Failure monitoring is a key part of any process system. The purpose of the Fail_Monitor composite in a phase is to alert the phase to the presence of a failure condition so that steps can be taken to put the process in a safe state if necessary. The failure monitor in a unit phase is active every time the module is scanned at its configured scan rate.
When a failure condition is detected, the FAIL_INDEX parameter is set, and the phase automatically goes to the holding state, from which it can go to the aborting, held, or stopping state according to the preconfigured state transition diagram. (Recall that FAIL_INDEX is one of the default phase logic parameters. A non-zero value indicates a failure.) The configured control logic determines how the phase proceeds. The operator may also intercede if necessary.