The standard is copyrighted. Free versions are either outdated, illegal, or incomplete. Some consultants offer summaries for free, but not the full PDF.
Formally titled “Petroleum, petrochemical and natural gas industries — Sizing, selection and installation of pressure-relieving devices,” ISO 23251 is the international twin of (6th Edition).
: Methodologies for determining individual relieving rates for safety-relief valves and rupture disks. Disposal Systems