When we talk about a tee NPS 3, a equal or straight tee is intended. With a tee NPS 3 x 2 a reducing tee is intended. Although, officially a reducing tee will be indicated by 3 diameters, namely 3 x 3 x 2 (A x B x C). The A & B measure stands for the Nominal Pipe Size of Run-Pipe, C measure stands for Nominal Pipe Size of Outlet.
This designation actually is superfluous, but comes from the time that tees were available in 3 different ends of diameters, such as
4 x 3 x 2 (A x B x C). In this implementation of course it is necessary, to specify 3 different sizes.
That kind of tees is probably no longer available. Applying a equal tee with a concentric or eccentric reducer is now the usual practice.
A straight or equal buttwelding Tee is available for all common diameters.
A Reducing Tee not, because many diameters are not produced, or can not be produced.
As an example: a reducing tee NPS 6 x 4 is a standard item for most suppliers, but a reducing tee NPS 16 x 2 is probably not available by any supplier.
It would also not economical to use a NPS 16 tee with a NPS 2 outlet; in such situations a Branch Fitting will be used or a Branch Connection will be made.
In addition to the defined tees, there are straight and reducing crosses.
Straight crosses are generally stock items, reducing crosses are often difficult to obtain.
I have never used that tees and I have they never seen in practice, at least not in petro and chemical industry. They will be used where space is limited, by revision work, in shipping industry et cetera.
