Technical Specifications

179
Exiting Failsafe
Failsafe Handling
CAUTION Setting the T2 Parameter to cause failsafe after a sustained
position error can cause unwanted process shutdown. Use
the T-2 cautiously. Avoid interaction with other configuration
parameters, particularly High and Low Position Limits.
Failsafe Examples Case Two and Four
Table 14 Failsafe Case Two
Case Two Failsafe Type Actuator
Action
Cause of Failure
Observable Behavior: Valve fails
closed with high actuator pressure.
After pressure problem is corrected the
valve is open in failsafe. As the
problem is corrected the valve can
move past setpoint and go to clear
latch briefly.
Deviation
Error
Air to Close
Fail Open
High actuator pressure
plugged nozzle?
XD Error: Deviation error appears in
the error status. The Failsafe Status
dialog box displays and shows Active,
Latched.
Table 15 Failsafe Case Four
Case Three Failsafe Type Actuator
Action
Cause of Failure
Observable Behavior: Valve fails to
open with low actuator pressure. After
pressure problem is corrected the
valve is open in failsafe.
Deviation
Error
Air to Close
Fail Open
Supply pressure failure or low
actuator pressure, plugged
nozzle?
XD Error: Deviation error appears in
the error status. The Failsafe Status
dialog box displays and shows Active,
Latched.