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About This Item


Full Description

Please note that any addenda or errata may not be included in translated editions of this standard.

This standard 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 specifies requirements and gives guidelines for the following:
  • examining the principal causes of overpressure;
  • determining individual relieving rates;
  • 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.

The portions of this standard dealing with flares and flare systems are an adjunct to API Standard 537, which addresses mechanical design, operation, and maintenance of flare equipment. It is important for all parties involved in the design and use of a flare system to have an effective means of communicating and preserving design information about the flare system. To this end, API has developed a set of flare datasheets, which can be found in API 537, Appendix E. The use of these datasheets is both recommended and encouraged as a concise, uniform means of recording and communicating design information.

Document History

  1. API Std 521


    Pressure-relieving and Depressuring Systems, Sixth Edition

    • Most Recent
  2. API Std 521

    Guide for Pressure-relieving and Depressuring Systems: Petroleum petrochemical and natural gas industries-Pressure relieving and depressuring systems, Fifth Edition (Includes 2008 Addendum)

    • Historical Version
  3. API 521

    Guide for Pressure-Relieving and Depressuring Systems

    • Historical Version