What Is a Positive Displacement Pump?

Positive displacement pumps transport liquids and maintain flow by utilizing physical drivers, making them highly adaptable to a wide variety of functions across multiple industries.

industrial diaphragm pump

Published May 24, 2021   |   4 minute read

What Is a Positive Displacement Pump?

A positive displacement pump transports liquid by utilizing fixed-volume chambers and pistons, plungers, diaphragms, or another driver.

The most important function of a PD pump is its ability to produce the same consistent flow, no matter the discharge pressure. That’s why it’s also called a constant flow machine and employed for a multitude of purposes, across various industries.

There are two main types of PD pumps: rotary and reciprocating. The former features one or more rotor(s), which rotate within a static casing or stator, which create discrete compartments of known volume and deliver relatively smooth flow. A reciprocating pump is analogous to a car engine or your heart. Liquids flow through a valving mechanism into a cylinder or casing and are then stroked back and forth with either one or more pistons, plungers, or diaphragms – or through peristalsis, in the case of a hose pump – releasing it through another valve mechanism in discrete flow pulses into the system.

There are numerous subtypes, including:

  • Diaphragm Pump
  • Rotary Gear Pump
  • Gear Pump
  • Rotary Lobe Pump
  • Peristaltic Pump
  • Screw Pump
  • Piston Pump
  • Vane Pump
  • Plunger Pump
  • Oscillating Piston
  • Progressive Cavity Pump
  • Circumferential Piston
  • Regenerative (or Peripheral) Pump
  • Reciprocating Disc Pump


How Do Positive Displacement Pumps Work?

Where a centrifugal pump relies on velocity generated by centrifugal force to transport liquid and maintain its flow, positive displacement pumps utilize physical drivers like gears, lobes, vanes, screws, diaphragms, pistons, plungers, or other means to create cavities of fixed volume displacement that expand and contract due to the force of the driver acting on the fluid within the cavities at a rate determined by the speed of the driver.

They feature two cavities and at least one chamber (depending on the type of PD pump): a suction or entry cavity, a transport chamber, and an exit or discharge cavity.

Typical of a rotary PD pump, liquid flows into the suction cavity, which has an expanding volume. The physical driver then holds and advances a fixed volume, regardless of pressure, to the exit cavity. This cavity has a reducing or collapsing volume, which then pushes the liquid out of the pump.


What Are the Advantages of Positive Displacement Pumps?

There are many advantages to using a positive displacement pump, but they can all be summed up in one word: adaptability.

PD pumps are useful for myriad purposes, thanks to their ability to handle the following:

  • Shear Sensitive Fluids
  • Solids
  • High Pressure Without Affecting Flow
  • Self-Priming
  • High, Low & Variable Viscosity Liquids
  • Sealless Pumping

How Do I Choose the Right Positive Displacement Pump for My Application?

The best way to ensure you select the optimal positive displacement pump for your application is to consult with Sunair Co. 

Sunair Co. is the tri-state area’s leader in pump sales for a reason: We’ve been developing and implementing custom industrial systems for nearly 50 years. Explore our brands or contact us today. 

When you need to maintain a consistent flow or move high-viscosity materials such as oils, resins and paints, a positive displacement (PD) pump is the ideal choice. With a wide and diverse range of these to choose from, however, picking the right one for your specific application can be confusing.

Here is the Sunair Co. primer on positive displacement pumps.

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