Plunger and piston pumps are positive displacement pumps in the truest sense. Their flow delivery is directly related to their speed and the volume swept, and head capacity is defined by vertical lines. The strength of the pump componentsthemselves determines the maximum permissible pressure rating, and top running speed is decided by the limitations on the piston speed, valve operation and cylinder fill time. If, for example, the fluid is very viscous, more time will be needed to fillthe cylinder, and so the pump must run slower. The capacity of the pump falls correspondingly.
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