For safety instrumented systems, only random failures shall be taken into account in the calculation of the probability of failure on demand (PFD). Finding sound random failure rates – especially for mechanical components - can be quite challenging due to a lacking statistical basis.
In the new VDI/VDE 2180-4 (2021) “Functional safety in the process industry” it is specifically discussed, how mechanical components shall be considered in PFD calculations. In principle, it is explained that for purely mechanical safety devices without an instrumented component, e.g. pressure relieve valves, burst disks, flame arresters, manually operated valves, random failure rates are negligible assuming a correct equipment selection, design, construction, installation, maintenance etc. This also applies to mechanical components within safety instrumented systems. In the case of components that show a failure behavior similar to safety valves, they are not taken into account in the PFD calculation.
Supporting the approach of VDI/VDE 2180-4, the NAMUR working group (WG) 4.5 Functional Safety has evaluated failure reports of valves of different companies. In addition, the first failure rates for valves have been derived from the failure database NAMUR.smart based on a period of about three reporting years. The data acquisition allows a differentiated estimation of failure rates for random failures delivering practical failure rates for valves.