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. Source: Internet
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. Source: Internet
A partial vacuum, or rough vacuum, can be created using a positive displacement pump that transports a gas load from an inlet port to an outlet (exhaust) port. Source: Internet
A relief or safety valve on the discharge side of the positive displacement pump is therefore necessary. Source: Internet
In this configuration the positive displacement pump serves two purposes. Source: Internet
Positive displacement pump behavior and safety Positive displacement pumps, unlike centrifugal or roto-dynamic pumps, theoretically can produce the same flow at a given speed (RPM) no matter what the discharge pressure. Source: Internet