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MCF for Displacement Pumps

MCF for Displacement Pumps

(OP)
Hello;

Minimum continious flow (MCF) is just for centrifugal pumps,

Why does not positive displacement pump operate under this princible?

Is there any another reason except design difference?

Thanks

RE: MCF for Displacement Pumps

Operating capacity of a centrifugal pump is controlled by the intersection of the system head curve and the pump characteristic (performance) curve. Each system has its own unique head curve, expressed in feet of liquid being pumped, and comprised of static head and friction head. A centrifugal pump has its own particular head-flow performance curve as supplied by the pump manufacturer for clear, cold water. Correction of this curve is necessary only when fluid viscosity differs significantly from water.

Centrifugal pumps can be damaged if operated at excessively high flow rates, even though this is not as harmful as operation below the minimum flow point. Pump operation should be limited so that the discharge pressure is never more than 10% below the design point. Operation at too high a flow rate can result in cavitation damage in the discharge of the pump, seal chattering, and leakage. When a centrifugal pump is operated at a very low flow rate, recirculation occurs within the impeller, and it surges at the natural frequency of the system.

http://www.sundyne.com/StaticFiles/Sundyne_Content...

Positive displacement pumps are of a different design and will different design limitations.

RE: MCF for Displacement Pumps

"Why does not positive displacement pump operate under this princible?

Is there any another reason except design difference?

You've answered your own question....

In essence centrifugal pumps are constant pressure (within 20%) from zero flow to max rated flow ( though head curves can vary)

positive displacement pumps are constant flow pumps ( +/- 10%) with pressure to match the system requirement from nearly zero up to max pump outlet pressure.

Thus if you don't allow for a full flow bypass / relief in the event of locked outlet flow, the PD pump can create a very high pressure until the motor stalls

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: MCF for Displacement Pumps

"Thus if you don't allow for a full flow bypass / relief in the event of locked outlet flow, the PD pump can create a very high pressure until the motor stalls." Right - if you are lucky. If not something else will break or burst and that can be much more dangerous than a stalled motor.

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