Dealing with possible defects in welds which are not examined
by Nicolas Kohout
This is not really a story but more of a questioning about what I think is recurring when dealing with welded equipment inspections and risk management.
I exclude aerospace construction because it's a field in which no defect at all is tolerated (in the extent of what it is possible to examine of course).
If you consider a pressure vessel, you have to follow strict design principles which are first dependent on where you plan to install it (local regulations) and then there's a bit of a choice from the customer and/or the manufacturer (which will chose an appropriate design code regarding the vessel, the local regulations, and so on...), additionally the manufacturer will often have to follow customer specifications.
Now, considering regulations, code and specifications require for example 20% radiographic examination on longitudinal welds and you find some repeated defects in the welds (let's be specific and say lack of fusion). If several repairs are done respecting all standards and specifications, and that you are able to clear all the defects, what posture should you have considering risk management of potential presence of other defects in remaining welds.
Codes require some control extension when you find defects but not always 100% examination so you could say you're good to go, assemble, test and install your vessel on site. On the other hand, you can reasonably believe that defects are remaining. But it's only a believing, big defects, small ones, of the same kind or not, on which length and so on, all of that you cannot know until you examine remaining welds, but the code does not require it. How do you decide to go on with manufacturing or perform additional examinations to clear your technical doubts? Even considering potential defects in some case (depending mostly on fluid, temperature and pressure cycles) it's highly improbable that the vessel will explode but you cannot be sure... The risk is big (explosion) but the probability of it to happen is uncertain...
What would be your position? Of course, depending on the field I guess it would not be the same, but what would be a good decision making attitude that you could use in many situations?