Evaluating Risk and Safety Integrity Levels for Pressure Relief Valves Through Probabilistic Modeling

The probability of failure on demand (PFD) for spring-operated pressure relief valves (SORVs) is estimated by applying the Fréchet and Weibull probability distributions using proof test data from the United States Department of Energy's Savannah River Site (SRS) in Aiken, South Carolina. The data can be accessed through the Center for Chemical Process Safety (CCPS) Process Equipment Reliability Database (PERD). The probability distributions enable the evaluation of risk, estimation of ANSI/ISA-84.00.01 Safety Integrity Levels (SILs), and the impact of potential modifications of the maintenance plan. Current SRS practices are reviewed, and recommendations are made for risk-based adjustments to the maintenance plan. Subsets of valves are identified in which maintenance times can be extended and in which increased safety margins may be needed.