Positive & Non-Positive Pumping

Objectives

  • Describe the difference in function between positive & non-positive displacement pumps
  • Categorize common models of pump as positive or non-positive
  • Explain the advantages of both styles of pump

Key Concept

There are similarities and important differences between positive & non-positive displacement systems. It’s very important to be aware of which type of system you are dealing with!

Positive Displacement Pump

A hydraulic pump is a positive displacement pump, which means that for every revolution of the pump shaft, the same amount of fluid should be displaced regardless of the system pressure.

Positive Displacement Pump:
The pump will displace the same amount of fluid on every revolution, regardless of system pressure.
Clock
Motor
1 GPM
Pump
Motor
1 GPM
Pump
1 Gallon Tank
1 Gallon Tank
Identical systems with one hardly workin' (left) and one workin' hard (right). Both systems still displace the same amount of fluid.

Non-Positive Displacement Pumping

A pump that is used to circulate coolant around an engine block, or a centrifugal pump used in an industrial plant to move fluid from one holding tank to another, is usually a non-positive displacement pump.

This type of pump allows for slippage to take place inside the pump whenever the system pressure starts to increase.

Non-Positive Displacement Pump

A non-positive displacement pump's output flow rate changes in response to restriction (pressure) on the outlet.

Pump running at high efficiency

Pump running against a strong restriction

Restriction

Pumps and Slippage

At very high pressures, some slippage will occur as fluid finds a passage in the fine clearances between pump parts back to the inlet side of the pump. This reduces the overall efficiency of the pump.

Slipping and Slippage:
Whenever a pump is putting out less than its maximum displacement, it is said to be "slipping". Slippage refers to the liquid that does not make it to the pump's outlet.
Demonstration of slippage in a non-positive displacement pump. These pumps are designed to slip.
Slippage in a positive displacement pump after severe damage. These pumps typically have little to no slippage if functioning correctly and are not overloaded.

Non-Positive Displacement Pumping - Continued

In such a system, the rate of flow from the pump, (gallons per minute), changes as the restriction on the pump outlet is changed. In other words, if the pressure at the pump outlet is decreased, the flow rate will increase, and vice versa.

This slippage is desirable in most simple transfer or circulation pump systems as it can help to limit the system pressures from rising to extreme and dangerous levels, and it helps to limit the amount of horsepower that is required by the prime mover (motor or engine).

Non-positive systems have their own problems related to fluid viscosity, system pressure and temperature, but they are somewhat different from a positive displacement pump.

Key Concept

A non-positive displacement pump can slip, making it a good choice for any application that does not put high pressure on the pump’s outlet.

Non-Positive Displacement - Example

If you designed a lift where a hydraulic cylinder is extended by flow from a non-positive displacement pump, the performance would be terrible.

  • The lift speed would vary according to the amount of weight to be lifted.
  • Different load weights would cause variations in pump outlet pressure and therefore different rates of slippage inside the pump.
  • The pump slippage would cause a serious heating problem at high pressures.

Centrifugal Pump

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