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
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.
The pump will displace the same amount of fluid on every revolution, regardless of system pressure.
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.
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
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.
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.
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.
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.
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
- 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.
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