The concept of minimum flow for a centrifugal pump can be complicated.  A number of standards have been used.  Sometimes the number is provided as minimum flow for thermal stability.  This means that the temperature rise in the fluid as it passes through the pump will not cause the fluid to vaporize (cavitate). There can be a minimum flow for hydraulic stability.  Below this flow rate the pump may experience suction recirculation or discharge recirculation that may result in cavitation, vibration and inefficiency.  Frankly, I am not sure what method is used for the minimum flow value published on an API datasheet.  We have changed our method for determining minimum flow over the past few years.  Both our former method and our current method involve calculating Specific Speed and Suction Specific Speed for the impeller.  Then a graph or table specifies what percentage of Best Efficiency Point (BEP) flow is required as minimum.  I believe that our original criteria was based on a Hydraulic Institute standard.  Our current criteria is based on a system published in a book by Heiz Bloch and Allan Budris titled "Pump User's Handbook".  If the total flow is throttled with a valve on the pump outlet, all that is needed is for the total flow from the pump to be greater than the calculated minimum.  If the flow is above minimum, it should not be necessary to use a recirculation line.  If the total flow needs to be pinched down below the minimum, the use of a recirculation line can be used to get the total back above minimum.  Pinching down on the discharge side of the pump changes the system curve.  The pump will always operate at the intersection of the pump curve and system curve.  One a recirculation line is installed, it can be used to make the pump more reliable.  It can be used to run the total flow from the pump closer to BEP than is required to satisfy the minimum flow requirements.  The cost of doing so is the extra energy needed to recirculate more flow.  But if the reliability of the pump is critical, it may be worth it.  A pump running close to BEP will be more efficient, with lower vibration, lower temperature rise, less shaft deflection and probably have better seal and bearing reliability.  We occasionally use our recirculation lines to improve reliability in this manner.Magnetic pumps

2011-05-26

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