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Two-scale stochastic modelling and analysis of leakage through metal-to-metal seals
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-3556-328x
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A seal is a commonly used machine element whose function is to preventthe flow of a fluid from a high to a low pressure region. Metal-to-metalseals, in particular, are used whenever extreme conditions prevent theuse of less expensive rubber seals. Situations where such extreme condi-tions may be encountered are found, for example, in oil wells and nuclearpower plants. In such applications, the failure of a metal-to-metal sealcan become catastrophic, as it might mean the leakage of hazardousfluids to the environment. In order to minimize the risk, it is critical tounderstand the mechanisms controlling the seal’s performance and,if possible, be able to predict capability to prevent leakage on before-hand. Not surprisingly, the surface topography plays a crucial role hereand therefore requires careful consideration when conducting studies ofthis kind. Indeed, it has been shown that even very small details in thetopography (of size of the order of micrometres) can have a large effecton the performance of the whole seal (of size of the order of centimetresor larger). Another complicating factor is the topography’s stochas-tic nature, which makes even the identification of the relevant detailschallenging. Modelling is, in this context, a desirable approach, as itprovides the possibility to easily zoom in those fine details as well asisolate individual parameters. Moreover, it can provide for a predictionon the expected leakage.This work focuses primarily on the development of a model suitablefor studying the mechanisms controlling the performance of metal-to-metal seals and to enable prediction of leakage. To accomplish this, amodel that follows a two-scale approach is proposed. More precisely,the small details in the topography are considered in a local problemconnected to a highly resolved local-scale domain, while the componentlevel features are considered in a global problem allowing for a coarsegrid discretisation of the corresponding global-scale domain. During the present work it was also found that realistic results can only be obtainedif the model explicitly considers the surface topography’s stochastic na-ture. The model was first developed for liquids and was based on theassumption of incompressible and iso-viscous flow. Further work, withthe objective to enable studies of more complex type of flow situations,resulted in a versatile transformation translating results for incompress-ible and iso-viscous to compressible and piezo-viscous fluids and viceversa. This means that, the flow of gases and other more complex fluidscan be studied by combining the model for the simplistic incompressibleand iso-viscous flow with this newfound transformation.Using the model developed, the sealing performance of metal-to-metal seals during load cycling, i.e., by gradually increasing the load toa certain value and then releasing it again, is studied. The scope of thisstudy is to assess how the plastic deformation that the metal surfacesundergoes during the loading phase can affect the leakage during unload-ing. It is shown that this results in a change of the original topographythat may lead to a better sealing performance during unloading. Themain result obtained is, however, that given the right conditions, the ap-plied load can be released considerably (even down to half of the reachedbefore starting the unloading) with only a small increase in leakage asa result. This shows the seal’s capability to prevent leakage evenif an unexpected reduction of load occurs and is therefore is a valuabledescription of the robustness of the seal.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
URN: urn:nbn:se:ltu:diva-66694ISBN: 978-91-7790-009-2 (print)ISBN: 978-91-7790-010-8 (electronic)OAI: oai:DiVA.org:ltu-66694DiVA, id: diva2:1159172
Public defence
2018-02-02, E231, Luleå tekniska universitet, 09:00
Opponent
Available from: 2017-11-28 Created: 2017-11-21 Last updated: 2018-05-22Bibliographically approved
List of papers
1. On the loading and unloading of metal-to-metal seals: A two scale stochastic approach
Open this publication in new window or tab >>On the loading and unloading of metal-to-metal seals: A two scale stochastic approach
(English)Manuscript (preprint) (Other academic)
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-66602 (URN)
Available from: 2017-11-16 Created: 2017-11-16 Last updated: 2018-05-22
2. On compressible and piezo-viscous flow in thin porous media
Open this publication in new window or tab >>On compressible and piezo-viscous flow in thin porous media
(English)Manuscript (preprint) (Other academic)
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Identifiers
urn:nbn:se:ltu:diva-66599 (URN)
Available from: 2017-11-16 Created: 2017-11-16 Last updated: 2018-01-13
3. On the flow through plastically deformed surfaces under unloading: A spectral approach
Open this publication in new window or tab >>On the flow through plastically deformed surfaces under unloading: A spectral approach
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part C, journal of mechanical engineering science, ISSN 0954-4062, E-ISSN 2041-2983, Vol. 232, no 5, p. 908-918Article in journal (Refereed) Published
Abstract [en]

