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Exceeded MAWT

Exceeded MAWT

Exceeded MAWT

(OP)
If the MAWT of an ASME stamped vessel is briefly exceeded, is there anything (like a fitness for service calc or ???) that would be required to continue using said vessel?

RE: Exceeded MAWT

Too many unknowns here. In general no. In your case specifically - it cannot be said. If you went 10 deg over the design temperature, no problem. If you had an exothermic runaway and don't know how hot the steel got but it was glowing red, different response.

jt

RE: Exceeded MAWT

(OP)
It's a hydrotreater with MAWT=850 F but the question is more hypothetical.  Say you have a pretty good array of thermocoules (both radial and axial) and the temperature excursion goes to 900 F. Or 950 F. Or even 1000 F.  At some point a more detailed analysis is justified.  I'm wondering where that point is or should be and was hoping the answer was as simple as "if it's less then 20% above MAWT it's ok, if it's greater than 20% above MAWT it should be looked at further", but that's probably a naive position...

RE: Exceeded MAWT

A typical hydrotreater material should easily withstand 1000°F for a few hours without metallurgical damage. But... you lose about half the allowable stress in the journey from 850°F to 1000°F. These allowable stresses are creep limited. Combined with the likely maldistribution of temperature which you should be able to confirm with your multiple bed thermocouples (provided by company whose name starts with a G?), the likely damage mechanism is bulging. Keep in mind that the reactor metal temperature will lag the process temperature. If you have skin thermocouples, they will be quite valuable in evaluating what happened to the steel. Post excursion check for bulging, perhaps localized weld cracking. Chances are that you will not find any. If you do find some damage call your company's materials and vessel experts in for a FFS vs repair decision.

jt
 

RE: Exceeded MAWT

Interesting variety of possibilities.
 A-All the "hydro" units I am familiar with were internally insulated; "cold wall" design. So it was very unusual for the shell temp to approach design. Is that your case ?
That being said , some of our reformer vessels had temperature sensitive paint so any hot spots could be located and measured.
B-For some vessels, coupons were cut out for creep-rupture testing (maybe a nozzle cut out); Expensive, but definitive.
C- Some bulging was tolerated (years ago) after hardness testing and crack/flaw testing were satisfactory.
D- For carbon and 1/2% Mo vessels, the possibility of high temperature hydrogen attack must be considered (that is where some of the Nelson Curve data came from). I don't think that is a concern for 2 1/4 Cr : 1 Mo; and likely 1 1/4 Cr : 1/2 Mo.

RE: Exceeded MAWT

(OP)
Thanks everyone.

No skin TC, yes the internals are by the "G" company.  Not cold walled, material is 321 SS clad on 1 1/4 Cr.  Will include a visual inspection following any temperature excursion.

RE: Exceeded MAWT

Not sure about ASME Section VIII Division 1 but...

I would be inclined to follow a similar approach to ASME B31.3 Appendix V, adjusting for the caveats and limitations therein specified.

Many years ago, I devised a guideline wherein the Larsen-Miller parameter was used to generate a plot of excursion temperature versus time (for a given pressure) for a carbon steel piping system that had been designed for saturated steam but occasionally experienced superheated steam conditions.  In that case, I limited the "zero time allowed" threshold to 427 C, which was the Code-accepted graphitization threshold.  This produced a curve that fell below the creep rupture curve, and to which excursion monitoring and safeguards were effected.  Basically, I created an envelope from which each excursion duration could be cumulatively deducted from the theoretical creep rupture life.  It was approved and adopted for use in a fairly large refinery, after meeting with approval from the local Regulator.  At that time, either "Appendix V" did not exist or I certainly wasn't aware of it.  Nor did I have any benefit of the paper entitled "Graphitization Of Steels In Elevated Temperature Service" - Foulds / Viswanathan, Journal Of Materials Engineering And Performance, Volume 10(4), August 2001.

In this case, I suspect that creep and other damage mechanisms are more a concern than graphitization.  I think ASME VIII Division 1 probably refers one to Section II Part D for properties at elevated temperature, but otherwise, I would anticipate that a creep analysis might be the course of action unless there is visually observable gross plastic deformation.
 

Regards,

SNORGY.

RE: Exceeded MAWT

RE the fitness for service issue, an "alteration"is needed under the NBIC
 

RE: Exceeded MAWT

There may be other areas to be included in the damage review, including:
- original desing diff expansion expectations may have been exceeded, and bumpers / supports limits  may have been exceeded with reslting high stress on attachments
- weld interface between SS cladding and 1.25% Cr shell has a thermal generated shear stress related to the difference in thermal expansion coeficients of SS and 1.25% Cr, may need to check for delamination at weld interface using UT
- if  the current overheat is judged OK, its publbication  may lead to operators allowing add'l overheats in future- a sort of "mission creep"- leading to later damage

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