Action of flow controllers, temperature controllers, level controllers and pressure controllers.
For level and pressure controllers where outlet flow is controlled, the controller OP will increase with increasing PV.
- For fail safe close valves (Air to open) like level controllers, the controller is direct acting
- For fail safe open valves (Air to close) like pressure controllers, the controller is reverse acting
For level and pressure controllers where inlet flow is controlled, the controller OP will decrease with increasing PV.
- For fail safe close valves (Air to open) like level controllers, the controller is reverse acting
- For fail safe open valves (Air to close) like pressure controllers, the controller is direct acting
For flow controllers, the controller OP will decrease with increasing PV (same as the case of level/pressure controller where inlet flow is controlled).
- For fail safe close valves (Air to open) like level controllers, the controller is reverse acting
- For fail safe open valves (Air to close) like pressure controllers, the controller is direct acting
For temperature controllers like temperature control using column reflux, the controller OP will increase with increasing PV.
- For fail safe close valves (Air to open), the controller is direct acting
- For fail safe open (most likely case) valves (Air to close), the controller is reverse acting
For temperature controllers like temperature control using steam in reboiler, the controller OP will decrease with increasing PV.
- For fail safe close (most likely case) valves (Air to open), the controller is reverse acting
- For fail safe open valves (Air to close), the controller is direct acting
Note: In modern DCS, the positioner can invert the signal making 0% for closed and 100% for open (no matter what the control valve is action is - Air to open or close). In these cases
- Controller OP increases with increasing PV - Direct action
- Ccontroller OP decreases with increasing PV - Reverse action