Hello all,

I would like some advice on a suitable control loop for the following scheme (attached sketch to better understand the system). I have three scenarios:

1) if the feed flowrate is less than 450 m3/h, FCV-002 is in closed position and all the flow is directed to the process.

2) if the feed flowrate is between 450 and 550 m3/h, FCV-002 is in closed position and all the flow is directed to the process (this extra 100 m3/h is acceptable as per the design of the downstream processes, subject to some quality parameters).

3) if the feed flowrate is more than 550 m3/h, FCV-002 and FCV-001 need to modulate to achieve 450 m3/h going to the process and the rest going through the bypass line. When we did the hydraulic analysis, FCV-002 will be in full open position, and FCV-001 needs to exert 0.5 bar pressure drop to achieve 450 m3/h going to process. However, we are assuming in real scenario both might need to modulate. Note: feed flowrate is maximum 600 m3/h.

I have two flowmeters, FE-001 will read for scenario 1 and 2 and initiate the opening of FCV-002 for scenario 3. While FE-002 will control FCV-001 to achieve 450 m3/h in scenario 3. If flowrate drops back to less than 550 m3/h, FE-001 will close FCV-002.

I am not very experienced in process control and any help would be greatly appreciate.

 

Can you control both valves off of FE-002 with two PID loops?

FCV-002 would be controlled with a direct acting PID at a setpoint of 450m3/h - so that as flow through FE-002 exceeds 450, the bypass valve will start opening.

FCV-001 would be an indirect acting PID, with a setpoint of 550m3/h. This valve would begin closing off as the flow exceeds 550.

 

Thank you MattyIce. I've been thinking about your suggestion but it seems like scenario 2 wont work. For example when the flow is 500 m3/h, FCV-002 will open partially and probably allow me than 50 m3/h to go through the pipeline. this will drop the flow below the 450 setpoint and signal FCV-002 to close again. Wont that cause an issue of hunting? Also I may have forgot to add an important detail, whenever the bypass line is operated, it needs to discharge > 100 m3/h to maintain a suitable velocity in the bypass line to prevent deposition of solids in the pipeline.