Pump flow from kW?
Pump flow from kW?
(OP)
I have a bunch of kW data (hour by hour)used by some pumps that are used for pumping water from one area to another. They are rather high volume pumps...around 3000 gpm each. What I want to do is take all the kW data and use that to estimate a flow rate for each pump during each hour to get a total of how much water has flowed. I have never done one of these types of calculations. I have some pump curves. Would the following steps be a correct way to go about it?
1. Determine total head, which is 236 ft.
a. I have estimated the friction head to be around 2 ft using a friction head calculator on the internet (10,000 ft of 30" ID steel pipe)
b. Water level is raised by 234 ft.
2. Using the bowl head vs. flow curve, find the flow rate.
I don't know where to go from here though and it seems as though I am missing something since from what I have assumed, the flow rate does not depend on power input. I presume I will have to multiply the kW number I have by some efficiency, but with the efficiency vs. flow curve, I need a flow to find efficiency.
I also have a curve for BHP vs flow. Can someone point me in the right direction?
1. Determine total head, which is 236 ft.
a. I have estimated the friction head to be around 2 ft using a friction head calculator on the internet (10,000 ft of 30" ID steel pipe)
b. Water level is raised by 234 ft.
2. Using the bowl head vs. flow curve, find the flow rate.
I don't know where to go from here though and it seems as though I am missing something since from what I have assumed, the flow rate does not depend on power input. I presume I will have to multiply the kW number I have by some efficiency, but with the efficiency vs. flow curve, I need a flow to find efficiency.
I also have a curve for BHP vs flow. Can someone point me in the right direction?





RE: Pump flow from kW?
bhp = gpm x head (in feet) x SG / [3960 x efficiency]
where
bhp - brake horsepower
gpm - flow rate in gallons per minute
head - total pump differential head in feet of liquid
SG - specific gravity of pumpage
efficiency - pump efficiency.
since you have the pump curves, you should be able to readily determine flow knowing rpm, impeller diameter, and pump total head.
good luck!
-pmover
RE: Pump flow from kW?
RE: Pump flow from kW?
RE: Pump flow from kW?
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RE: Pump flow from kW?
To know total head you need to know the total flow in the system, as friction head is proportional to square of the velocity, so I'm not sure how you figured 2 ft friction head.
You have BHP vs flow for each pump. The BHP is pump BHP, so you need to convert the kW readings to HP for each pump(divide by .746), multiply that by motor efficiency, then read what flow that gives from the BHP/flow curve.
For the overall system you can develop a System Curve taking account of the static suction and discharge heads, and the friction head as it increases with flow. Against this can be plotted the combined curve for all the pumps operating in parallel to give the operating point, and hence an indication of the total flow in the system. Use this to back check whether you are in the right ball park with the individual pump flow estimates.
Things can be a little bit out if the pumps are not behaving exactly as their curve predicts, but you may get a reasonable idea of flows.
Cheers,
John
RE: Pump flow from kW?
Firstly, you would be a certified test curve for each pump unit and assume that the pumps are still in exactly in the same condition as when tested.
Secondly, you will need to accurately measure total head.
Thirdly, you will need to measure the pump speed at the operating duty.
Fourthly, you will need motor test data to assentain the motor efficiency and speed at which it was operating on test and which needs to be speed corrected to the actual duty operating speed.
If you had this information, ie test curve, total head, and motor efficiency and speed, you could then extrapolate this information onto the "known" pump performance curve to give you a fairly accurate flow rate.
However, it is much easier if you measure the flow rate directly which also gives you an accurate answer.
You can measure the discharge trajectory at the discharge point which can give a reasonably accurate flow rate.
Or get an "expert" to look at the discharge point - "yeah -that looks like about 3000 gpm" -- about as accurate as guessing without all the necessary information.
Naresuan University
Phitsanulok
Thailand
RE: Pump flow from kW?
Of course, if the pumps are old then efficiencies may have deteriorated and the published curves may no longer tell the truth, but it all depends on how accurate an estimate one requires. I'll stop rambling now.
Cheers,
John
RE: Pump flow from kW?
Also, the accuracy you need dictates the method to use.
Naresuan University
Phitsanulok
Thailand
RE: Pump flow from kW?
Without knowing all the ins and outs of the above problem it is difficult to provide precise advice. I agree that the accuracy required will dictate the method to use, and conversely, the method you use will dictate the accuracy of the result.
Cheers,
John