API 521 is applicable to pressure-relieving and vapor-depressuring systems. Although intended for use primarily in oil refineries, it is also applicable to petrochemical facilities, gas plants, liquefied natural gas (LNG) facilities and oil and gas production facilities. The information provided is designed to aid in the selection of the system that is most appropriate for the risks and circumstances involved in various installations. This standard is intended to supplement the practices set forth in API RP 520-I or ISO 4126 for establishing a basis of design.

API 521 specifies requirements and gives guidelines for examining the principal causes of overpressure; determining individual relieving rates; and selecting and designing disposal systems, including such component parts as piping, vessels, flares, and vent stacks. This Standard does not apply to direct-fired steam boilers.

This edition of API 521 is the identical national adoption of ISO 23251, Petroleum and natural gas industries-Pressure-relieving and depressuring systems