Definition of Classes 5

[STALguy]

I have been searching high and low for an explanation on how the pressure Classes are defined.

In ASME B16.5 (and others) the flanges are divided into Class 150, 300, 600 etc. Could someone please explain to me where these numbers stem from?
I erroneously assumed they where nominal pressures in PSI for some basic steel at room temperature (as PNs are) but I don't see the connection.

 

[pennpiper]

STALguy,
Since you are listed as "Industrial" and asking this question you must not know much if anything about piping. I will try to give you an answer that is simple to understand.
In piping we have the term "Nominal pipe size" (NPS). Example 6"NPS. This does not neccessarally mean that the size (6" NPS) of the pipe is actually the outside or the inside diameter. It is in name only.

Flange pressure Classes are somewhat the same. A Nominal 150# line class is not exactly, only, or limited to 150 psig.

The internal pressure of a line class will depend on the material of the pipe (Carbon Steel, Alloy, etc.) and the operating temperature.

For Carbon Steel 150#
@ -20 to 100degrees F is good for 285 PSIG
@200 degrees F it is good for 260 PSIG
@ 500 degrees F it is good for only 170 PSIG
etc.
@1000 degrees F it is only good for 20 PSIG

You notice that as the temperture goes up the pressure allowable goes down however the "Nominal Flange Rating" stays the same.

There are charts available that cover the basics of this, but you should only use the services of a qualified Piping Material Engineer to advise you or develop proper line class specifications.

I hope this helps.

 

[JLSeagull]

Anyone can use the tables in ASME B16.5, even those of us from the electronic end of the business. Everyone who pays any attention to anything would be curious about this issue.

Clues exist in the forward of ASME B16.5. Until 1973 the flange classes were called 150 lb, 300 lb, etc. Perhaps the pressures and temperatures for the classes in the pound rating era corresponded to steam. In other words, Class 300 or 300 pound carbon steel flanges may have beens suitable for 300 psig saturate steam applications. Perhaps those interested in the history of these standards can contribute.

 

[STALguy]

pennpiper, thanks, sorry for being unclear but as you could guess my native language isn't English.

I do understand about P/t ratings and material types but I think JLSeagull is more on target. That is, I would like to now the source of the Class numbers. How did they come about in the mists of history?

 

[jte]

STALguy-

There is actually some consistency with the ratings but... there's always an exception and in this case its with Class 150. Take a look at B16.5-2003 Annex B2 (Annex D2 in the 1996 ed). The ratings are a function of the class and the allowable stress at temperature. In psi units, the rated pressure at temperature pt is simply the flange class times (the allowable stress / 8750).

So for a CL-300 flange in material group 1.1 (SA-105) at 400°F the allowable stress (B2.2) is governed by yield and is 60% of 30.8 ksi or 18 480 psi. So pt = 300 psi (18480 / 8750) = 633.6 psi. Compare that with the rating listed in Table F2-1.1 which is 635 psi at 400°. Close enough for me...

The system is the same but different for Class 150. I don't know why - probably an experience based situation. For CL-150 the pressure basis is 115 psi instead of 150 psi, and the other (ceiling) limits are different.

jt

 

[GregLamberson]

Don't forget aboput API classes, mainly for higher pressure, but API (based on API 6A) also has 7 classes.

They are:
Class 2000
Class 3000
Class 5000
Class 10000
Class 15000
Class 20000
Class 30000

Each "class" designation corresponds directly to the MAWP, for example, Class 2000 has an MAWP of 2,000 psi, Class 3000, 3,000 MAWP, etc. That is predicated on an API temperature class designation of U (0F-250F) and a retained fludi rating of A (general service).

API's are used for high pressure, well head applications, manidolds, etc. and they are generally more expsnive.

There are dimensional compatabilities:

API 2000 = ANSI 600
API 3000 = ANSI 900
API 5000 = ANSI 1500

When mating we always use the ANSI flange for rating.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com