Yokogawa YVP20S - Manual (Page 167)

17-6
<17. FAILSAFE HANDLING>
IM 21B04C50-01E
WARNING
Setting the T2 Parameter, to cause failsafe after a sustained position error, can cause
unwanted process shutdowns. The T2 setting should be used cautiously. Care must be
taken to avoid interaction with other configuration parameters, particularly High and Low
Position limits.
● Deviation Error, Air to close, High or low actuator pressure (Cases 2 and 4)
Table 17.2 Failsafe- Cases 2 and 4
T17.2eps
Case 2
Case 4
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 may move past setpoint and go to clear latch briefly.
XD Error: Deviation error appears in the Error status.
The Failsafe Status dialog appears and shows Active (Latched).
Failsafe type
Deviation Error
Actuator Action
Air to close
Fail Open
Cause of Failure
Valve fails open with low actuator pressure.
After the pressure problem is corrected, the valve remains open in failsafe.
XD Error: Deviation error appears in the Error status.
The Failsafe Status dialog appears and shows Active (Latched).
Failsafe type
Deviation Error
Actuator Action
Air to close
Fail Open
Cause of Failure
High actuator pressure
Observable
behavior
Supply pressure failure or low
actuator pressure
Observable
behavior
Figure 17.8 Failsafe Case 4 Steps 1 and 2