Why?
A read of the foreword of ASME B 36.10 gives you an official history of such things.
"The original intent of the Committee was to establish a system of Schedule Numbers for pipe
size/wall thickness combinations which would have an approximately uniform relationship equal
to 1000 times the P/S expression contained in the modified Barlow formula for pipe wall thickness as defined in the Appendix* [no longer there] to this standard. The resulting Numbers departed so far from existing wall thicknesses in common use that the original intent could not be accomplished. The Schedule Numbers were then adopted strictly as a convenient designation system for use in ordering."
So take the last sentence (my bolding)as the Key one....
As for std, XS, XXS etc
"It was the hope in 1939 that the designation of pipe used commercially by all industry as
Standard weight, Extra-Strong, and Double Extra-Strong would gradually be replaced by Schedule
Number designation. However, owing to customs of over 50 years’ standing, demand and production
of pipe to these traditional dimensions is undiminished. Consequently, in response to a
demand from users, accepted practice for dimensions and weights of commercial wrought steel
and welded wrought iron pipe were added. These changes were designated an American Standard
on February 23, 1950."
Note the date here - 1939~!! And still we use them.
So for 8" (NPS 200) pipe there is no difference between XS and Sch 80. HOWEVER this is not valid for all pipe sizes, 2" to 8" yes, but then 10"XS = sch 60, 12 -14"XS is it's own thickness, 16" is sch 40 .....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
