Safety Integrity Level in Valves

Abstract

Safety integrity level (SIL) is a part of an international standard such as IEC 61508 that provides suppliers and end users with a common framework to design products and systems for safety-related applications. SIL provides a scientific and numeric approach to specifying and designing safety systems, enabling risk of failure to be quantified. The probability of failure on demand and probability of dangerous failure per hour on safety functions for each SIL level are given in the IEC standard. This paper reviews a 20″ pneumatic actuated ball valve in low-temperature carbon steel material ASTM A352 LCC and pressure class 1500 equal to 258.6 barg at ambient temperature (38 °C) that has an emergency shutdown (ESD) function. The ball valve is installed on a gas export pipeline that transfers the gas from an offshore platform to onshore. The ball valve is usually open, but it will be closed in case of failure in the pipeline to stop production and allow maintenance on the pipeline. The system includes an integrated valve, actuator, and control panel as a single unit that corresponds to one-channel (1oo1) architecture in international standards such as IEC 61508. Associated failure possibilities such as dangerous detected and undetected as well as safe failure types were assigned to the valve, actuator, and control panel separately. Calculating the safe failure fraction for the whole system as well as each single component separately proved that the safety level of the system is categorized as SIL 2, which is according to the client demand.