Flow Balancing Valves - Control Theory

I'm thinking a periodic addition to all valves, or a cascaded loop that will try to increase "average valve position" etc

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Yes, close. You don't really care about the valve position; you care about the flow.

I think a cascaded PI solution is needed. The outer loop would control the total flow using he VFDs. The outer loops total flow should be divided by 8 to get the average set point for each of the flows through the valve. The valves are butterfly valve so they won't be linear.

Note that none of the valves are fully opened, but flow is perfectly balanced. My question was about creating a bias, so that at-least one valve was always at maximum position. (aiming to minimize unnecessary resistance to flow)

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Yes, you do this by reducing the total flow set point when the any valve goes over 100%. I would find the max of all the valve openings and use that like yet another PV for another loop that increase the flow if the max valve opening is below 98% and decrease the total flow if the max valve opening goes above 98%. If Maybe 95% would be better. It is a number to play around with. I would use a SLOW integrator like function to reduce the set point as the peak flow goes over about 98%.


So that is 10 PI controllers in the PLC. 8 for the valve positions to control flow. 1 for the total flow to control the VFDs and 1 for the keeping the max valve position at about 98% or whatever works. I am assuming the valves have their own internal controls so if you give the valve a signal of 50% the valve opens 50%. In servo hydraulics the valves have their own built-in controllers. You could say there ae 18 closed loops.

My company is used this kind of control structure before but to control press forces instead of flows, but the general idea is the same.

How much can the resistance of the filters change from the state of the new/clean to the fact that the filter will be replaced?

The total flow can change 20 times (10 - 200 l/s), i.e. the gain on the valve can change by 20 times. How much can the valve gain change, taking into account the resistance changing of the filters?

The objective is to always have one valve at the max position to minimize friction losses through the valve. We do this all the time with air in aeration systems.