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API publishes 7th edition of API Standard 521

Published by , Senior Editor
Hydrocarbon Engineering,

API has published the 7th edition of API Standard 521 for use in addressing risks of overpressurisation at refineries and a wide range of other facilities.

The standard provides guidance, recommendations and alternatives for the design of pressure-relieving and vapour de-pressuring systems to be used at LNG terminals, petrochemical facilities, gas plants and other petroleum production facilities.

API Standard 521, Pressure-Relieving and Depressurizing Systems, addresses causes for overpressure as well as controls and mitigation measures for high pressure relief when the maximum allowable pressure of a vessel, piping system, or other equipment is exceeded.

Refineries and petrochemical plants are complex systems that use temperature and pressure together in an integrated way to transform crude oil and other feedstock into the value-added products that make modern life possible. To assist the natural gas and petroleum industry in enhancing plant safety and sustainability, API 521 recommends that a hierarchy of measures are used to ensure that equipment is not subject to excessive pressure. The standard is designed to aid in the selection of the most appropriate overpressure protection system for addressing the risks and circumstances at a variety of petroleum and petrochemical installations.

This standard specifies requirements and recommends guidelines for the following:

  • Examining the principal causes of overpressure.
  • Determining individual relieving rates.
  • Engineering controls such as pressure-relief devices.
  • Procedural controls.
  • Selecting and designing disposal systems, including such component parts as piping, vessels, flares (used with API Standard 537, Flare Details for Petroleum, Petrochemical, and Natural Gas Industries), and vent stacks.

API 521 is a voluntary standard that has been developed through API’s consensus-based process overseen by the American National Standards Institute (ANSI). The standard is being published this month by API’s Global Industry Services (GIS) division and is made available through

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