PetroStreet

In what cases, piping mill tolerances are set to other than default 12.5% value?

There are cases in procurement where tolerance other than a default value is considered but within +/-12.5%.
Like they can go to a tight one -10%.
ASME B31.9M specifies to be -10%, as that is expensive, SS.

mechcolor,
Tolerance would not be defined by ASME B31.9.
Actual tolerances are governed by the material specification, such as:
ASTM International ASTM A312 (seamless & welded SS pipe)
ASTM A358
ASTM A790 (duplex SS)

Thanks all.