Fluid Handling

Fundamentals of positive displacement pumps

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

By Hydraulic Institute PD Pump Members

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.

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

High viscosity

Certain rotary technologies and air-operated piston pumps easily handle viscous fluids, whereas frictional losses degrade centrifugal pump flow rate and efficiency when used with fluids that have a viscosity above 500 SSU. Flow and efficiency in a rotary pump, however, typically increase with viscosity. In fact, PD pumps can handle fluids with viscosities of several million SSU.

Low and variable viscosity

PD pumps, such as vane or air-operated double-diaphragm (AODD), often are used for thin fluids. Other liquids, such as oil, have viscosities that vary with temperature. With variable viscosity liquids, a moderately small change in viscosity can have a large effect on centrifugal pump efficiency but little effect on PD pump efficiency.

Low-shear pumping

In many fluid applications, liquid shear isn’t a problem; however, in some applications it is critical. PD pumps excel in the handling of shear-sensitive fluids.

Solids-handling capability

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.

Progressing-cavity pumps handling the high-solids-content sludge in a waste treatment plant and coal slurry pipelines use reciprocating pumps to handle fluids with a solids content as high as 40% by weight. You might find this to be a surprising PD pump characteristic, but widely varied applications serve as examples.

Multi-phase flow

A centrifugal pump needs a constant source of liquid, but not all processes can provide it. If there’s insufficient liquid at the pump suction, a gas bubble forms, the pump loses its prime and the fluid stops moving. Such isn’t the case for PD pumps because they’re capable of handling a high percentage of air or gas entrainment.

High pressure

Beyond the range of centrifugal pumps are many chemical, sandblasting and high-pressure water-cutting applications in which PD pump technology dominates. Figure 2 provides an overview of the pressure and capabilities among pump technologies.

Figure 2. This chart shows the regions where the three pump technologies can function at their best.
Figure 2. This chart shows the regions where the three pump technologies can function at their best.

Low flow

PD pump technology easily handles flows below 100 gpm and pressures greater than 200 psi.

Efficiency

For viscous fluids that both PD and centrifugal pumps can handle, the positive-displacement option often can be 10 points to 40 points more energy-efficient than the centrifugal pump.

High-pressure, low-flow, efficiency demand

Any of the previous three individual characteristics are reason enough to use PD pumps. But, if the application features all three simultaneously, the PD pump solution becomes the obvious choice.

Sealless pumping

Magnetic drives and canned motor pumps, which require no shaft seals, are available in PD pump designs. Other designs that have no shaft penetration include peristaltic and diaphragm pumps.

Self-priming

The PD pump’s ability to self-prime is a useful feature because it allows substantial flexibility in system layout and eliminates the need for suction-priming systems. PD pumps aren’t only self-priming; they also have excellent suction lift capabilities — raising liquids on the suction side — and are capable of drawing down to near vacuum.

Constant flow/variable pressure

At a constant speed, PD pumps deliver practically constant flow. This is true even if the system pressure varies, which is a desirable condition in certain systems.

Accurate, repeatable measurement

Because a PD pump is a constant-flow device, certain designs that limit slip are useful for metering fluids into or out of systems. This application, of course, requires accuracy and repeatability. It might also need flow variation, which typically is obtained mechanically or electronically by speed variation.

There’s a universe of standard PD solutions in addition to the baker’s dozen described here. Because these pumps also must meet many other requirements, manufacturers provide products with special options, such as jacketing, non-corrosive materials and built-in pressure relief valves, to name a few. Some PD units have duty-cycle limits that users should investigate. It’s important to note PD pumps are constant-torque devices. In variable-speed applications, VFDs must be rated with that understanding.