Single Controller for a Flow and Pressure Control Valve


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I need to make a Regulatory control point in TDC-3000 DCS system that behaves like a flow controller during normal. and when pressure of the line exceeds a specified pressure, it should control the pressure of the line. When pressure becomes normal, it should again behave as a flow controller (with some offset in the pressure setpoint). Any related information sharing would be valuable?
of course simulteous control is not possible,

but to talk about valves in a specific application such as this pressures, temperatures, flows, metering accuracy, and coordination with the safety systems is also needed, in otherwords the engineering details.

DCS conf. without these sort of considerations typically ends up costing your customer more than just poor performance.
I am assuming that the final output of the flow controller is a position command to a control valve. I am also assuming that in order to reduce pressure you have to reduce the controller output. If that is the case, then you can set this up with two controllers, one for flow and one for pressure, with the pressure limit as the setpoint of the pressure controller. The outputs of the two controllers would go to a minimum value gate (also called a low selector), which then provides the final output to the control valve positioner. You do need to set this up to make the higher output controller track down to the lower output controller to avoid reset windup. I have not worked with the TDC-3000 DCS, so I can't be more specific.
Sound a lot like a split range control that you need to do in this application. In a split range configuration you will only use one controller to control two valves. The easiest and most stable method is to use two identical valves installed in parallel but you may also install a small and a larger valve in parallel, depending on how drastic the change will be from normal to abnormal conditions. All you will be doing then is controlling the pressure in the line with one valve during normal operations and the second valve will kick in to help if the first valve reached its limit during a major upset or demand. You may also use the same to do from normal to very large flow control but to try and do a switch over from flow to pressure control is definitely not advisable. Doing one tuning set on your PID controller for a small/big valve setup is difficult enough. To try and find a tuning set on one controller to be suitable for flow as well as press control might be asking too much but I suspect that it might not be completely imposable to do in certain applications, since press and flow are both fast controls unlike temp and level which are slow control loops.