A crankcase explosion is a product of primary and secondary explosions occurring in the crankcase of the main engine. In fact, it is one of the greatest threats to safety on the ship.
It happens primarily because of the ignition of oily mist present within the engine crankcase.
While the source of mist and heat is the presence of a hot spot. A hot spot can appear at various parts of the crankcase; often due to negligence or poor/improper maintenance and housekeeping.
If not checked its consequences can be dangerous and devastating; including but not limited to extensive damage to the engine, loss of life, injury, and property.
Here the majority of damage occurs due to the secondary explosion; which follows the primary one. The presence of oil, heat, and air triggers the first explosion.
This adds up the heat and further evaporates the oil; which then explodes again in contact with the fresh charge of air. Just like any other explosion, it does not happen accidentally and has many tell-tell indications.
For example, just before the explosion, the engine will show sudden noise, irregular running, high exhaust temperature, a sudden increase in load and smell of oil mist.
Thus safety device in the form of an oil mist detector is installed to check and avoid crankcase explosion in the first place. Even if it does happens its effects are minimized with the help of crankcase relief valves.
How Crankcase Explosion Occurs?
All it needs is a combination of heat, air and flammable substance to catch fire or explosion. Unfortunately, all three are always present in the crankcase space.
There is lots of lube oil, air, and pockets of the hot spot. Because of the reciprocating action of the engine; there is a lot of churning of lube-oil producing smaller oil particles up to 200 micrometers in dia.
Now when this small particle gets in contact with the hot spot; it evaporates and converts into the oil mist after reaching cooler reasons. Under normal conditions, the mist generated is too little to ignite.
But in the event of multiple hot spots, the mist formation is larger. Now if this air-fuel fixture is well within the flammable region it self ignite when it came in contact with another hot spot.
The first explosion then propagates within the crankcase as a flame front with a shock wave. This then aids in the formation of more small oil particles vaporizing oil in its path.
If not relieved in time this can rapture crankcase door and relief valves. The low-pressure area that followed then suck in fresh air from the damaged door and leaky relief valves.
This then leads to the formation of another flammable mixture. Which then in contact with hot spot causes a secondary, more violent explosion. This explosion will damage the main engine, injure crew member or even cause death.
What Is Hot Spot And Why Does It Occur?
The term hot spot refers to a localized heat source originated mostly due to rubbing of parts ( friction between two surfaces ), spark, blow past, etc. It is one of the main causes of equipment failure.
In the marine diesel engine, a hot spot can occur due to overheating of parts; improper maintenance, hot gas blow past, failure of lube oil bearings, insufficient clearance, poor quality of lube oil and scavenge fire.
A localized heat source is termed as a hot spot; only if its temperature is above the ignition point of working fuel. For example, the hotspot temperature in the crankcase is between 200 to 400 degrees Celcius.
One can detect the presence of a hot spot by simply touching the surface of the crankcase door. Some other methods to detect the presence of hot spots in an engine are:
- Heavy noise with the abnormal operation of the engine.
- Oil mist detector alarms, which indicate the presence of oil mist.
- High lube oil temperature alarms or just too high lube oil temperature.
- The appearance of dense mist from the breather pipe.
- Irregular operation of engine sudden surge in exhaust temperature.
How To Prevent Crankcase Explosion
Crankcase explosion can be prevented by avoiding the generation of a hot spot in the first place. If such a hot spot does exist and oil mist form necessary action must be taken to avoid an explosion.
The formation o hot spot can be avoided by adequate cooling of the engine. Furthermore, lube oil filters must be changed regularly, ODM pipes blow though and the quality of lube oil must be maintained.
Any abnormality in terms of noise or operation must be looked after. In addition, we also need to ensure there is proper cylindrical lubrication in the engine; no oil leaks, all alarms and safety trips working.
Other things that need to be looked for to avoid hot spots are; proper drainage of scavenging space, maintenance of stuffing box, proper operation of lube oil purifier, etc.
Now if the hot spot appears oil mist detector inform us of the presence of oil mist. Then engine speed is reduced or sometimes stoped and lubrication increased to counter heat formation.
After 30-40 minutes when the engine colls down, the crankcase door is being opened under supervision. The crankcase is inspected for hot spots especially at bearings, piston, piston rod, guides and stuffing box.
The crankcase door should not be opened till the engine is sufficiently cooled. Seniors, the bridge must be informed and fire fighting equipment is kept on standby. After inspection, if noting found engine is restarted and the incident is logged.
Crankcase Safety Devices
There are various safety devices such as the crankcase exhaust fan, relief valve, bearing temperature sensor, breather pipe, oil mist detector, lube oil temperature sensor, and alarm.
Together they play an active role in the safety and protection of the main engine from the crankcase explosion.
Out of them, there is two safety device; “oil mist detector” and “crankcase relief valve”; that plays a greater role in the protection against the possible crankcase explosion.
1 ) Oil Mist Detector
Oil mist detector is a safety device fitted in marine diesel engines to check and monitor crankcase environments for the presence of oil mist. It can detect the presence of oil mist within its flammable range.
Thus it helps in early indication preventing primary crankcase explosion. An ODM ( Oil mist detector ) consists of six parts; a reference tube, sampling point, extraction fan, light source, measuring tube and rotary vane.
These two parallel tubes of equal length ( reference and measuring tube ); with its photoelectric cell and common light source comprise the main detector element.
Here, the light source is reflected through mirrors to the photocell placed on another side of these tubes. The reference tube is filled with fresh air while the mearing tube has a connection for sample air to be tested.
The sample air is drawn into these tubes through sampling points with the help of the extraction fan.
Rotary vane then switches between the sampling points at a fixed interval. This allows for the detection of oil mist in different cylinders using a single oil mist detector.
Under normal conditions, light can pass through both these tubes; thus producing no potential difference. Now if the concentration of oil mist increases; less light will pass through the measuring tube.
This then leads to the formation of a high potential difference between the two photoelectric cells raising alarm. Then rotary vane stoped to indicate the position of the cylinder unit having issues.
2 ) Crankcase Relief Valves
The crankcase relief valve is a non-return valve fitted on the crankcase door. It releases any sudden rise in pressure as in the case of a primary crankcase explosion.
Being a non-return valve it only allows the air to go from inside to out and not vise versa. All engines except the ones which are too small must be fitted with a similar safety device.
The valve must be spring-loaded to ensure the instantaneous operation and positive closing.
The valve opens in event of a primary explosion dissipating energy. Then it closes instantly as the pressure drop. The recommended number, size and operating pressure of these valves are specified by the classification society.
This on many occasions is purely based on risk assessment, power, and volume of the marine engine.
The relief valve itself consists of a nonreturn disc valve, a valve cover, deflector, a dome-shaped flame trap, and the valve spring. The disc valve is made of aluminum alloy while the cover of aluminum.
In event of extreme pressure build-up inside; the spring is compressed allowing passage for the gas to escape. To avoid any damage to the crew the deflector is made such deflect them downwards away from operating personnel.
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