Pump Expert? How to read pump head using pressure gauges.

RayWohlfarth

I was asked about a B&G pump and need your advice. The gauge on pump suction reads 15 Psi and the discharge gauge reads 55 Psi. Would you add them together or use the pressure differential to determine head pressure? I was told each way was correct. I know you mutilply the pressure by 2.31 to convert pressure to feet. The owner said the discharge pressure used to be around 45 Psi. Thanks
Ray

RPK

Use the differential

Tinman

Someone much smarter than I am once told me that there's more involved than that. I'll see if I can find my notes tonight.

RayWohlfarth

Thanks @RPK and @Stephen Minnich

Jamie Hall

Stephen Minnich said:

Someone much smarter than I am once told me that there's more involved than that. I'll see if I can find my notes tonight.

Really not, @Stephen Minnich -- just subtract. Of course, assuming that the gauges are both near enough to the pump to eliminate friction losses and the pipe sizes are about the same (there is a velocity head consideration in there, if you want to get complicated)... now with air or gasses, it's a different story.

Tinman

In my conversation with this guy, a well respected mechanical engineer specializing in hydronics, he brought up what I thought to be valid points but I cant find my notes. That said, my suggestion would be to pipe a single pressure gauge in parallel with the pump with a ball valve on each side of it. Less margin of error with a single gauge. My .02

hot_rod

ideally use the same gauge to eliminate gauge error
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf

EBEBRATT-Ed

reading with the same gage is preferable. diff is 40 psi x 2.31'/lb

Gordy

Differential x 2.31, and single gauge setup @hot rod , and @Stephen Minnich posted. The closer to the discharge, and suction side of the circulator with the gauge piping the more accurate the reading will be. Pete’s plugs in the flanges are ideal.

hot_rod

the temperature of the water changes the calculation a bit if you are trying the be exact

Tinman

That's one of the things that the M.E. mentioned. Specific gravity of water. M.E.s tend to be very precise.

ch4man

differential all the way
remember the term "pumping away" and what it means?

Gordy

Here is a thought if you are pumping away from the PONPC. And that point is right at the return of the circulator.

Theoretically all you would need is a pressure gauge on the discharge side of the circulator. Then use the difference of system psi, and discharge psi. However you are back to two separate gauge inaccuracies.

RayWohlfarth

Thank you everyone for your help. I was watching this video on line by a guy with several initial after his name. He made me second guess my thoughts as he said the suction and discharge were to be added.I appreciate all your input.

SuperJ

One thing to watch out for is zero or negative suction pressure, or a clogged inlet strainer. The gauge may not read vacuum accurately in most cases and the pump is probably cavitating.

Most pumps have a specified minimum positive suction pressure requirement that depends on temperature to avoid cavitation.