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Sizing a pump to nozzles

  1. Nov 10, 2009 #1
    This may seem like a stupid question, but what is the relationship to flow and pressure in a pump to flow and pressure in nozzles. Specifically i have 5 nozzles that i am pumping into rated at 3000 Psi @ 0.84 GPM, what do i size my pump to? Do i need a pump rated for 3000 Psi @ 4.2 GPM or do i need a pump rated for 15000 Psi @ 4.2 GPM?

    I was under the impression that as long as my flow is the same my pressure should be the same, so if i have 4.2 GPM into the 5 nozzles at 3000psi and .84 GPM out of each nozzle totaling 4.2 GPM then my pressure shouldn't change and i should have 3000psi at each nozzle. Is this correct?

    Thanks in advance.
     
  2. jcsd
  3. Nov 10, 2009 #2

    stewartcs

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    Depends on the nozzle. There will be a pressure drop across the nozzle. The OEM of the nozzle should provide you with the discharge coefficient.

    Use Bernoulli's Equation and account for your system losses to determine the head required by your pump (i.e. the head added).

    There are various ways to do this depending on what information you know.

    Note: Just because your flow rate is constant doesn't mean the pressure is constant. In fact the pressure normally decreases as it flows along the pipe due to frictional losses.

    CS
     
  4. Nov 10, 2009 #3

    russ_watters

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    Welcome to PF.
    You need a pump rated at 3000 Psi @ 4.2 GPM. When the loads are in parallel to each other, the flow rates are additive and the pressures are equal.
    I'm not sure the explanation is quite right, but the answer is. Just to be sure, consider if you had one nozzle rated for 3 gpm at 3,000 psi and another rated for 1 gpm at 3,000 psi. What performance would you look for in your pump?
     
  5. Nov 10, 2009 #4

    russ_watters

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    Based on the wording of the problem, I assume 3,000 psi is the pressure drop across the nozzle.
    That doesn't have anything to do with the question. The question is just about pump sizing. When sizing a pump, you take the required flow rate and add up all the pressure losses in the system. The system must have already been designed for the appropriate pressure drops.

    That said, the OP will want to ensure that if his pump generates 3000 psi and his nozzles need 3000 psi that he doesn't have much in the way of pipe and fitting loss.
     
  6. Nov 11, 2009 #5

    stewartcs

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    Apparently not if he is asking how to size the pump...the system losses should include all losses including the losses due to the nozzles. So if the system was already design for the appropriate pressure drops including the nozzels then he should already know the required pump size.

    I would not assume the system was already design for appropriate pressure drops.

    Which was my point to begin with.

    CS
     
  7. Nov 11, 2009 #6
    The pump is pumping into a machined block to distribute the flow to the 5 spray nozzels, and all my calculations for the fittings show the pressure loss for them to be irrelevant. The block is only a few feet from the pump with negligible hieght difference. I am trying to get a 3000psi spray out of each spray nozzel. Since the nozzle is the end of the system spraying to atmosphereic, does that mean its a 3000psi drop across 1 nozzle, so there is no way i can get 3000 psi spray out of all 5 at once with a 3000 psi pump?
     
  8. Nov 11, 2009 #7

    The pump i am looking at using is actually rated for 3700 psi @ 7gpm.
     
  9. Nov 11, 2009 #8

    stewartcs

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    Since you have determined that there are negligible losses, the nozzles discharge to atmosphere and are in a block, essentially the 3000 psi will be dropped across each nozzle (since each nozzle has 3000 psig at its input and 0 psig at the outlet).

    So like you originally thought, you'll need around 3000 psi at 4.2 GPM.

    CS
     
  10. Nov 11, 2009 #9

    FredGarvin

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    Yes. The 3000 psi is the drop across the nozzles. As long as your pump can provide 5X the flow required for each nozzle at that pressure you will be OK. You should be able to get the flow numbers from the nozzle supplier.

    Perhaps to make it a bit easier you could run your pump at the higher pressure of 3500 psi or so and put a back pressure regulator at another port on the manifold block. That way you would be ensured to maintain 3000 psi at the manifold block.
     
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