Positive displacement pump get usually used for specialised applications, such as pumping viscous fluids or liquids containing suspended or brittle solids. Generally, these pumps do not have the high performance of centrifugal pumps but can produce significantly higher pressures. Pumping is cyclic and gets driven by pistons, bolts, gears, rollers, membranes or vanes.
Characteristics of positive displacement pump
Compared with a centrifugal pump or a rotary pump, a positive displacement pump can theoretically produce the same flow rate at a given speed (rpm) regardless of the delivery pressure. The piston pump could get regarded as a constant flow device. The internal leakage and the slight increase of internal leakage can avoid a consistent flow rate.
In use, a direct-acting plunger pump should not work with a shut valve on the pressure side of the pump because it does not have a cut-off pressure head like a centrifugal pump. If the pump runs with the outlet valve closed, it will proceed to run. You can generate flow and increase the pressure in the delivery line until the hose gets torn or the pump gets severely vitiated, or both.
To avoid this situation, a pressure relief valve or relief valve gets required on the pressure surface of the positive displacement pump. The safety valve gets located inside or outside the pump. Pump manufacturers can choose to implement internal or pressure relief valves. Usually, the inner valve gets used only as a safety measure. The external pressure relief valve in the pressure line can give additional safety and retreat to the suction line or storage tank.
Positive displacement pump is very suitable for applications requiring constant flow. They produce medium to high pressures and are usually ideal for pumping oil and other viscous liquids. They are also very useful for applications that require a mixture of low flow and high pressure. For example, moving liquids containing suspended solids or brittle solids.