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What happens when fluid flows through a static hydraulic pump? Will there be a negative RPM to impede the flow? By static pump, I mean a pump with initial RPM = 0.0 and is not driven by external forces during the transients of flow.

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  • $\begingroup$ Depends on the type if pump and where it is in the cycle - even though there may be pressure at the inlet it may not cause rotation. $\endgroup$
    – Solar Mike
    Nov 4 '20 at 15:16
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First I would state the question differently. What happens when there is a negative pressure difference across an unpowered hydraulic pump? This is actually a common situation occurring when a pump's electric motor is turned off but pressure still exists downstream because of a compressible accumulator or gravity storage device.

There are 3 possibilities in this situation. I'm using numbers for a typical application to help explain the effects. The actual numbers would vary for different pumps and fluids.

  1. 0 - 50 psid (0 - 3.5 bar) pressure difference: Likely no pump rotation. Some leakage is able to flow through clearances in the pump from outlet to inlet. Fluid will also exit the case drain.
  2. 50 - 200+ psid (3.5 - 14 bar) pressure difference: Pressure is sufficient to overcome static friction in the pump seals and motor bearings. Pump rotation will occur until supply pressure is depleted. Leakage flow through clearances will be additive to the positive displacement flow.
  3. Any pressure difference against a check valve: Negligible flow. Many pump and system designs include check valves to prevent any flow traveling from pump outlet back to the inlet. Generally forcing a pump to rotate backwards (motoring mode) is undesirable unless the pump was designed to allow bi-directional flow. Also there must be some way to deal with the incoming energy from pump shaft back to electric motor. Storing in a battery / dissipate through resistors / turning other rotating devices / etc.
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