[Sealing Knowledge] The history of mechanical seals

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A mechanical seal is a device designed to prevent fluid leakage, consisting of at least one pair of end faces perpendicular to the axis of rotation, which remain in close contact and slide relative to each other under the action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, as well as with the assistance of auxiliary seals. The elastic loading mechanism and auxiliary seal constitute the mechanical seal for metallic bellows, which we refer to as a metal bellows seal. In light-sealing applications, rubber bellows are also used as an auxiliary seal; however, their elastic strength is limited, so springs are generally required to provide the necessary elastic force. ““A mechanical seal” is usually abbreviated as “mechanical seal”. The emergence of mechanical seals: A mechanical seal is a type of seal for rotating shafts, or alternatively, a seal for the end faces of rotating machinery. It was patented in the UK in 1885, but it wasn’t until 1900 that it began to be used in practice. In 1920, it was used in refrigeration units; prior to that, it served as a bearing seal, and at that time the operating parameters for mechanical seals were very low. It was not until 1930 that it began to be used in centrifugal pumps for internal combustion engines. From 1939 to 1945, due to the development of the petrochemical industry and the emergence of materials such as graphite, ceramics, and cemented carbides, surface roughness in processing techniques improved, and the operating parameters were enhanced. Mechanical seals became widespread in the United States and were applied in the petroleum and chemical industries, giving rise to balanced-type seals. In the mid-1950s, the former Soviet Union began using mechanical seals in the centrifugal oil pumps of refineries, and mechanical seals were also incorporated in the centrifugal oil pumps used at the Lanzhou Refinery during its construction in China. In the early 1960s, our country developed mechanical seals on its own and began to use them; other oil refineries also adopted these seals one after another and made improvements to them. In the mid-1960s, mechanical seals were used in almost all of the imported platinum reforming units, which greatly contributed to the development of mechanical seal technology in the petroleum refining industry. With the development of China’s petrochemical industry, various factories, mines, as well as related schools and research institutions have conducted in-depth research on mechanical seals. China now has its own series of mechanical seals, with nearly a hundred specialized manufacturers operating in this field. However, mechanical seals are continuously evolving and being updated in terms of structure, materials, and auxiliary measures. Structural features of mechanical seals: A mechanical seal, or more precisely, an end-face seal for rotating machinery, is an axial sealing device that achieves sealing through the pre-tensioning of the friction pair between the stationary and rotating rings by elastic elements, as well as through the compressive force exerted by the medium pressure and the elastic element pressures. The main components of a mechanical seal include: the end-face friction pair (stationary ring and rotating ring), elastic elements (springs), auxiliary seals (O-rings), transmission elements (transmission pins, screws), anti-rotation elements (anti-rotation pins), and fastening elements (springs and gland nuts). The functions of the basic components of a mechanical seal are as follows: (1) The end-face friction pair (rotating and stationary rings) remains in close contact to form a sealing surface that prevents leakage of the medium. It is required that both the moving ring and the stationary ring have good wear resistance. The moving ring can move axially, automatically compensating for wear on the sealing surface so as to ensure a good fit with the stationary ring, while the stationary ring has a degree of flexibility that serves as a buffer. (2) Elastic elements (springs, corrugated plates, serpentine sleeves, etc.) mainly serve to compensate, preload, and cushion, and they are also factors that determine a proper specific pressure on the sealing surface. It is required to maintain flexibility at all times to overcome the friction of the auxiliary seals and driving components, as well as to utilize the compensating effect of the start ring. The materials are required to be corrosion-resistant. (3) Auxiliary seals (O-rings, V-rings, wedge rings, and other custom-shaped seal rings) primarily serve to seal the stationary ring and the moving ring; they also provide floating capacity and cushioning effects. The auxiliary sealing element of the stationary ring is required to ensure sealing between the stationary ring and the gland, thereby allowing the stationary ring to have a certain degree of flexibility ； The auxiliary sealing element of the rotating ring ensures sealing between the rotating ring and the shaft or shaft sleeve. The material is required to have heat and cold resistance similar to that of the medium. (4) Transmission components (transmission pins, transmission rings, transmission seats, transmission sleeves, transmission keys, transmission lugs, or spline couplings), whose function is to transmit the torque of the shaft to the moving ring. The materials are required to be corrosion-resistant and wear-resistant. (5) Fastening elements (set screws, spring seats, gland plates, assembly sleeves, shaft sleeves) serve to position and secure the stationary and rotating rings. It is required that the positioning be accurate, so that the sealing surfaces of the friction pair are in the correct position and maintain good contact; moreover, an appropriate spring compression force is necessary ； It is also required to be easy to assemble and disassemble, to fit in place easily, and to be reusable. At the interface with the auxiliary seal, attention should be paid to the chamfering and compression amount of the sealing ring; special care must be taken at the area where the moving ring seal meets the shaft sleeve, as corrosion and wear resistance are required – in such cases, a hard-coated layer may be used. Mechanical seal leakage: There are mainly three possible pathways for leakage in a mechanical seal: (1) Leakage at the sealing surfaces of the face friction pair A. This is the primary sealing surface; it plays a key role in determining the friction and sealing performance of the mechanical seal, as well as its service life. Therefore, the requirements for the contact surface are very high: the roughness must be close to that of a mirror surface, and the flatness must reach 0.0009. For different media, it is necessary to use appropriate combinations of friction pair materials, paying attention to wear and corrosion resistance, as well as selecting suitable geometric parameters (load coefficient, width-to-diameter ratio, etc.) and performance parameters (specific pressure, spring constant, pressure ratio, etc.). (2) Leakage at the seal between the stationary ring and the gland is denoted as A, while leakage at the seal between the rotating ring and the shaft (shaft sleeve) is denoted as D. These are auxiliary sealing surfaces that play a key role in determining the sealing performance of the mechanical seal as well as the flexibility of the rotating ring. In particular, the seal surface between the rotating ring and the shaft (shaft sleeve) must prevent rust, scale, or the accumulation of chemical substances from causing the rotating ring to become stuck and unable to move freely. (3) The static seal C between the gland and the sealed housing, and the static seal E between the bushing and the shaft – both of these are static seals, and compatible materials can be selected based on the sealing medium. Furthermore, if the moving ring adopts an inlaid structure, leakage may occur at the inlay joint, so attention must be paid to the fit there. The leakage rate of mechanical seals is much lower than that of packing seals, typically ranging from 0.01 to 3 mL/h; it is a sealing method that cannot be ignored. A proper understanding of mechanical seals is essential to effectively prevent leaks and ensure the proper operation of equipment.