What are the basic requirements for safety shutdown valves?

Safety shutdown valve requirements: fail-safe action (typically fail-closed for isolation, fail-open for relief), tight shutoff (metal-to-metal or soft seat based on leakage class), fast response time (specified stroke time, often <2 seconds), fire-safe design (API 607 certification), position feedback (limit switches for closed position verification), partial stroke testing capability (for proof testing without process shutdown), and SIL-rated actuator and accessories. Valves should be separate from control valves where possible. Documentation includes safety manual with failure rate data, and valves require periodic proof testing per SIS requirements.