There are numerous quality issues related to the specification of a PSL2 product. I advise your read API-5L to understand the differences between PSL-1 and PSL-2 and discuss these differences with the Owner before accepting the lower quality PSL-1 product.
I've used Belleville washers in very high vibration environments to maintain bolt pre-load and prevent the nuts from backing off due to vibrations. The use of Belleville washers normally isn't a Code issue, but an operations or maintenance issue (how often do you want to have maintenance crew...
Weldolets are required by MSS-SP-97 to be made from a forging rather than being machined from bar stock. ASME also requires pressure parts such as B16.5 flanges, B16.11 couplings and fittings, etc. to also be made only from forgings. The reason, as previously mentioned, has to do with strength...
