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

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RayWohlfarth
RayWohlfarth Member Posts: 1,518
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

Ray Wohlfarth
Boiler Lessons

Comments

  • RPK
    RPK Member Posts: 109
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    Use the differential
  • Tinman
    Tinman Member Posts: 2,808
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    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.
    Steve Minnich
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,518
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    Thanks @RPK and @Stephen Minnich
    Ray Wohlfarth
    Boiler Lessons
  • Jamie Hall
    Jamie Hall Member Posts: 23,436
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    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.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Rich_49Gordy
  • Tinman
    Tinman Member Posts: 2,808
    edited October 2018
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    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
    Steve Minnich
    GordySolid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 22,256
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    ideally use the same gauge to eliminate gauge error
    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Rich_49Gordy
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,620
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    reading with the same gage is preferable. diff is 40 psi x 2.31'/lb
  • Gordy
    Gordy Member Posts: 9,546
    edited October 2018
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    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
    hot_rod Member Posts: 22,256
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    the temperature of the water changes the calculation a bit if you are trying the be exact
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tinman
    Tinman Member Posts: 2,808
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    That's one of the things that the M.E. mentioned. Specific gravity of water. M.E.s tend to be very precise.
    Steve Minnich
  • ch4man
    ch4man Member Posts: 296
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    differential all the way
    remember the term "pumping away" and what it means?
  • Gordy
    Gordy Member Posts: 9,546
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    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
    RayWohlfarth Member Posts: 1,518
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    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.
    Ray Wohlfarth
    Boiler Lessons
  • SuperJ
    SuperJ Member Posts: 609
    edited October 2018
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    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.