Caracterisation et modelisation des garnitures de presse-etoupe

In the chemical and nuclear industries, valves are used to control fluid circulation. The reliability and the efficiency of this mechanical component are evaluated by their ability to confine fluid inside and prevent escapes to the outer boundary. In the valve, the sealing function is ensured by a packing made up from superposed braided rings. The axial packing deformation generates radial contact pressures on the internal housing wall and the external stem wall. The level of the valve sealing depends on the values of these radial contact pressures. Several researchers proposed analytical models, supported by experimental studies, to characterize the behaviour of the stuffing box packing. These models do not take into account the mechanical and physical properties of the packing rings and the stuffing box assembly.;In the first part of the study, an analytical model is developed to evaluate the lateral pressure coefficients and to evaluate the radial contact pressures at the interface between packing and stem and packing and housing. This model is based on the thick walled cylinder theory. The packing is considered as an elastic but rather incompressible material that follows Mooney-Rivlin law. The model takes into account the mechanical properties of the different assembly elements and their geometrical characteristics. The results of the analytical model were compared successfully with those of a 2D axisymmetric finite element model. While the study showed that the lateral pressure coefficients ratio is close to one, the contact pressures at the packing-stem and packing-housing interfaces depend on several factors including the assembly geometrical characteristics, the packing material and the friction coefficients.;The second part of the study concentrates on the short term relaxation behavior of the packing and its influence on the lateral pressure coefficients and the distributions of axial and radial stresses at the packing-stem and packing-housing interfaces. An analytical viscoelastic model based on the elastic interaction model was used to determine the stress decrease with time. The method used is based on elastic analogy which accomodates successfully the creep-relaxation behaviour of the stuffing box packing. The results of this approach were compared to those of a finite element model with the same viscoelastic properties. The developped analytical model is capable of predicting accurately the change of the axial and radial stresses of the stuffing box assembly with time.;Finally, the last part of the study, concentrates on the characterization of the stuffing box packing using a hybrid method based on experimentation and FE analysis to evaluate the lateral pressure coefficients and the two mechanical characteristics E and v of the two most used packing types in the industry namely flexible graphite and Teflon. The experimental bench takes into account the geometrical characteristics of the real assembly components and therefore incorporates their flexibilities. The second phase of this part was devoted to conducting leak tests. The influence of the compressive load, the fluid pressure and the number of packing were examined. These tests showed that the leak rate is larger when the initial stress is lower or the pressure of the gas to be confined is larger or also when the number of the packing rings is smaller.;This work proposes a new approach to model the true mechanical behavior of the stuffing-box which includes the stem, the stuffing box, the gland and the packing rings and their elastic interaction. The evaluation of the operating parameters and their change with time represent the main objective of this study. To achieve this goal, both analytical and numerical models were developed. An experimental study was undertaken to caraterise the mechanical and sealing performance of packings and validate the developped models.;Keywords: stuffing-box, packings, leak, sealing, mechanical caracterization...