Chapter 21. Evaluation of Flange Leakage due to Thermal Bowing and Shock
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Published:2022
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During a flaring event, some operating conditions in an LNG plant cause liquid refrigerant to be introduced into a flare header causing large temperature gradients between the bottom and top wall and lead to vertical and lateral displacements along with rotation of the horizontal header. This thermal bowing is a serious problem with risk of flange leakage due to differential thermal expansion between bolts and flange and pipe cracks at high stress concentration location as it lifts off the supports. This paper evaluates the introduction of warm gas into the flare header to prevent a flange leakage as a mitigation attempt.
This paper recounts the use of Fluid Structure Interaction (FSI) to study bowing and flange leakage mitigation. The liquid flowing from the lateral into the main header pipe is multiphase in the dispersed, stratified, slug or annular flow régime. Multiphase flows with heat transfer are analyzed using Computational Fluid Dynamics (CFD) and conjugate heat transfer for steel. The temperatures on the metal are then transferred to a Finite Element Analysis (FEA) and analyzed for thermal stresses and deformation due to bowing. Elastic plastic Service Criteria Assessment per ASME BPVC Sec-VIII, Div.2 Part 5.2.4.3 (b) is performed to calculate Von-Mises, peak stresses and derive membrane and bending stresses on the hub, bolts, and gasket. These stresses are compared to ASME standard allowable limits. Based on these results, risk of flange leakage is evaluated, and mitigations are recommended to reduce the risk.