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I have been tasked with designing a control strategy to control both the pressure and flow of a USP water loop. USP requirements demand a minimum flow rate in order to insure water quality. Use point requirements demand a minimum pressure.
Basic System Description:
A storage tank supplies the distribution loop with USP water. A VFD pump is used to pump water through the use points. A flow meter and pressure transmitter monitor the flow and pressure of the loop as the water returns to the storage tank. There is a back-pressure control valve at the end of the distribution loop just before returning into the storage tank.
If I implement pressure control using the pressure transmitter and control valve and flow control using the flow meter and the VFD, the control loops will constantly fight.
If I fix the control valve (eliminating it from the equation), I can control either pressure or flow using the VFD. But, what is the best way to implement this option?
If I use two separate PID control loops (one for pressure and one for flow) and select the greater of the two CV to run the VFD at, then one of the PIDs will not be calculating correctly (and, therefore, not control correctly) since it is not really controlling the PV.
If I use a single PID control loop and select between pressure and flow as the PV, pressure and flow have different tuning parameters, how do I tune the loop?
Has anyone implemented an effective control strategy on a water distribution system like this?
Basic System Description:
A storage tank supplies the distribution loop with USP water. A VFD pump is used to pump water through the use points. A flow meter and pressure transmitter monitor the flow and pressure of the loop as the water returns to the storage tank. There is a back-pressure control valve at the end of the distribution loop just before returning into the storage tank.
If I implement pressure control using the pressure transmitter and control valve and flow control using the flow meter and the VFD, the control loops will constantly fight.
If I fix the control valve (eliminating it from the equation), I can control either pressure or flow using the VFD. But, what is the best way to implement this option?
If I use two separate PID control loops (one for pressure and one for flow) and select the greater of the two CV to run the VFD at, then one of the PIDs will not be calculating correctly (and, therefore, not control correctly) since it is not really controlling the PV.
If I use a single PID control loop and select between pressure and flow as the PV, pressure and flow have different tuning parameters, how do I tune the loop?
Has anyone implemented an effective control strategy on a water distribution system like this?