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What does OMD stand for?
An overheated diesel engine can become a source of fire and extreme havoc if periodic maintenance and proper practices are not carried out. Oil Mist is created in the crankcase when the lubricating oil is splashed by moving and rotating parts of the engine. This oil mist reduces the flash point of the oil, allowing it to catch fire in presence of a hot spot. It is important that this concentration of oil is kept under control and incase its presence is detected, the engine should be stopped or the speed lowered. But how will an engine detect that the level of oil mist has increased in the crankcase?
Oil Mist detectors are used for this purpose. Crankcase of each cylinder is connected to the OMD, which continuously checks the air sample from each cylinder. If the amount of mist increases, OMD raises an alarm. Let's see how it detects the mist.
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Generally only only OMD is fitted in each engine. OMD doesn't reduce or prevent the formation of mist, but only give warning in case the concentration rises above the level at which an explosion can take place.
The arrangement of OMD consist of two tubes of equal sizes. Both these tubes are places parallel to each other. At one end of each tube, a photo-electric cell is fixed. Photo-electric cells generate an electric current when light falls on their surface. The amount of electric current generated is directly proportional to the intensity of light falling on it. The other ends of both the tubes are sealed by fitting lens that allow light to pass through them.
Equal intensity of light is reflected on the photo-electric cells using a lamp. Light passes through the lenses after being reflected by mirrors. One of the tube has an inlet and outlet connection for introducing oil mist.
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Out of the two tubes, one is called the reference tube and the other is called the measuring tube. Measuring tube has a connection for oil mist, which is extracted from the crankcase with the help of an electric extractor fan. The reference tube is filled with clean air and is used as a reference for measuring the level of mist in the measuring tube. Samples from each cylinder is monitored by using a rotating selector valve, which connects each cylinder in sequence to the OMD.
If the concentration of Oil mist in the measuring tube rises, the intensity of light reaching the photo-electric cell reduces. Now as both the tubes are electrically connected, reduction in the generation of electric current will induce an electrical imbalance between the two cells, which will lead to ringing of the alarm.
When Oil mist is detected, the rotating selector valve immediately stops to indicate the cylinder with high concentration of mist.On indication of an alarm, the engine should be slowed down or stopped to prevent damage or explosion.
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It is important to carry out routine maintenance of OMD to prevent false alarms. The sensitivity of OMD should be checked on a regular basis. As all the samples contain a small amount of mist, the lenses and mirrors tend to get dirty and thus require periodic cleaning. The extractor fan and the rotating valve should be checked to avoid chocking of a particular sampling tube. The sampling tubes that connect cylinders to the OMD should not have any loops and also shouldn't be of length more than 12.5 meters.
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Diesel Engines by A.J Wharton