Re: Why Class 150 ?

>But as materials and steel manufacturing improved the allowable pressure
>rating of the flange was able to be increased.
I think this was strictly a matter of material improvements. All flanges have to conform to ASME Code rules, and the rules for flanges really haven't changed since the early 60's (At least). Extending the operating limits by corporate research wouldn't have made sense, since the rules and particularly the requirement that only materials conforming to ASTM specifications can be used with the code rules.

ANSI ratings are conservative by Code standards, I think because of conservative assumptions for gasket loading and possibly to accommodate over-bolting. You can go through the Code (Appendix Section VIII) calculation for any ANSI flange and get a higher rating. Code rules allow for the use of ANSI flanges at their rated temperature without verification, because the ANSI pressure classification is conservative. I should add that the flange rules are fairly straightforward until you get to gasket loading and metal-to-metal contact which can generate prying

loads on bolts. Flange design is not something for the amateur, but it
can be learned. And if anyone is thinking that FEA will do all the work,
he's mistaken....

Bonney and Taylor didn't go out of business because someone took advantage of their research. They were bought by conglomerates like Gulf and Western and ITT. Industry, such as it is these days, supports research by the PVRC and WRC in this area.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw@skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)