Fundamentals of positive displacement pumps

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.

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.

Rotary pumps constitute the second major positive-displacement category. In this case, a rotating pumping element inside a pumping chamber drives the fluid. This family is distinguished by its lack of inlet or discharge valves. These pumps are available in a number of pumping principles, each with its own features and benefits that provide specific pumping solutions.

The third major category is the controlled-volume metering pump (CVMP). These often are called chemical-injection feed pumps or dosing pumps. Essentially, these are reciprocating positive-displacement pumps configured to dispense an accurate volume of liquid during a specified time period using any of several mechanisms for varying the effective volumetric displacement. This pump type is used where highly accurate, repeatable, adjustable flow rates are required.

Pumping solution products

Positive displacement pumps often are called “pumping solution products,” because they perform that function for applications across a range of process conditions. For example, rotary PD pumps handle viscous products (3 million SSU), whereas reciprocating pumps handle water-thin liquids. 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. It’s important to emphasize that PD pumps, at constant speed, are constant-flow devices, whereas centrifugal pumps are variable-flow devices. And PD system design requirements are very different from those for centrifugal pumps. For example, PD pumps require some type of pressure protection, and certain designs require pulsation control.

PD pumps can be used almost anywhere, but the generally accepted view is that more than nine out of 10 PD applications are in six industrial markets:

• oil and gas
• water and wastewater treatment
• chemical
• food, beverage and pharmaceutical
• power
• general industrial (marine/medical/OEM).

Many of these industries are representative of multiple markets. Oil and gas, as an example, has distinctly different applications for PD pumps across its segments: exploration, production, pipeline, processing and distribution marketing. The food and beverage market is another key positive-displacement market with multiple segments that include beverage, bakery, confectionary, dairy and meat packaging.

A dozen benefits

Some applications clearly should use positive-displacement pumps and others should use centrifugal pumps. It’s important to recognize, however, the broad range of applications in which both types should be considered with selection being based on the user’s desired results. Be aware of the reasons that positive-displacement pumps might be the best solution to a specific pumping problem. Twelve reasons to select PD pumps, grouped by fluid characteristics, pressure conditions, environmental system requirements and flow control are summarized below. Table 1 provides a matrix of the 12 reasons compared to the primary markets of PD pumps.

Table 1. These are the pump-application characteristics that prompt specific industries to select positive-displacement pumps.