Increasing demand for seals that can withstand ever higher pressures in severe operating conditions need advanced numerical modelling to simulate these conditions and predict leak tightness.
The prediction and evaluation of leakage and leak tightness is an important issue in a multitude of high-pressure applications, such as valves, flanges or threaded connections. Using finite element techniques, it is in general possible to determine the local contact conditions at the seal on a macroscopic level: contact area and contact pressure in this area.
Macro- and microscopic level
However, the leak tightness of a contact area depends also on the surface topology, which is a microscopic characteristic. Therefore, the assessment of leak tightness requires an evaluation criterion relating both scales.
Over the years, a lot of empirical evaluation criteria have been developed, each with their own application domain. In terms of modelling the leakage a big step forwards was made only quite recently. The Persson method models the contact area microscopically using contact models developed in the field of tribology. This contact evaluation is then combined with results from percolation theory, which state that for a sufficiently large contact area and a uniform contact pressure leakage will occur beyond a well-specified threshold.
Beyond uniform contact
This has yielded a way of evaluating leakage, but the application is limited to uniform contact pressure only. In many applications, such as valves or O-ring seals, the contact is Hertzian and the contact pressure distribution is not uniform but parabolic.
OCAS therefore developed a dedicated test set-up for leakage experiments. A set of samples was produced with varying surface topology. The surface of these samples is measured and the leakage behaviour under high pressure is evaluated.
At the same time a cellular automata model was built and used to model percolation under non-uniform contact pressures in an effort to adapt the Persson model. Finally, the experiments and the modelling results are brought together via a finite element model and compared to each other.
Out of roundness (measured)
Contact pressure (modelled)
Percolation (predicted using cellular automata model)
Click images to enlarge
Related scientific papers
Van Wittenberghe J. ,Vande Voorde J., “Prediction of the leakage threshold for Hertzian contact seals: an experimental approach”, Proceedings of the ASME 2018 Pressure vessels and Piping Conference PVP 2018, July 15-20, 2018, Prague, Czech Republic, PVP2018-84622
Vande Voorde J. en Van Wittenberghe J., “Prediction of the leakage threshold for Hertzian contact seals: a cellular automata model”, Proceedings of the ASME 2018 Pressure vessels and Piping Conference PVP 2018, July 15-20, 2018, Prague, Czech Republic, PVP2018-84404
Ernens D., Pérez-Ràfois F., Van Hoecke D., Roijmans R., van Riet E. Vande Voorde J., Roggeband S. van Haaften W. Vanderschueren M. and Pasaribu H., “On the sealability of metal-to-metal seals with application to premium casing and tubing connections”, Society of Petroleum Engineers SPE/IADC International drilling conference and exhibition, The Hague, 5-7 March 2019.
”Modelling of sealing is highly challenging. There are so many influencing factors. It’s really been an exciting journey.”
