Process safety time
Process safety time (PST) is the sum of early response time, operator reaction time, and SIF (Safety Instrumented Function) response time1.
Early response time
The time after a failure has occurred where the process variable is changing and no alarm has been triggered.
Operator reaction time
The time between alarm and SIF activation in which the operator could take manual corrective action.
SIF response time
SIF response time is the time between the SIF setpoint being reached, and the hazardous condition occurring. It should consider sensor response time, and time to fully operate final elements such as shutdown valves.
Info
A common rule-of-thumb for shutdown valves is to assume 1 second per inch of nominal bore for preliminary assessment.
Footnotes
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Is defined in BS EN 61511-1. ↩