Crankcase Explosion

Question 1

What are they?

Answer 1

A hot spot within the sump due to blow by or overheating bearings or fuel dilution, reducing the flash point of the oil.

Question 2

How do they occur?

Answer 2

• Oil comes into contact with the hot spot and vaporises.
• The vaporized oil circulates to a cooler part of the engine and condenses, causing a white mist.
• Creating a good air/ fuel ratio
• The oil mist now lands back on the hot spot and ignites
• After ignition a pressure wave forms, this vaporizing further oil droplets
• This can build up so much pressure that it will rupture the crankcase doors, hence crankcase explosion doors.

Question 3

OMD, how do they work?

Answer 3

Two types of OMD. Light scattering and Opacity measurement

Light Scattering: Most oil mist detectors work on the principle of light scattering. When oil mist particles are present in the air, they scatter light. The detector uses a light source (typically a laser or LED) and a photodetector to measure the amount of scattered light, which correlates with the concentration of oil mist in the air.

Opacity Measurement: Some detectors measure the opacity or turbidity of the air. A light beam is passed through the air, and the amount of light that reaches the detector on the other side is measured. The presence of oil mist reduces the transmitted light, indicating the concentration of oil mist.

Question 4

OMD, how are they tested? How does it take samples?

Answer 4

By using smoke to trigger the light scatter

There are sampling points near likely locations that are constantly drawing samples. This then goes to the detection chamber and will return to the crankcase

Question 5

What can set off the oil mist detector? False alarms caused by?

Answer 5

• Dust and dirt
• Electrical interference
• Incorrect calibration
• High temperatures and humidity
• During rapid load changes and start up/ shut down

Question 6

Draw a mist detector distributor?

Answer 6

See one note

Question 7

Full procedure in the event of an OMD alarm?

Answer 7

• Shut down engine
• Monitor it cooling down
• Prepare for fire teams/ hi fog
• Once cooled down look over trends to see a change and start investigating

Question 8

How would you secure the area?

Answer 8

Ensure all personnel are out of the engine room by completing muster. Remind all crew not to enter

Question 9

When would you stop that engine?

Answer 9

Straight away but if not navigational safe to do so then slow down till safe

Question 10

Draw a crankcase explosion valve? How is it fitted to the engine?

Answer 10

See one note

Question 11

Primary and secondary explosion?

Answer 11

Primary explosion is caused by the first concentrated mist to ignite.

The secondary is when the primary pressure wave explodes, it leaves an area of low pressure, causing a rush of new air back into the crankcase and mixing with the vapour, causing a secondary explosion

Question 12

SOLAS explosion doors info?

Answer 12

Outlines specific requirements for the design and installation of explosion relief doors - Chapter II-1 Reg 47.

• Constructed to withstand engine crankcase
• Must seal quickly after primary to stop secondary explosion
• Size and number calculated on crankcase volume
• Held close with a spring mechanism and flame arrestors

Question 13

Thought process on whether it is navigable safe to shut machinery down in the event of OMD alarm?

Answer 13

Yes, as long as the vessel is safe to do so. If not then reduce speed to minimum until in a safe location.

Question 14

Other indications?

Answer 14

Physical signs - Increase vibration, Oders and fumes, Exhaust smoke
Abnormal Readings - Sudden fluctuations or abnormalities in engine parameters, such as temperature, pressure, or RPM