The norm ISO 4126 consists of several individual parts and relates to safety devices against excessive pressure. Among the safety devices covered by the norm are safety valves, including pilot operated safety valves as well as Controlled Safety Pressure Relief Systems (CSPRS), and rupture discs. Among other things, the content consists of requirements regarding the sizing and testing of safety devices, the operating behaviour depending on the pressure on the inlet and the outlet side, and the design of the inlet and outlet lines. Related to the sizing and testing, methods are described for the calculation of the capacity of the safety devices, and requirements are defined for the test procedures by which the capacity is determined experimentally.

The norm contains a list of terms and corresponding definitions in order to ensure a consistent understanding when multiple parties interact. These include, among others, terms such as set pressure, relieving pressure, reseating pressure, and back pressure.

Inlet and Outlet Lines

The pressure losses occurring in a pressure relief event via the inlet and outlet lines due to the fluid flow have a direct impact on the pressure conditions existing at the position of the safety device and therefore on its operating behaviour. To ensure stable operation, the norm defines limits for the permissible pressure losses via the inlet lines.