We recommend three essential steps that extend beyond just safety integrity level (SIL) certification and the probability of failure on demand (PFD). Moreover, these steps help in evaluating and selecting a safety valve to ensure that your plant will be safe.
The most important step in choosing an ESD valve is to use the application-based valve selection process. This step has not been observed often enough in recent years. Instead, functional safety calculations have attracted excessive attention. There are a number of different ESD valve types available, each designed for a specific application. Correct ESD valve selection needs to consider valve type, pressure class, temperature, materials and other application details. If the valve does not match the application in question, systematic valve failures will occur. A typical systematic error that may affect the valve’s operation in a real process is incorrect seat selection. A rough categorization includes seat types that have contact between the sealing elements all along the stroke and seat types that lose contact while operating. In polymer service, for example, the selection of seat type that loses contact leads to polymer accumulation between the surfaces. The valve does not fully work or starts to jam, which is a disaster for the ESD valve. The problem can be solved using a seat design that keeps contact between the seat and trim. Also a closed-seat design will solve the problem of seat jamming against open types of seats where impurities can enter critical areas of the seat.