Pump efficiency prediction

[RE]

The similarity rules for centrifugal pumps dictate that homologous operating points of a certain pumps (changing velocity) have same efficiency. Starting at BEP of a pump runing, say, 1750 rpm (max. efficiency = 80%), the similarity rules predict that there must be an homologous operating point at lower velocity, say, 1500 rpm with the same max. efficiency = 80%. But practice indicates that at lower velocity the max. efficiency is also lower, then 80% eff. is no possible al 1500 rpm. Theory fails?

 

[smckennz]

Hi RE
the similarity laws assume constant efficiency which is clearly incorrect but normally acceptable for small changes in operating variables.

Cheers

Steve

 

[RE]

Hi, smckennz
Thank you very much for your reply. I found that theory is aprox. OK for flow rates larger and lower than BEP. At BEP same efficiency is inconsistent.

 

[25362]

According to pump manuals the efficiency at the BEP doesn't change much with small changes of rotating speed. If a all, it tends to improve a little at higher speeds.