*Exploded crankcase
Primary Explosion:
When
lub oil falls on hot spot, because of high temperature it vaporizes
into fine oil vapors.This vaporized oil moves in crankcase into a low
pressure and relatively cold area and condense to form white mist.
This
white mist is a perfect mixture of oil and air.This white oil mist
circulates in crank case and when it comes in contact with hotspot gets
ignited and BOOM - Primary explosion occurs.
Secondary Explosion:
Pressure
waves created by primary explosion break open the crankcase.Thereby
vacuum get created.Because of this air from outside rushes into
crankcase and cause secondary explosion which is highly destructive than
primary explosion.
Minimizing or Avoiding Hazardous condition:
By design and equipment-
- Bearing material chosen should have strength to carry maximum load and minimum generation of heat.Material shouldn't readily deteriorate.
- Adequate lubricating oil supply to bearings
- Thermometers to be provided in strategic locations
- Bearing temperature indicators to be installed
- Oil mist detectors to be installed
- Crankcase relief door to be installed
By operating personnel-
- Use of good quality oil
- Proper treatment and purification of oil
- Regular lub oil analysis for quality check
- lub oil purifiers to be always running
- lub oil temp shouldn't go high
- regular crankcase inspection[pay attention to wiping of metals,overheating]
- regular check of bearing clearances and crankshaft deflection
- ensure all warning devices and safety devices are working properly
- checking of oil lub oil filters for evidence of bearing material.
*Crankcase relief doors
Regulations concerning crankcase safety:
- All engines with bore dia > 300mm should have crankcase relief doors for all units
- Crankcase relief door fitted to be greater than 115sq.cm for 1cub.m of crankcase
- For engines between 150mm to 300mm bore dia, crankcase relief doors to be fitted at either ends of the engine
- All engines greater than 2250KW or bore dia > 300mm should have oil mist detector unit [OMD]
Indications of overheating other than Oil Mist Detector:
- High bearing temperature
- High lub oil temperature
- Unusual noise
- Smell of hot carbonizing oil
- White smoke from crankcase breather
- Feel over the crank case
Actions to be taken in the event of oil mist detector alarm actuation:
As per MAN B&W:
- Call all engineers alarm
- Inform wheel house.Reduce speed if auto slowdown has not activated
- Stop the engine at the earliest
- Keep blowers, lub oil pump and all cooling on
- Evacuate the engine room
- After 1hr crankcase should cool down
- Feel the crankcase and stop l.o pump
- Open crankcase keeping door as shield while opening
- Fire extinguisher to be ready
- Enter crankcase, investigate and rectify problem
- Turn engine with lub oil ON for 30 mins
- Start engine and slowly raise to full load.feel over the crankcase.
- After stop for a while and inspect again
Actual actions or response to be taken:
- Call all engineers alarm
- Inform wheel house and reduce rpm if auto slow down has not activated
- Check O2 and span setting of OMD
- If alarm is still coming follow MAN B&W actions
- If alarm has vanished then problem is with OMD
Severity
of crankcase explosion can be reduced by crankcase relief valves,
general housekeeping of ER,purifier rooms enclosed [will reduce fire
spread]
*Understanding
crankcase relief door valve -designed to open at a pressure not more
than 0.2bar[0.02–0.1bar]. valve disc material is Aluminium.
Oil Mist Detector:
- Based on light scatter principle
- Individual sensors are fixed at each crank throw space and chain case
- A suction fan draws sample through each detector
- In detectors[ D1,D2….Dn] Light is transmitted at one end.Directly opposite to transmitter is compensating/check receiver
- A measurement or alarm receiver picks up scattered light produced by oil mist particles
- Monitor compares received signal against a set point[2.5% LEL]. Alarm condition is reached then alarm is set out.Auto slowdown is initiated.
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