Insight and reactions to key industry coverage


“Pump Ed 101,” November 2011


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In Joe Evans’ November 2011 “Pump Ed 101,” an error appeared in one of the equation for suction specific speed. The correct formula is:

S = N√Q/NPSHr3/4

We apologize for any inconvenience this may have caused.



“HI Pump FAQs,” February 2011 & “Readers Respond,” March 2011


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Can I have a complete design process of how the formula for the shaft deflection is derived? From the March reader respond, I though magazine was to re-publish the complete formula.

Jack Terlecki


Lee Ruiz (Pumps & Systems’ reader and letter writer in the March issue) responds:

The step-by-step derivation of the H.I. FAQ shaft deflection formulas is made more complex because of the three different shaft sizes. In lieu of using the double-integration method with boundary conditions, I have employed the principle of superposition and have assembled the pertinent elementary beam equations for the solutions. An Excel file is attached with the formula development steps shown (available on 

The derivation assumes: the shaft material is homogeneous, the maximum stress is less than the material elastic limit (therefore, superposition can be used), the shaft deflection due to vertical shear is considered insignificant and not included.

For the elementary beam formulas used to develop these solutions, refer to a mechanics of materials text or go to



"The History of Pumps," January 2012


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I always look forward to the new issue of P&S, the January 2012 issue is unique not only for the usual informative and interesting articles but for all the good information of the World Pump History, congratulations this issue is a keeper. I always recommend P&S to my friends and clients in the chemical process industry. This issue is a fine example of the high quality of your magazine. What a way to start the year!

Keep up the good work.

Jose M. Sentmanat


Liquid Filtration Specialist, LLC



"Pumping Prescriptions," July 2011


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If for some reason, the flow has been reduced to 50 percent of the original design, a better energy saving may be obtained by installing a variable frequency drive (medium voltage for this application). Impeller trimming is permanent.

Mallur R. Nandagopal, Ph.D., P.E. 

Senior Engineer, City of Spokane Water Department


Lev Nelik responds:

You have a good point. Speed reduction via a variable frequency drive (VFD) is more flexible versus irreversible impeller trim. However, especially for municipal applications and for systems where friction head is small in relation to the static head (mostly lift), speed reduction by VFD (or trim) could be problematic. The reason for that is, per pump affinity laws, speed reduction carries with it not only flow reduction but head reduction as well. With relatively small speed change, the entire pump curve can easily dip before the system head, making a pump effectively shut-off hydraulically. I see VFD systems for such applications all over the U.S. and internationally and must admit many have issues.


Mallur Nandagopal responds: