Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
(OP)
This probably has been posted before.....I note the Code doesnt really doesnt provide much guidance with regard to when you use the high or the low allowable stress values for 316 and 316L SS, aside from advising you to use your best engineering judgement and to consider the lower values when designing flanged joints etc. Generally I have just used the temperature of operation and / or the magnitude of the design pressure as a basis.
Anybody got any a more defined criteria or useful words of wisdom?
Anybody got any a more defined criteria or useful words of wisdom?





RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
Using this basis, flanged connections and shaft-seals become really the only components where I use the lower stress values (these are just off-hand examples, I'm sure there are many more).
Pressure and temperature don't really affect my choice.
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
As had been said, the lower values are typically used for deformation-sensitive components (flanges, etc). However, you need to have an understand of why that is.
From Appendix 1:
At this point, you may still be wondering why this is an issue. Well, the stress-at-a-point in a vessel component is not uniformly at or less than the allowable stress. In fact, the localized stresses can be substantially more - as much 3 times more (or even greater, in certain circumstances). So, in these types of situation, it is acknowledged that there will be higher stresses - and in the case of the "higher" allowable stresses, almost assuredly, the stresses will exceed yield.
(Of course, an appreciation of the full elastic-plastic stress-strain curve of these materials in needed, as they are usually quite ductile, yet exhibit a low proportional limit).
So, you need to understand the strain-sensitivity of your component. If some plastic deformation is acceptable/permissible, then you are very much welcome to use the "higher" allowable stress values for your design, and still be confident that your vessel will have an equivalent design margin against burst as any other vessel. And, based on experience, the "lower" allowable stress should be used if your component is strain-sensitive. In fact, you should probably do some calculations (FEA is NOT required) to demonstrate to yourself that the selected allowable stress is appropriate; sometimes for strain/deformation-sensitive equipment, even the "lower" allowable stresses are not low enough.
So, figure out, first, what the potential failure modes for your component are, and then select the allowable stress accordingly. And, of course, read the footnotes and the Appendices.
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
In section II part D
Recommendations given for not to use higherstess values for gasketed joint and other application where slight amounts of distortion can cause leakage.
Read general note G5.
Correct me if I am wrong
Thanks
Yp3135
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
TGS4 - Other situations where I have used the low stress values over the high stress values are situations for example where the the design and operating temperature is close to max permitted by ASME (ie. SA240 316L operating around 800 F with design at 825 F...max permitted by ASME VIII is 850 F) with pressures in the 800 psi and up range). Perhaps this is an over design but I sleep better at nite having used the lower values!
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
So, think long and hard before you derate your material. You may be doing more harm than good...
RE: Distinction Between Low and High Allowable Stress Values For Austenitic Stainless Steels
That is a good point, I agree there are situations where stout is certainly not the best approach (cost has generally not been an issue at least in the instances I mentioned). Anyway what I take from this thread is that aside from leakage issues (and potentially stress sensitive areas) most use the higher stress values for the SS in most other situations...so that really answers my original question....in the reverse of what I expected