Ali,
Have you looked in API? I don't know if it will answer your question or not.

Lew

If I understand your question you are seeking an explaination for why a particular pecentage of NDE (RT) would be required. The Codes of Construction do not elaborate on the reasons. It is a safe assumption that as the percentage required increases, the risk associated with (failure of) the particular application is also increasing. Hence, more assurance is needed to justify the factor of safety applied in the design of the piping run.

The problem is, it is easy to assume that 100% NDE means the component (piping run) is 100% acceptable if the NDE is satisfactory. I have seen a pretty decent discussion of the pros & cons of NDE (from a low percentage/random application to 100% required). I think it is in one of the many books that seek to explain the ASME Codes. I'll look for it when I return to the office.

The explanation covers the fact that in the design philosophy, sometimes (often) it is justified to specify no NDE. A logical assumption is that if nothing is examined, something probably contains defects that would be rejectable if discovered. The discussion goes on to explain that some applications may run for the entire predicted life with defects that do not catestrophically fail. The question the designer must answer: Is the cost of discovering a potential defect through failure more of a burden than the cost of discovery and repair by NDE? In a perfect (cash abundant) world, everything would be examined to the maximum extent possible. In the real world, the cost is always a very significant contributing factor in the decision to require NDE.

The other extreme is also considered: On the surface it seems that if 100% of the welds are examined and accepted, nothing contains rejectable defects. The component should perform flawlessly for the predicted life. We all know this is not necessarily true.

Basically, through an appropriate application of safety factors, joint efficiencies, NDE, operating conditions, etc; the designer attempts to provide reasonable assurance that the system will be safe for the duration of it's expected life. Sometimes that means that defects are likely to exist, yet the risk is considered acceptable.

This is a topic I have to visit every so often. I hope my explanation is not too simple minded or boring to the members. The good news is, in almost every case you should be able to specify an acceptable Code of Construction for whatever system you are concerned with. The Code will contain specifications that you may very assuredly rely on. In the words of one of my Navy Nuke instructors (as he was trying to teach us about reactor design): "People far more intelligent than you & I designed this...we just have to know how to use it and make the right decisions based on training..."

Charles Hall

ASME B 31.1 refers to Power Piping. Depending on the service you are thinking of you should go to other divisions, i.e., B 31.3 (Oil refinery and chemical plant piping), B 31.4 (Oil pipelines), B 31.8 (Gas transmission and distribution) etc.
Now, those Codes establish the MINIMUM x-ray requirements that are to be obeyed when erecting a particular kind of piping. For some justified reasons, the project engineering firm and/or the owner may require a larger amount of radiographs to be taken in a particular job. In this case, this requirement will be stated on the piping specs. issued by the project engineering firm and/or the bid documents issued by the owner. In any case, the exact x-ray amount which satisfy the enginering firm and/or the owner requirements should be clearly stated on the contract documents attached to the purchase order so as to prevent future claims.
Now, there are some very particular piping systems which fall under standards other than ASME ones. Two examples are:
a) oxygen piping, for which there is a standard issued by The Compressed Air Institute
b) chlorine piping, for which there is a standard issued by The Chlorine Institute.
Couldn`t these piping be considered as Chemical Plant Piping, under ASME B 31.3? No, because oxygen and chlorine are a very special kind of chemical substances which deserve a special treatment.
Giovanni S. Crisi
Sao Paulo - Brazil