrication.
4. Maintain the cooling flow through the Jacket and piston
5. Shut the scavenge drain cock.
6. Cut off the Fuel injection pump of the affected unit.
7. Monitor the exhaust and scavenge temperatures.

For Major Fire

1. Stop the engine immediately
2. Engage the turning gear and start turning the engine to prevent the seizure of significant components.
3. Attempt to extinguish the fire with the fixed fire fighting system or the
4. Open and inspect the scavenge space after proper cooling down once it is confirmed that the fire is extinguished.
5. Clean the scavenge space, scavenge box, and drain regularly.
6. Avoid excessive cylinder lubrication.
7. Regularly inspect the Piston rings, cylinder liners, and piston skirt.
8. Carry out the proper honing of cylinder liners.
9. Prevent the Engine vibrations
10. The trunk should be cleared of improperly burnt oil and carbon particles.
11. Periodically drain the scavenge drains and keep the drains cleared.
12. Install the high-temperature alarm to provide the high-temperature indications.
13. Test the pressure regulating valves regularly.
14. Test the fire alarm regularly
15. Avoid prolonging engine overloading.
16. Open the drain cocks during the low load operation to clear the oil accumulation in the manifold.
17. Check the piston alignment.
18. 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.
19. Inspect the spring of the safety relief device.
20. Check the tension of bolts.

Crankcase explosion

In this article, we will discuss the Crankcase explosion. Crankcase Explosion is a most disastrous explosion that takes place in the marine Engine.

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

Three essential elements are required to complete the fire triangle. These three elements are Oxygen, Heat, and fuel. In the presence of these elements in equal proportionate, the fire is caused and results in a 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. This serious explosion will result in damaging the Engine and fatal to human beings.

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 vapors traveling 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

Hot spots are created due to the following reasons

1. High temperature is created due to the reciprocating moment of pistons.
2. The high temperature of the bearings.
3. Heavy blow by in the crankcase from the combustion chamber.
4. Fire in the scavenge trunk.

Methods to detect the presence of hot spots in an( engine Safety Precautions)

1. Abnormal Engine operation with obnoxious noise.
2. The alarm on Oil mist detector.
3. High Lubricating oil temperature.
4. Dense smoke from crankcase breather.
5. High exhaust temperature with sudden surging.

Blowy is the exhaust gases leaking from the combustion chamber to Crankcase through poor sealing between Piston rings and cylinder liner

• 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.
• 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.
• 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.

Following guiding points will help in preventing the Crankcase explosion.

1. Avoiding the high temperature of the parts by providing the appropriate lubrication.
2. Engine not to be overloaded.
3. Using proper bearing with proper material composition.
4. Use of oil mist detector
5. Crankcase doors with pressure relief valves.
6. Periodical testing of crankcase pressure relief valves.
7. Check the vent pipe for any choking etc.
8. Proper standby fire extinguishing system

A list of safety devices is produced below.

1. Crankcase relief door.
2. Crankcase exhaust fan
3. Bearing temperature sensor
4. Breather pipe
5. Oil mist detector
6. Lube oil temperature sensor
7. Lube oil temperature alarm

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 (Crankcase Explosion and Scavenge Fire)The oil mist detector is the essential device fitted on Marine Diesel Engines for the safety of Engines. This device checks and monitors the mist in the crankcase.

Image source-google/image by — marinegyan.com

This device can identify the mist within its ignitable territory. An oil mist detector consists of flowing parts

1. A sampling Point
2. Reference tube
3. Extraction fan
4. Light source
5. Measuring tube
6. Rotary vane

Reference and measuring tube are the main detector elements of a mist detector. 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

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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 airline explosion on ships

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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)

Checks on starting valve for leakages.

1. Regular draining of air bottle
2. Proper maintenance of Starting valve
3. Maintenance of Airline relief valve
4. Care of bursting disc.
5. Maintenance of flame arrester.

Safety devices fitted in starting an airline.

1. Relief valve
2. Bursting disc
3. Flame arrester
4. Nonreturn valve

Relief valve

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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

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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. This is also having a protective cap that will cover the hole after the rupture of the disc and will continue the air supply in need.

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. The simplest and the best device to prevent Explosion And Fire In Marine Engines

I hope the post Crankcase Explosion and Scavenge Fire will provide some valuable information to the reader. We, Will, welcome the valuable feedback for the improvement of this post on our mail id — [email protected]

Originally published at https://www.marinediesel.co.in on April 19, 2021.