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Safety Instrumented Systems (SIS) have been a cornerstone of safeguarding the Process Industry for decades. These systems are intended to protect against pressure, temperature or other excursions outside of process design parameters. During design, the Safety Integrity Level (“SIL rating”) requirement of the Safety Instrumented Systems is determined by Safety Objective Analysis (SOA); however confirmation of the protection system response time is sometimes lacking. This evaluation ensures that the process has the hardware and software capability to reach a safe state before the failure condition of the process is reached. Whilst the Process Industry has extensive experience, published standards and calculation methods for sizing Pressure Relief Valves, limited documentation exists for calculating the Process Safety Time (PST) for a system that is protected using a Safety Instrumented System.
Process Safety Time calculations require detailed knowledge of the Process System, skilful selection and use of appropriate software and careful application of simplifying assumptions. These requirements can make determination of Process Safety Time challenging, particularly early in design, when understanding the required SIS response time would be most advantageous.
An introduction to Process Safety Time key concepts is presented in this paper, together with discussion of calculation approaches, common problems and potential mitigations of PST-related issues. The paper highlights why Process Safety Time should matter to International Operating Companies (IOCs) and how Process Safety Time considerations can be incorporated early in design to ensure smooth project execution, and is further illustrated with examples from real project experience.