Hello Fellow Stressers!!

We have 3 sets of coke drums (2 coke drums per set). The coking time for each set is 15 hrs & decoking time is 2 hrs.

Please correct if my understanding of the coking & decoking process is correct:

1) Coking & Decoking ----> Line is pressurized & hot (Total 17 hrs)

2) Calculation of thermal/pressure cycles for 25 years:

First Case:

1 year = 365 x 24 = 8760 hrs
25 years = 219000 hrs

No. of Cycles = 219000/17 =12882 cycles

Second Case:

Set 1 Coke Drum is coking & decoking for 17 hrs then stop

Set 2 Coke Drum is coking & Decoking after 17 hrs (after Set 1 stop Decoking)

Set 3 Coke Drum is coking & Decoking after 34 hrs (after Set 2 stop Decoking)

Then cycle repeats:

In this case, the line is hot & pressurized for 17 hrs then cold/non pressurized for 34 hrs, therefore 1 cycle = 51 hrs

Cycle for 25 years = 219000/51 =4294 cycles which is less than 7000 cycles.

Please share your experience for this coking/decoking fatigue analysis. Are you considering both temperature/pressure is cycling or thermal alone. Normally, the expansion stress already considers 7000 cycles by default but this is for cyclic temperature only.

Your opinion is highly appreciated.

Just correct any wrong statement I've made.

Warm Regards,

We are in the same page in this case...

Cheers!!

I won't comment to the day-to-day activities on de-coking operation for any given specific application.

It makes sense generally to de-coke one at a time, since it's a batch process, though I'd expect the intent could be that the third is an emergency spare and is generally unused.

However, we're obligated to assume worst case scenario in the absence of better information, so you have no guarantee which 2 they'll favor at any given point in the future, or even abandon the sparing philosophy if one even exists.

Dialogue with the end user / operators is important.

I wouldn't step out on any limb to say duration and frequency, since every facility is like a snowflake, it seems.
These folks offer a range of 10-24 hours.

With respect to the question of considering pressure vs temperature vs pressure and temperature: If you can demonstrate that the differences between the 3 cases are substantially different/worse from simply combining P&T, then it's one of those situations where it becomes an obligatory assumption in the absence of better data.

I would expect by and large the results to be comparable, though. There will be a minority of cases where pressure or temperature will work to cancel out one-another's ill-effects that might be picked up by looking at SUS -> P, P -> P&T, P&T -> T, T -> SUS versus SUS -> P&T.

At the end of the day, CAESAR will report total permissible cycles for each of these fatigue cases, and those permissible cycles can be what's ultimately approved, in lieu of total life cycle usage.

Thanks Michael for the advise... I will confirm to the operations team regarding their operation philosophy...

Any other opinion is highly appreciated.

Cheers!!