Containers that have reached their fatigue life
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Seeking expert advice: 1. For containers that have reached their fatigue life and are still in use, if they are to be used beyond that lifespan, how should they be inspected (tested) and evaluated? 2. Could you also introduce some advanced practices abroad? Thank you.
TSG21-2016 7.1.7 Fatigued equipment is generally scrapped beyond its design life.
Only qualified parties should carry out fatigue testing and assessment.
Zhongte Inspection is currently drafting a group standard for the inspection of containers at the end of their design service life. For such containers, my personal recommendation is that 100% MT testing should be carried out on the welds on the inner and outer surfaces, along with RT testing on the butt welds
For ultra-long-life equipment, TSG21 only provides a general direction: inspection, and safety assessment in accordance with 8.9 when necessary (evaluation for suitability for use). However: Clause 8.9 specifies that the evaluation for usability should be carried out in accordance with GB19624, but this standard applies only to defects; in other words, even if you engage a qualified organization, its work still falls outside the scope defined by this standard. For example, in the case of a fatigue-prone container, if no defects are detected during inspection, does that mean it can still be used until its fatigue life is reached? I think not; an evaluation is still necessary. How should this evaluation be carried out? Which one is a qualified organization
I’ve seen this group standard; it requires conducting a risk assessment first, followed by surface inspection. However, I am highly skeptical that for containers that have reached their fatigue life (number of cycles), it is possible to determine the inspection interval based on the test results without conducting a fatigue analysis.
Regarding fatigue containers, there is an important detail. A pressure vessel manufactured according to fatigue design, used from its brand-new state up to the specified fatigue life, does not require further fatigue analysis if, upon inspection, no fatigue cracks are found and no degradation of the material is detected. If defects other than wall thickness thinning, which can be simply quantified, are detected, it is currently impossible to conduct any analysis using existing fatigue analysis methods and software. Because the current ones are all for newly built containers.
Personal understanding: Fatigue generally starts at defects and stress concentration areas; once the number of stress cycles exceeds the design limit, fatigue can occur even if no defects are detected through inspections. So fatigue analysis is still necessary. Please correct me, thank you. Additionally, can the fatigue section of API579 be used as a reference?
The fatigue analysis we are talking about does not refer to the same thing. The fatigue analysis I am referring to involves designing and manufacturing a vessel in accordance with the user’s technical requirements, following standards such as 4732 or ASME VIII-II. What you're referring to is analyzing, for any device that is in use, whether that device is likely to experience fatigue failure. This is really difficult. Currently, the method in China that is closest to your requirement is RBI, which stands for Risk-Based Inspection. As for API 579, it cannot be used at present for the inspection of pressure vessels in our country.
First, check the condition of the equipment, its thickness, and whether the non-destructive testing results are consistent with the original design. Next, contact the designer, as the fatigue life specified in the calculations is often higher than the actual fatigue life.