Bernard Tournerie

TEHD Lubrication of Mechanical Face Seals in Stable Tracking Mode: Part 2—Parametric Study

In this paper the influence of the design and operating parameters on the TEHD behavior of Mechanical Face Seals (MFS), in steady dynamic tracking mode, is analyzed for two different types of applications. First, an extensive parametric analysis of a typical MFS with very low leakage is presented. Then, the influence of rotational speed, sealed fluid temperature and pressure, rings materials, shape, waviness and misalignment of the rotor, are successively examined. The use of an original dimensionless parametric analysis leads to a very simple and overall description of the results. It is shown that ignoring the thermoelastic distortions of the rings could be misleading as far as the evaluation of MFS performance is concerned. In the final part, a hydrostatic MFS with a very large gap and flow rate is studied. The increase of the rotational speed induces a progressively turbulent radial flow. In this case, it is shown that neither thermal effects nor fluid flow regime significantly affect seal behavior.

Three-dimensional modeling of THD lubrication in face seals

In this study, the seal is supposed to operate in laminar flow, under steady-state conditions and without phase changing in the lubricant film. The model tackles the three-dimensional general case of misaligned faces and wavy rotor face in stable dynamic tracking regime. The general equations of the THD lubrication and the equations of heat transfer through the rings are established. Different types of heat exchange on the external frontiers of the ring are considered. The equations are numerically solved by using the finite difference method in an iterative procedure which ensure the heat transfer conditions on the boundaries of the fluid film and of the solid rings. Complete and approximated solutions are compared and the validity of the approximations is discussed, A parametric study is carried out in the three-dimensional cases corresponding with misaligned, wavy, and notched faces. The results show the influence of the interface geometry, the nature of the lubricant, the fluid flow, the ring materials, and the different heat transfer conditions on the ring surfaces.

A Deterministic Mixed Lubrication Model for Mechanical Seals

Mechanical seals are commonly used in industrial applications. The main purpose of these components is to ensure the sealing of rotating shafts. Their optimal point of operation is obtained at the boundary between the mixed and hydrodynamic lubrication regimes. However, papers focused on this particular aspect in face seals are rather scarce compared with those dealing with other popular sealing devices. The present study thus proposes a numerical flow model of mixed lubrication in mechanical face seals. It achieves this by evaluating the influence of roughness on the performance of the seal. The choice of a deterministic approach has been made, this being justified by a review of the literature. A numerical model for the generation of random rough surfaces has been used prior to the flow model in order to give an accurate description of the surface roughness. The model takes cavitation effects into account and considers Hertzian asperity contact. Results for the model, including Stribeck curves, are presented as a function of the duty parameter.

Analysis and Modeling of the Topography of Mechanical Seal Faces

The aim of this article is to provide some relevant statistical parameters for mechanical seal faces and to present some methods of modelling them in order to study mixed lubrication. Three mechanical seals with faces of three different material combinations were analyzed at three operating times (unused, run-in, and worn). Surface roughness and waviness were analyzed. Generally speaking, the amplitude of the waves tends to increase with time, whereas roughness height shows the opposite trend, except in the case of the hard faces combination. The seal surfaces are extremely skewed, this phenomenon being enhanced by wear. The surfaces are nearly isotropic with a slightly higher correlation length in the sliding direction. Two models with two different autocorrelation functions (ACFs) were used to simulate surfaces. These models, based on the Patir approach (Patir (1)), used the Johnson translation curves to impose non-Gaussian height distribution. Even if the models are able to reproduce experimental tendencies, there are some difficulties with the modeling of large structures, leading to the overestimation of summit density and underestimation of the radius of the asperities’ curvature. Generally speaking the exponential ACF gives better results than the bilinear ACF.

Influence of Fluid Flow Regime on Performances of Non-Contacting Liquid Face Seals

Some non contacting mechanical face seals are running near the laminar boundary flow limit. A modification of operating conditions leads to a non laminar fluid flow in the seal interface while inertia forces remain negligible. A numerical model has been developed to determine pressure and velocity fields in the sealing dam for laminar to turbulent regime. The turbulent viscosity determination is based on the Elrod and Ng model. Evolutions of seal characteristics (opening force, friction torque, leakage rate...) and fluid film dynamic coefficients versus running conditions are presented. Numerical results show that great variations appear in the transition to turbulence.

