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How do I calculate a pumps psi output?

  1. Mar 9, 2012 #1
    I have a pump that puts out 396 gal/hr of water. The output pipe on the pump is 9/16 inch in diameter. Is there a way to calculate the psi?
  2. jcsd
  3. Mar 9, 2012 #2


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    Staff: Mentor

    It can be done but it isn't easy: you have to add up the pressure losses of all of the components of the piping system
  4. Mar 9, 2012 #3
    Ok, I thought maybe there was a simple formula that could be applied. Thank You anyway.

    On second thought, the piping is the same size as the outlet and it is only about 6 feet long, but the pump will be pumping the water up. It's for an aquarium.
    Last edited: Mar 9, 2012
  5. Mar 9, 2012 #4


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    Pumps have a charcteristic that they will produce a certain flowrate at a certain head. I assume you mean 396 gal/hr at zero head. As the head is increased by adding more pipe to the pump exit, the flowrate decreases, up to a point where there will be a maximum pressure at the pump but no flow. By plotting several points on a graph of flow versus head, one obtains a pump characteristic curve.

    The system curveon the other hand of all the losses in the pipe is more difficult to obtain as Russ mentioned.

    The intersection of the system curve with the pump characteristic curve will give you the point at which the pump is operating.

    The 6 foot pipe section upwards will give only the elevation head loss for the system curve.
  6. Mar 9, 2012 #5
    The 396 gall/hr is the flow rate with the 6 feet of head according to the chart that you are mentioning. So I guess it is still not possible to calculate the psi?
  7. Mar 10, 2012 #6
    We can roughly approximate the head if we know the pump input power and the efficiency.

    The efficiency of pumps are usually between 60% and 70% and of the driver ( motor) is above 90%.

    The output power of the the pump is the flow rate times head.

    If the driver is running with for example 50 W, for the flow rate of 396 gal/hr ( about 0.0004 m3/sec), we will get

    [itex]\Delta P=\frac{50\times 0.65 \times 0.9}{0.0004}=0.7 KPa[/itex]

    which is about 7 meters of water .
  8. Mar 10, 2012 #7
    These specs may help in your calculations. The pump is a 3.5 horsepower.

    The watts are 68, voltage =120, amps=.7
    The Q max GPH = 687 gph
    The Q min GPH = 449 gph
    Head max =12.5 feet
  9. Mar 10, 2012 #8
    The pump horsepower seems too much! According to your number, your pump can do more work as 3 horses can do. Such a pump needs a powerful driver not a 68 watt one.
  10. Mar 10, 2012 #9
    You mean it is not a 3.5 HP pump as the specifications say? According to your calculations, you believe it to be a 3 HP....Correct?
  11. Mar 10, 2012 #10
    I expected a much less hp. Something like 0.1 HP. Or perhaps its 3/5 hp rather than 3.5 hp.

    See this online calculator http://www.engineeringtoolbox.com/pumps-power-d_505.html
  12. Mar 10, 2012 #11
    In the calculator I entered q=687, and h=12.15 and shaft bph =3.52
  13. Mar 10, 2012 #12

    The calculator gets the flow in units of gallon/minute. Yours is per gallon/hour
  14. Mar 10, 2012 #13
  15. Mar 10, 2012 #14

    jim hardy

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    That's a picture of an aquarium pump not a lawn sprinkler pump.
    That much power would wreck an aquarium.
    HP must mean something else. Probably "High Performance".

    Figure the power it takes to lift the weight of 687 gallons of water twelve and a half feet in an hour.
    F X D / T .
    (8.3 X 687) X 12.5 / 3600 = 19.8 ft-lbs/sec = .036 hp by my back of envelope, ~26 watts.

    To OP's question about discharge pressure - it's given in your spec.
    Centrifugal Pump "head" is the discharge pressure given in units " height of fluid flowing."
    Convert 12.5 feet of water to PSI.
    Were the pump instead pumping mercury, its discharge would be 12.5 feet of mercury and input power would be much higher.
  16. Mar 10, 2012 #15

    jim hardy

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  17. Mar 10, 2012 #16
    I bought this pump in pet shop and they had one in use in an aquarium. I do want to use it in a large tank. I would still like to know the approximate PSI output curve if possible.

    The posted curve is not plotted correctly. The column with all the Syncras needs to be lifted up 2 increments and the H/m shifted to the right.
  18. Mar 10, 2012 #17
    The plot seems fine. The column of Syncras is there just tell you ,by color , that which curve is for which pump. It has nothing with the vertical axis.

    3.5 meters is not enough for your purpose? by the way, isn't pressure in unit of meters/ft of water easier to understand? I wonder why you keep asking the psi.
  19. Mar 10, 2012 #18
    I want to attach another device to the pump and it requires a certain psi to function properly.
  20. Mar 10, 2012 #19
    Thanks for the clarification.

    The chart gives you the pressure just at the discharge of the pump. If after the pump, your pipe goes upward, The pressure on the top is less than that. Add friction too, though friction can be reduced to a small value. If the pipe goes horizontally, The psi of the end of the pipe is something between 5.0 and 6.0 psi. If the pipe goes upward, reduce 1.5 psi per meter from the value of horizontal case.
  21. Mar 10, 2012 #20
    Thank you for resolving this problem. As you said the chart gives you the answer automatically if you know how to interpret it correctly. That's all I guess and have a good day.
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