FROM: LEE RUIZ
To: Lev Nelik
Thank you for your September 2025 Pumps & Systems article, “What Is NPSHr Stonewalling?”* The NPSHr condition that arises by operating a centrifugal pump significantly to the right of BEP was an interesting topic. After looking over the article’s curves, I had some comments.
Per Image 1, if the pump is operating where the NPSHr is located on the steeply rising portion of the curve, there is a benefit limit to an increase in rotation speed. Beyond this limit, increasing pump speed will require more NPSHr, at the original flow rate, than the lower-speed requirement (similar to operating in a reasonable flow range). When choosing a higher operation speed, it might be a good idea to plot the new (estimated?) higher-speed NPSHr curve versus the original-speed curve for comparison. If the new, higher-speed NPSHr is less than the original requirement at the original flow rate, the chosen speed increase might indeed improve the situation.
In other words, a speed-increase benefit limit occurs at the intersection of the higher-speed NPSHr curve with the original low-speed NPSHr curve at the original flow rate. As an example, if the 200 rotations per minute (rpm) pump in Image 1 operates at about 53,000 gallons per minute (gpm), then there would be little or no NPSHr advantage to increasing the speed to 400 rpm. The 300 rpm speed option might be a better choice.
To operate at a higher speed, the pump must be throttled back to match the original flow rate. This may result in the higher-speed pump operating to the left of its BEP. Hopefully, it will still be operating in its preferred or allowable operating range, and the resulting efficiency is acceptable. The increase in the high-speed pump pressure head might also be considered.
Of course, if the existing low-speed pump could be throttled back to, say, its BEP with a lower flow rate and higher-pressure head, then all the better. Hopefully, all pumping cases occur within a “normal/reasonable” flow range.
Thanks again for your not-often-addressed article topic.
Lee Ruiz
Oceanside, CA
FROM: LEV NELIK
To: Lee Ruiz
Thank you, Lee, for your insightful comments on net positive suction head required (NPSHr) at high flows and speed control, which, when speed is reduced, may make it worse. This does come as a surprise to most. The reason is that, in practice, pumps do not often operate that far to the right of the best efficiency point (BEP), and thus such a situation remains mostly theoretical.
In my years of work in the field, I came across all sorts of natural problems. For example, many years ago, I was asked to consult on net positive suction head (NPSH) issues when cavitation was so severe that net positive suction head available (NPSHa) forced pump head drops not just a few percentages (as even 3% head drop is what defines NPSHr), but to a 50% head drop!
My first question was: Why worry about such a heavy cavitation regime? Obviously, the pump should not be operating even close to that. Was that not obvious?
The surprising answer was that a nuclear plant wanted to know what happens when a sudden event damages pipes and discharge lines are broken (for example, by an earthquake) and the flow gushes out of a pump, making it run out to very high flow, at a severely cavitating situation.
Yet, a nuclear plant cannot quickly shut down the pump, as in most other cases operators would, because a reactor needs to be called—at least at some water flowing, for at least 20 minutes or so, until they can shut down the pumps and reactor in a controlled fashion.
So, at 50% head loss during cavitation, there is still some flow, and the plant wants to know how much, and pumps would survive these 20 minutes.
The world has some interesting and unusual pump situations, and your comments support this.
Best regards,
Lev Nelik
*Note: “What Is NPSHr Stonewalling?” appeared in the September print issue of Pumps & Systems under the title, “NPSHr vs. RPM, Part 2.”
For more guest columns from Dr. Nelik, visit pumpsandsystems.com/tags/lev-nelik.