A Simple and Easy-To-Use TEHD Model for Non-Contacting Liquid Face Seals

Numerous studies have shown that the properties of non-contacting liquid face seals depend greatly on thermal effects. A complex numerical TEHD (ThermoElastoHydroDynamic) model is needed to accurately determine seal temperature. Moreover, this temperature is influenced by a significant number of parameters (such as operating conditions or seal ring shape). This makes the task of design and optimization of mechanical face seals difficult. This paper presents a simple and easy-to-rise TEHD model, which allows for the determination of mean face temperature, power loss, leakage rate, axial stiffness and fluid film thickness. It is demonstrated that the results depend on a single dimensionless parameter. The simplified model is validated by comparison with an accurate numerical TEHD model.

A Multiscale Approach to the Mixed Lubrication Regime: Application to Mechanical Seals

This paper presents a multiscale approach to solve the problem of mixed lubrication in mechanical seals. In fact, the lubricating fluid film developed between the faces of mechanical seals is usually a fraction of a micron in thickness, leading to a mixed lubrication regime. However, over a velocity threshold the fluid film can completely separate the faces because of the hydrodynamic effect due to the surface roughness, even if the surfaces are nominally parallel. To study this phenomenon, a deterministic model is preferable because the stochastic theory based on flow factors is unable to reproduce this effect. Unfortunately, a deterministic approach needs a prohibitive amount of nodes and computation time. This is why a multiscale model is proposed. It is composed of a micro-deterministic model working on a small area coupled with a macro model giving the pressure distribution on a macro-mesh. The results of the multiscale model are compared to those of a pure deterministic model in terms of accuracy and computation time when the area of the macro-cells is varied.

Thermoelastohydrodynamic Behavior of Mechanical Gas Face Seals Operating at High Pressure

A numerical modeling of thermoelastohydrodynamic mechanical face seal behavior is presented. The model is an axisymmetric one and it is confined to high pressure compressible flow. It takes into account the behavior of a real gas and includes thermal and inertia effects, as well as a choked flow condition. In addition, heat transfer between the fluid film and the seal faces is computed, as are the elastic and thermal distortions of the rings. In the first part of this paper, the influence of the coning angle on mechanical face seal characteristics is studied. In the second part, the influence of the solid distortions is analyzed. It is shown that face distortions strongly modify both the gap geometry and the mechanical face seals performance. The mechanical distortions lead to a converging gap, while the gas expansion, by cooling the fluid, creates a diverging gap.

The Effect of Inertia on Radial Flows—Application to Hydrostatic Seals

A theoretical study of thin fluid film flows between rotating and stationary disks is presented. Inertia terms are included using an averaged method. It is assumed that inertia effects do not influence the shape of velocity profiles. It is shown that this assumption applies in many cases encountered in fluid film lubrication. The model is validated by comparison with experimental data and previous theoretical studies. A thermoelastohydrodynamic analysis of a hydrostatic seal is performed. The substantial influence of inertia terms on leakage rate prediction is demonstrated.

Numerical Modeling of Thermohydrodynamic Mechanical Face Seals

The aim of this study is to present a numerical model of thermohydrodynamic mechanical face seals to analyze the influence of operating and design parameters. This type of seal is equipped with a notched rotating face and is generally used in heavy-duty applications. The experience showed that thermohydrodynamic mechanical face seals are efficient for high pressures and at high velocities by reducing the friction coefficient. However, when parameters such as the number of notches and size are not optimized, extensive damage to the seal faces can be observed. The operating principle for this type of seal has been qualitatively explained but not demonstrated. The notches are assumed to create periodic thermoelastic deformations of the rotating face, thereby improving the hydrodynamic lift and avoiding contact of the faces. To analyze the validity of this principle, we developed a finite element model, resolving the Reynolds and energy equations and taking account of cavitation, face deformations, and heat transfer in the seal rings. The results show that waviness appears on the notched face but that the other face, made of carbon, exhibits a very similar wave pattern due to elastic deformation. This therefore reduces the expected hydrodynamic effect. The model is then used to carry out a parametric study in order to analyze the seals behavior and performance.