Severity of explosions vary between a puff which may lift a relief valve to a violent explosions whi...
causes major damage and may injure personnel and cause a fire. Evidence indicates that the longer the
combustion path, the more violent the explosion. This has become an area of concern with the large two
strokes of today which may have a crankcase volume of 500m3.
When an explosion occurs a flame front travels down the crankcase with a pressure wave in front of it.
The turbulence caused by moving engine components causing churning and mixing of vapors increase the
speed of the flame front and its area, which contribute to the increase in pressure. Turbulence caused by
venting of the pressure through relief valves can also influence the explosion.
Following the venting of the explosion through the relief valves, there is a drop in crankcase pressure
to below atmospheric pressure. This can cause air to enter the crankcase resulting in another flammable
mixture to be developed resulting in a secondary explosion to occur. The secondary explosion is more
violent and can result in crankcase doors being blown off the engine, and fires starting in the engine room.
If the relief valves do not reseal after lifting, or if they do not lift at all in the primary explosion ( due to
lack of maintenance etc), then door(s) may be blown off in the primary explosion, giving a ready path for
the ingress of air, which will make a secondary explosion more likely. Air can also be sucked in via the
crankcase vent, although rules state that this must be as small as practicable and new installations must
have a non return valve fitted.
If a primary explosion occurs, the pressure wave may send a large amount of oil mist out into the
engine room. Although the flame arrestors on the relief valves should prevent ignition of this oil mist by the
flame front, the mist will be sucked up towards the turbocharger where it may be ignited by an un-lagged
hot exhaust manifold. This ignition of oil mist can cause severe damage to plant and personnel.
参考解析:
1. This passage is mainly about
A. severity of explosions
B. violent explosions
C. implosion of crankcase
D. primary explosion & Secondary explosion
2. Which of the following is TRUE?
A. The longer the combustion path, the less violent the explosion.
B. The volume of the crankcase of present large two strokes engine may exceed 500m3.
C. Turbulence caused by venting of the pressure through relief valves has nothing to do with the explosion.
D. After the venting of the explosion through the relief valves, there is an increase in crankcase pressure.
3. Which of the following is FALSE?
A. The secondary explosion is much weaker than the primary explosion.
B. The rules state that air in the crankcase must be as small as practicable.
C. New diesel installations must have a non return valve fitted in crankcases.
D. In spite of the flame arrestors on the relief valves, the mist will still be sucked up towards the turbocharger.
4. An un-lagged hot exhaust manifold is an exhaust manifold
A. whose insulation is damaged
B. whose insulation is very thin
C. whose insulation is very thick
D. which has no insulation
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