单项选择题

The usual symptoms of cavitation in a centrifugal pump is （）.

A.noise and vibration
B.an increase in discharge pressure
C.an increase in suction pressure
D.lifting of the relief valve

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One cause of high head pressure occurring in a refrigeration system can be （）.

A.insufficient cooling water flow to the condenser
B.a low refrigerant charge in the system
C.the liquid valve is open too much
D.high evaporator superheat

单项选择题

The control mode in which the final control element is moved from one of two flexed positions to the other is known as （）.

A.dead band action
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单项选择题

Pitting in the suction areas of a centrifugal pump bronze impeller is usually caused by （）.

A.cavitation
B.electrolysis
C.abrasion
D.corrosion