PD pumps captures confined amounts of liquid and transfers it from the suction to the discharge port.
The main reason positive displacement pumps are not more common than dynamic pumps is because they cannot achieve the high flow rates often required for industrial applications.
Flow is constant with changing 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. Progressing cavity pumps handling high solids content sludge in a waste treatment plant and reciprocating pumps are applied on coal slurry pipeline with solids contents as high as 40 percent by weight. This is sometimes a surprising PD pump characteristic, but widely varied applications serve as examples.
When low flow rates are adequate or preferred, PD pumps are often a better choice. They are generally more efficient for applications involving moderate to high viscosity fluids, high pressures, or variable system conditions.
PD pumps can handle fluids with viscosity of several million SSU. Efficiency increases with increasing viscosity. Selected rotary technologies and air operated piston pumps easily handle highly viscous fluids. Due to high friction losses in centrifugal pumps, their flow rate and efficiency start to drop above 500 SSU. Flow and efficiency in a rotary pump, however, typically increase with viscosity. With variable viscosity liquids, a moderately small change in viscosity may have a large effect on centrifugal efficiency but little effect on PD pump efficiency.
Efficiency increases with increasing pressure.
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.
In many fluid applications, liquid shear is not a problem; however, it is critical in some applications. PD pumps excel in the handling of shear sensitive fluids.
Negative pressure is created at the inlet port. A dry pump will prime on its own.
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.
PD pumps are capable of handling a high percentage of air or gas entrainment.
PD pump has more or less constant flow regardless of pressure. PD pumps, such as vane or air operated double diaphragm (AODD), are often applied on very thin fluids. Other liquids, such as oil, have viscosities that vary with temperature.
Effect of viscosity on PD pump has on the capacity of the pump. You will notice in the flow rate chart hoe the centrifugal pump loses flow as the viscosity goes up but the PD pump’s flow actually increases. This is because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency.