单选题Purging air from a hydraulic system is necessary when（）A adding small amounts of oil to the systemB the system has been overheatedC the system has been drained and then filled with new oilD the system has been idle for a long period of time

One function provided by a hydraulic accumulator is to ___.

A.provide an area where air can separate from the oil

B.provide an area to separate solid contaminants from the oil

C.act as an oil and water separator

D.absorb shocks occurring in the system

The fresh air intake of the inert gas system ______，

A.Prevents the flue gas from falling below 3% content of oxygen

B.Allows the inert gas piping to be used for gas freeing the tanks

C.Opens when there is excessive vacuum on the deck water seal

D.Enables outside air to mix with and cool hot flue gasses

Frequent ______ is important: fixing or changing faulty parts of the propulsion system when necessary.

In an air conditioning system, moisture is removed from the air by ______.

A.filters

B.separators

C.ducted traps

D.dehumidifiers

第二组

Under normal operation, some lubrication oil mist may be discharged from the air compressor to the air start system. This oil may be from excess compressor cylinder lubrication, from faulty oil scraper rings, or may even be suspended oil vapor contaminating the engine room atmosphere and drawn in at the compressor suction. Oil discharge is kept to a minimum by draining the after cooler, air receiver and starting system. If small quantities of lubrication oil do get passed into the starting air system, they will deposit as thin moist film over internal pipe surfaces but are not readily combustible. If a cylinder non-return valve leak while the engine is in operation, some hot gas, possibly with unburned fuel and cylinder lubrication oil, may be blown through the valve to the adjacent air manifold. With further heating from the leaky valve, this, together with the already deposited oil film, will carbonize and form. incandescent carbon. If starting air is applied to the system while still hot, the high pressure air coming into contact with the burning carbon may cause an explosion. Such an explosion will cause a flame to pass back through the air start pipe system, evaporating the deposited oil film and igniting it in the presence of air. Very high velocities and shockwaves are generated which may rupture pipes and fittings.

Alternatively, if excessive oil has entered the air start system, a mixture of air and oil droplets may be discharged through the open cylinder non-return valve during starting. This spray may ignite due to high temperatures in the cylinder, causing a flame through the still open valve to the air manifold.

To prevent an explosion, air start valves must be correctly maintained and lubricated to ensure correct timing and free movement with positive closing. Oil in the system must be kept to a minimum, pipe lines must be drained and cleaned internally when necessary and oil discharge from air compressors must be kept to a minimum by good maintenance.

The passage is mainly about

A.crankcase explosion

B.scavenge box fire

C.starting air pipe

D.the danger of explosion in the starting air system