I am looking for some suggestions on this control scheme.
I have a multiple burner process heater with an outlet temperature controller (Master) that cascades to a fuel gas flow controller (Slave). The fuel gas being burner is a refinery gas which has a variable BTU content. The heater is equipped with an Safety Instrumented System. One of the many shutdowns is a high fuel gas pressure trip at the burners. The temperature controller does not care what the fuel gas pressure is. So if a temperature set point is increased aggressively, charge to the heater is increased aggressively, a burner is taken offline, or multiple other scenarios can cause a high fuel gas pressure trip.
I have configured a high fuel gas pressure override. It is a standard PID controller who's output is sent to the fuel gas flow control module. In the fuel gas flow control module the flow controller PID output is sent to a Control Selector. The Control Selector has these two inputs. One being the flow controller output and the second being the high fuel gas pressure override output. The Control Selector is configure to take the low signal.
I have some calculations on the high fuel gas pressure override on the OUT_HI_LIM so that it will not wind up during normal operation trying to get quicker response.
I am having problems trying to tune it to be robust. If I tune the high fuel gas pressure override for a fast response the loop can oscillate and cause a trip. If I tune it slower the controller does not catch the initial bump and the heater will shutdown on high fuel gas pressure. These occurrences do not happen very often, so it is tough to experiment with different tuning sets. I currently have the high fuel gas pressure override gain set ~0.85 and reset set at 20 seconds.
Any thoughts or suggestions would be welcome.