— Relief Valve (PSV) Sizing — Gas/Vapor, Critical Flow
Required minimum effective orifice area for a gas/vapor relief valve under critical (choked) flow, from the compressible-flow mass-flux equation for an ideal gas at a choked orifice, combined with the Kd/Kb/Kc correction factors defined in API Standard 520 Part I. This sizes the orifice area only — final valve selection (standard orifice letter, inlet/outlet sizing, and installation) must follow API 520/526 and be verified by a qualified engineer.