POSITIVE DISPLACEMENT PUMPS
What is a positive displacement pump?
A Positive Displacement Pump is more commonly known as a PD pump, they a broad category of pump, second only to centrifugal pumps in terms of their popularity and use.
They are generally used for special applications such as viscous liquids or liquids containing fragile solids. Compared to centrifugal pumps they are not capable of as high a flow rate, but are able to produce much higher pressures. Positive displacement pumps move a liquid and pressurize it to allow it to pass through the system by drawing the liquid into a chamber, then contracting the chamber to force the liquid out of the pump at the necessary pressure to move through the piping system. PD pumps do not have impellers, but rather rely on rotating or reciprocating parts to directly push the liquid in an enclosed movable volume, until enough pressure is built up to move the liquid into the discharge system. Because they don’t rely on raising the velocity of the fluid as the centrifugal pump does by moving the liquid through the impeller, the fluid velocity inside a PD pump is much lower than a centrifugal pump, which is often a desirable feature for certain applications such as pumping fragile solids.
Positive displacement pumps generally work within the following ranges:
Flow rate ranges between 0.4 and 50,000 L/min
Total head (pressure) ranges between 10 and 100,000 psi
Horsepower ranges between 0.5 and 5,000 hp
Types of positive displacement pumps
There are two main types of PD pumps:
Rotary pumps – including gear, screw, vane, peristaltic, lobe, and progressive cavity
Reciprocating pumps- including plunger, diaphragm, piston, hydraulic,
Advantages of PD pumps
!n general, positive displacement pumps are ideal for applications where a constant flow is needed. They create medium to high pressure, and are often an excellent way to pump oils and other viscous fluids. PO pumps can also be used in low flow and high pressure combinations, to move fluids containing fragile solids, or for other application niches.