I have a startup issue in need of a solution, the problem involves 3 valves on a evaporation system. The three valves are a evaporator feed water level valve, evaporator feed water temperature valve and the evaporator pressure valve. The level and pressure PID's are fast loops the temperature is a slow loop all are tuned for steady state running of the system. The problem that is seen during startup is as follows the feed level drops, causing the level valve to open to maintain the running water level, because of the slow tuning of the temperature valve it can't get the temperature of the water up to setpoint fast enough this causes the pressure to drop drop rapidly and shut the system down.
The old control philosophy was to set the low limit of the temperature valve to a percent greater then zero for a configurable amount of time. This would ensure that the temperature valve would not close during startup of the system. At one point in the past both the time and CV percent were configurable by the operator. Over the years the CV limiting also became complex with the percent open having 3 different setpoints the one used dependent of the percent open of the feed level valve the greater open the level valve the greater held open the temperature valve. The percents open value varied from unit to unit and programmer to programmer, it was all pretty much a guessing game.
I have started to use the feed forward of the PID to assist the temperature and pressure valve. For the pressure valve I use the percent error of the level valve as the feed forward control value. For the feed temperature I use whichever percent error is greater of the level and pressure valves. Because of the nature of the percent error once the system is stable the value is near zero and has little effect on loop it is being used as the feed forward on.
Using the feed forward allowed me to eliminate the need to use the startup CV limiting on the temperature valve, this worked great on a unit with larger/perfectly sized steam valves. On a system recently with valves that were slightly undersized and the temperature valve during steady state was operating in the upper rages of it's CV we end up with startup issues again. This lead me to have to add a configurable startup CV that is sent to the feed temperature valve for 1 second upon the system starting up.
What I'm looking to do is get away from the guessing game of a startup CV. I do not want 2 different tuning parameters either since startup only last about 5 minutes.
I've looked at cascading ratio but in all of the applications I've seen it used in it is 2 different process values dependent on 1 valve. I have 3 different process variable and 3 different valves.
Thoughts and ideas would be greatly appreciated.
The old control philosophy was to set the low limit of the temperature valve to a percent greater then zero for a configurable amount of time. This would ensure that the temperature valve would not close during startup of the system. At one point in the past both the time and CV percent were configurable by the operator. Over the years the CV limiting also became complex with the percent open having 3 different setpoints the one used dependent of the percent open of the feed level valve the greater open the level valve the greater held open the temperature valve. The percents open value varied from unit to unit and programmer to programmer, it was all pretty much a guessing game.
I have started to use the feed forward of the PID to assist the temperature and pressure valve. For the pressure valve I use the percent error of the level valve as the feed forward control value. For the feed temperature I use whichever percent error is greater of the level and pressure valves. Because of the nature of the percent error once the system is stable the value is near zero and has little effect on loop it is being used as the feed forward on.
Using the feed forward allowed me to eliminate the need to use the startup CV limiting on the temperature valve, this worked great on a unit with larger/perfectly sized steam valves. On a system recently with valves that were slightly undersized and the temperature valve during steady state was operating in the upper rages of it's CV we end up with startup issues again. This lead me to have to add a configurable startup CV that is sent to the feed temperature valve for 1 second upon the system starting up.
What I'm looking to do is get away from the guessing game of a startup CV. I do not want 2 different tuning parameters either since startup only last about 5 minutes.
I've looked at cascading ratio but in all of the applications I've seen it used in it is 2 different process values dependent on 1 valve. I have 3 different process variable and 3 different valves.
Thoughts and ideas would be greatly appreciated.