Mission time is the period over which PFDavg is averaged, typically the expected service life of the function or the interval between full renewals. It matters whenever some failures are not revealed by proof tests: those failures accumulate over the mission time.
Renewal
Overhaul or replacement of a device returns it to "as new", clearing everything, including failures the proof test cannot see. For a valve whose proof test coverage is 80 %, overhauling every 5 years instead of running 20 years cuts the hidden contribution by four.
In First Out AI the "Renewal / overhaul" field per group sets this interval; left empty, it equals the mission time.
Useful life
Constant failure rates are a model of the flat middle of the bathtub curve. Electronic components and mechanical parts wear: manufacturers state a useful life (often 8 to 15 years for electronics). A calculation over 20 years with constant rates assumes devices are replaced before wear-out. State it in the SRS and in the maintenance plan.
Practical guidance
- Use the actual planned life of the function or the turnaround-based overhaul cycle.
- For devices with significant hidden failures (valves, analysers), set a renewal interval and put it in the maintenance plan.
- Re-verify when the plant's life is extended.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.