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A positive-displacement pump moves fluid by
trapping a specified amount and displacing (forcing) that
trapped volume into the discharge line. Many positive-displacement pumps employ an expanding cavity on the suction head and a decreasing cavity on the discharge head. Liquid passes into the pump as the cavity on the suction head expands, and the fluid passes out of the discharge side as the cavity collapses. The volume is fixed in each cycle of operation. |
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Positive-Displacement Pump
Behaviour and Safety Unlike centrifugal, positive-displacement pumps can theoretically provide the same flow at its speed (rpm) no matter what the discharge pressure. So, positive-displacement pumps are constant flow devices. However, a small increase in internal leakage as the pressure grows limits an exactly constant flow rate. A positive-displacement pump should not work against a closed valve on the discharge head of the pump as it has no shutoff head, the same as centrifugal pumps. A positive-displacement pump in a closed discharge valve proceeds to produce flow, and the pressure in the line increases until the pipes burst, the pump is seriously damaged, or both. A safety or relief valve on the discharge head of the positive-displacement pump is necessary. The pump manufacturer typically provide internal relief or safety valves. The internal valve is regularly used only as a safety precaution, while an external relief valve in the discharge pipe has a return line back to the suction position or supply tank and provides increased safety for human and machine. |
| Types of Positive-Displacement |
A positive-displacement pump can be
categorized according to the mechanism of moving the fluid:
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TVET COLLEGE SUPPORT
PUMPS
Positive displacement pumps