VFD lead lag water pumps

A couple of thoughts:

Running identical pumps at the same speed produces the same flow in an ideal world, but not necessarily in the real one. You should probably also monitor power or flow to provide early warning of problems like impeller wear or pipe blockage.

There is a minimum speed for centrifugal pumps, below which the pump won't produce enough head to move any water. That should be set at your VFD or controller.

I don't like continuing to run one pump at 100% after bringing on the lag. When you bring on the lag you will go from 100% capacity to 130% or so - a jump that may disturb the system. I prefer to ramp down the lead to 50% and bring on the lag at 50% for a bumpless transition. Then whether you ramp one or both is a matter of personal preference.

lesmar96 said:

Would it be better if they would both be running at 70%?

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It is definitely better to run two pumps at the same speed.

Commenting so that I can follow. I'm about to do something like this.

kittydog42 said:

It is definitely better to run two pumps at the same speed.

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I would appreciate an explanation on why this is true. So it is never a good idea to vary the speed independently?

lesmar96 said:

I would appreciate an explanation on why this is true. So it is never a good idea to vary the speed independently?

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It has to do with where the pumps operates on the pump curves. Google "running pumps in parallel" and you will see a lot of diagrams showing the pump curves showing head and flow.

ramping the pumps

Tom Jenkins said:

A couple of thoughts:

................
I don't like continuing to run one pump at 100% after bringing on the lag. When you bring on the lag you will go from 100% capacity to 130% or so - a jump that may disturb the system. I prefer to ramp down the lead to 50% and bring on the lag at 50% for a bumpless transition. Then whether you ramp one or both is a matter of personal preference.

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In my application both pumps run up to 50% then the lead pump cycles above 50% as necessary.


When I switch Lead pumps, if I am over 50% on the lead I am going to:
#1 - begin ramping the lead pump down toward 50%.
#2 - release the clamp on the lag pump (will be new lead) and let the PID control bring that pump up.


FOR the pump I am ramping back to 50%......
I want to temporarily switch that output to a separate PID with a 50% setpoint, and let that PID ramp that pump down. When it gets back to 50% I will "switch" the speed control back to the single PID controlling both pumps and this pump will now be the lag with a 50% clamp.....


so lets say the Outgoing lead pump is @ 100% and I want to ramp it back down to 50%.



the question is: I am using a SLC500. assuming this "standby PID" setpoint is 50%....can I ONS the CV and PV to 100% and then put the PID in auto and let it ramp the CV down to the 50% setpoint (tying the CV back to the PV)? I am trying this in RSLogixEmulate with only moderate success as far as how the PID is acting. (Might just be the lack of a controlled PV though).



I was wondering if any has done this kind of "handoff" with PIDs previously? that is to say poking initial values and then letting it go.