轮机英语【大管甲类】
考试试题
[问答题]Severity of explosions vary between a puff which may lift a relief valve to a violent explosions which
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.
[问答题]In the event of a main generating system failure an emergency supply of electricity is required for
essential services. This can be supplied be batteries, but most merchant ships have an emergency generator.
The unit is diesel driven and located outside of the machinery space.
The emergency generator must be rated to provide power for the driving motors of the emergency
bilge pump, fire pumps, steering gear, watertight doors and possibly fighting equipment. Emergency
lighting for occupied areas, navigation lights, communication systems and alarm systems must also be
supplied. Where electrical control devices are used in the operation of main machinery, these too may
require a supply from the emergency generator.
Usually only one emergency switchboard is installed, however, for safety reasons large passenger
vessels may have two emergency sources of power, thus requiting two emergency switchboards.
Emergency switchboards normally consists of a generator control panel, a bus tie and distribution
panel and 24-volt, 120-volt and 450-voh distribution panels in size and number as required.
Emergency generator switchboard includes those devices noted in the foregoing for main switchboard
generator control panels plus the following:
1. A white light to indicate that the normal supply is available.
2. A green light to indicate that all devices are “set up” for automatic operation.
3. A normal supply circuit breaker, unless the automatic bus transfer is of the contactor type.
4. A feed back switch when the automatic bus transfer is of the contactor type.
It is not usual for an emergency generator to require paralleling, so no equipment is provided for this
purpose. Automatic start up of the emergency generator at a low voltage value is usual on modern
installations.
