I am refering to B31.3 ASME committe page-B31 Case 178 "Providing an equation for Longitudinal stress for Sustained Loads In ASME B31.3 Construction" where committe has specified the equation for Sl.

http://cstools.asme.org/csconnect/pdf/CommitteeFiles/15243.PDF

However I read on Becht's site A Code Case(Sustain) is presently under discussion that would provide a specific equation that can be used for the longitudinal stress(Sl) evaluation.

As per my understanding, now we are calculating Sl by summing up Slp, Sb and F/A. This equation seems to be different to me. Can B 31.3 experts throw more light on this ? Does it affect our present sustained stress calculations ?

Best Regards,

Hi Tushar,

I checked the 2004 edition of B31.3. It does not specify the equation for sustained stress.Only thing: The weld joint strength reduction factor has been mentioned( you can get more information about this factor at Dr. Becht's website).So in my opinion we will continue to do it like the old way i.e. slp,sb and F/A.

Regards

Yes, B 31.3-2004 still gives just defination of sustained stress in Para 302.3.5. However if we read the referred document(B31 Case 178), it clearly says " In opinion of the committee, in absence of more applicable data and in accordance with Para 302.3.5, Sl shall be calculated by following equation."

Does it mean commitee interprets the para 302.3.5 by the given equation ? In that case, shall we include torsional effects and use the equation provided by the committe ?

My another question is where is PD/4t (stresses due to internal pressure) component in this equation ?

Best regards,

Tushar,

I agree with your view on "Does it mean commitee interprets the para 302.3.5 by the given equation ? In that case, shall we include torsional effects and use the equation provided by the committe ?".This is the eqn. we have to go in for, including torsion.

PD/4T ( or in whatever way you want to express the longitudinal pressure stress)is a part of Fa/A.

Regards

Hi Tushar,

Hi Tushar,

It is an interesting question.

As Anindya points out, an inquiry from a Code user will result in an interpretation OR A CODE Case as a response from the Committee. An interpretation is the Committee's answer (yes or no) to a question about something that is already written in the Code. A Code Case is issued when a question is asked about something that is not currently in the Code. When the Committee issues a Code Case, it has a finite amount of time to either write one or more new rules into the Code addressing the issue addressed by the Case or to withdraw the Case. This allows the Committee sufficient time to give due consideration to the question regarding something that is not in the Code and to develop new rules consistent with the overall philosophy of the Code. Sometimes the Committee will, at the end of the day, decide that the issue should not be addressed by the Code and then they will withdraw the Case. In this Case, the Committee issued Case 178 and put the issue on their agenda for a good look. The Committee will take some time and they will (in my opinion) write one or more new rules in which they include an explicit equation (but I could be wrong).

When a Code Committee does not provide an explicit equation for calculating any specific stress, it is the responsibility of the piping engineer to use a creditable equation from a reliable source. The source of the equation should be documented.

The issue of calculating stresses due to sustained weight and pressure (also know as "additive stresses") has been discussed at length in all the B31 Pressure Piping book section committees. What are the sustained loadings that act together (additive) in the pipe? Well, obviously there is the sustained internal pressure (resulting in a tensile stress), there is the bending stress due to weight (resulting in a compressive stress at the extreme fiber at the top of the pipe and a tensile stress at the extreme fiber at the bottom of the pipe) and there may be some other compressive or tensile stress resulting from some other axial force acting along the longitudinal axis of the pipe.

As B31.1 (Power Piping) is fairly creditable, I have used the their sustained stress equation for years in the absence of an explicit B31.3 equation.

B31.1 gives us the equation for Sustained Stress as:
Ss = ((P x Do) / (4t)) + ((i x Mb) / Z))

For internal longitudinal pressure we have:
Slp = (P x Do) / (4t)
Remember that the circumferential pressure ("hoop") stress was addressed in choosing the wall thickness.

For bending stress due to weight we have:
Slb = (i x Mb) / Z
.....and the question has always been should the moment (Mb) be SRSS of just bending or should it include torsion as B31.1 and B31.3 Case 178 do.

One thing that interests me about the equation that is provided by Case 178 is that it leaves the calculation of the “stress due to axial loads” (Sa) open to discussion. Case 178 defines:

Sa = Fa / Ap
Where Fa is defined as the TOTAL longitudinal force due to pressure, weight and OTHER SUSTAINED LOADINGS.
And Ap is defined as the cross sectional area of the pipe wall.

The B31.1 equation considers pressure and weight (as discussed above) but the B31.3 Case 178 does it in a different way and it asks us to also evaluate any other sustained “longitudinal” loadings (e.g., forces). This would close a “loop hole” that we have discussed for years – sustained stresses due to axial forces other than internal pressure. An example: what if you had a vertical pipe with a very large (heavy) valve located along its length. If there is a support on the vertical pipe above the valve that section of the pipe would be in (axial or longitudinal) tension. If the support were on the vertical pipe below the valve that section of the pipe would be in (axial or longitudinal) compression. Axial forces of that kind have eluded being captured into a Code equation for a long time. Another issue here is that if there is a large enough axial compressive force it might cause buckling before the calculated stress (F / A) would be near the Code allowable stress. It is hard to picture a piping system where there would be a large sustained axial force in a horizontal run of pipe (other than internal pressure, especially if there are expansion joints). It is not so hard to picture a large axial force caused by thermal expansion.

So if the equation in B31.3 Case 178 goes into the "book" it will break new ground (but B31.3 has been doing a lot of that lately).

Regards, John.

The Code Case does not change the code rules. The existing base codes rules provide a verbal description which does not address stress indexes for sustained loads.

The word you must pay attention to in the inquiry is "may." The Code Case was written to introduce a specific equation that may be used. It does not replace the Code rules, it is in addition.

There are a number of good reasons to introduce it as a Code Case, including getting the change in sooner, giving writers of piping stress programs time to add that equation to their programs, and giving people time to become familiar with it.

It may be brought into the base code in the future.

Note that Appendix P was added in a similar fashion, but as an Appendix, as alternative rules.

This Code Case is what was being referred to in the article on the Becht website.

Thanks to all for valuable reply.

Best Regards,