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JasonM163 (Mechanical) (OP)
1 Mar 13 20:21
I am trying to determine the characteristic curve of a small unknown pump. I have set this up with the large tank of water with piping connected to the pump. I have a pressure gauge directly before the inlet of the pump and also right after the discharge. After the second pressure gauge coming off of the discharge I have a globe valve and then a flow meter.

The globe valve obviously is to control the flow rate which is then read from the flow meter. So my flow points are easily attainable. What I am curious to is lets say I have the flow at (making up numbers) 10 GPM and pressure gauge 1 is reading 5 PSI, and pressure gauge two at the discharge is reading 12 PSI. Differential pressure of 7 PSI. How can I find the head of the pump at this flow rate based on the readings.

The suction and discharge are the same size diameter so velocity head should cancel itself out I believe.
Sorry, I am new to centrifugal pumps etc and not sure what all needs to be considered.

Thanks,
Jason
bimr (Civil/Environmental)
1 Mar 13 20:40

When a centrifugal pump is operated with a positive suction head, the resulting discharge pressure will be the sum of the suction pressure and the pressure normally developed by the pump when operating at zero suction pressure. So you should subtract the suction pressure from the discharge pressure to obtain 7 psi as the discharge head.

http://www.pacificliquid.com/pumpintro.pdf
rmw (Mechanical)
1 Mar 13 20:57
Wow, you seem to have all the tools, but ned to know how to use them. First, correct the gauge readings to the centerline of the pump suction and discharge, respectively, meaning that if the gauge is like a lolly pop standing on top of the pipe, you have to add head to your pressure reading to account for the difference in elevation of the gauge above the pump nozzle centerline.

Second, pump curves are usually always stated in feet of head and therefore the gauge reading is a function of the density of the fluid you are pumping. Density is a function of the temperature. So in your case it is water, and let's assume it is at or close to standard conditions. And for the following, let's assume that your gauges are mounted on or already corrected to the pump nozzle centerline.

For water, since you are using English units, one PSI = 2.31 ft. of head which means that the differential pressure of your pump at 10 gpm flow is 7 X 2.31 = 16.7 ft. Now you have a point to plot on your curve. You can also use the 2.31 value to correct your gauge readings. Say your gage is standing like a lollipop 2.30 ft above the centerline of the pump nozzle. That means that the static head of the water at the pump nozzle centerline is 2.31 or 1 PSI higher than it is at the gauge. Now you can correct your gauge readings.

Close an isolation valve on the discharge of the pump and read the deadhead pressure. (dont do this for very long) That will be the highest pressure point on your curve.

Now start varying the flow and recording the head for each flow and soon Excel will plot you a curve.

Piece of cake. Good luck.

rmw
JasonM163 (Mechanical) (OP)
1 Mar 13 21:16
Okay. This is exactly what I planned on doing and thought I had everything correct. Thanks for the help guys. It is much appreciated.

Always best to double check. If you have time to do it twice you had time to do it right the first time.

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