Quiz Corner: Calculating pump speed, pressure and mechanical load

What are the approximate pump speed, pressure and mechanical load at 50% flow?

EtiAmmos/iStock
EtiAmmos/iStock

What are the approximate pump speed, pressure and mechanical load at 50% flow?

Various reference materials contain information describing the Affinity Laws associated with reducing the speed of a centrifugal pump. Mathematically, they show that: pump flow is proportional to pump speed; pump discharge pressure is proportional to the square of pump speed; and mechanical load on the pump is proportional to the cube of pump speed. 

Using the information given, 50% flow would correspond to 50% speed, 25% pressure and 12.5% mechanical load. Note that the mechanical load is closely related to electrical energy consumption.

Of significant interest is that the electrical energy consumption would be reduced by approximately 87.5% at half flow. Similar calculations show that electrical energy consumption would be approximately 99.9% less at 10% flow.

Additional complicating factors

Not so fast — pumps almost always pump uphill, so the pump needs to overcome the static head of the piping system in order for the liquid to start to flow. If the static pressure needed to start flow is 64%, the pump must operate at 80% speed just to start flow — at which point, its electrical energy consumption would be approximately 50% to produce zero flow. 

If it looks too good to be true, it probably is.

 

David W. SpitzerDavid W. Spitzer

 

David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.

More in Pumps, Motors & Drives