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Pressure Rating for Dual Cert. Flange (304/304L)?

Pressure Rating for Dual Cert. Flange (304/304L)?

Pressure Rating for Dual Cert. Flange (304/304L)?

I'm hoping someone can verify my understanding of the pressure ratings of dual rated flanges ASME B16.5.  I only have the 1996 ed. so bear with me on the paragraph numbers.  

My understanding is that either of the applicable flange rating charts may be used (per paragraph 2.7 and 4.1.2(d)) as long as the material meets the requirements of both material specifications.  So for example, a dual rated 304/304L flange can be rated per Table 2-2.1 or 2-2.3.  

Is this correct?



RE: Pressure Rating for Dual Cert. Flange (304/304L)?


Yes, for a generic B16.5 application.

If this is going on a Section VIII vessel, then you need to choose one of the two material grades and stick with it throughout the vessel documentation. For example, you cannot use the L grade for welding related issues and the straight grade for pressure rating.


RE: Pressure Rating for Dual Cert. Flange (304/304L)?

jte, gotta disagree.  304/304L is both, at the same time.  Strength calc's can be made using the stronger 304-plain table values [up to 1000-deg F], and weldability and corrosion calc's can be based on the 304L material properties.  All the difference is, is that without using extra Carbon - 304L - this material has achieved the properties of 304-plain.

Caveat:  For 304L to be used above 1000-deg F at the same strength as 304-plain and 304H, it must have at least 0.04% carbon '4 points'.  That is what "Note (5) of of Table 1A" in the interpretation is telling us.  Thus a careful reading of each CMTR for each piece of material is critical.

RE: Pressure Rating for Dual Cert. Flange (304/304L)?


Can you back up that statement? It would seem to contradict the Code.

Quote (II-D_App_7):

If a material is marked with specifications, grades, classes, or types, it may be used with the allowable stresses, design stress intensities, or ratings appropriate for any of the markings on the material, as long as the material specification, grade, class, and type is permitted by the code of construction governing the boiler, vessel, or component in which the material is to be used. However, once the designer has selected which marking applies (specification, grade, class, type, etc.), the designer must use all the design values appropriate for that selection and may not mix and match values from any other specifications, grades, classes, types, etc., with which the material may be marked.


RE: Pressure Rating for Dual Cert. Flange (304/304L)?

jte:  the section of the code quoted is related to mechanical properties only- the "design values" mentioned.  You can't mix and match these properties between grades, but clearly you're going to design using the grade which has the superior mechanical properties.  Dual certified 304/L or 316/L meets all the requirements of BOTH grades, so it is safe to design using the straight grade mechanical properties (i.e. the higher SAS values).  

The low carbon content is there to avoid sensitization due to chromium carbide precipitation in the HAZ during welding, and you can count on this for corrosion resistance from dual grade material.  Neither grade can be used above 1000 F any more- the H grade is required.

RE: Pressure Rating for Dual Cert. Flange (304/304L)?


I think we're in agreement. My point is simply that any formal code calc's and procedures must be consistent as to a single grade. I would not expect to see a U-1 form listing the shell as being fabricated of 304 / 304L material.

Yes, as a vessel designer I tend to choose the one with the higher allowable stress values. At the same time, I may be in communication with my materials / welding engineer and they are happy since I'm using the dual grade. But those materials considerations are non-Code issues.


RE: Pressure Rating for Dual Cert. Flange (304/304L)?

Thanks to everyone for their input.  Just for furhter clarification - the application is process piping designed to ASME B31.3 (attached to a section 8, div. 1 vessel).  L-grade piping and flange materials are required for corrosion resistance to the process fluid.  

There are a few instruments within the piping system, but the flange rating (which ever applies) is the limiting component in terms of allowable pressure... I'm just trying to figure out if that pressure is 600 psig @ 200ºF, or 505 psig @ 200ºF for dual rated material.  Based on everyone's feedback, it appears that it's my choice and I could select 600 psig if I wanted to maximize, or 505 psig if I wanted to minimize... correct?

jte - regarding your last post, I understand your point with respect to using a specific vessel material for code calc's and assume the same would apply to each specific component in a pipe system (e.g. if you had two dual rated flanges, the designer could choose to design one per 304 and the other per 304L, but could not change the selected spec. mid design without starting over completely).  Am I understanding this correctly?

What about a situation where a 304L vessel had 304/304L dual rated flanges?  For the sake of the example, lets say we were trying to determine the limiting component and the head/shell MAP fell between the 304 and 304L flange rating (say 550 psig @ 200ºF)... If all the flanges were the same material / rating, what would govern the MAP?  It seems like the designer could simply choose... if they wanted the head / shell to limit the design, 304 allowables could be used.  If head / shell were not to limit, then 304L could be used.  Is this correct?  Seems arbitrary....

Thanks again!


RE: Pressure Rating for Dual Cert. Flange (304/304L)?

ya know, the piping service specs require 304l...that's a given

if it were me, i would use the 304l flange rating.

after all, you may get a l grade flange that is not dual certified.

most of the time, the flange ratings set the max allowable pressure for a piping spec.

RE: Pressure Rating for Dual Cert. Flange (304/304L)?

Customer specifications can also limit your ability,
minor overviews can make your vessel costly.
I got a vessel order for 316L,
of course I can use dual cert and design to 316 with thinner
materials, but how do I show the customer that his vessel will meet specifications since the customer require docs not just words. If designed to 316 then the Data report document will show 316.
So I have to use 316L and spend a little more money to compensate for T.

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