FFS Forum: Understanding Part 8 Assessment of Weld Misalignment and Shell Out-of-Round

Introduction

Shell imperfections such as bulges, cylinder out-of-round, and weld misalignment can result in high bending stresses when the component is pressurized. If the load is cyclic, fatigue problems can develop. But even under static loading, high bending plus pressure stresses can create a fitness-for-service (FFS) problem. 

Evaluation of shape imperfections can be difficult to calculate by hand and often a finite element analysis (FEA) is required. FEA is a great tool, but for a few simplified (but very common) geometries, Part 8 provides valuable techniques for simple and conservative closed-form solutions.

Overview of Part 8

Part 8 addresses:

• weld misalignment – including both peaking and mismatch, and
• shell distortions – including out-of-round and bulges.

Figure 1 shows a sketch of each type of defect.

Peaking and mismatch are cases where the plate centerlines do not line up. It could be because the plates simply are not aligned, as seen in Figure 1(a), or it could be because the plates are of different thicknesses and are aligned on the ID or OD. In either case, the result is an offset in the centerlines of the plates, which creates a moment that puts bending loads on the weld joint (Figure 2).