The words fire and explosion spell danger under any circumstances, much more so in case of ships which sail across the deep seas often in isolation from the rest of the world. We have been studying about various situations involving fire, fire fighting equipment and so forth in our previous articles. We have also talked about crankcase explosions and their cause.
As we learned earlier, crankcase explosions impose heavy threat to the ship’s engine room and ship personnel. It is imperative to take necessary precautions to prevent such explosions on ship. The main reason for crankcase explosions is the accumulation of fuel mist in the crankcase which gets ignited due to sparks for hot spots. A large pressure is generated inside the crankcase due to this, which eventually bursts out the doors and casing.
As a practical safe guard against these explosions, explosion relief valves and doors are fitted. These valves serve two functions
- To relieve excess pressure inside the crankcase thereby normalizing the pressure
- To prevent the flames inside the crankcase from coming out and causing further damage
The picture below shows such safety relief valve doors as fitted on the side of the main diesel engine of the ship. We will learn about the actual construction and operating principles in the subsequent section
Construction and Working
All the explosion safety relief valves are spring loaded self-closing valves, which stop the entry of atmospheric air in to the crankcase. Moreover, the design, size and arrangement of the valves depend on the size and capacity of the engine. For example, a large slow speed diesel engine consists of two door type relief valves fitted to each crankcase, whereas in a medium speed diesel engine only one valve is used. So basically the relief valve is a one-way entry which only allows inside pressure to escape when excess, without allowing the other way round.
As shown in the figure, an explosion relief valve consists of a light spring that holds the valve closely against its seat. The whole arrangement is enclosed in a cover, which is tightly bolted from all the sides. The assembly is mounted on the door of the crankcase. On the outside of the valve a deflector is fitted which safeguards the personnel from escaping pressurized gases. On the inside of the engine, an oil wetted gaze acts as a flame trap to prevent any flame leaving the crankcase.
As the pressure inside the crankcase increases the valve raises compressing the spring. The excess pressure is released to the atmosphere. The Oil wetted Gauze assembly prevents the flame in the crankcase from escaping. The valve closes automatically under the tension and action of spring once the pressure reduces. Hence we can see that though the basic principle of operation is quite simple, yet it is a very useful mechanism which gets activated during times of need.
Mere presence of these valves is not sufficient and will not guarantee their operation during times of actual need. Therefore in order to be safe and sound in case of a real emergency, routine inspections should be carried out for determining the working condition of the valves. Some of the following points need to be considered
- The valve springs should be checked for any jamming or obstruction.
- The gauze assembly should also be properly maintained and prevented from corrosion.
Needless to say, if anything is found lacking in the above observations and checks, immediate steps need to be taken to rectify the fault. Remember that Safety First Is A Must and nothing is more important than life itself.