PLC Programmer Meets Pump

They are good ones.

The pump will have a different performance curve for every particular run speed. If the only curve available is the one for 100% speed then it will prove fruitless to use this one to establish the operating point at 50% speed. Anyway, one point on the 50% speed curve is already to hand!

Simply put, the current operating point of 12m3/h and the difference in pressures between the pump inlet and outlet is one point on the pump's 50% speed operating curve, were one to be to hand. Plotting the curve could only be done by varying the flowrate and observing the pump pressure change, though it might prove quicker to telephone the pump manufacturer and ask for a 50% speed performance curve instead.

Duty point? Operating point? Two terms for the same thing?

Thank you for your reply. As it happens I actually spoke to the pump supplier and they told me that they did not have a curve for the various speeds - perhaps they realized that I did not know what I was talking about so they decided to save themselves the bother in trying to explain.

James

The performance curves for less-than-100%-speed are usually only of casual interest, as there is something on the system that is modulating the speed to 50%, even if it is only a manual speed setpoint. If the device is some sort of instrument, be it a flow instrument, pressure instrument or whatever, then the actual performance of the pump is only of real interest if it is doing 100% and it can't keep up with whatever the instrument is saying and the system setpoint is asking for.

And if the pump has been correctly selected, then it will have sufficient performance to achieve the system demands. Modulating it to 50% speed merely saves energy.

If it is doing fine at 50% then there's nothing to worry about. Enjoy the energy savings!

Thank you Codey for you reply. From what I understand (noting once more that I am the PLC programmer and not the mechanical/ pump engineer) there is a spray ball on the return to the tank that is causing the back pressure (we wish to maintain an overpressure at the end of the loop).

I will bang in those numbers into XL and see what it throws out. We are also going to vary the pump speed (via the inverter) and note the flowrate and the discharge pressure in an attempt to draw the system curve. Unfortunately we don't have much information from the design phase of the project so what we are trying/ wishing to do is to see what the system looks like. We will be making some modifications to the loop (adding in additional take off points and perhaps extending the loop) and I think it would be good to get a base line to work from.

No doubt I will be back with some more daft questions.

Again thanks for your assistance.

James

OK, but as PWSlack said, each pump curve is at a fixed speed. Get a range of flow and head at that speed (by throttling the discharge) then repeat for a different speed if you want.

Cheers..........Codey

The technique will establish the system curve. It won't establish a pump curve, as the speed is varying. Just so long as everone is aware of that!

PWSlack and Codey,

Thank you very much for your assistance. I have now sketched in the various pump curves for the various pump speeds (40% upto 90%). I will hopefully get to "play" a bit and actually vary the pump speed and then plot the system curve. I am sure it will not be 100% accurrate but it will be close enough for someone like me.

Again thanks for your assistance.

James

It's easy enough to plot the system curve. You have readings for flow Q_m and corresponding ΔP across the pump H_m, which = system ΔP. That gives you one point on the curve.

Then system curve is H = H_m*(Q/Q_m)². In the more general case there is static head to consider, but for your system this is zero (provided the return to the tank is below water level). You can put this in your XL and add it to the pump curve graph.

Cheers..........Codey

PS to Ed Weldon - thanks for the GA

Good answers, codey.

Perhaps our Guest will disclose the manufacturer, whose sales and marketing managers obviously need to find new careers. This ignorance of basic centrifugal pump application engineering principles in the sales office and even out in the dealerships is totally inexcusable.

Ed Weldon