Doing Load Case Analysis in Correct Way
Up to B31.3-2012, when using eq. (1b) in para. 302.3.5 (d) to get liberal “Allowable Displacement Stress Range, SA“, a typical piping stress program sets “SL” as sustained stress at installation condition (with all supports active). This practice basically fails to satisfy the intent of the code, because apparently, a “hot” sustained case might have a different support condition, as illustrated in B31-3 Appendix S, Example 2.
In B31.3-2014 and 2016, “SL” in para. 302.3.5 (d) is precisely defined as “the maximum value of sustained stress, considering all support conditions” (quoted from B31.3-2016). That essentially declares that the above practice for liberal expansion allowable is wrong.
In WinPIPE, we have designed a new scheme to comply with the code requirement:
- All operating cases should be processed to get support status at all operating conditions
- “Hot” sustained cases might be required, depending on each operating case:
a. If there is no lift-off, a “hot” sustained case might not be required.
b. If there is lift-off, a “hot” sustained case needs to be set up using new support status.
c. In some situation, engineers need to manually adjust restraints, creating a “hot” sustained case to check different scenarios.
- All “cold” and “hot” sustained cases should be analyzed to meet sustained stress requirement.
- Different Max-SLs should be required using combination of “cold” and “hot” sustained cases
- Expansion/Fatigue analysis should select one of these Max-SLs, depending on its range
All these can be done easily in WinPIPE, as shown in Products section “Improved Load Case Analyzing Facilities”.
Most of time, a hot sustained case could be easily set up by just getting support status directly from an operating case. However, there are situations that call for engineers’ experiences and judgement. For example, if a pipe model has a lot of +Y supports, although an operating condition doesn’t lift off the supports, it does alleviate the weight on the supports (therefore reduces friction force in horizontal directions). In this scenario, a sustained horizontal force might cause a hot spot in the pipe system. Turning off horizontal restraints manually could help engineers to catch this problem.