Impact of Testing Frequency on the Reliability of SIS for Flare Load Mitigation

Many plants use safety instrumented systems (SIS) to reduce their flare load. This has been done for a variety of reasons including meeting emissions permit limitations, reducing flare equipment investment cost, and accommodating capacity expansion. To ensure that these systems are capable of limiting the flare load during all feasible scenarios, they must be designed to, and maintained at, the appropriate safety integrity level (SIL). A fault tolerance is built into the system design based upon an assumed reliability for each of the components. Inherent in this assumed reliability is the requirement that the components be tested at, or more frequently than, specified intervals. It is essential that each plant be aware of the testing frequency that was assumed when calculating the basis for the reliability of their system and that plants adhere to that testing frequency schedule. This paper will present an overview of the principles related to flare load reduction design and provide simplified examples of the impact that testing frequency has upon the reliability of the system to emphasize the importance of this issue. Suggestions will be presented to assist operating plants with confirming the expected reliability of their system.