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Pump and nozzle output flow rate

  1. Dec 6, 2014 #1
    If I have a pump of 50psi and 2L/min flow rate and it is connected to a nozzle. The nozzle catalogue specs for example shows that :
    • 40psi = 0.5L/min
    • 50psi = 0.6L/min
    • 60psi = 0.7L/min
    Is flow rate at the nozzle output 0.6L/min? Regardless of the pump flow rate, as long as pressure are same? Please reply asap thanks
     
  2. jcsd
  3. Dec 6, 2014 #2

    Q_Goest

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    Yes, the nozzle produces 0.6 L/min when 50 psi is on the upstream side and discharge is to atmosphere, regardless of what flow rate is in some other upstream pipe.

    However, if you have a system with a pump as described flowing into this nozzle and there is no where else for flow to go, the nozzle pressure won't be 50 psi. If the pump is 2 L/min at 50 psi (I'm assuming you have a centrifugal pump), then pressure will rise to some point above 50 psi depending on the pump's flow curve and the nozzle pressure will increase until the pump flow rate and nozzle flow rate are equal. Note that generally there's a pressure switch on the discharge of a pump to shut it down at some pressure. There's also generally a relief valve there to prevent the system pressure from exceeding some value.
     
  4. Dec 6, 2014 #3
    Hi thanks for replying! Does upstream side means the part where the hose connecting the pump and nozzle?

    And I do not quite understand second paragrpah. Do you mean that the flow will stay constant but the pressure will be higher than 50psi? If 50psi is the pump max pressure/will be cut off when 50psi is reach. Wouldn't the pressure will always be less than 50psi as the liquid still flows through the nozzle unless it is blocked.
     
  5. Dec 6, 2014 #4

    Q_Goest

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    Hi. Upstream is relative to something. Upstream of the nozzle is the pipe that water is going through before it reaches the nozzle. Your pump is also upstream of your nozzle. Upstream of the pump is the pipe that water is going through before it reaches the pump.

    A typical pump system will have some means of control such as a pressure switch. The well water pump system in my house for example, has a pressurized accumulator that the pump charges up to some pressure. It takes a few minutes for the pump to charge the accumulator at which point the pressure reaches a high point and shuts the pump off. The water in our house can then be supplied from the accumulator until the pressure drops to a low point. When that happens, the pressure switch turns the pump back on. So your system probably has something similar. Do you know what your system has in it and how it's controlled?
     
  6. Dec 7, 2014 #5
    I'm doing this for design calculation. The pump will be a 12V diaphragm pump for liquid such as pesticides. The only feature I know is probably that it will cut off when there is no flow. From the market example 50psi and 2L/min, does this mean that the pump will be at 2L/min at 0psi and 0L/min at 50psi? (flow pressure performance chart) If according to the chart say 0.6L/min is at 20psi. Won't the pressure be unbalance or am I wrong in this case.
     
  7. Dec 7, 2014 #6

    Q_Goest

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    Diaphragm pumps are positive displacement machines so there won't be much of a change or drop in flow rate as pressure goes up. If you're designing this yourself, it won't work to simply put a diaphragm pump upstream of the nozzle you're proposing. The pump is too large and you haven't mentioned any way of turning the pump on and off nor any way of storing the liquid temprarily once it's up to pressure. If you could be more specific about what you're trying to do, it would be much easier to help you. Can you explain in detail what you are trying to do and provide a diagram showing how you envision this system working? Can you provide part numbers and links to the equipment you'd like to use?
     
  8. Dec 7, 2014 #7
    http://www.ebay.com.au/itm/25L-ELEC...pray-Pump-Tank-Knapsack-Battery-/221258527209
    http://www.ebay.com/itm/16L-RECHARG...ing_Agriculture_Equipment&hash=item3f29db83b9
    My design is like the product. The liquid flow to pump which is connect to a spray lance to the nozzle tip. Assuming no pressure loss at hose. Since I want to get the nozzle output flow rate.
    And as observed here
    http://www.knapsacksprayers.co.uk/pdfs/Knapsack_Nozzles.pdf
    That's the only info I can get.
    And here is an example of pump.
    http://www.ebay.com/itm/New-DC-12V-..._Accessories_Gear&hash=item259325703e&vxp=mtr
    Most of the information just provide the pressure and flow rate. And I am trying to find what is the flow output value when I connect them together. Some say the flow rate will be the nozzle flow rate as long as the pressure is meet. Which I don't quite get it
     
  9. Dec 7, 2014 #8

    Q_Goest

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    The nozzle and pump are completely independent devices. They don't adjust their flow rates just because the other part exists. They don't 'care' nor do they 'know' that the other part is there. The flow through the nozzle depends only on what pressure it has on the inlet. If there's 40 psi on the inlet, it will flow 0.5 L/min, no more, no less. It doesn't matter if there's a pump upstream or a reservoir of water. All the nozzle cares about is what the pressure is. If the pressure increases to 60 psi, the flow will increase to 0.7 L/min. Again, it doesn't matter if there's a pump upstream, or an enormous dam with an infinite amount of water. It will flow 0.7 L/min when 60 psi is at the inlet.

