I’ve uploaded the pump curve from the manufacturer’s site for the particular model pump in question. From the nameplate on the pump, I have the following related information:
H = 137.5 ft
H_max = 193.2 ft
Q = 30.38 US gpm
3452 rpm
2 HP motor

I expected the pump curve and nameplate information would be very close, but from the pump curve I should expect closer to 150 ft of head for a flow of 30 gpm. Are there some common reasons for this mismatch, such as particular unit changes that would only be reflected in the documents issued with purchase?

Edit: the pump is used to keep a building water loop in constant recirculation - the water is at ambient temperature

You are using what we would call a “book” curve that is general for this model of pump. Pumps are often delivered with impellers cut down to a smaller diameter to meet the process requirements. Verify that the installed impeller diameter and running speed match the curve you are using.

@JJPellin
I’m not familiar with the tests to verify the running speed, do they depend on only measuring certain motor parameters while the pump is running? I can ask the site maintenance if they are familiar with this task.

Would it be reasonable to use the affinity laws and the standard impeller and speed info if I can measure the output volumetric flow rate of water? I have an old P&ID that says there is a flowmeter on the pump discharge, but if that isn’t there I could have someone take data on fill rates of the recirc tank.

The nameplate on the motor is accurate enough. No need to measure motor speed. Ideally, you would measure flow rate, suction pressure and discharge pressure. Yes. If your impeller diameter does not match the curve you have, adjust the curve using the affinity laws.

If I know those values you listed then I would be able to set up the affinity law relation for flowrate vs head such that the new head values would be a function of the measured head (diff. head the pump develops) and flow as a ratio (a constant) and the independent variable of flowrate. Is that correct?

I am not sure what you mean. You have never told us if the installed impeller is the same diameter as the one specified on the book curve you are using. If not, you can take the performance data (flow and head) and correct them to the curve diameter to plot them. Or, you can correct the pump curve to the actual installed diameter and then plot the measured performance data as measured. I have attached a copy of the spreadsheet I use to adjust pump curves for different speed or impeller diameter.

The vendor said they don’t offer trimmed impellers for this pump and that the diameter is 73 mm and there are 6 stages.

However, in further proof that it’s usually the obvious thing that makes things hardest, I come to know that the loop has a back pressure regulator set to maintain the loop at 30 psig which is only 4 psi higher than the mismatch between my pump and system curve intersection point. Since I’ve been ignoring minor losses till now, I think this revelation resolves my question.

Thanks for the spreadsheet @JJPellin, I will review it to see if it confirms my thoughts on how to do the math. I should have just written out my thoughts with formulas rather than trying to write it out with words.