B31.3 allowable stresses are governed by the creep strength at the higher temperatures and the basis of allowable stresses are time dependent (e.g. 67% of the average stress for rupture at the end of 100,000 hours)

Is the design life of a piping system at high temperature (in creep range) is limited by those many number of hours of operation?

Or, By number of displacement cycles , N ?

Hello, I am convinced that it is governed by appendix V https://whatispiping.com/creep-rupture-u...erature-service

This is conjecture, but I would believe that the amount of creep life left would be a function of both creep life expended and number of cycles spent, and that number of cycles left would also be a function of creep life expended and number of cycles spent.

Both are likely acting in tandem in actively destroying your metal.

But without the lab equipment and materials needed to find fatigue curves at various temperatures and times, it'd be difficult to correlate the two.

Appendix-V deals with pr-temp variations only

I was expecting someone to provide an answer in terms of if we meet the code design what would be the life expectancy.

As always happens, it depends...

If your Design Allowable Stress was established based upon 100000 h averaged rupture stress, then 100000 h is the Design Lifetime. You should be aware that Design Lifetime correspond to effective operating service conditions (e.g. continuous work under design loadings action), and it is not a calendaristic period. All the shut-down periods are excluded.
However, if the System was operated for a longer period than 100000h, then you need to adjust/de-rate the allowable stress, by decreasing the averaged rupture stress in accordance with the effective operating lifetime (example: 130000 h > 100000 h). Such assessment is generally done using Larson-Miller parameter (B31.3 Appendix V)

Then, you need to clarify/define which is the Operating Service regime. What kind of pressure & temperature loading/unloading cycles are applied to your system. Is it one single set of P & T parameters, or you are dealing with several cyclic loads (example: reaction and regeneration regimes, each of regimes defined by an Operating Temperature and an Operating Pressure).

In addition, you need to clarify which are the frequencies and time-durations of these cyclic actions? In general, for simplicity, transient portions are conservatively included in full-loading periods.

Based on the answers to the questions raised above, you'll be able to estimate the effective numbers of loading/unloading cycles. For several cyclic loads, cumulative damage principle (Palmgrin-Miner) may be used to establish the equivalent no. of cycles.

Then, using the screening criteria provided by ASME VIII-2 Part 5, or API 579-1/ASME FFS-1, you'll be able to decide if fatigue check is mandatory or not.
If Fatigue assessment is required, then, as pointed out by Michael, you'll need to check the overall/resultant cumulative damage ratio, by superposing Creep damage mechanism and Fatigue damage mechanism.

I suggest to have a look on ASME VIII-2 Part 5, and/or API 579-1/ASME FFS-1, Parts 10 (Creep) and 14 (Fatigue). The methodologies provided by these Codes are equivalent.

Good Luck!