When To Use Rotary Positive Displacement Pumps

We’ve spoken about reciprocating types of positive displacement (PD) pumps and how the different types of reciprocating pumps can be useful for precise metering and dosing. Today, we’re going to look at rotating positive displacement pumps, which transfer fluids as the gears rotate, and when to use them.

A gear pump is a rotary positive displacement pump at its simplest, and the two terms are often used interchangeably. However, it’s important to first understand how a rotating pump works before we go into the different types of gear pumps. Basically, a rotating element seals with the pump casing and creates a suction effect at the inlet. The rotation of the gears displaces fluid drawn into the pump from the reservoir and into the discharging pipe.

The rotating pump has several advantages:

• Self-priming
• Can be used in many applications
• Operate at low speed
• Easy to maintain
• Well-suited to handling viscous fluids

There are two main types of gear pumps: Internal and external.

External gear pump

How does it work?

An external gear pump consists of two interlocking, identically sized gears supported by separate shafts. When the gears rotate, fluid is trapped between the teeth, moving it from the inlet to the discharge and around the casing.
The reason fluid doesn’t leak anywhere is that gears are interlocked – close tolerances between the gears and casing create suction at the inlet and stop fluid from leaking back from the discharge side.

When should an external gear pump be used?

An external gear pump is well suited for high viscosity liquids, polymers, chemical additives and fuels as leakage becomes more likely with low viscosity fluids.
Our tip? Don’t use an external gear pump in applications where solids, abrasive materials or high temperatures are the norm.

Internal gear pump

How does it work?

An internal gear pump operates on the same principles as the external gear pump, except for one important factor. The two interlocking gears, instead of being identical, are completely different sizes, with one smaller gear rotating inside the other. The cavities between the two gears are filled with fluid at the inlet and transported to the discharge port, where it is expelled by the action of the smaller gear. Simply put, the space between these gears draws in the fluid and displaces it.

When should an internal gear pump be used?

The close tolerances between the gears and casing mean that internal gear pumps are susceptible to wear and tear as well as unsuitable for abrasive fluids or feeds containing entrained solids (large solids can block or damage the pump).

Our tip? Never let internal gear pumps run dry – they should be kept lubricated by the pumped fluid, which is why oils and other high viscosity liquids are ideal.

When should you use internal and external gear pumps?

We recommend internal gear pumps for a wide variety of applications especially viscous fluids or resins, bitumen or chemicals. External gear pumps are useful for sampling and metering applications and we generally suggest using them in applications where high-pressure and low to medium temperatures are required.

Are you unsure if a rotating positive displacement pump is the right tool for the job, or what type of gear pump you should be using? Call our toll-free number on 1-800-367-4180. We have experts on hand to help you choose, install and maintain a variety of equipment. And to answer questions about things you’ve previously tried gone wrong.