What is partial stroke testing (PST) and how does it improve SIS reliability?

Partial Stroke Testing moves a shutdown valve a small amount (typically 10-20% of travel) during normal operation to verify valve freedom and actuator function without causing process shutdown. Benefits: detects stuck valves and actuator problems between full proof tests, allows longer proof test intervals while maintaining SIL, improves availability by reducing unplanned shutdowns from valve failures. Implementation: smart positioners with PST capability, solenoid arrangements allowing partial venting, position feedback verification, and diagnostic software. Credited PST coverage is typically 60-70% (doesn't verify full closure or seat integrity). Full proof test still required periodically.