PID Pump control Help

Hi All,

I have been asked to modify our contact tank pumping station controls. Currently pump speeds are manually entered (as %) by operators. There are 4 pumps. 2 large ones and 2 small ones. The operators start/stop pumps or up/down the speeds to maintain a level in the Contact tank.

I understand how i would do this using PID if all 4 pumps were of the same size, but not sure how to go about it using different sized pumps. Im think Large group Duty/Assist and small group duty/assist. Take the output from a PID loop (looking at the level meter compared to a setpoint) and divide that between the ratio of pump sizes (ive not yet got together all the details of the pumps yet). But when to start the assist pump? If 1 big and small pump isnt enough to make up the level, do we stop the small pump and run the assist big pump?

I know the pumps arent efficient or really pump anything below 60%. The usual running is two big pumps at 80%, but when were doing high flows, a smaller pump is started aprox 80% with the large ones up to 90%.

Ive done projects with PID's with the same sized pumps, its all this large and small pumps.

All pumps are on VSD controlled by the PLC (manual entered speed) with feedback already into the PLC. As is the level instrument.

EDIT: Its a PLC5 control system.

Start by setting up Stages and determine which pumps will be running during each Stage. Something like -

Then create (2) PIDs, one used to control a small pump the other used to control a large pump. I would work on the principle that one pump is controlled by the PID while the other is running at full speed. So if I'm running 2 large pumps then one of the pumps would be full speed the other would be controlled by the PID.

Then determine when to switch Stages.
You would switch Stages by looking at the speed of the PID controlled pump. If pump is running at full speed and level is going down, jump up to the next Stage. If pump is running at min speed (whatever you determine that to be) then jump down to the next Stage. Note - You'll need to adjust the speed of the PID controlled pump when bringing a full speed pump on or off.

Alternate the PID control loops.
After the PID controlled pump as been off for a certain amount of time, then switch the PID to control the other pump.

Hi,
I am not sure how helpful that can be but I have to control feedwater pumps to fill steam boilers. For both steam boilers and pump I am using sequencer cascade control. Boilers and pumps can be different size. in sequencer I am putting devices in queue, so it can be from the biggest to the smallest or vice verse. Cascade is using single PID loop and CV output to determinate output to pumps or boilers. That mean PID will send e.g. 60% modulation / speed so depend on your pump size you will have different flow but pumps will work on 60% efficiency. In cascade I have rules when too start first lag and second lag and when to switch them off. Usually when my PID goes over 80% modulation I enable first lag and then when both dev work to the same speed when that is not enough and PID still sits above or 80% the I enable secondary lag. When PID goes down and stick below 50% I am staring switching pups off 2nd lag and then 1st lag etc.I hope that does make sense.