How does hp change with flow for a "centrifugal pump"

By @electricpete

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 sometimes called “centrifugal” pumps, even though that terminology is dubious).

In general (there are exceptions - see manufacturer’s curve for highest assurance):

  • 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 thumbrule peak BHP is near BEP so current increases when adjusting flow in a direction toward BEP.

Goulds Pumps has excellent resources for educating yourself on 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 may be mixed flow (high flow pumps) or radial flow (high dp, low flow pumps).

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 only a small fraction of the total pressure… and considering that density is proportional to pressure the gas will act like incompressible fluid if this assumption is met.

Although it is commonly done, I think it very bad practice to call an axial flow pump “centrifugal”. This is because axial flow pumps do not use the principle of centrifugal force and therefore are definitely not “centrifugal pumps”. Centrifugal and axial pumps fall under the category of “rotodynamic” pumps which are of mechanically similar construction. A shaft turns an impeller or a propeller. So the same manufacturer can easily make either type of pump and they look pretty similar.

However, the operating principles, and the pump curves and equations are very different. So it is really wrong to call an axial flow pump a centrifugal pump. It just leads to confusion and miscommunication.

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For centrifugal pumps (radial flow) it’s convention to simply say centrifugal pump, once Specific Speed (Ns) increases it is prudent to change terminology to mixed flow or axial flow to correctly describe the hydraulic style, to be pedantic there is a further designation between centrifugal and mixed flow - Francis vane (rarely designated as it performance is very similar to centrifugal).
Therefore correct designation to remove doubt on pump style would be:
150 x 100 centrifugal pump.
150 x 100 mixed flow pump.
150 x 100 axial flow pump.