Fundamentals of positive displacement pumps

The Hydraulic Institute provides an overview of positive displacement pumps' performance characteristics.

By Hydraulic Institute PD Pump Members

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In brief

  • PD pumps handle flow rates from less than 1 gpm to 15,000 gpm and pressures from a few psi to more than 70,000 psi.
  • Be aware of the reasons that positive-displacement pumps might be the best solution to a specific pumping problem.
  • Positive displacement pumps perform across a range of process conditions, including viscosities to 3 million SSU, flow rates from less than 1 gpm to 15,000 gpm, and pressures from a few psi to more than 70,000 psi.
  • The 12 reasons to select PD pumps are grouped by fluid characteristics, pressure conditions, environmental system requirements and flow control.

Positive displacement pumps are used in myriad applications across multiple industries. Users have found them to be the solution to many specific pumping challenges. However, because of their size, simplicity and ruggedness, they often aren’t as well understood as other pump types.

Technologies within the extensive positive displacement family enable coverage of a broad range of horsepower, fluid and pressure applications (Figure 1). These products, therefore, merit increased consideration in the pump selection process. To help in the understanding of the definitions, applications, installation, operation, maintenance and testing procedures, the Hydraulic Institute published 10 ANSI/HI Standards covering air-operated, controlled-volume-metering, reciprocating and rotary PD pumps.

Figure 1. There are many types of positive displacement pumps that can find application in the plant.
Figure 1. There are many types of positive displacement pumps that can find application in the plant.

Centrifugal versus PD pumps

In simple terms, the impeller in a centrifugal pump moves a stream of liquid from the pump suction to a discharge cone, where the gradually decreasing kinetic is converted to pressure energy. A positive-displacement pump, however, doesn’t rely on a velocity change. Pressure is obtained as liquid and is forced through the pump discharge into the system, thereby converting shaft work into pressure. An example of this principle is reciprocating motion, in which a moving piston forces liquid out of a closed cylinder through inlet and outlet valves.

Reciprocating pumps represent one form of PD technology. In portions of their operating range, reciprocating pumps are the single technology that can successfully provide the necessary pumping solution.

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