Relief Valve Standard API Effective Orifice Area API has generated the standard API Effective Orifice Area with corresponding designation letter (following table).
Besides, standard effective orifice area, it also has a series of corresponding inlet size, outlet size combination for various pressure classes of flanged relief valves.
All these information are tabulated in API Std 526 Flanged Steel Pressure Relief Valves.
One shall remember above effective areas are valid only when used with the sizing equations contained in API RP 520 Part I - Sizing, Selection & Installation of Pressure-Relieving Devices in Refinery.