Difference between a positive and non-positive displacement pump

What is a positive displacement pump?

A positive displacement pump is a pump that operates by transferring liquid repeatedly by enclosing a specific amount and moving it via a system. The pump transfers fluid via screws, gears, vanes, diaphragm, piston, or plunger. The pump is called “positive” since its design allows fluid flow only in the forward direction. The pump’s internal components are designed such that no fluid can recirculate backward to the casing or inlet. There are two main types of this pump: the reciprocating and the rotary positive displacement pumps. Each pump design has unique internal components to accommodate fluids of different characteristics.

Figure: Positive displacement pump (screw pump).

Types of positive displacement pumps

  • Gear pump. This pump uses two gears to transfer liquid. One gear connects the pump to the prime mover (driver gear), and the other gear is driven by the driver gear. The two gears are in constant mesh. As the gears rotate, they displace a specific amount of fluid towards the discharge end.

Figure: Gear pump.

  • Screw pump. This pump uses two or more screws. The screws rotate, creating a vacuum that draws liquid from storage into the pump. The screw rotation helps to displace the fluid to the discharge end for transportation.

Figure: screw pump.

  • Diaphragm pump. This pump uses two diaphragm components that move back and forth to move fluid.

Figure: Diaphragm pump.

What is a non-positive displacement pump?

A non-positive displacement pump is also known as a centrifugal pump. This pump operates by generating centrifugal force. The pump has a shaft that transmits power from the prime mover. The power on the shaft is transmitted to the fluid via a component known as an impeller. The impeller is a circular disc with curved vanes on its sides. The impeller helps to add pressure and kinetic energy to the fluid. They are very common in many industrial applications due to their simple and compact design.

Types of non-positive displacement pumps

  • End suction pump. This pump is designed with the suction and discharge ends oriented at 90 degrees to each other. This pump is common due to its simple and compact design, easy installation and repair, and high reliability and durability.

Figure: End suction pump.

  • Self-priming pump. This pump is designed such that it can prime itself. It relieves the operator the manual work of filling the pump with liquid to eliminate air bubbles before starting normal pumping.
  • Figure: Self-priming pump working to remove air bubbles.
  • Split case pump. This pump type has its casing divided into two parts (the bottom and top casing). The two parts are mounted together using bolts and nuts. This design makes it easy to access the internal parts of the pump for easy repair and maintenance. This pump has high flow rates of around 3500 m3//h.

Figure: Split case pump.

Differences between positive and non-positive displacement pumps

  • Positive displacement pumps operate by drawing the liquid into a cavity or by displacing it. The fluid is then forced from the pump cavity towards the discharge end. On the other hand, non-positive displacement pumps use a spinning impeller to move liquid into the pump. The fluid is then forced out of the pump by reducing its flow velocity while increasing pressure energy.
  • Positive displacement pumps transport highly viscous media such as honey, jam, grease, juice, etc. They can increase the flow rate even when the fluid gets thicker. Non-positive displacement pumps are designed for use in low to medium-viscosity liquids. The flow rate reduces when viscosity increases.
  • Positive displacement pumps can operate without being primed. In contrast, non-positive displacement pumps need to be primed to eliminate air bubbles to create enough vacuum to initiate fluid flow.
  • Positive displacement pumps produce a constant flow rate even when pressure changes. On the other hand, non-positive displacement pumps have the flow rate varying depending on the amount of pressure.
  • Positive displacement pumps have high efficiency over a wide range of pressure and flow rates. They keep their efficiency high even when working on highly viscous fluids. The efficiency of non-positive displacement pumps depends on the head and flow rate. As such, the pump becomes efficient when handling lower heads at high flow rates.
  • Positive displacement pumps are primarily used in metering liquids, hydraulic systems using high pressure, dosing chemicals, transportation of oil and gas, and viscous fluids in foods, paints, and adhesives. Non-positive displacement pumps are used in applications such as wastewater treatment, fire protection, irrigation, HVAC systems, and water supply, among others.
  • Positive displacement pumps can produce high pressure by displacing the liquid directly. On the other hand, non-positive displacement pumps are capable of producing moderate pressure. Where liquid transfer needs high pressure, non-positive displacement pumps with more than one impeller are used.

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