Pumps Galore! Dispelling Some Centrifugal Pump Fallacies
An article appeared in the November 1997 issue of an engineering trade publication that discussed centrifugal pump applications.
In this article the author stated, "Increasing a motor's horsepower will increase discharge pressure, which will have no effect on lift
but will increase head." This statement provoked a campaign of "Letters to the Editor" that lasted for three issues. A lot of ink was
spilled in the name of physics because of a poorly worded statement. During the ensuing feeding frenzy, professional engineers
wrote to the magazine ruthlessly brandishing theory and calculation. Contributors offered calculations for the maximum theoretical
lift of a pump at sea level accounting for temperature, relative humidity and air pressure (down to two decimal places of course).
Others got shots in explaining induction motor slip factors and affinity laws. Equations and physical laws are the tools of the
engineering profession. Apparently, they make pretty good weapons, too.
The centrifugal pump is one of the most popular members of the turbo-machinery family. It has applications in the areas of
potable water distribution, wastewater transfer, the pumping of chemicals, slurries, pulp and seawater. They pressurize sprinkler and
standpipe systems. They are used to circulate purified water for pharmaceutical applications. They deliver feed water to boilers
and pump steam condensate, and so on. Every engineer and designer in the mechanical, plumbing and fire protection disciplines
must be a master of centrifugal pump selection. Unfortunately, as a dynamic machine involving the interplay between the physics
of momentum, thermodynamics and fluid dynamics, the centrifugal pump can be a potential source of confusion and
misunderstanding. Far too many designers and engineers proceed with pump selections unfettered by the facts. This practice of
"eyeball" engineering leads to gradual degradation of technical knowledge among practicing engineers and designers. It is
worthwhile to review pump fundamentals every so often. It is equally important to keep these fundamentals in perspective when
considering practical applications.