Positive Displacement (PD) Pumps work by trapping a fixed amount of fluid, then displacing or forcing it into the discharge pipe. PD Pumps are engineered to use a reciprocating, rotary or diaphragm method to move a variety of fluids. These pumps are most commonly used for pumping viscous liquids and those that contain fragile solids. PD pumps are commonly used for pumping high viscosity fluids such as oil, paints, resins or foodstuffs. They are preferred in any application where accurate dosing or high pressure output is required.
PD pumps produce the same flow at a given speed (RPM) no matter the discharge pressure, thus why positive displacement pumps are often referred to as a “constant flow machine.” PD Pumps must never operate against closed valves on the discharge side of the pump because it has no shut-off head. A pump operating against closed discharge valves continues to produce flow until the pressure in the discharge line is increased until the line bursts or the pump is severely damaged – or even worse… both. This is why a relief valve or safety valve is 100% necessary on the discharge side of any and all positive displacement pumps.
In the long run, faster pumping isn’t always the right choice for all applications. For example, the transfer of high-viscosity fluids commonly found in the food or pulp and paper industries, or variable-viscosity fluids commonly found in chemical processing, requires a highly efficient pump that is capable of consistent flow rates. And fluids with suspended or fragile solids, such as those found in paints and pigments, need a pump that will minimize shear. Positive Displacement Pumps combine mechanical efficiency with steady flow rates to deliver fluid transfer capabilities that are both cost-effective and consistent.
The best way to classify positive displacement pumps is by the mechanism they use to move fluid and the two main types are reciprocating and rotary. There are a wide range of positive displacement pumps available out there, each with its own benefits and specific usage to provide consistent product transfer for challenging media.
TYPES OF POSITIVE DISPLACEMENT PUMPS
- Air-Operated Diaphragm (AOD)
- Eccentric Disc
- High Temperature
- Magnetically Driven
- Progressive Cavity