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Freewheeling centrifugal pumps



Q  -  Are there technical reasons for not flowing through a non-running pump
equipped with adjustable speed drive (ASD)? A case in point would be to control
flow with downstream control valve during periods in which the flow demand is
less than the minimum continuous stable flow (MCSF) for the pump running at
minimum ASD speed.


A  -  We do not expect negative consequence if the pump were freewheeling in the
right direction over a short period of time, depending on the speed of its rotation.

But over a longer period of time it could damage the wear parts and mechanical
seals of the pump. Since the pump would not develop differential pressure, there
would be no lubricating leakage flow across its wear parts, and the mechanical
seal flush would not function. The heat build-up and lack of lubrication could be
damaging.

The pump would also act as a "throttling valve" that would break down its suction
pressure. If the liquid had entrained or dissolved gas, some amount of gas could
be released downstream of the pump. Depending on the product this might, or
might not, be detrimental to the on-going process.

We do not have information if the freewheeling rotor would cause an electrical
charge to be induced into the ASD, and what effect, if any, it would have on the
equipment if the ASD were powered up. (
Any comment from our readers?)

A solution to this scenario is to install a low flow by-pass line to divert the flow, and
isolate the pump and ASD, during periods of low flow demand.



Q  -  Are there consequences resulting from starting a pump that is freewheeling
in the right direction? (I understand there are consequences when it is spinning
backwards.)


A  -  We do not expect negative consequence if the pump were re-started from its
freewheeling condition in the right direction. On a positive note, the pump would  
require a lower starting torque.



R: 1109-FRCE
C: operation
F: freewheeling pump


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