CRANKCASE EXPLOSION
For a explosion to occur there must be source of air (oxygen),fuel and ignition. Under normal conditions the oil/air concentration is too weak and the possibility of ignition by heat source is too low.
Crankcase oil should normally have a high flash point (above 200 C) and this must be maintained to reduce risk of explosion.
CAUSES :
The cause of the crankcase explosion is a hot spot or overheated part within or adjacent to the crankcase of an operating engine. A local hot spot may arise due to overheating of
bearings, piston rod gland, timing chain,
hot combustion gases or sparks from piston blow-past in engines where no diaphragm is fitted, or
from fires in scavenge spaces.
PRIMARY EXPLOSION :
If a hot spot exists, some oil will come in contact with it and will be vaporized.
It will circulate to cooler parts of the crankcase and will condense to form a white oil mist.
The oil droplets in this white mist are very small. If this oil mist should now circulate back to the hot spot in such concentration, it will be ignited and a primary crankcase explosion will occur.
SECONDARY EXPLOSION :
The explosion causes a flame front and pressure wave to accelerate through the crankcase, vaporizing further oil droplets in the path.
The pressure shock wave may build up sufficiently to rupture crankcase doors, if not relieved.
Also, if relief valves do not reseal after lifting, it will cause air to enter into the crankcase resulting into another flammable mixture to be developed leading to secondary or major explosion.
The secondary explosion is more violent and can cause crankcase doors to blow off and start fire in engine room.
INDICATION :
Detection of hot spot can be by
use of temperature sensitive probes within the crankcase near the bearing oil returns.
More commonly, the white mist can be detected by Oil Mist Detectors, which operates visual and audible alarms.
ACTION :
In case of detection of hot spot,
the engine must be slowed down and stopped and bridge should be informed accordingly.
The engine must be turned on turning gear with lubrication oil supply on.
Under no circumstances the crankcase doors should be opened unless the engine is sufficiently cooled. Once the engine is cooled, stop main LO pump, the crankcase should be opened and ventilated.
Inspect all bearings and running surfaces for any hot spots.
Inspect the bottom of crankcase for any for any signs of bearing metal.
Investigate the cause for the hot spot and engines to be started only after the fault is rectified.
Author marineGuru
Crankcase oil should normally have a high flash point (above 200 C) and this must be maintained to reduce risk of explosion.
CAUSES :
The cause of the crankcase explosion is a hot spot or overheated part within or adjacent to the crankcase of an operating engine. A local hot spot may arise due to overheating of
bearings, piston rod gland, timing chain,
hot combustion gases or sparks from piston blow-past in engines where no diaphragm is fitted, or
from fires in scavenge spaces.
PRIMARY EXPLOSION :
If a hot spot exists, some oil will come in contact with it and will be vaporized.
It will circulate to cooler parts of the crankcase and will condense to form a white oil mist.
The oil droplets in this white mist are very small. If this oil mist should now circulate back to the hot spot in such concentration, it will be ignited and a primary crankcase explosion will occur.
SECONDARY EXPLOSION :
The explosion causes a flame front and pressure wave to accelerate through the crankcase, vaporizing further oil droplets in the path.
The pressure shock wave may build up sufficiently to rupture crankcase doors, if not relieved.
Also, if relief valves do not reseal after lifting, it will cause air to enter into the crankcase resulting into another flammable mixture to be developed leading to secondary or major explosion.
The secondary explosion is more violent and can cause crankcase doors to blow off and start fire in engine room.
INDICATION :
Detection of hot spot can be by
use of temperature sensitive probes within the crankcase near the bearing oil returns.
More commonly, the white mist can be detected by Oil Mist Detectors, which operates visual and audible alarms.
ACTION :
In case of detection of hot spot,
the engine must be slowed down and stopped and bridge should be informed accordingly.
The engine must be turned on turning gear with lubrication oil supply on.
Under no circumstances the crankcase doors should be opened unless the engine is sufficiently cooled. Once the engine is cooled, stop main LO pump, the crankcase should be opened and ventilated.
Inspect all bearings and running surfaces for any hot spots.
Inspect the bottom of crankcase for any for any signs of bearing metal.
Investigate the cause for the hot spot and engines to be started only after the fault is rectified.
OIL MIST DETECTOR
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| Oil Mist Detector |
An Oil Mist detector is fitted to monitor samples of the air and vapour mixture taken continuously from the crankcase of the diesel engine. Such a device will detect the presence of oil mist at a concentration well below the level at which explosions may occur, giving a warning in time to take necessary action.
The detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end, which generates an electric current directly proportional to the intensity of light. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tubes onto the cells which are then in electric balance.
One tube is sealed to contain clean air and is termed the reference tube. The other, the measuring tube, has connections through which samples of the crankcase vapour are drawn by an electric extractor fan.
Sampling points should be fitted to each cylinder crankcase and their connections are brought to rotating selector valve which is driven from fan motor. This repeatedly connects each sampling point to the measuring tube in sequence.
If a concentration of oil mist is present in the sample, light will be obstructed before reaching the cell of measuring tube. Electric balance between the two cells will be disturbed and an alarm will be operated. The rotary valve stops to indicate which sampling point has high concentration of oil mist.
The detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end, which generates an electric current directly proportional to the intensity of light. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tubes onto the cells which are then in electric balance.
One tube is sealed to contain clean air and is termed the reference tube. The other, the measuring tube, has connections through which samples of the crankcase vapour are drawn by an electric extractor fan.
Sampling points should be fitted to each cylinder crankcase and their connections are brought to rotating selector valve which is driven from fan motor. This repeatedly connects each sampling point to the measuring tube in sequence.
If a concentration of oil mist is present in the sample, light will be obstructed before reaching the cell of measuring tube. Electric balance between the two cells will be disturbed and an alarm will be operated. The rotary valve stops to indicate which sampling point has high concentration of oil mist.
Author marineGuru
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1 comments:
Nice article, using great equipment will fasten your work, besides it will produce you with smooth and tidy result!
ReplyNice article, using great equipment will fasten your work, besides it will produce you with smooth and tidy result!
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