In flexibility analysis, the response of the system to loads is calculated. The objectives of flexibility analysis are to calculate stress in the pipe; loads on supports, restraints, and equipment; and displacement of the pipe. It is essentially a beam analysis model on pipe centerlines. The fundamental principles include the following:
1. The analysis is based on nominal dimensions of the pipe.
2. The effect of components such as elbows and tees on piping flexibility and stress is considered by inclusion of flexibility factors and stress intensification factors.
3. For thermal stresses, only moment and torsion are typically included. Stresses due to shear and axial loads are generally not significant. However, the Code, via para. 319.2.3(c), directs the analyst to include these stresses in conditions where they may be significant. This paragraph states the following:
“Average axial stresses (over the pipe cross section) due to longitudinal forces caused by displacement strains are not normally considered in the determination of displacement stress range, since this stress is not significant in typical piping layouts. In special cases, however, consideration of average axial displacement stress is necessary. Examples include buried lines containing hot fluids, double wall pipes, and parallel lines with different operating temperatures, connected together at more than one point.”
4. The modulus of elasticity at 21°C (70°F) is normally used in the analysis. As a result, no elastic modulus adjustment is required. For a more detailed discussion, see below, Section 8.8.