interpretation of asme b31.3 341.3.4

Assuming you are referring to the minimum requirements of Normal Fluid Service and these are on the same piping system, then you have met the code after 1) the first random progressive sampling. If this is indeed the case, and the owner/engineering design requires more than the minimum first testing, where you have proceeded onto more stringent inspection - than this is not dictated by the code, but rather the owner specifications which are, allowably, more than the code - then this case is covered in interpretation 16-02, and the 3) set may be treated as standalone.

So please provide more information/background (particularly what type of testing was performed, what fluid service is this, what extra requirements are being dictated by the engineering design).

A quick question regarding the selection of samples .... is it a safe assumption that the progressive samples are to be taken from previously completed welds of the same type (welder, size, etc) and not "wait" for future weldments of that welder to sample.

Example, a welder may only perform one weldment of a specific size and not perform another on that "product", or may or may not perform more at some unforseable date in the future. How would one determine progressive samples in this scenario. Certainly if the welder performs no additional welds of that type then there is nothing to sample, but what if it is not necessarily known if or when this welder may perform similar welds in the future?

The code is fairly silent on that scenario, except for, each welder/operator must be represented . That is, at a minimum, at least one welder/operator weld must by examined. This would be counted for in hold points of number of welds. The engineering design must choose a lot size (not explicitly laid out in code) and work from there. For example, you would know that there are going to be 100 welds from the beginning. With 5% RT you must examine 20 welds. Arbitrarily choose a lot size of 5 welds - when five welds (regardless of who has done them) have been completed - stop and examine, at which time you will know who has performed the welding.