Answer MEP Question 53

Q. A. Describe the events leading to a crankcase explosion. B. State how overheating might be indicated other than by a mist detector. C. Discuss the procedure to follow in the invent of overheating being indicated. D. State how severity of a crankcase explosion is limited.  

A. Describe the events leading to a crankcase explosion. 

Normal running condition:  

- In normal running condition the atmosphere inside the crankcase doesn't support fire or explosion as there is no heat or fuel. 

Production of oil vapour: 

- Sequence of CC explosion starts with the production of heat and combustible atmosphere which is a hot spot due to lubrication failure or any such reason which cause the lube oil to vaporize and build combustible atmosphere inside the CC.

Oil vapour turns into combustible mist:

- Evaporated oil vapour cools down in a relatively cooler spot and produce mist, the resultant white mist is within the explosion envelope and is thus flammable.

Primary eplosion:

- The next event is the ignition of this white mist by either the same or another hot spot within the crankcase. When the oil mist is ignited, a crankcase explosion will occur, which will raise the pressure within the crankcase. 

- This explosion causes a flame front and pressure wave to accelerate through the crankcase, vaporising further oil droplets in its path. 

Major or Secondary explosion:

The Pressure shockwave may build up sufficiently by the time it reaches the crankcase causing to rupture crankcase doors or panels, unless otherwise relieved. If the pressure wave reaches an 

opening through which it call escape to the atmosphere the pressure pulse is immediatly followed by a suction pulse of lower magnitude but greater duration. 

- This suction pulse can he responsible for drawing in a charge of fresh air to take the place of which has been burned by the initial explosion. A secondary explosion or major explosion of such intensity as to cause widespread damage then follows. 

B. State how overheating might be indicated other than by a mist detector. 

1. Main bearing temperature reading may be high. 

2. Individual unit water / exhaust outlet temperature is one of the major key to indicate tha - - particular unit may be undergoing some temperature rise. 

3. By feeling the CC door on a regular watch, the affected door may feel warmer than the rest. 

4. The paint peeling off the CC door. 

5. Feel of heat and smell of smoke like smouldering are some of the indications 

6. Scavenge fire in a unit is most likely to be the cause hence following a scavenge fire its suggested to keep a good eye on the temperature of the running gear inside the CC.

C. State how severity of a crankcase explosion is limited. 

- The severity of the crankcase explosion can be limited by reducing the pressure shockwave traveling through the crankcase; This can be achieved by subdivision of crankcase which will inhibit the build-up of high velocities and pressures of flame propagation through the crankcase from a primary explosion. 

- The pressure build up must be relieved without allowing air to be drawn back into the crankcase thus preventing a secondary explosion. To do this, the crankcase is fitted with explosion doors, which are in effect a spring loaded, low inertia non-return valve of sufficient size to relieve any build-up of pressure (115 cm2 per cubic metre of crankcase volume) The doors are fitted with a  

gauze flame trap, wetted by crankcase oil, and a deflector on the exterior to prevent harm to personnel. 

D. Emission of flame has in the past caused severe burns to personnel present during a crankcase explosion. Despite the addition of flame traps. Discuss the procedure in the invent of overheating being indicated. 

- Once oil mist accumulation is observed, Move away from the crankcase doors immediately. 

- Reduce speed to slow, and ask the bridge to stop. - When the engine has stopped, close the fuel supply stop the auxiliary blowers 

- Open the skylight and / or stores hatch - Leave the engine room - Lock engine room entry doors and keep away from them prepare the firefighting equipment.  - Do not open the crankcase for at least 20 minutes after stopping the engine, and 

- Ensure that the oil mist detector alarm (or bearing temperature monitor) has reset stop the LO circulating pump. 

- Shut the starting air, and engage the turning gear. Locate the "hot spot" (source of the oil  overheating) make a permanent repair.