B31 philosophy question: operating case "allowables" and OBEs?

[KernOily]

Here's a question for you guys. As far as I know, the B31 codes do not require a calculation of allowable stress for the operating load condition (W+D+P). What do you guys do when checking the operating load case? What I've seen done was to check the maximum operating stress magnitude against the B31 allowable calculated for the displacement (expansion) load condition. If the operating < displacement allowable, the system was pronounced to be OK as long as all other Code requirements were met.

Secondly, do you guys ever consider a seismic (or other occasional load) as acting concurrently with the operating load? The nuke codes call this an operating basis earthquake but the B31 codes do not require this check and thus there is no allowable; as you know only an occasional allowable is calculated. Again, similar to the operating load, what I've seen is the operating-occasional load is calculated (W+D+P+U) and then compared to the displacement allowable.

Your thoughts? Thanks!
Pete
pjchandl@prou.com

 

Hi Pete and all.

At this time B31.1 (Power Piping) and B31.3 (Process Piping) do not include guidance for the evaluation of combined operating case loads (T+P+W). The original Mechanical Design Committee's &quot;Flexibility Committee&quot; looked at the loadings ine the early 1950's and they developed a report to the Main MDC recomending separate evaluations of primary (sustained pressure and weight) and secondary (thermal expansion stresses, aka displacement stresses, aka P+W). So, in 1955 the &quot;new&quot; B31.1 Code (that was the only Pressure Piping Code then) incorporated fatigue rules for secondary (expansion) stresses for the first time. From this time (1955) on, the rules for stress evaluation have been the same. If you can find a copy of the Fifth Edition of the Piping Handbook by Sabin Crocker and Reno C. King, you will be able to read some of this &quot;Flexibility Committee&quot; history.

Of course the way that the computer programs calculate the thermal expansion stresses is to &quot;de-superpose&quot; loading vectors. That is, they calculate the T+P+W case and then the P+W case and they subtract the later vector from the former vector leaving the &quot;T&quot; case for comparison to the &quot;allowable stress range&quot;, aka Sa. The &quot;T&quot; case is arrived at this way in recognition of the fact that weight is a significant &quot;restraint&quot; against thermal expansion movements and its effect must be considered in the evaluation of thermal expansin stress range. If someone is interested in the derivation of the allowable stress range equation (S = f(1.25 x Sc + 0.25 Sh)), go to the Caesar II discussion board where that and other Code allowable stress discussions have beaten the subject into submission. The Codes say that if you do not &quot;use&quot; all the allowable stress (Sh) with your primary (P+W) loadings you may add the &quot;surplus&quot; to the second term in the Sa equation, giving you more secondary stress &quot;allowable&quot;. But now I am off on a tangent.

Getting back to the operating case load, the B31 MDC is currently considering a proposal from one of our esteemed members for including an evaluation of the operating case in future editions of the Codes. Stayed tuned.

The seismic loadings (B31 calls them &quot;Occasional&quot; loadings) are only coupled to the primary (P+W) loadings.(see B31.1, paragraph 104.8.1) This is done by adding a third term to the &quot;sustained&quot; (primary) loading case (see B31.1, Paragraph 104.8.2). The chairman of the MDC has been working on proposed rules for seismic loadings for about 15 years (recently he has made some progress). One of these years, those rules are going to be &quot;out of committee&quot; and will be sent up to Main Committee for the review and approval process. As has been observed in other threads, we all use the B31.1 stress equations for both B31.1 and B31.3 (which includes no explicit equations for &quot;sustained&quot; or &quot;occasional&quot; loadings.

Interesting subject (to some of us), thanks for raising the issues.

Regards, John.

 

[kstaylor]

I use the operating case for determining the actual loads on restraints and supports, including equipment loads (Caesar II provides a restraint report that provides the correct magnitudes and signs of the X, Y, Z forces and moments that act on the restraints, signs are opposite those listed in the operating case) and for determining real displacements that I can verify in the field. Until the Code committee(s) develop and publish allowable stress limits for the operating case that is about all you can do with it.