My Boss threw an idea at me which while sounds correct in theory, I'm not sure how accurate it is practically. We have 6 vertical pumps in parallel and there's a discharge gauge on each. We have no individual flowmeters, just a total flow of all 6 pumps from the header. My boss was suggesting to check the power required by the pump and use this value on the power curve to obtain a flow value and then plot this on the performance curve with the pressure value from the gauge. Is using the power to obtain a flow value accurate?

Probably not.  It depends on the power curve. We have used exactly the technique that you describe for some big cooling tower pumps where it is impossible to get individual flow measurements.  And, in general, it has worked to help us understand which pump is the weakest of the bunch. But, we don't use it to do detailed analysis of each pump condition. It is more a screening tool.  If the total flow drops below some limit, we use the motor amps to decide which pump to pull for overhaul.  But, there are a number of potential problems.  Some big vertical turbine pumps have a power curve with a hump in the middle. There could be two points with the same power.  Some of these pumps have a horsepower curve that increases to shut-off.  Some of them have a hump plus a rise to shut-off. There could be as many as three points with the same power.  So, in order to know where you are on the curve, you need to trend the motor power on an ongoing basis.  If you have a baseline reading on a freshly overhauled pump, and trend it monthly, you can distinguish where you probably are on the curve as the amps decrease and then increase again.  But, if you wait until you have a low total flow problem and then go out to take amps, you may not be able to tell.  Of course, if your power curve is continuously rising as flow increases, then it is easier.  If the power curve has a long flat section, it may be difficult to distinguish where you are at.  In any case, beware of inaccuracy.  The power curve is based on a pump in new condition, running over a range of flows.  You are looking at a pump that may be in degraded condition with extensive internal recirculation through open clearances. You cannot get an accurate measurement of pump performance.  You can only get a relative measurement of which pumps are probably stronger and which ones are probably weaker.   

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