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What is a positive displacement pump used for?

By Thiemo Schuff (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
By Thiemo Schuff (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Positive displacement pumps are used in a variety of applications in a range of fluid handling industries.

They are deployed in situations where a specific amount of liquid is required to be moved into a system. Due to their ability to handle highly viscous liquid, they are also commonly used in heavier duty services.

Positive displacement pumps are a popular choice for applications with variable pressure conditions. This is because they deliver near constant flow, allowing the flow to be matched to the process requirements.

Two types of positive displacement pumps are commonly used for two major applications in the oil and gas industry. Progressive Cavity Pumps (sometimes referred to as eccentric screw or single screw pumps), are used to transfer difficult liquids, such as those containing solids or highly viscous fluids.

Twin screw pumps can handle non-homogenous fluids and operate through any level of abrasiveness, lubricity and viscosity. Twin pumps are most often used in situations that contain high gas volume fractions and fluctuating inlet conditions. They are also able to comfortably perform in situations where there is a changing viscosity.

 

How do positive displacement pumps work?

Positive displacement pumps move fluids by trapping a fixed amount and displacing that trapped volume into a discharge pipe.

They usually have an expanding cavity on the suction side and a decreasing cavity on the discharge side. Fluid flows into the positive displacement pumps as the cavity on the suction side expands and flows out of the discharge as the cavity collapses. The volume is a constant for each cycle of operation.

According to an article for Pump Scout by Ross MacKay, president of Ross MacKay Associates, the piston pump is the "oldest and best known positive displacement pump." Piston pumps use a piston or plunger to force liquid from the inlet side to the outlet side of the pump.

 

What is the difference between a positive displacement pump and a centrifugal pump?

The vast majority of pumps used in industry fall into either the positive displacement or centrifugal pump category.

Examples of positive displacement pumps include screw, peristaltic, piston and rotary lobe pumps.

The key feature of a positive displacement pump is that it is a ‘constant flow machine’, which means it will deliver the same flow at a given speed (RPM) no matter what the discharge pressure is. Another key feature of positive displacement pumps is that their flow actually increases as the viscosity of a fluid increases. The opposite is true of centrifugal pumps.

Pump School provide an example of the key performance differences between positive displacement pumps and centrifugal pumps here.

 

Setting-up a positive displacement pump

Because positive displacement pumps must not operate against a closed valve on the discharge side of the pump. If a positive displacement pump is operated against a closed discharge valve, it continues to generate flow, increasing pressure in the discharge line until it bursts. 

By Thiemo Schuff (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons