The latest version, , supplements API Standard 521 (6th edition, 2014). It focuses on several critical safety functions:
Petrochemical facilities, gas plants, Liquefied Natural Gas (LNG) facilities, and oil and gas production sites. iso 23251 pdf
ISO 23251:2019 - Petroleum, petrochemical and natural gas industries — Pressure-relieving and depressuring systems The latest version, , supplements API Standard 521
This standard does not apply to direct-fired steam boilers. Key Technical Focus Areas The latest version