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Floating ring seal Floating ring seal, referred to as floating ring seal, is used for shaft seals of centrifugal compressors, hydrogen-cooled turbine generators, centrifugal pumps, etc. The sealing methods available for medium and high pressure centrifugal compressors include: Mechanical seals, labyrinth seals and packing seals. However, since the heat dissipation and lubrication conditions of gas are not as good as those of liquid, packing seals are only used in small and low-speed applications, and mechanical seals are difficult to adapt to when the peripheral speed is greater than 40m/s and the temperature is higher than 200°C. Only labyrinth seals and floating ring seals are the two most commonly used methods. Floating ring seal has the following advantages: (1) The sealing structure is simple, with only a few simple-shaped rings, pins, springs and other parts. Multi-layer floating rings also only have a combination of these simple parts, which have fewer parts than mechanical seals. (2) It is not sensitive to the operating status of the machine and has stable sealing performance. (3) The seals do not wear out, are reliable in sealing, simple to maintain and easy to repair. (4) Because the seal material is metal, it is resistant to high temperatures. (5) Multiple floating rings can be used in parallel to form a multi-layer floating ring, which can effectively seal high pressures above 10MPa. (6) It can be used in high-speed rotating fluid machinery of 10,000 to 20,000 r/min, especially in gas compressors. Its allowable speed is as high as 100 m/s or more, which is unmatched by other seals. (7) As long as the floating ring is made of corrosion-resistant metal materials or lined with corrosion-resistant non-metallic materials (such as graphite), it can be used for sealing highly corrosive media. (8) Since there is a liquid film in the sealing gap, the friction power is extremely small, making the machine more efficient. Disadvantages of floating ring seals: The manufacturing precision of seals is high, and the non-concentricity of the ring, the non-verticality of the end face and the roughness of the surface have a significant impact on the sealing performance. In addition, this seal cannot be leak-tight against liquids. Although the gas can be sealed tightly, it requires a complex and expensive automated oil supply system. 1. Floating ring sealing mechanism Floating ring seal is a flow resistance type non-contact dynamic seal, which relies on the fluid resistance effect in the sealing gap to achieve the purpose of preventing leakage. Due to the existence of gaps, solid friction is avoided. It is suitable for high-speed situations and can block liquids and gases. However, the leakage volume is large, and in some cases, a more complex sealing auxiliary system must be configured. 2. There are many types of structures of floating ring floating ring sealing devices. The main types are:: Wide and narrow rings, smooth and split rings, liquid film and dry floating rings. (1) Wide Ring and Narrow Ring The width of the wide ring is larger than its diameter, and its ratio l/D=0.4~0.6. The characteristic of this ring is that the hydrodynamic force acting on the ring during operation is greater than that of a narrow ring, and no centering accessories are required. Under a certain pressure difference and leakage amount, the number can be less than that of narrow rings. In this way, the structure of the sealing device can be simplified and it is easy to assemble, disassemble and repair. The disadvantage of a wide ring is that there will be a large pressure difference on both sides of the ring. In this way, the pressure acting on the end face of the ring will be greater, and the end face friction that must be overcome when floating freely is greater, that is, floating is more difficult. The width of the narrow ring is smaller than its diameter, and its ratio l/D=0.1~0.2. The gap between the narrow ring and the shaft is small. When working, the fluid power formed in the gap is small, so its ability to automatically concentrically is poor. Rubber O-rings are mostly used to help align the center. Due to the use of this auxiliary measure, the eccentricity is smaller and the parking time is shorter. In this way, although the ring is narrow, the leakage is not large. Narrow rings can also be positioned without O-rings and instead use springs. The ring is pressed against the end face of the isolating ring under the action of spring force. When the pressure of the sealing fluid is reduced, the ring can still maintain its centered position. Since the pressure difference acting on each narrow ring is smaller than that on the wide ring, the pressure acting on the end face of the isolation ring is also smaller, that is, the narrow ring is easy to float. (2) Smooth rings and split rings. The inner hole of the smooth ring is smooth. ； The inner bore of a slotted ring is slotted throughout its entire length or partially slotted. Since the hydraulic friction in the gap between the smooth ring and the shaft surface is small, a large amount of leakage occurs during use. The inner hole of the slotted ring is processed with many annular grooves. The hydraulic friction in the gap with the shaft is larger. Under the same pressure difference and the same width, the leakage amount is smaller than that of the smooth ring. Especially at high speeds, it can be completely leak-proof. The liquid film formation is also very stable and can effectively play a sealing role. Therefore, for high-speed rotating shafts, slotted rings are better than smooth rings. For example, comparing smooth floating ring seals with mechanical seals, the leakage of mechanical seals is less at low speeds, while the leakage of smooth rings is less at high speeds. Therefore, smooth rings should be used for high-speed rotating seals. However, when the rotation frequency is too high, the sealing oil will generate heat due to the viscous blocking effect of the sealing oil. In order to dissipate heat, a certain amount of leakage is often intentionally maintained. The leakage amount is not only related to the form of the ring, but also related to the movement speed, oil characteristics, inlet oil temperature and atmospheric temperature, etc. (3) Liquid film and dry floating ring floating seals can seal both liquid and gas. Liquid film floating ring seal used to prevent liquid leakage ； The type used to prevent gas leakage is called a dry floating ring seal. Because the floating ring is usually made of solid self-lubricating materials such as graphite, it is also called a graphite floating ring seal. Graphite floating ring seal: The elastic force of the corrugated spring and the gas pressure make one end face of each floating ring closely fit with one end face of each isolation ring, preventing gas from leaking in the radial direction, and relying on the friction of the end face to prevent the ring from rotating. A small amount of gas that leaks in the axial direction through the floating ring seal is discharged from the drain hole, or is led to the gas inlet of the host machine. The working gap of the graphite floating ring seal is not a fixed value, but adjusts itself according to the friction and heating conditions, so it is called "thermal self-adjusting gap seal". Graphite is both corrosion-resistant and heat-resistant, but it is too brittle and easily breaks under radial loads. In centrifugal compressors, graphite is used as a floating ring. In order to prevent breakage, a metal ring is often placed around the graphite ring. The graphite ring is put into the metal ring by cold shrinking method, and then the inner hole of the graphite ring is processed to reach the specified size. When the temperature of the shaft seal rises, if the materials of the insert ring and the shaft are the same or similar, their expansion amounts will be the same or not much different. without affecting sealing performance. This structure has been successfully used for gas sealing at temperatures up to 400°C.