API 520, Sizing, Selection, and Installation of Pressure-relieving Devices Part I - Sizing and Selection, was written for use in the process industries in gas and liquid service.

CGA S1.3, Pressure Relief Device Standards-Part 3-Stationary Storage Containers for Compressed Gases was written for fixed equipment in gas service.

Which standard to use depends on the choice by the owner/designer considering regulations that may apply. In the USA, the API standard is used most often in process plants where local regulations for relief valve sizing usually do not apply. For other industrial facilities and for commercial and residential facilities, regulations frequently apply. The CGA standard is more often referenced in regulations.