Positive Displacement Pumps
Positive Displacement Pumps displace a known quantity of liquid with each revolution of the pumping elements. This is done by trapping liquid between the pumping elements and a stationary casing. Pumping element design include gears, lobes, rotary pistons, vanes and screws.
Positive Displacement Pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation.
In general, Positive Displacement Pumps, unlike centrifugal pumps, will produce the same flow at a given speed (RPM) no matter the discharge pressure.
A Positive Displacement Pump must not be operated against a closed valve on the discharge side of the pump because it has no shut-off head like centrifugal pumps. A Positive Displacement Pump operating against a closed discharge valve will continue to produce flow until the pressure in the discharge line has increased 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 absolute necessary. The relief valve can be internal or external. The pump manufacturer has normally the option to supply internal relief or safety valves.
The internal valve 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.
The positive displacement pumps can be divided in two main classes, reciprocating PD pumps and rotary PD pumps. For information on our ranges of rotary and reciprocating positive displacement pumps, please follow the links below:-