Frequently Asked Questions
by Andrew C. Whyte, BSc, MSc, CEng, M.I.Mech.E.
- Q. In the oval pipe analysis the stresses are non-linear with pressure, are the material properties also non-linear?
A. No. The analysis is applicable to materials with linear-elastic properties.
- Q. In the oval pipe analysis what sort of materials bulge or become oval when subjected to internal pressure?
A. The solution is limited to those isotropic materials that exhibit linear-elastic material properties and
that the deflections are small compared to the radius of the cylinder. This essentially limits the solution to metals. Although the aim is to produce a perfectly round cylinder.
In practice the cylinder is often out-of-round as a result of: manufacturing tolerances, welding distortion, pipe bending. As internal pressure is applied the out-of-round cylinder
tends to become more round. In doing so it induces bending stresses across the wall of the cylinder.
- Q. Can the oval pipe analysis be used at pipe bends where the ovality can be high compared to the rest of the pipework system?
A. Although "Haigh's" analysis is strictly for straight pipe I know that it is often applied to pipe bends. You would have to give consideration to
combining the results with other factors, e.g. the pipe bend stress intensification factor, in the assessment.
- Q. Can the oval pipe analysis be used for external pressure or vacuum conditions?
A. No. "Haigh's" analysis is strictly for internal pressure systems. Although in principle there is only a sign change for external pressure the problem is more complex
as cylinders subject to external pressure can collapse due to buckling, especially with increasing amounts of out-of-roundness. The major pressure vessel codes have rules for cylinders subjected
to external pressure.