Positive displacement pumps, unlike centrifugal pumps, will displace the same flow at a given speed (RPM) no matter what the discharge pressure. Thus, positive displacement pumps can be considered “constant flow machines". A positive displacement pump operating against a closed discharge valve will continue to produce flow and the pressure in the discharge line will increase, until the line bursts or 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 internal valve, if supplied, should in general only be used as a safety precaution, an external relief valve installed in the discharge line with a return line back to the suction line or supply tank is recommended.