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.
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