Uncertainties in Equipment Reliability Data and Its Impact on Safety Integrity System Calculations – a Case Study of Process Shutdown Valves

Risk assessment to predict plant equipment reliability is reliant on external factors like operational environment and maintenance. Major hazard prevention often dependent on reliable equipment and its availability upon demand. Major accident events like Buncefield, Fukushima, Deep Water Horizons have in one way or other depended on equipment performance upon demand.

Process shutdown valves have high reliability expectations due to the inherent nature of protection they offer. A case study of process shutdown valve is presented where in a site data for failure rate is analysed, which is then compared with failure rates provided by vendor, Offshore Reliability Database (OREDA) and PDS Handbook ( Norwegian acronym for “reliability of computer based safety systems” ) . Analysis of dangerous undetected failures and failure on demand has indicated variation in the risk levels. The variations in the risk levels upon further analysis have indicated that the vendor supplied data is often not conservative.

The failure rates when compared in this case study are in relatively close proximity and this could be attributed to pro-active reporting culture and stringent regulatory requirement for reporting equipment failures. On the contrary, there have been other sources of reporting regimes where the site data has varied up to two orders of magnitude. In either cases, there could be similarities found and is detailed in the paper.

The intent of this case study is to demonstrate through Safety Instrumented Systems (SIS) calculations the impact of failure rate on predicted risk levels, likely contributing factors for varied risk results and the fit for purpose equipment failure rate data usage in undertaking risk predictions. This paper establishes, through the case study of process shutdown valve, a cascade effect of using improper failure rate data on maintenance strategies and equipment performance standard.