The development of oil mist detector started way back in 1947 after crankcase explosion onboard the “ Reina del Pacifico” causing 28 deaths. As a result, first oil mist detector was marketed in 1960’s. In Nov 2010, there was a crankcase explosion and subsequent fire on board Carnival Splendor, the estimated cost of repairs, loss in revenue and passenger refund was approx. $65 million. Recent data of crankcase explosion are not available, but old record of 2001 shows 143 incidents of crankcase explosion between 1990 and 2001 under only LR classified vessels.
How to avoid it? – To avoid such occurrences, prime importance is that the crew must take due precaution during maintenance like; inspection of crankcase, gear case, camcase, clearances check of running parts and replacement of running parts. Crankcase relief doors to be tested (lifting test) on regular basis, Oil mist detector to be tested as per maker’s instruction on weekly, monthly, yearly and 5 yearly basis. Most important is to act as quickly as possible upon receiving high mist alarm, never ignore the alarm and never bypass this alarm. OMD is one of the most unique, innovative and most important sensor fitted on engine to protect the engine as well as life onboard.
**In one of the DNV casualty cases, Electromagnetic Interference, it is recommended that new communication equipment should not be tested near oil mist detector. It has caused 2 incidents of propulsion failure when newly received high frequency communication equipment was being tested near aux engine.
Causes of oil mist – Lack of periodic maintenance, misaligned parts, wrong clearances of running parts, lack of lubrication, wrongly tightened parts and material failure, piston crown hole and combustion gas passing through piston cooling oil pipe to crank case and this list is not exhaustive.
Any overheat in diesel engine crankcase causes oil mist in crankcase, if this is not attended immediately, then it can cause damage to engine as well as life on board. In some case the crank case explosion has caused damages in the tune of millions of dollars for repair of engine, loss of life cannot be estimated. So the purpose of OMD is to predict serious failure as early as possible, some units are detecting 2.0mg/ltr in ≤ 1 second.
Usually 200micron size oil mist is always suspended in crankcase warm atmosphere, when the temperature of internal atmosphere rises due to failure of moving parts in the crankcase, hotspot causes the oil droplets to vaporise to 5 to 10 micron size oil mist. These oil mist at 13% oil mist/air ratio, 47mg/liter approximately, reaches to lower explosion limit (LEL) and any contact with hot spot at 850deg C will cause explosion. The engine running parts consists of any bearing, crankshaft, gears, chain, camshaft bearings, piston rod and pistons in cylinder liners(in 4 stroke engine).
The primary explosion is caused by internal atmosphere of the crankcase; secondary explosion, caused by fresh air entering into crankcase through any means such as unsettled relief doors, are more violent. The primary explosion is released through relief doors, but secondary explosion is extremely severe and in some cases blown away the crankcase doors, causing subsequent fire in engine room. In most of the cases, a burn trail mark can be seen in the engine room, starting from engine to the funnel exit door, it indicates how pressure pulse and flame has travelled from high pressure to low pressure.
Action required on alarm – engine speed should be reduced or stopped as per IACS requirement for the engine type.
SOLAS requirement for need of oil mist detector – Engines of 2,250 kW and above or having cylinders of more than 300mm bore shall be provided with crankcase oil mist detectors or engine bearing temperature monitors or equivalent devices.” Chapter II-I, Regulation 47.2 (1981)
As per IACS M10.8 Oil mist detection arrangements (or engine bearing temperature monitors or equivalent devices) are required:
- for alarm and slow down purposes for low-speed diesel engines of 2250 kW and above or having cylinders of more than 300 bore
- for alarm and automatic shutoff purposes for medium- and high-speed diesel engines of 2250 kW and above or having cylinders of more than 300 mm bore.
**From January 2009 all engines has been specified with OMD on the manoeuvring side, for improved crew safety.
Makers – There are many approved Oil mist detectors available in the market, basic principle of operation is same, despite some differences in method used.
- Oil mist detector Graviner MK7. Make: Kidde Fire Protection
- Oil mist detector Graviner MK6. Make: Kidde Fire Protection, Measuring range – 0-30mg/ltr, Forced ventilation
- Oil mist detector Visatron VN 215/93. Make: Schaller Automation,Measuring range – 0.2-50mg/ltr, Forced ventilation
- Oil mist detector QMI., Multiplex, Make: Quality Monitoring Instruments Ltd., Measuring range-0-2.5 mg/ltr, forced ventilation
- Oil mist detector MD-SX. Make: Daihatsu Diesel Mfg. Co., Ltd.Measuring range –0-2 mg/ltr,Natural ventilation, so sucking mechanism or fan. Optical dispersion system
- Oil mist detector Vision III C. Make: Specs Corporation, Measuring range – 0-2.5 mg/ltr, forced ventilation
- Oil mist detector GDMS-OMDN09. Make: MSS GmbH
- Oil mist detector Triton. Make: Heinzmann
Detection or sensing of mist – There are 2 types of detection, for 2 stroke engine comparator system is used, whereas for 4 stroke engine level detector is used. Comparator uses various chambers to be measured one after another and one time with fresh air. Level sensor is used for 4 stroke smaller engines because there is no individual chambers as in 2 stroke engines.
Basic principle of working of detector is scattered light falling on photoelectric cells. There are 2 equal sizes of tubes , one works and measuring and other works as reference cell are placed parallel to each other, there is a photo electric cell fixed on one end of the tube and light source on other end. Reference cell is sealed and uses clean air, on the other hand measuring cell uses mist from crankcase through an extractor fan. A set amount of electric current is generated when light falls on the fixed cell without any obstruction, however when the light is obstructed the light on the photocell falls in proportion with lesser intensity and this deviation in current causes the alarm to activate.