單項選擇題Which of the following statements concerning butterfly valves is correct?（）

A.Special tools are required for lapping or grinding
B.It is impossible to throttle flow with a butterfly valve
C.To close the valve, it is only necessary to turn the handle a quarter of a turn
D.The butterfly valve should never be used in a freshwater system


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1.單項選擇題The simplest method to use for determining if a centrifugal pump is operating as designed, is to （）.

A.closely observe the pump discharge temperature
B.close off the discharge valve, and watch for a rise in pressure
C.momentarily close off the suction valve, and watch for a rise in pressure
D.use a clamp on ammeter and compare the readings to past records

2.單項選擇題Air compressors are equipped with unloading systems for the purpose of（）.

A.removing all but the frictional load during starting
B.relieving intercoolers of high temperature buildups
C.providing high starting torque in the drive motor
D.seating valve plates during compressor shutdown

3.單項選擇題When a refrigeration compressor motor fails to start, the FIRST thing that should be checked for is a （）.

A.loose expansion valve control bulb
B.low differential setting on the H-Pcutout
C.blown fuse in the motor circuit
D.faulty suction pressure regulator

4.單項選擇題The purpose of the wearing rings used in a centrifugal pump is to （）.

A.prevent an internal explosion in the pump when it is overheated
B.enable a visual inspection of the pump while it is running
C.insure a proper alignment of the pump coupling to the driver
D.accommodate for friction between the impeller and the easing

5.單項選擇題The lowest temperature required to cause self-sustained combustion of a substance independent of any outside source of ignition is called （）.

A.explosive range
B.flash point
C.ignition temperature
D.combustion temperature