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Pump NPSHA (net positive suction head) question

  1. Apr 15, 2015 #1
    Simple (I hope) pump question.

    With the pump not even running, would like to know what my static NPSHa is if I had a flooded pump suction with 10 feet suction head of water and a vacuum of 27" Hg above the water?

    Basically it is the hotwell from a condensing turbine exhaust after the steam is condensed.
    We hold a 10 foot suction head and it is under vacuum with steam temp about 115 degrees above it in the hotwell tank

    Thank you for your time
     
  2. jcsd
  3. Apr 15, 2015 #2

    SteamKing

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    The head available at the pump inlet will be 10 feet of water + about 3" Hg or

    10*62.4/144 = 4.33 psi + 1.43 psi = 5.76 psi less any friction losses in the piping when the condensate starts to flow.

    Here's a better description of calculating NPSHA:

    http://www.pumpschool.com/applications/NPSH.pdf
     
  4. Apr 15, 2015 #3
    Thank you SteamKing!
    Excellent info from the link also.
    I take it the 1.43 psi is from the 3" Hg , (29.9 =14.7, roughly 2 to 1 ratio) and the total is what I thought it should be.
    Now, two follow up questions.
    If I open the blow off valve on the pump suction strainer, it will suck air in, so it is under a vacuum at the pump inlet, (Agree?)

    Also, I understand why a pump can only "lift" water 34 ft (suction lift, not flooded) due to atmospheric pressure pushing the water up to the pump.
    so why isn't the vacuum above the water trying to lift it like a pump lifting water by producing a vacuum?
    Uhhh, I think I just answered my question with the "atmospheric pressure pushing the water up to the pump.
    There is no pressure to push it up, correct?

    Thank you so much for the help. I believe the last question is critical to my understanding
     
  5. Apr 15, 2015 #4

    SteamKing

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    It's not clear why you would want to do that.

    You want to make sure that the NPSHA > NPSHR for the pump, in all flow regimes. If NPSHA is not sufficient, the pump will cavitate and damage the impeller.
     
  6. Apr 15, 2015 #5
    "It's not clear why you would want to do that"
    I don't want to but I know if I do, it will suck air, (I've done it quickly to confirm my suspicion) so if I installed a pressure gauge, what would it read? 5 to 6 psi? or vacuum?.

    I understand the A has to be greater than R, just wondering why if I produce a vacuum to lift water with a pump, why this water doesn't "lift" and starve the pump.
    Ex. a well pump on top producing suction lift, lifts the water by producing a vacuum, but the vacuum in my hotwell doesn't "lift" away from my pump below.
     
  7. Apr 15, 2015 #6

    SteamKing

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    I can't really say, since I don't know how your system is laid out.

    If a well gets too deep, then the pump must be placed at the bottom of the well and force the water to the surface. At least, that's how it was done when my household water was provided by a well (about 90 feet deep) drilled in my backyard.
     
  8. Apr 15, 2015 #7
    Thank you, I have a much better understanding now!
     
  9. Apr 17, 2015 #8

    jim hardy

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    Boring Anecdote ∞-1...

    When our power plants were small ,,,
    "priming" the condenser waterbox doused the turbine deck with salt water, as the steam jet priming ejector was located only twenty feet or so above the seawater intake .
    When starting the first "huge" 160 mw plant , operators were flustered: " Cant get the waterbox primers to blow saltwater, but vacuum is fine.. ? ?"
    Resident engineer shouted "Eureka! Let's go back and move all our priming jets up above 32 feet !"
     
  10. Apr 18, 2015 #9
    Jim,
    Good story!
    I believe it was Galileo and Torricelli who figured out that due to atmospheric pressure, a pump can only lift water a max of 34 ft .
    That I fully understand. It's actually the atmospheric pressure pushing the water up the well to the pump and not the pump "sucking" it up.
    Thanks!
     
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