Lev Nelik responds to a reader response about determining static components of system curves.
by Lev Nelik
September 6, 2019

Hi, Dr. Nelik,

Your latest article in Pumps & Systems magazine, “Determining Static Component of System Curves in Tricky Situations,” (July 2019, read it here) was an intriguing read.

In the article, the conditions for a particular boiler feed application were described and analyzed. I was curious if the sum of the static plus operating friction pressure components were approximately the same as the differential pump pressure.

I found that the sum of the article components was about 24 pounds per square inch (psi) higher than the operating pump component.

That is, static + friction pressures, 889 psi + 465 psi is 1,354 psi compared with the differential pump component of 1,380 psi - 50 psi is 1,330 psi.

The assumed constant boiler and deaerator static pressure differential is indeed 915 psi - 26 psi = 889 psi.

However, if there were an elevation difference between the boiler and deaerator liquid levels, I assume that it would be added to the static pressure value for a total static, pressure plus elevation, component of the system curve.

So, that leaves the operating friction pressure component that may be something other than 465 psi. The friction pressure component can be determined by using the energy method where the static elevation difference is assumed to be zero, and the minor velocity-head differences are disregarded. Then, the revised operating friction pressure component turns out to be the pump-discharge-to-boiler pressure differential minus the pump-suction-to-deaerator pressure differential. That is, 1,380 - 915 - (50 - 26) = 465 psi - 24 psi = 441 psi.

Now, the static pressure + revised friction pressure, 889 psi + 441 psi, equals 1,330 psi. The sum of the static and revised operating friction pressures now agree with the operating differential pump pressure.

The system curve equation is then:

H(psi) = 0.011 Q^2 + 889 psi.
Equation 1

Thanks again for your articles.
Regards,

Lee Ruiz
Oceanside, CA

Deaerator and boilerImage 1. Deaerator and boiler at same elevation (Image courtesy of the author)

Dr. Nelik’s response:

Always a pleasure hearing from you. Thank you for your comments and sharp eye! I appreciate the time you took to review the system curve and come up with a system curve equation. This is usually helpful when people take pump test data and enter it on a spreadsheet for analysis. Then, consecutive tests are spared time to re-enter all data as much of the originally entered material can be easily modified and a tabulation pointed to a graph, which will conveniently update the curve for a clear comparison.

Best of luck!
Thank you, and keep on pumping!

Dr. Nelik

To read more Pumping Prescriptions columns, click here.