ATI
Q46H/64 Dissolved Ozone System
Part 8
– PID Controller Details
57
O&M Manual
Rev-J (11/18)
The easiest processes to control with closed-loop schemes are generally linear, and symmetrical,
in nature. For example, controlling level in tank where the opening of valve for a fixed period of
time corresponds linearly to the amount that flows into a tank. Chemical control process’ can be
more problematic when the nature of the setpoint value is non-linear relative to the input of
chemical added. For example, pH control of a process may appear linear only in a certain range
of operation, and become highly exponential at the extreme ranges of the measuring scale. In
addition, if a chemical process is not symmetrical, that means it responds differentially to the
addition and subtraction of chemical. It is important in these applications to study steady-state
impact as well as step-change impact to process changes. In other words, once the process has
apparently been tuned under normal operating conditions, the user should attempt to force a
dramatic change to the input to study how the output reacts. If this is difficult to do with the actual
process input (the recommended method), the user can place the control in manual at an
extreme control point such as 5% or 95%, and release it in manual. The recovery should not be
overly oscillatory. If so, the loop needs to be de-tuned to deal with that condition (reduce P
and/or I.)
