Allowable stress for SA-240 304

[MikeG7]

Can anyone shed light on why there are 2 listings in ASME Section II D for SA-240 304 with different values of allowalbe stress.

I guess I'm guilt of not reading the lengthy preamble and references (and cross references) to notes etc., but anyway I would appreaciate some input. For example, the first listing at 100 deg C gives allowable stress of 113 MPa and 137MPa for the next listing of the "identical" material??

Thanks in advance.
Michael

 

[gr2vessels]

Michael,
Read the notes related to each material and come back with questions for clarification if required. Basically, when the application allows, you can use higher allowables.
Cheers,
gr2vessels

 

[MikeG7]

Thanks gr2vessels

Having now read the notes, it basically says you can use the higher values if your design permits greater deformation which is what you pointed out. This kind of leaves the door open to interpretation would'nt you say? It leaves it open to the designer to use their judgement that they can use the higher values for aspects of the the design that permit greater deformation. Now that's a good thing because good engineering judgement is a pre-requisite for pressure vessel design but what I'm trying to do is document my design methods in a formula for use repeatedly like a spreadsheet. I also have "cookbook" programs like Compress but I like to work through the calcs in my way to get a greater understanding first.

I suppose the answer to my own question is to include, by default, the lower values in my spreadsheets (to be on the safe side) and if the outcome is marginal, then I can apply the higher values with judgement.

Another point which I find highly confusing is why the SA-240 304 listings I refer to fall under the table 1A for "Ferrous metals" - it's news to me that 304stainless steel is a ferrous metal. Very confusing!

 

[SnTMan]

MikeG7, in general, the higher allowables are used for shells, head, tubes, nozzle pipe and so forth while the lower allowables are used where a gasketed joint may exist such as flanges and tubesheets. Some customer specs address the usage as well, such as that only the lower allowables may be used for any component. Unless a specification requires otherwise it is as you say a matter of engineering judgement.

As to ferrous / non-ferrous, most of the stainless steels contain iron as the major alloying element. Not that there are not some grey areas such as AL-6XN (N08367) which may be classified as ferrous by ASTM but as non-ferrous for ASME pressure vessel use.

Regards,

Mike

 

[MikeG7]

Thanks SnTMan (Mike)

 

[EdStainless]

These days even AL-6XN is ferrous, N number not withstanding, since it has more iron in than any other element.
ASME will catch up soon, the B (non-ferrous) specs are being eliminated from ASTM for these stainless grades.

Only two different allowed stress levels? Most tubing specs have three or four different stress levels depending on the notes.
In many heat exchanger applications the decision on the "deformation note" is based on temperature and pressure cycling. Applications that cycle a lot or rapidly tend to prohibit it use, steady state application tend to allow it.


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