Crankcase explosion is one of the most dangerous reasons that can lead to massive accidents and fires on a ship. It is therefore imperative to prevent all the reasons that can lead to crankcase explosion on a ship. In this article we will learn about the various methods to prevent crankcase explosion on a ship.
The first and foremost thing to avoid any type of explosion on a ship, it is necessary to take the preventive steps right from the basic roots. In a main engine crankcase also there are safety features provided to detect the causes of explosion.
There are two main features provided on the crankcase to prevent crankcase explosion. They are as follows:
1. Oil Mist Detector
The Oil mist detector takes continuous samples from the main engine crankcase and check whether the sample concentrations of mist are well below the level at which a crankcase explosion can take place. The oil mist is drawn into the instrument with the help of small fan which takes suction from each crankcase through sampling tubes provided on each crankcase.
The oil mist detector consists of a small rotator with which it takes sample from one cylinder at a time and the rotator then turns to the next after approximately 4 seconds. The sample from the rotator goes to the measured cell and the reference cell takes sample from rest of the crankcase to evaluate the difference in oil mist.
An overall mist density of the crankcase is also measured by comparing the samples with the fresh air once every rotation of the sampling valve is done. A beam of light from a common lamp is reflected through mirrors and output is measured from a photo cell.
Under normal conditions the output from the reference and measured contact is same and hence no deflection is measured. However, a deflection in the output gives an alarm indication and the valve rotator stops at position to know which chamber has high mist concentration.
Some engines are even fitted with slowdown alarms so that when the oil mist alarm rings, the engine automatically slows down to prevent crankcase explosion.
2. Crankcase relief doors
The Crankcase relief doors are also fitted to prevent any damage to the crankcase and ingress of fresh air inside the crankcase.
The crankcase doors are spring loaded valves which lift up in case there is any rise of pressure inside the crankcase. Once the pressure is released they re-seat to prevent any ingress of fresh air. This helps especially in case of any ingress of air that can lead to a secondary explosion followed by a lot of surge and damage to the crankcase.
The opening pressure and sizes of the valves are specified by different classification societies, depending on the volume of the crankcase. The number of doors to be present also depends on the bore of the cylinder.
Reference: Instrumentation and control engineering by G.J Roy, Operation and maintenance of machinery in motor ship by N.E Chell
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