    The pump doesn't know what the nozzle is doing either. The pump doesn't care. If you turn it on, it produces some amount of flow. If this is a diaphragm machine, the flow rate won't change much when you change the pressure. So if you try to pump into a sealed volume, the pump will try to put 2 L/min into that volume regardless of what pressure it is. But if there's no place for the water to go, the pressure will rapidly rise. Almost instantly in fact. As pressure rises, the diaphragm in the pump has to push harder so the motor has to work harder. But the motor can only produce so much torque. As pressure goes up, the motor will stop because it won't be able to turn the crankshaft with so much force on the piston.

    That's what's going to happen when you try to use this pump to pump water or other liquid through this nozzle is that the nozzle will only produce some relatively small amount of flow compared to the flow of the pump and pressure will rapidly increase. When pressure gets to some point, the motor will stop and your electrical current will increase dramatically. It will probably blow a circuit breaker at that point or just burn the motor out.
     
  10. Dec 7, 2014 #9
    Yes, I just read about the diaphragm pump. You are correct. Can you help me understand how the equipment model work? Since most of the equipment uses diaphragm pump connected to the nozzle without any regulator. Now I cannot understand how they connect the pump to the nozzle without it breaking!

    Edit: The diaphragm pump has cut off feature if a certain pressure is reach. Is this why it can still continue to be use without breaking? Or it will be safe as long as there is some output. I read that it should not be operated on closed end but nothing after that. Maybe it can self adjust the power of the pump?
     
    Last edited: Dec 7, 2014
  11. Dec 7, 2014 #10

    Q_Goest

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    A cut off feature won't help in this case. Liquid is relatively incompressible so as soon as the pump shuts off (ie: when it meets the pressure required) the pressure will almost instantly drop again. With an incompressible liquid, the pump won't be able to control pressure by turning on and off since there's no 'buffer' to prevent pressure from rising or falling too quickly. The power to the motor can't be adjusted either, that actually requires changing the rotational speed of the motor - which can be done but isn't because of the cost to do so.

    For a pump and nozzle set up like this, the pump and nozzle have to be sized to complement each other. Using just any pump isn't going to work, you need the system to operate under a specific set of conditions known as the "operating point". You gave 3 different flows for your nozzle in the OP. Note that as inlet pressure is increased, the amount of flow through the nozzle also increases. This increase in flow with an increase in pressure continues as pressure is increased, so at 100 psi, you might get 0.85 L/min out of the nozzle and at 200 psi, you might get 1.2 L/min out of the nozzle. To get 2.0 L/min out of the nozzle, you might need 550 psi.

    Most engineers would draw a curve on a graph with flow on the X axis and pressure on the Y axis. The point at which that curve meets the flow of the pump is called the operating point as given for example, here:
    http://www.engineeringtoolbox.com/pump-system-curves-d_635.html

    Here's a graph that might be a bit more helpful:
    pump-curve-relative-to-hydronic-system-curve.jpg
    In this graph, there's the "System Curve" which starts at 0,0 and rises up to the operating point. There's another curve called the "Pump Curve" which starts out high on the Y axis and drops down to cross the System Curve. Where they cross is the point that the system will naturally operate at. That point has a flow and a pressure. So what you need is to find a pump that flows roughly what you want the nozzle to flow at the inlet pressure your nozzle needs to produce that flow.
     
  12. Dec 7, 2014 #11

    russ_watters

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    Why are you insiting on a diaphram pump here anyway? A properly sized centrifugal pump will work much better. They are much more forgiving when it comes to flow restrictions like this nozzle.
     
  13. Dec 7, 2014 #12

    Q_Goest

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    Hi Russ,
    I can imagine that to get the velocity from the nozzle required for the application, a certain amount of pressure will be required. In this case, 50 psi produces the flow needed but also produces a certain velocity for spraying. Centrifugal pumps can easily produce the pressure (50 psi) but producing it at 0.5 L/min or there about would be nearly impossible. There are 2 options, the impeller would either need to be exceedingly small in diameter and spinning at some horrific speed, or it would need to have a series of these small impellers in series to produce the pressure (ie: a multi-stage pump). I don't think this application lends itself well to a centrifugal.
     
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