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Pump RPM question

Pump RPM question

Pump RPM question

Hi everyone,

I am new to the industry and have a quick question. I have 3 sets of pump data.
RPM Discharge Pressure(PSI) Flow Rate(GPM)
1570 370 300
1280 250 250
1020 150 200

Background information: Pumping water at 25 DEGC and with the discharge pressure listed above and corresponding flow rate through a tube bundles, the outlet pressure is atmospheric pressure.

I have built a hydraulic model, but my question is with the same system (same number of elbows, bends, pipe lengths, etc.), why the last two sets of data will give an outlet pressure less than atmospheric pressure?

Thank you,


RE: Pump RPM question

Hold on, You said that all discharge to atmosphere.
SO your system has 370psi delta P at 300gpm, and so on.
You can easily model the system.
What about the pump, where do these points fall on the pump curve?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Pump RPM question

Hi Edstainless,

Thanks you for your response.

I have modeled the system, unfortunately, with the above data, the last two sets of data does not give an outlet atmospheric pressure.

I do not have pump curve because it is not our company's pump. We used it for pigging.

Thank you,


RE: Pump RPM question

Because the pump data is just a single point in the curve. If your pressure drop is more than the pump can supply then you're trying to put too much flow. You will get a lower flow rate that matches pump discharge pressure with hydraulic losses.

You can find a pump curve from the vendors website

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump RPM question

Sounds like your model in constrained somehow or not characterized correctly such that the exit pressure of the final discharge pipe/fitting is not defined as atmospheric pressure no matter the flow.

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Pump RPM question

So these are not real data points?
I don't get the question.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Pump RPM question

If the data that you presented is correct, then the ensuing hydraulic model that you wrote is faulty.

RE: Pump RPM question

It seems that the points data are correlated with fan laws (within a relative error <5% roughly which may be because we are looking at real OEM map and data points). So I think the answer to OP is as follows:

- Plot OEM curves at each speed P vs flow
- Plot the system curve of quadratic shape by taking several sample points on the Q axis and calculate the corresponding pressure
- Plot the three set of pumps data. The first set (at highest rpm) should intersect with your system curve, the two others not.

It can mean that in reality for the lowest rpm's, the pump will operate at a different set of flow and discharge pressure than the one stated to satisfy your final outlet of atmospheric pressure.

It can otherwise mean that the set of data points are derived based on real system curve, and your theoretical model is deviating from the real losses behavior through your tube bundle.

We need to know better the purpose of your exercise, it is a hardware modification of the downstream pump discharge system, or an attempt to test a theoretical model that models system losses based on real/measured set of data?

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