PetroStreet

At our plant, we faced a leakage of one of the radiant tubes in primary reformer furnace (Foster Wheeler) a week ago. This was the first tube leakage after the service life of around 18 years (versus the estimated design life of some 12 years). Leaking tube has been isolated successfully thru online nipping of inlet & outlet pigtails after extensive reviews, dry run demonstration and careful safety precautions.

During the course of activity, a very useful paper on the subject was found. This is being shared as below: Surprisingly, the leaking radiant tube was of creep grade-1 (as per the ultrasonic attenuation). Why it happened so? What would be the outlook of 20% creep grade-2 tubes?

Have you shared this with the inspection company which provided you the results of ultrasonic attenuation?
Any replica testing / metallography?

Remaining life assessment must be done for the heater tubes based upon the inspection findings and the actual incidents, if any, like in your case. API 530 provides you a complete guideline for remaining life assessment with which you may limit the temperature for continuous operation till next shutdown / planned inspection.
Having said that, ultrasonics are capable of providing you the right data but sometimes the degradation takes lesser time for a certain tube which may have taken the effect of severe operating conditions just because of its location in the tube pattern. The temperature / heat profile inside the furnace which has been in operation for this much years is so complex that you cannot say with complete authentication about any such failure.
For such cases, more stringent inspections must be carried out with some mechanical testing (destructive) also. Only after this a complete assessment must be made generating recommendations for the concerned sections on the next operating cycle (till next inspection).

Go through para 12.3 of API 573 and Appendix E of API 530 to assess compliance of your inspections.