3 pumps and 1 flowmeter

Hello!

I have a project with 3 water pumps, controlled by ABB ACS500 drives, and only 1 flowmeter at the exit of the pipeline, that is commom to all. Normally I will work only with one pump, but I can have the situation where I need two pumps.

So, I think that best solution is:
Desired flow = 50 m3/h
Pump 1: PID closed loop for 25m3/h with the return analog value from flowmeter
Pump 2: Fixed frequency for 25m3/h

Is this the best solution?

I have read thies thread:
http://www.plctalk.net/qanda/showthread.php?t=40590&page=2

And is quite nice but just want to confirm if I am one the right track.

Regards,
Bruno

Hi Blima,

First of all, do you have no-return-valves on all of your pumps?

If not, you might be "forced" to run them at the same frequency.

Also look at your pump curves. It might be beneficial to treat your pump as one unit, and just run them in parallel.

If you have flat curves for your speeds, and no-return-valves, you are spot on .

IF these are centrifugal pumps and IF the discharge head is not absolutely constant then the flow rate vs. frequency relationship isn't constant. Furthermore, "identical" pumps never are, and discharge piping resistance isn't often identical.

I suggest you run the two pumps at approximately the same frequency. I also suggest that you monitor as a secondary parameter pump power draw from the VFD and trim the speed to maintain equal power - assuming the discharge pressure seen by each pump is nearly equal that will mean each pump is at nearly equal flow.