Why Class 150 ?

Ok, all you pipe nerds.

Here's a dumb question.

I'm not an engineer, just a gasket salesman. But I enjoy reading this forum and have enhanced my knowledge thereof. I've asked this for the last few weeks to all my engineer customers and no one knows.

Usually when a pipefitter (in the USA) refers to a "150 lb. flange" he means an ASME B16.5 Class 150 flange. It's amazing how many think the "pressure rating" of that flange is 150 psig. Obviously, that's not the case. It's material and temperature dependent. For example, per table 2 of B16.5, a 304 Stainless Class 150 flange has a pressure rating of 235 psig AT 200 deg. F. But here's my question:

Why DO they call it a "Class 150" flange? From whence was the figure "150" derived?

It's not the weight! It's not the pressure rating! I've even run some numbers from table 2 and it doesn't even seem to be the AVERAGE! (I ran an average on the table for the 4 material groups of carbon steel, which is the highest pressure rated group: 130 psig)

Here's hoping someone can clear up a long-standing and nagging question.

-Davis

Thermoseal Contact Information

     Area Representative:
         Davis McConnico
         South East District Manager
         Thermoseal Inc.
         Pensacola, FL.  32504
         voice: 850 / 477-0960
         fax: 850 / 477-0106
         email: <a href="/group/PipingDesign/post?postID=CTVU3CIClcBw_KBCmVXuN92cpHN4LnqL_kBdOpdIWKRUUVOeGK2mygzCxlCJeci1RC7WAJwIdcRreNKvdJ7aa8qGRA">dmcconnico@thermosealinc.com</a>