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Load characteristics

How does hp change with flow for a "centrifugal pump" by electricpete
Posted: 13 May 09 (Edited 6 Jan 10)

Most people think increased flow on a centrifugal pump always means increased current.  It is true for radial flow centrifugal pumps, but not for mixed flow or axial flow pumps (which are also called centrifugal pumps in common terminology).

In general:
* for a radial flow centrifugal pump, opening up on a system throttle valve will increase BHP demand and current draw.
* for an axial  flow centrifugal pump, opening  up on a system throttle valve will decrease BHP demand and current draw.
* for mixed flow centrifugal pump, the curve is non-monotonic.  As a very rough thumrule  peak BHP is near BEP so current increases when adjusting flow in a direction toward BEP.

The following link provides more info on pump BHP vs flow for these three kinds of pumps:

Which type pump do you have? The best answer is to check the documentation... however some general thumbrules:
Single-stage high dp pumps, low flow pumps are typically pure radial flow design.
Single stage low-dp, high flow pumps are typically pure axial flow design.
Multi-stage pumps are typically mixed flow design.

By the way the pump laws and the above behavior for radial and axial flow pumps also applies to fans IF the system pressure is close to atmospheric and the dp is small (several inches water).  In this case the change in pressure is aonly a small fraction of the total pressure... and considering that density is proportional to pressure the gas will act like incompressible fluid if this assumtpion is met.  

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