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!
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.
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.
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.
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.
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