I'm on a petrochemical project which happens to produce steam as part of the process as well. This steam is even sold to battery limit but is still not the main application of the plant. There is a steam drum and superheaters in the plant and is point of argument whether should be designed as ASME I or ASME VIII; consequently the piping would get split partially in B31.1 and B31.3 if ASME I was selected.
Does this have to be considered as part of the plant and therefore B31.3 or we should split them for before and after first shut-off valve for B31.1 and B31.3 spec break, stress analysis, etc.?
(By the way client has no obligation for either way)

Any thoughts?

This can be a confusing issue indeed. You and your project team need to discuss it with the PIDs and the B31.3/B31.1 scope sketches in the front part of the codes.

Sometimes a section of the piping is classified as BEP (Boiler External Piping) which means the piping is purchased/fabricated as "SA" material (VIII) but designed to B31.3 (or B31.1). Many outcomes are possible depending on valve qty and placement.

Thanks Bob,
As I am getting from your reply you are considering more as B31.3 case with a bit of severity in terms of connecting to a boiler(which I do agree as well)
The point for our process people is that they want to consider whatever produces steam as "power boiler" especially when they sell it as biproduct and therefore scope of ASME I;consequently B31.1 would be applicable.(by the way the position of valve is after superheater and this may end up to take the whole steam drum to superheater piping and a bit of superheater outlet as B31.1)
My argue is when whole plant is B31.3 application and we have couple of equipment items, even though producing steam, but part of the stream of petrochemical production, I take it as a case that B31.3 dominates it. Hence the equipment would be ASME VIII and piping would be (as you say) either SA material designed by piping or B31.3, rather than ASME I and the hassle of variety of commodities for identical composition BUT B31.1 and B31.3, concerns of matching them stressing them as two calcs or one,etc.
thoughts are very welcome.