Every probability-of-failure calculation for a safety loop rests on a quiet assumption: that the devices fail at a constant rate. That assumption is only true for part of a device's life, and once a component ages past its useful life the rate starts climbing and the whole calculation stops being valid. Mission time is the related planning number, the interval after which a device is meant to be replaced. Together these two ideas explain why a safety valve or transmitter carries a manufacturer-stated life limit, and why ignoring it silently erodes the integrity you thought you had.
Mission time / useful life in one line: Useful life is the period during which a device's dangerous failure rate stays roughly constant, which is the period SIL calculations assume. Mission time is the interval you plan to operate a device before mandatory replacement, and it should sit within the useful life. Running a device past its useful life invalidates the constant-failure-rate assumption behind PFDavg, because wear-out pushes the failure rate up.
The equations used to compute average probability of failure on demand treat the dangerous failure rate as a single fixed number. That is a modeling convenience that is only accurate during a particular phase of a component's life. The classic bathtub curve describes the reality: an early period where manufacturing defects cause elevated failures, a long flat middle where failures occur at a low and roughly constant rate, and a final rising period where wear-out mechanisms take over and the rate climbs.
Useful life is that flat middle section. During it, the assumption of a constant failure rate holds well enough that the standard PFDavg formulas give trustworthy answers. The SIL verification for a loop is, in effect, only valid within this window. Outside it, on either side, the real failure rate departs from the number the calculation used, and the departure is not conservative in the wear-out region because the rate goes up, not down.
This is why the end of useful life matters so much for safety. As a mechanical device such as a valve or actuator ages, seals harden, springs weaken, and moving parts wear, and the probability that it fails dangerously on demand rises above the flat-line value the SIL math assumed. The loop may still look compliant on paper, using the original constant rate, while the physical hardware has quietly moved into a region where that rate no longer describes it.
Mission time turns the useful-life concept into a schedule. It is the operating interval a design assumes a device will run before being replaced, and it should be set so the device stays within its useful life for the whole interval. Because PFDavg accumulates over the operating period, mission time also appears in the reliability calculations themselves: a longer mission time between replacements generally means a higher average probability of failure, all else equal, so the number is both a maintenance input and a reliability input.
Manufacturers support this by publishing useful-life limits for safety-relevant devices, frequently in the range of ten to fifteen years for many valves and transmitters, though the figure varies widely by product and service. That published life is not marketing; it is the boundary of the region in which the failure rate the manufacturer quotes remains valid. Once a device passes it, the quoted rate can no longer be relied on, and the honest response is replacement rather than a hopeful extension.
Good replacement planning treats mission time as a hard commitment rather than an aspiration. It means tracking the installation date and accumulated service of every safety device, scheduling replacements before the useful-life boundary rather than after a failure, and recognizing that a device surviving past its stated life is not evidence of extra reliability but of accumulating, unquantified risk. Replacing on time is one of the few maintenance actions that directly protects the validity of a SIL calculation.
The practical difficulty with useful life is not the concept but the bookkeeping. A facility may run hundreds of safety-relevant devices installed at different times, each with its own useful-life limit and its own accumulated service. Knowing which ones are approaching the end of their validity window, and doing so before they cross it, is an inventory and records problem that paper systems handle badly.
A cloud SCADA platform helps by keeping each device's identity, installation history, and operating record in one place, so that age against useful-life limit becomes a queryable fact rather than something buried in a filing cabinet. It can also surface early behavioral signals, a valve stroking more slowly, a transmitter drifting more often, that hint a device is entering its wear-out region ahead of the nominal date, letting engineers replace on evidence rather than purely on the calendar.
Because Merobix reads field devices into a single browser-based view and retains their history, it lets an operator see the fleet's aging at a glance and plan replacements around genuine useful-life boundaries. That keeps the constant-failure-rate assumption honest: the loops on paper stay valid because the hardware in the field is retired before it drifts out of the region the calculations assumed.
The SIL verification quietly stops being valid. Those calculations assume a constant dangerous failure rate, which only holds during the useful life. Past that point, wear-out drives the real failure rate above the value the math used, so the loop's actual probability of failure on demand is worse than the compliant-looking number on paper. The device should be replaced rather than run on.
They are related but not identical. Useful life is the period during which the failure rate stays roughly constant, a property of the device. Mission time is the operating interval you plan before mandatory replacement, a planning choice that should sit within the useful life. Mission time also appears directly in reliability calculations, since PFDavg accumulates over the operating period.
It varies by product and service, but manufacturers commonly publish useful-life figures in the range of ten to fifteen years for many safety valves and transmitters. The exact number is specific to the device and the conditions it operates in, so you should use the manufacturer's stated life for the actual model rather than a generic figure.
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