Session: DA-08-03 Practical Applications of FFS

Paper Number: 107570

107570 - Numerical Analysis of Permanent Deformation in Pressure Vessels Due to Weld Overlay 

Pressure vessels are regularly exposed to corrosive chemicals and oxidizers which could cause reduction of material from vessel internal diameter and reduce the wall thickness. Weld overlay (WOL) is performed to deposit material layer on either the internal or external diameter to compensate for lost material so that the pressure vessel satisfies API 579 Fitness for Service guidelines. In addition to being a remediation for general and local metal loss, WOL is also employed as a remediation against crack like flaws developed in pressure vessels. The WOL process deposits a layer of molten material on remaining shell base material. During this process the base material shows rapid heat buildup, which is then subsequently, dissipated through conduction and convection. The change in the temperature causes localized expansion and contraction of shell which induce thermal stress and permanent deformation. It is important to analyze the permanent deformation as it can impact the performance of pressure vessel and, thus, to ensure that the pressure vessel satisfies the API 579 FFS guidelines. In addition, the material buildup also results in a change in the shell geometry and hence API 579 – 4.6.2.4 also recommends performing a follow up analysis to ensure continued acceptability of the design.

Several factors such as material composition, material melting point, weld speed, weld zone thickness, voltage, thickness of existing and deposited material etc. can affect the resulting thermal stress and permanent deformation. Testing to analyze the impact of these parameters is expensive in terms of time and money and, thus, computer simulations could be advantageous. In addition, a computational parametric approach could also help identify optimal overlay patterns which would minimize shell distortions while minimizing overall process costs.

This study presents methods to analyze the effect of weld overlay on the thermal stress and permanent deformation in the pressure vessel. Both the thermo-mechanical sequentially coupled, and fully coupled thermal-structural approaches are analyzed and compared. Effect of variation in the size of heating zone along with two different overlay patterns on the permanent deformation and its impact on the RSF (remaining strength factor) of the structure are studied. The method introduced could further be expanded to perform parametric studies to understand the effect of each input parameter on the permanent deformation in pressure vessel at the end of weld overlay along with its impact on the overall acceptability of the structure post the remediation process.

Presenting Author: Mandar Kulkarni Stress Engineering Services, Inc.

Presenting Author Biography: Dr. Mandar Kulkarni has over 10 years of engineering experience and has been working with Stress Engineering Services since 2012. He mainly practices in the areas of solid mechanics, fatigue and fracture mechanics covering a range of materials and applications straddling different industries ranging from oil & gas to the medical devices and automotive. He also holds a professional engineering license in the state of Texas.

As a finite element analysis expert he has performed a number of static, dynamic, steady-state and transient heat transfer and coupled thermo-mechanical evaluations with geometric and material nonlinearities on systems such as pressure vessels and piping, and medical devices.
He has experience in establishing credibility of the simulations per industry standards and help with regulatory submissions. His efforts are supported through extensive material characterization and design qualification testing.

Authors:

Akshay Dandekar Stress Engineering Services Inc
Andrew Gordon Stress Engineering Services, Inc.
Mandar Kulkarni Stress Engineering Services, Inc.