The results from a project to investigate the design and optimisation of a metallic seal ring as part of an end cap for a 140 tonne pressure vessel are presented. A vessel loading condition of 4483 bar (65,000 psi) internal pressure was stipulated. The design of a metallic seal of 762 mm (30″) bore diameter provided a challenging task, due to the extremely high pressure application.

A detailed Finite Element Analysis (FEA) was carried out to evaluate a proposed seal and the initial conclusion was that it required complete re-design. In this work, analysis and interpretation were used to establish the modelling method for the seal. The FEA model that was developed for seal optimisation contained elasto-plastic material response, frictional contact modelling and advanced pressure field application.

To maintain sealing contact pressure, the seal has to overcome differential radial deflections between the vessel and the end cap during pressure loading. The optimisation process developed a novel profiled seal contact that reduced contact stresses by 48%. This eliminated seat damage occurring during pressure loadings, whilst maintaining an adequate contact pressure ratio during a complete loading cycle.

Structural deformations cause the seal contact point to shift, providing the opportunity for liquid to penetrate up to the contact point. Limitations were recognised within the FE software, which allowed unidirectional pressure penetration only. To overcome this limitation, a bespoke user subroutine was written to revise the contact algorithms within the Abaqus FE program. This enabled the model to capture ‘contact pressure zones’ to correctly progress by opening and (for the first time) close in response to seal and seat deflections.

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