單項(xiàng)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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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

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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