Editing Pump (section)

==== Positive-displacement pump behavior and safety ====
Positive-displacement pumps, unlike [[Centrifugal pump|centrifugal]], can theoretically produce the same flow at a given rotational speed no matter what the discharge pressure. Thus, positive-displacement pumps are ''constant flow machines''. However, a slight increase in internal leakage as the pressure increases prevents a truly constant flow rate.

A positive-displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps. A positive-displacement pump operating against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is severely damaged, or both.

A relief or [[safety valve]] on the discharge side of the positive-displacement pump is therefore necessary. The relief valve can be internal or external. The pump manufacturer normally has the option to supply internal relief or safety valves. The internal valve is usually used only as a safety precaution.  An external relief valve in the discharge line, with a return line back to the suction line or supply tank, provides increased [[safety]].