F1 ! F1 !
Help ! Help !
I am trying to tune a PID conroller for a pH control system.
the inlet stream comes at pH=1.5-2.5, and caustic is added to get a pH = 6.5 (ideally 7.0, but I will refrain to discuss about that right now). I have a Equal % Open Control valve, and the Tanks has a residence time of 15-20 mins (DEAD TIME). What I see is the following:
When PV = 3.5, CV tries very hard and opens 100%, and remains like that, as teh Residence Time is 15 mins (pH is measured at the TANK Exit). By the time the new pH information reaches and shuts off, caustic addition is too much- thus resulting in high pH around 8.5. too much offshoot.
What I have done:
I set intially manual mode to get the pH around 6.2, then turn on AUTo. It did nto do a good job. I changed the Gain to 0.01 from 0.5, turn off the Integral action (originally 20), and set K_D=0. I do not see any improvement. Any pointers how to TUNE the loop?