Reverse and Direct acting controllers
Reverse and direct acting actions for a controller is really confusing. Most of the definitions we hear from chemical engineers are wrong. It is not as simple as.
- If an increase in the process variable (increase in devation from setpoint) results in an increase in output (Electric signal or Instrument air flow) , the controller is direct acting .
- If an increase in the process variable results in a decrease in output, the controller is reverse acting .
We need to take into consideration the controller action, i.e. whether it is fail close or fail open. The above definition will hold good for fail close (air to open). It is the opposite for fail safe open (air to close) controllers.
Direct acting means increase in process variable will increase in controller output and visa versa. And opening and closing of valve will depend on valve type AFC (Air fail - CLOSE) or AFO. (AFO valve normally use for cooling water, antisurge valve etc.)
So we can conclude,
- If the PV is increased and the air to controller is also increased, then the controller is direct acting.
- If the PV is increased and the air to controller is decreased, then the controller is reverse acting.