The influence of fluid properties on single and two-phase two-component flows in safety valves

Spring-loaded, self-actuating safety valves are frequently used in chemical, petrochemical and nuclear plants to avoid human injuries, economical losses and environmental disasters following the rupture or the explosion of a pressurized unit due to fire, an extraordinary exothermic reaction, malfunction of a control valve, loss of coolant, etc. In the simplest form the safety valves can be considered as a nozzle topped by a spring-loaded disk, which begins to open at a predetermined set pressure and discharges the media into either a collection reservoir at a lower back pressure or the environment. An adequate sizing postulates that the disk reaches the final position or maximum lift when the pressure in the unit to be protected is the relieving pressure. The difference between the relieving and the set pressure is called overpressure. A minimization of the overpressure occurs when the safety valve is mounted the closest as possible to the protected item. In practice constructive constraints of the facility may impose the installation of the valve at some distance from the protected item, which must be limited to a value causing an irreversible pressure loss within 3 % of the set pressure. The correct installation of safety valves, the sizing and the required certification tests are standardized; for instance in the European Union ISO 4126-1 replaces previous national standards like DIN 3320.