This study considers flow through the gap left between two surfaces during unloading, in other words, when an applied load is gradually reduced after loading to a state where plastic deformation occurs. In particular, the permeability of the gap is studied. It was found that a substantial reduction of the applied load is required before the permeability starts to increase significantly. The explanation for this phenomenon is given by the combination of components with different wavelengths present in the surface. Components with long wavelengths deform elastically and those with shorter wavelengths may also deform plastically. We found that plastic deformation acts to keep the permeability nearly constant at the beginning of the unloading and elastic spring-back is responsible for the rapid increase at lower loads. This principle constitutes a basis for the strategy that was developed in order to predict the load at which the rapid increase of permeability starts.

Place, publisher, year, edition, pages
Sage Publications, 2018
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-62112 (URN)10.1177/0954406217690180 (DOI)000429913000015 ()
Note

Validerad;2018;Nivå 2;2018-02-26 (svasva)

Available from: 2017-02-22 Created: 2017-02-22 Last updated: 2018-04-27Bibliographically approved
4. Modelling of leakage on metal-to-metal seals
Open this publication in new window or tab >>Modelling of leakage on metal-to-metal seals
2016 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 94, p. 421-427Article in journal (Refereed) Published
Abstract [en]

Surfaces in a typical seal exhibit both waviness and roughness. The influence of the interaction between these two scales on the leakage behaviour is expected to be relevant. Therefore, a model, which can study it, is developed here. The model is composed of state-of-the-art models for the contact mechanics between rough metal surfaces and for the liquid flow through the rough aperture in-between them. Correlation between percentage real contact area and actual contact topology and leak rate was confirmed through numerical analysis. Small changes in relative position between the contacting surfaces showed large deviation in leak rate. The validity of the model was justified by comparing results from numerical simulations using the model and experimental results found in literature qualitatively.

National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-6971 (URN)10.1016/j.triboint.2015.10.003 (DOI)54c9146f-0295-467a-a864-3d65128b9088 (Local ID)54c9146f-0295-467a-a864-3d65128b9088 (Archive number)54c9146f-0295-467a-a864-3d65128b9088 (OAI)
Note
Validerad; 2015; Nivå 2; 20151026 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-02-20Bibliographically approved
5. A stochastic two-scale model for pressure-driven flow between rough surfaces
Open this publication in new window or tab >>A stochastic two-scale model for pressure-driven flow between rough surfaces
Show others...
2016 (English)In: Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences, ISSN 1364-5021, E-ISSN 1471-2946, Vol. 472, no 2190, article id 20160069Article in journal (Refereed) Published
Abstract [en]

Seal surface topography typically consists of global-scale geometric features as well as local-scale roughness details and homogenization-based approaches are, therefore, readily applied. These provide for resolving the global scale (large domain) with a relatively coarse mesh, while resolving the local scale (small domain) in high detail. As the total flow decreases, however, the flow pattern becomes tortuous and this requires a larger local-scale domain to obtain a converged solution. Therefore, a classical homogenization-based approach might not be feasible for simulation of very small flows. In order to study small flows, a model allowing feasibly-sized local domains, for really small flow rates, is developed. Realization was made possible by coupling the two scales with a stochastic element. Results from numerical experiments, show that the present model is in better agreement with the direct deterministic one than the conventional homogenization type of model, both quantitatively in terms of flow rate and qualitatively in reflecting the flow pattern.

National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Fluid Mechanics and Acoustics Mathematical Analysis
Research subject
Machine Elements; Fluid Mechanics; Mathematics
Identifiers
urn:nbn:se:ltu:diva-14714 (URN)10.1098/rspa.2016.0069 (DOI)e22df646-269f-4f4e-80d7-43e89b5a630a (Local ID)e22df646-269f-4f4e-80d7-43e89b5a630a (Archive number)e22df646-269f-4f4e-80d7-43e89b5a630a (OAI)
Note
Validerad; 2016; Nivå 2; 20160816 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-02-20Bibliographically approved

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Perez Rafols, Francesc

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