Answer SSEP Question 37

Q37. In engine rooms that are operated under UMS conditions describe with the aid of sketches how the following are monitored:

A. The perforation of a high-pressure fuel pipe;

B. The imminence or possibility of a scavenge fire;

C. Condition that may be conducive to a crankcase explosion.

Answer: A. The perforation of a high-pressure fuel pipe; -
Sheathed or double skinned fuel Pipes between the fuel pump and injector are used as a safety measure to protect against the risk of fire in the case of a fuel pipe failure. It is a requirement for engines running under UMS conditions. To be effective, high pressure must be /correctly fitted and sealed against external leakage at fuel pump and injector by tightening it to the correct torque value with proper 0-rings.
- A leak off at the high-pressure fuel pipe or ends from the annular space is led through a leak off line to an alarm pot or leak off tank and then to the fuel oil leak off tank. Outer sheath of the pipe prevents spillage of leak off oil to the surroundings. This leak off tank or alarm pot Is usually common to all units or individually fitted to each cylinder.
- Small amount of leakages are led through a needle valve to the fuel leak off tank without activating the float, When the leakages are high, flow through the valve will not be sufficient to drain all the oil. Now level of the leak Off tank rises, which lifts the float fitted in the leak off tank. The float is connected to an alarm circuit.

B. The imminence or possibility of a scavenge fire; -
In ships, where the engine room periodically U.M.S, Temp sensors are fitted at critical point within scavenge space. a monitoring device can be installed in the scavenge air space, which gives an alarm at abnormal temp rise. 
- In inflow scavenged engine sensors are fitted around the cylinder liner just above the scavenge ports; temp higher than reference will actuate the alarm system.
- In U.M.S. ship it will first actuate slowing down procedure; increase of abnormal temperature rise will cause the main engine to slow down.
- These Sensors Fitted in scavenge spaces sense a rise in temp and actuate an alarm in engine room as well as respective remote station.
- Engines are usually provided with a firefighting system like steam smothering. CO2 or any suitable water mist etc. 

C. Condition that may be conducive to a crankcase explosion
- In engines of power 2250 Kw and above or cylinders having bore more than 300mm shall be provided with oil mist detector for crankcase or bearing temperature monitor or both.
- Formations of oil mist in the crankcase can lead to crank case explosion. A more reliable and prompt warning of the formation of oil mist the crank case can be obtained with an 'oil mist detector' which gives an alarm at a mist Concentration of 2.5% of the in flammable quantity.
- The instrument is used to continuously sample the air oil mixture in a diesel engine crank case and detect any concentration due to a hot bearing etc., well below the level at which an explosion may occur, oil mist is drawn into the instrument through sampling tube connected to the top of the respective crank chamber of the crank case.
-A rotating sampling valve driven off the crank case tube in turn for 4 seconds to the measuring tube, whilst a reference tube has a sample from the remaining crank chamber passing through it, so it can evaluate the difference in oil mist level.
- The overall mist density of all crank chambers is also taken once every rev of the sampling the sampling valve, and compared with fresh air.
- A beam of light from a common lamp is reflected by Mirrors along the axis of the parallel measuring and reference tube energizing silicon—photo electric cells connected electrically back to back, so that the output from the circuit is the difference between their individual current under normal condition the oil mist level is the same in both tubes and the output is zero.
-A rotating indicator stops at the crank chamber within the abnormal oil mist condition.