Union are generally socket welding?

union is always is the socket welding? I just check the slipknot about socket welding head related standards. According to my understanding, the union should have butt welding, the butt welding of union is what standard? Is American standard size and gb size should have? Union seems to be originated from American standard ASME B. , lb forged fittings, don't know if I understand right? Slipknot is generally socket and thread, I know MSS_SP - is the standard - __BL socket welding and threaded union, do not know to have other standard of butt welding slipknot, who know that provide ah. And national standard and chemical mark forged pipe fittings in the standard union and American standard size, is there any national standard size of the union? Such as 'American standard pipe diameter pipe. Mm, national standard pipe diameter is mm. I now is the problem of union is a national standard of the pipe, one end is American standard short single head wire, slipknot slipknot is the target, so that I can plug into single head wire end slipknot welding, but due to the national standard pipe end diameter pipe less than target, so feel the gap is very big, connection have a problem.