Applying Process Safety Time Concepts in Process Plant Applications

In order to be considered effective, a Safety Instrumented Function (SIF) must be able to sense when a failure has occurred, and respond quickly enough to mitigate the hazard in question. Most frequently, this detection involves sensing that a measured process variable has exited the normal control range. The terms Process Safety Time and the related allowable Response Time are calculated and used to quantity the required quickness of the safety system’s response.

Due to the differences in the process variables measured, the technologies used to measure those variables, and the system dynamics associated with different types of unit operations, multiple approaches must be utilized for determination of the Process Safety Times and allowable Response Times for SIF’s, depending on the process variables and types of unit operations in question. This paper examines three types of SIF’s, and provides guidance for determining the Process Safety Time and the allowable Response Time for each.

The SIF types discussed include vessel liquid overfill protection, high pressure protection from abnormal heat input, and high temperature protection. These SIF’s provide the reader with examples using three different types of initiators (level, pressure, and temperature), one of which pose special challenges, as well as a variety of equipment types which results in different constraints on action completion times.