Fundamental research on mechanical seals

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Sealing is a technology or component that prevents or restricts “leaks”. Leakage mainly refers to the leakage of fluids that possess certain pressures, temperatures, and other physical and chemical properties (such as flammability, explosiveness, toxicity, corrosivity, radioactivity, etc.). There are three driving forces for leakage: first, pressure difference; second, concentration difference (diffusion); third, surface tension (seepage). To achieve sealing, there is generally a small gap or direct contact between the sealing pairs. Friction and wear are inevitable between the friction pairs in dynamic seals, especially since the sealing elements are in direct contact with the medium inside the machine (unlike bearings). As a result, dynamic seals are generally the most vulnerable components among all mechanical parts, with lifespans and reliability that are often lower than those of bearings. Therefore, sealing is directly related to important issues such as energy saving, environmental protection, safety, and long-term reliable operation of the entire large-scale equipment. Seals can be either components or parts; some even come equipped with relatively complex systems for ensuring performance and monitoring. Examples include dry gas seals in oil, gas, and petrochemical systems, as well as mechanical seals for the main pumps in pressurized water reactor nuclear power plants – all of which are complex high-tech products. Although the seal is small, it involves many disciplines, including: fluid mechanics, solid mechanics, dynamics, thermodynamics and heat transfer, materials science, tribology, measurement and control technology, safety and reliability engineering, precision and specialized manufacturing processes, and so on. The fundamental research on mechanical seals includes: the friction conditions between the end faces and their mechanisms ； Mechanical and thermal deformation of the end face and their impact on friction conditions ； Temperature distribution on the end face ； The dynamic characteristics of sealing components (i.e., the “following ability” issue), etc. Conclusions regarding the friction conditions between mechanical surfaces: First, ordinary mechanical seals (with sealing rings made of materials such as carbon graphite, which have a low elastic modulus, and using low-viscosity media such as sealing fluid) are generally in a state of boundary friction, resulting in low leakage rates. Second, when the stiffness of the sealing ring is high and there is a certain amount of net waviness, significant leakage occurs. Third, under certain conditions (such as high viscosity and high rotational speed), along with the presence of a certain amount of net waviness, a significant hydrodynamic pressure effect can arise (alongside a hydrostatic pressure effect); in this case, the interface may be in a state of full liquid film lubrication, leading to higher leakage rates. Basic conclusions regarding mechanical and thermal deformation in mechanical seals: First, the mechanical deformation of the sealing surface (including pressure-induced deformation as well as deformation caused by springs and driving mechanisms) and thermal deformation have a significant impact on the friction conditions at the interface. Second, with regard to the radial deformation of the surface, it is necessary to achieve a very small converging gap (in which case hydrostatic effects are significant) or to maintain good contact (which may result in boundary lubrication); an expanding gap, on the other hand, is harmful. Third, concerning the circumferential deformation of the surface, a slight net wavy pattern (whether caused by mechanical or thermal deformation) can, under certain conditions, generate dynamic pressure effects to varying degrees (along with hydrostatic effects). Fourth, an increase in either dynamic or hydrostatic effects leads to an increase in leakage rate (at least for ordinary face seals).