Do We Need Schedule Replacement for Field Critical Instruments?

How about something like 0.65 x MTBF? (That's somewhat arbitrary, but at least sensible.)

Equipment wears out as location, and service variation determine.

Many places buy extras when the installation was commissioned for use as swap-in spares as needed, the swapped in parts are repaired/replaced. As time goes by they keep service logs of the installation and can determine the approach of the points at which ongoing maintenance has mounted to the point where some sort of facility overhaul or refurbishment is needed. Often these proceed in a segmented way in large plants to maintain production in various aspects as new tech replaces some parts, and old tech is scrapped out.

Pipe corrosion can reach the point at which large portions get scheduled to be scrapped. In large facilities the replacement is built close by and piped in after test, so production continuity is assured.

Sad to say, a number of companies neglect things, and insurance companies that make annual surveys of insured plants might deny insurance or up the rate to accept the increased risk.

So what size is your facility?

What is stated in your QC/QA standards, ISO perhaps? It would be a prerogative of the operations management and bean counters, I pretty sure.

But have you consult, manufacturer's recommendation on this? Their say on the life cycle might be worth than the standard that you are looking for.

If I were you, I'd be using statistics(monitoring, faults and defects of vital equipment). Probability analysis would be a key in determining, either defects are attributed by equipment wear or may be by the type of service.

Critical systems are usually redundant systems with at least one backup and sometimes two, barring that then proactive maintenance is called for...this would mean replacing any components before failure , and that would be specific to the application and component....

See predictive maintenance procedures...

https://www1.eere.energy.gov/femp/pdfs/OM_5.pdf

Most manufacturers have recommended Maintenance/replacement intervals for their equipment.

When in doubt, stock a spare.

I worked for an OEM for many years. We published that information in our equipment manuals for critical items.

It is mainly an economic consideration, and that is a function of what and how the plant is manufacturing/processing. If plant down time costs $50,000 per hour and you predict that in years 7 to 14 you will get 100 wear out failures, then spending $3million on a complete swap out is actually a predicted saving of $2million. I've worked on several plants where complete swap outs make sense. A liquid sodium manufacturing plant runs continuously for several years. One stoppage and the sodium solidifies in the pipes and there is no way to get it out. That calls for 200% redundancy on all equipment and because the environment is so corrosive a complete pipe/vessel/instrument rebuild at most planned shut downs. I don't know enough about gas processing to advise in your situation but it is probably the accountants rather than the engineers who will decide the issue.

"An ounce of prevention is better that a pound of cure."--Written on the wall inside of a missile silo.