单选题In order to properly remove air from the casing of a centrifugal pump when starting, the pump should have a （）.Apositive headBnegative suction headCmechanical sealDvent valve attached to the casing at the top of the volute
单选题
In order to properly remove air from the casing of a centrifugal pump when starting, the pump should have a （）.
A
positive head
B
negative suction head
C
mechanical seal
D
vent valve attached to the casing at the top of the volute
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