Positive Displacement Pumps
What are positive displacement pumps?
Also known as PD pumps, they a broad category of pump, second only to centrifugal pumps in terms of their popularity. 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 of 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 .1 and 15,000 gpm
- Total head (pressure) ranges between 10 and 100,000 psi
- Horsepower ranges between 0.5 and 5,000 hp
- Viscosity limits vary with pump type, but some types can handle millions of SSU viscosity
Different types of positive displacement pumps
There are two main types of PD pumps:
1. Rotary pumps - including gear, screw, vane, peristaltic, lobe, and progressive cavity — use rotating parts to move the liquid in and out of the pump chamber. Some rotary pumps, such as gear pumps, must have very tight clearance between the rotating elements and the walls of the chamber, and between the rotating parts, which means they generally can’t be used to pump large solids or abrasive fluids that may wear the parts. Other types such as lobe pumps and progressive cavity pumps are designed to move liquids containing solids.
2. Reciprocating pumps - including plunger, diaphragm, piston, hydraulic, and many others - use a repetitive reciprocating mechanism to expand and contract the chamber at regular intervals. Reciprocating pumps incorporate one or more sets of check valves at the inlet and outlet of the pump to help guild the liquid through the pump and to prevent reverse flow.
Primary advantages of PD pumps
In general, positive displacement pumps are ideal for applications where a constant flow is needed. Note that many types require pressure protection via a relief valve to safeguard the pump and system from over pressurization. They create medium to high pressure, and are often an excellent way to pump oils and other viscous fluids. PD pumps may also be needed for low flow and high pressure combination, to move fluids containing fragile solids, or for other application niches. Some types are inherently self-priming, and several types are seal less.
Here are some positive displacement pump manufacturers to consider.
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