Modelisation d'une vanne a tournant spherique munie d'un joint de polymere viscoelastique

The VELAN-Inc. Company, which is a leading global manufacturer of industrial valves, has remained competitive through continuous improvement of its products. The valves are elements of a plant that must be especially strong in advanced fields such as nuclear or chemical. Seals used in valves are elements likely to deteriorate over time. These seals are mostly made of polymer materials and have a viscoelastic behavior which makes them susceptible to creep. A particular valve called ball valve was studied and depending on the conditions of use, three materials are proposed for the seals (PTFE, TFM or PEEK). VELAN had previously developed an elastic finite element model of this valve, however, this model does not take into account the viscoelastic nature of the materials of the seals and no simulation of the evolution of seals over time was therefore possible. VELAN therefore wished to develop a new finite element model that can better predict the behavior of seals over time and in particular the sensitivity to creep. The hypothesis of using a linear viscoelastic model was taken to describe the behavior of seal materials. The development of this model required the knowledge of the behavior law of each material that we determined by creep tests in compression. These tests are the method of loading the most representative of the actual solicitation of a seal.;A creep device in compression has been developed to obtain the viscoelastic properties of materials. Tests under different stress levels were conducted during a period of one week. These experimental data have shown that PTFE and TFM did not have a linear viscoelastic behavior. However, data from each creep test was approximated by a linear viscoelastic model. Additional tensile and compressive tests helped us determine the Poisson ratio. The parameters of the behavior of each material previously identified were then entered as required by the finite element analysis software used. The finite element model of the valve has been developed based on the plans of the geometry of the different parts. The simulation results were compared with experimental data from tests that VELAN had previously performed on seals mounted in the valve. The correlation between the model and testing is not as good as expected because there is an initial deflection difference between the simulations and tests. Moreover, the elastic return of the seal is greatly overestimated by about a factor of two in the simulations. However, many factors explain these differences. Plasticity was involved in the seals at the pressure used for testing but, plasticity is not taken into account in the model. Moreover, seals and samples may have not been machined from the same material. Nevertheless, the assumption that the materials follow a linear viscoelastic behavior cannot match the experimental data supplied by VELAN. A new viscoelastic plastic model would probably be more appropriate but it would require to redo many creep-recovery tests.