API 688 is the industry standard for the design, selection, and operation of pressure-relieving devices. While API 520 covers the sizing and API 521 covers the pressure relief philosophy , API 688 focuses on the mechanical design, installation, and maintenance of the devices themselves.
The standard codifies complex design approaches (formerly "M" nomenclature like M2-M5) into a structured process to prevent fatigue, structural damage, and cavitation: api 688 pdf
Note: API 688 essentially consolidated and updated older recommended practices (like portions of API RP 520 Part II) into a formal standard. API 688 is the industry standard for the