API 520 Part II

Key requirements (9)

• Installation of pressure-relieving devices in refineries — companion to API 520-1 (sizing).
• Inlet line pressure loss ≤ 3% of set pressure at rated flow (rule preventing chatter).
• Outlet (built-up) backpressure ≤ 10% set pressure for conventional valves; ≤ 50% for balanced bellows.
• Inlet line schedule: typically same as PRV nozzle, no reductions.
• Block valves under PRVs: car-sealed open (CSO) or locked open (LO) per OSHA 1910.119.
• PRV elevation: discharge piping should self-drain back to disposal header.
• Reactive force calculation required for open atmospheric discharges (especially relief to atmosphere).
• Tail pipe sizing: avoid two-phase flow regime transitions.
• Multiple PRVs: staggered set pressures to prevent simultaneous popping.