Hello,
I have a program that monitors pressure in a manifold discharge pipe. Pressure in the pipe should be less then 7.5 PSI. If the pressure is greater then 7.5 psi for 10 seconds, a 5% increase is added to the CV low limit for the air header valves. This effectively increases my CV by 5%. The problem is that this logic has a one shot, and 5 % may not always be enough to get that discharge pressure below 7.5. At less then 7 PSI for 10 seconds the original CV low limit value is placed back into the PID parameter. The pressure transmitter is not the PV for my PID that controls the air header valves. But it's monitored for safety.
A request has been made to continue to add 5 % increments until the pressure drops below 7.5 PSI. Because after this initial increase if it does not drop below 7.5 PSI it will not increase 5% again.
My first thought is to reset the timer that triggers the ONS and ADD statements.
Second, if the Pressure transmitter fails high I do not want to keep triggering the add statement that adds 5 % to the CV low limit. I was thinking of allowing the CV low limit a max of 95% in a GRT MOV clamp as a safety because I do not know what what happen to the PID if the low limit parameter exceeded the high limit that is set at 100% in some kind of weird situation . This is very a rare occasion when the system pressure goes above 7.5 usually if a DO probe fails..
Can anyone give me clarity on the output low limit? Would this be considered railing?
If the pressure increased to the point where my low limit clamped at 95% the valves are in auto so my air handler valves would be opened at 100 % anyway.
Thanks.
I have a program that monitors pressure in a manifold discharge pipe. Pressure in the pipe should be less then 7.5 PSI. If the pressure is greater then 7.5 psi for 10 seconds, a 5% increase is added to the CV low limit for the air header valves. This effectively increases my CV by 5%. The problem is that this logic has a one shot, and 5 % may not always be enough to get that discharge pressure below 7.5. At less then 7 PSI for 10 seconds the original CV low limit value is placed back into the PID parameter. The pressure transmitter is not the PV for my PID that controls the air header valves. But it's monitored for safety.
A request has been made to continue to add 5 % increments until the pressure drops below 7.5 PSI. Because after this initial increase if it does not drop below 7.5 PSI it will not increase 5% again.
My first thought is to reset the timer that triggers the ONS and ADD statements.
Second, if the Pressure transmitter fails high I do not want to keep triggering the add statement that adds 5 % to the CV low limit. I was thinking of allowing the CV low limit a max of 95% in a GRT MOV clamp as a safety because I do not know what what happen to the PID if the low limit parameter exceeded the high limit that is set at 100% in some kind of weird situation . This is very a rare occasion when the system pressure goes above 7.5 usually if a DO probe fails..
Can anyone give me clarity on the output low limit? Would this be considered railing?
If the pressure increased to the point where my low limit clamped at 95% the valves are in auto so my air handler valves would be opened at 100 % anyway.
Thanks.
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