Modelling and numerical analysis of leakage through metal-to-metal seals

Sammanfattning: Metal-to-metal seals are critical components as their failure can lead to leakage of hazardous fluids to the environment or to fatal failure of the systems they operate on. Most systems are subjected to increasingly more demanding conditions and deeper knowledge about how different parameters affect the leakage is necessary to design seals with the desired performance. Fundamental knowledge can be obtained by means of numerical simulations, since it can provide in-situ information which would be extremely difficult, if not impossible, to obtain by means of physical experiments only. Moreover, in the virtual experiments it is possible isolate the effect of variations in a single parameter. However, no model that can serve as a predictive tool and thus has been tested against experimental results has been found in literature. The reason for this is the complexity in accounting for both the multi-scale nature of surface roughness and its intrinsic randomness. This lack have defined the main objective of this work, i.e., to develop a model for the leakage through metal-to-metal seals, which can output quantitative results that can be used for comparison against experimental work. This has been accomplished by including the stochastic nature of the surface topography explicitly in a two-scale method. The model constructed following this approach fulfills the requirement of giving a quantitative prediction of metal-to-metal seals. Moreover, it also provides new insight on the expected variability in leakage introduced by the stochastic nature of the roughness. A secondary objective has been to investigate the seal behaviour during unloading, i.e., when the applied load is gradually released after having caused significant plastic deformation. The reason for assessing this topic is that metal-to-metal seals subjected to a certain load cycle exhibit, at any given load, a significantly larger leakage during the first loading than it does during the subsequent unloading for the same load. The numerical simulations of the seal behavior during unloading also confirmed the smaller leakage during unloading. Moreover, it was observed that a substantial load release was required before a significant leakage increase could be detected and that the leakage remained nearly constant up to that point. This is an important finding that can be used when designing seals in order to account for stress relaxation during service live.