First, each company has their own criteria, this is a fact, the reasons are many (e.g. weather were they have their plants, national regulations, local markets, etc), in second, they are options for different things. With an o’let in high pressure you probably are going to have a very high heat input in the weld, not with the TEE (you have much more complication if you are thinking in use a 45°branch), but on the other hand if you have a big run and a small branch and you want to use TEE red. and another red… and so on, your spool piece is going to be very big with no sense because you could use an o’let reducing the space. One important thing is that the most part of companies that construct this kind of pieces not have their engineering designs studies and they copy from others very well known that have the studies of each part. Remember that these pieces are non std. and there are no ASME/ANSI that regulate their design. I think that using a TEE or a TEE red. up to one half of the NPS of the run is the best option, if you have less, you could think in an o’let branch.
