Incestious PID loop

Ok here's the situation:

Common header splits into 3 water lines, each branch has a control valve, and a flow meter.

Now the flow coming into the common header is unknown, but we want to make sure that the flow through each line is equal. My first thought was 3 PID loops, CV = valve, PV = flow and SP =(all three flows )/3. The more i think of it, the more I'm scared of the SP being based around on the PV. I'm thinking that this is going to go out of control and all three valves are going to open 100% or shut completely.

Any thoughts?

You want the overall flow setpoint to be the water required for the process the pipes are feeding. After you have established that then divide by 3 and send to the three loops for each pipe.

You've got tremendous potential for instability in this system - the phrase "incestuous" describes the problem perfectly! Every adjustment of one valve will screw up all three flows.

I've made a lot of money over the years solving exactly this problem, so I can't give you the full solution. However, I can get you going in the right direction.

First off, the source of the water flow will make a big difference to the solution. If you are drawing from a stand pipe or water tower your problem is simplified because it is essentially a constant pressure source and there isn't a limit on upper and lower flow rates. If your source is a centrifugal pump the total flow is going to vary as the valves throttle and change the backpressure. If your source is a positive dislplacement pump total flow will be constant but the system pressure will swing as the valves throttle. If you identify which you are using I can give a little more guidance.

Your tuning must be very non-agressive to minimize hunting problems.

You need to clamp max and min valve positions to avoid having all of the valves throttling down and creating a lot of unecessary backpressure.

The response time of the system will have a significant impact. Too slow and you lose control, too fast and you get more hunting.

Rather than take total over three as the setpoint consider making one flow, perhaps the one with the most open valve, the setpoint and have the other two match it.