Why are auxiliary devices needed for mechanical seals?

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In fluid machinery (such as pumps, compressors, mixers, and centrifuges), the component subject to the harshest operating conditions is the mechanical seal. It bears axial forces like a thrust bearing; it transfers the heat generated to the outside, just as a heat exchanger does. Achieving a static seal in environments with high temperatures, high pressures, and corrosive conditions is already quite difficult; it is even more challenging to achieve a dynamic seal on a sealing surface just a few millimeters wide while the component is rotating at high speeds. To address this tricky problem, through long-term practice, people have found two approaches: one is to start with the sealing structure and materials to make them suitable for harsh working conditions. This approach comes at a high cost, and yet it still fails to resolve the problem fundamentally. Once the seal fails, the consequences are severe. Another approach is to improve the working environment through auxiliary facilities. Currently, mechanical seals that do not require any auxiliary equipment in actual operation are truly rare. Mechanical seals used under high pressure, such as those in series, as well as those in high-temperature pumps, all require corresponding auxiliary facilities. By using auxiliary facilities to transform certain harsh working environments into ones that are acceptable for sealing, the performance of mechanical seals is **improved**, allowing for a wider range of applications. The frictional heat generated during the operation of mechanical seals raises the temperature of the sealing surfaces; if no appropriate measures are taken, this can lead to many adverse consequences. 1) Rising temperature causes the liquid film between the sealed surfaces to vaporize, leading to increased wear and subsequent failure of the seal ； 2) Rising temperatures cause thermal deformation of the stationary and rotating rings, leading to increased leakage and greater wear ； 3) Rising temperatures also exacerbate the corrosion of mechanical seals by the medium ； 4) Rising temperatures cause the auxiliary sealing ring to age, deteriorate, and become ineffective ； 5) As the temperature rises, the graphite impregnated with synthetic resin sees its performance decline due to the carbonization of the resin, while the graphite rings impregnated with metal leak as a result of the melting of the metal. It is evident that the allowable temperature rise is a major enemy of mechanical seals. The purpose of auxiliary devices is to reduce this unwanted temperature rise, maintain good lubrication of the sealed surfaces, and ensure that all components of the mechanical seal function properly; in media that are prone to vaporization, it helps to maintain the pressure within the sealing chamber so that the medium does not vaporize; in the case of pumps with low sealing performance, it can serve to insulate and provide heat; for seals exposed to crystallizing solids or highly corrosive media, it also helps to prevent damage to the seal. Therefore, some **refer to auxiliary systems as protection systems. In 1983, Sinopec organized a survey on the pump and compressor seals of seven major petrochemical enterprises and concluded that: \"The shaft seals of centrifugal pumps and compressors imported from abroad come equipped with various auxiliary devices depending on the specific conditions.\" Practice has shown that a well-structured design and high-quality materials are often overlooked in favor of auxiliary facilities, resulting in difficult working conditions and early failure of the seal. On the other hand, in some cases the structure and materials are not sufficient to meet the requirements of the working conditions; by using effective auxiliary facilities, the sealing can meet those requirements and allow for smooth operation. Some mechanical seals operate at temperatures, pressures, and speeds that are not very high, and can function without any auxiliary equipment. However, from the perspective of seal stability and service life, it is advantageous to use auxiliary facilities. Therefore, auxiliary facilities should be regarded as part of mechanical seals, with different auxiliary facilities being used depending on the working conditions. The internationally recognized American Petroleum Institute standard for centrifugal pumps (API610) specifies these requirements in detail. The proper and rational selection of auxiliary facilities is of great significance not only for the stability of sealing but also for extending service life. It plays a role in ensuring safe production, reducing leaks, minimizing maintenance efforts, and lowering production costs, and therefore deserves adequate attention. Auxiliary facilities are sometimes referred to as a sealed circulation protection system, or simply as a sealing system or protection system.