What's behind ANSI B16.5 Class Ratings 1

[PDPumpGuy]

I've always been curious into what's behind an ANSI B16.5 flange pressure rating and the "class" that it's normally referred to. For example, and CL600, or 600#, flange actually is rated for about 1,440 PSI in typical materials and services. Where did the 600# come from?

Thanks

 

[heymans]

Dear,

You have overlooked the fact that all material (and pressure) -test in ASME code are done at high temperature (temperature where material yields without additional force). Other values at lower temperature are derived from these high temperature results .( Thus the basis of ASME are these high tmeperature values.) This temperature-point is for standard Fe materials 800-850 °F.
Look at this in mind at the pressure temperature ratings in B16.5 for materialgroup 1.1, 1.2, 1.3, 1.4,...
For example 600# ==> you will see that for 850°F the pressure is +/- 600 psi (600#). The same counts for all tables. (This high temperature point is for every material different!)

The only exception is the 150# class. The reason for this is for me still a misery.

 

[Ashereng]

150# used to be a lot cheaper than 300#.

Today, the gap is closing/not as great.

Don't know if that is the original reason, but it may be one reason why people specify 150#.

 

[xcrosby]

I believe that the class designation are based on saturated steam. For example, when these class designations were established, with the flange materials in use at the time, that a class 600 flange was good for 600 psi saturated steam. As materials improved the actual pressure ratings increased but the class designation remained.

 

[Ashereng]

I believe xcrosby is correct regarding a Class 600 originally being suitable for saturated steam at 600 psi (which has a temperature of around 1100 °F I think).

I am not sure what xcrosby meant by "As materials improved the actual pressure ratings increased but the class designation remained.".

ANSI flange tables have a whole host of materials, and temperature/pressure relationships. Different materials, of course have different temperature/pressure relationships - not so much better/worse, just different.

Also keep in mind, the ANSI flange tables by class has taken into account that different pressure class flanges are constructed differently (eg 300# flanges are much more stout, have more material, than 150# flanges, and different bolting patterns too).



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[JLSeagull]

Perhaps steam was the original basis. The word "steam" is not found in the current ASME B16.5. The boiler code is mentioned in the forward.

"... comments from users and changes in the ASME Boiler and Pressure Vessel Code led to significant revisions in the Class 150 rating basis, and, in the ratings of stainless steel and certain alloy steel flanges and flanged fittings in all rating classes. Extensive public review comments led to addition of considerations for flanged joints, for bolting and gaskets, and of marking requirements. To avoid
frequent and confusing changes in ratings as further changes in Code allowable stresses are made, it was agreed with Subcommittee N to leave ratings alone unless the relevant Code stress values are changed by more than 10%."

 

[jte]

Take a look at the Annex to B16.5 which is titled Methods for Establishing Pressure-Temperature Ratings[-i]. In the 2003 edition its Annex B while in the 1996 edition its Annex D. Some of the calc's are based on using the flange class (basically) as the design pressure. Other considerations include deflection limits.

As to how the original classes were chosen... I also believe that it goes back to steam pressures/temperatures.

jt