15
58-651 Pulsar
®
Model R86 SIL Functional Safety Manual
7.4
Report: Lifetime of Critical Components
According to section 7.4.9.5 of IEC 61508-2, a useful life-
time, based on experience, should be assumed.
Although a constant failure rate is assumed by the proba-
bilistic estimation method, this only applies provided that
the useful lifetime* of components is not exceeded. Beyond
their useful lifetime the result of the probabilistic calcula-
tion method is therefore meaningless, as the probability of
failure significantly increases with time. The useful lifetime
is highly dependent on the subsystem itself and its operating
conditions.
This assumption of a constant failure rate is based on the
bathtub curve. Therefore, it is obvious that the PFD
avg
cal-
culation is only valid for components that have this con-
stant domain and that the validity of the calculation is lim-
ited to the useful lifetime of each component.
It is the responsibility of the end user to maintain and oper-
ate the R86 per manufacturer’s instructions. Furthermore,
regular inspection should show that all components are
clean and free from damage.
The R86 has an estimated useful lifetime of about 50 years.
When plant experience indicates a shorter useful lifetime
than indicated in this appendix, the number based on plant
experience should be used.
*
Useful lifetime is a reliability engineering term that describes the operational
time interval where the failure rate of a device is relatively constant. It is not a
term which covers product obsolescence, warranty, or other commercial issues.
