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POSITIVE DISPLACEMENT PUMP

A positive displacement pump is defined by its ability to move fluid by trapping a fixed amount of fluid and then forcing that amount into a discharge pipe. The mechanisms used to move the fluid can be classified as rotary – type, for example the lobe, external gear, internal gear, or reciprocating type, for example piston or diaphragm pumps. The positive displacement occurs because there is an expanding cavity on the suction side and a decreasing cavity on the discharge side. Also, positive displacement pumps are ‘constant flow’ machines, and unlike Centrifugal or RotoDynamic pumps will produce the same flow at a given speed.

The positive displacement pump must not have a closed valve on the discharge side because unlike a Centrifugal pump it has no shut off head. This being the case there needs to be a relief or safety valve on the discharge side, and there is normally an option to supply an internal relief or safety valve.

Positive displacement principles apply to rotary lobe pumps, progressing cavity pumps, rotary gear pumps, piston pumps, diaphragm pumps, gear pumps, vane pumps, regenerative (peripheral) pumps, Peristaltic pumps. The rotary pump use rotation principles and the vacuum created by the rotation of the pump captures and draws in the liquid. What is more, air is naturally removed from a rotary pump line, so it is not necessary to bleed the pump annually. However there are weaknesses to a positive displacement pump. For example, because there is little clearance between the rotating pump and the outer edge the pump must rotate at a slow steady speed otherwise the fluids will cause erosion. If this happens then it is inevitable that large clearances will be formed, and then fluid can leak and the efficiency of the pump is reduced.

The reciprocating positive displacement pumps are classified as plunger pumps or diaphragm pumps. Typically a plunger pump is a cylinder with a reciprocating plunger in it. The suction and discharge valves are situated in the head of the cylinder, and the plunger retracts on the suction stroke and sucks fluid through the suction valve into the cylinder. In the forward stroke the liquid is pushed out of the discharge valve.

Diaphragm pumps have a plunger that pressurises hydraulic oil which is used to flex a diaphragm in the pumping cylinder. This type of pump is used to pump hazardous or toxic fluids.

The positive displacement pump has the advantage over the centrifugal pump when it comes to high viscosity fluids because the positive displacement pump has increased flow with higher viscosity whereas the centrifugal pump has decreased flow with higher viscosity.

Your choice of positive displacement pump must depend on the purpose for which it is required, and take into account all the variables that can affect performance.