Local pressure testing of spherical vessels

Abstract

This work pertains to the numerical investigation of the feasibility of a proposed local pressure testing to verify structural integrity of nozzle-to-shell junctions in repaired/altered spherical pressure vessels. The “local pressure testing” involves the use of a small temporary testing closure at the nozzle-to-shell junction to get around the inconvenience of the conventional industry-wide pressure testing of the entire pressure vessel. However, it is essential in deciding on the reliability of such testing approach, to understand the influence of dimensional ratios between the nozzle, vessel and testing closure on achieving equivalent behavior, in terms of stresses and deformations, as compared to those associated with the full conventional testing.The paper presents the findings of a finite element study of the effect of cap size on the stresses near the junction of a cylindrical nozzle with a spherical vessel under internal pressure. The numerical model was verified by comparing its results to available analytical solutions of similar problems. The study focuses on the determination of the minimum required cap radius that will result in a local pressure testing that is equivalent to the conventional full pressure testing, mainly in terms of peak stresses at the junction. Results in the form of plots and empirical equations are presented for a parametric study covering a wide range of dimensions. The results show that the minimum required cap size is linearly related to the nozzle size, but also its value is usually much larger. This leads to the main conclusion that a reliable local pressure testing must use relatively large cap sizes, and that the caps with sizes slightly larger than those of the nozzle may not be acceptable.