Yokogawa YVP20S - Manual (Page 176)

App-7
IM 21B04C50-01E
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Appendix C Setting a Failsafe Strategy
The user must design a strategy for safe handling of fault conditions that will be safe and
effective for each control valve. Several levels of failsafe action can be configured using
YVP and ValveNavi to support the safety strategy.
As levels of safety are increased, more actions are required to restart a system that has
gone into failsafe. The most secure level will cause the positioner to close the valve and
cause it to remain closed until an operator manually reopens the valve. In the least secure
level, the valve will close and will reopen as soon as the cause of the failsafe event has
cleared.
On non-critical processes, the highest level of security may cause excessive nuisance
shutdowns. In these cases, it may be preferable to allow the valve to restart after failsafe.
When a critical process that has the potential to release toxic substances or to result in an
accident, it may be necessary to force full shutdown with manual restart.
WARNING
Do not set Position Lower Limit equal to or larger than the Deviation Error Band value. It will
not be possible to clear the Failsafe condition.