What are the key considerations for sizing a pressure relief valve?

Sizing considerations: determine relieving scenarios (fire case, blocked outlet, runaway reaction, etc.), calculate required relieving rate for each scenario, select worst case, calculate required orifice area using API 520/521 methods accounting for fluid properties, back pressure, and set pressure. Types: conventional (sensitive to back pressure), balanced bellows (tolerates back pressure variation), pilot-operated (tight closure). Verify inlet pressure drop <3% set pressure and outlet pressure within valve rating.