Basic knowledge of sealing

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1.1 Leakage: Leakage is one of the common faults that occur in mechanical equipment. There are mainly two reasons for such leaks: First, as a result of mechanical processing, the surfaces of mechanical products inevitably have various defects as well as variations in shape and size; therefore, gaps inevitably form at the joints between mechanical parts ； Second, a pressure difference exists on both sides of the seal, causing the working medium to leak through the gap. Reducing or eliminating gaps is the main way to prevent leakage. The function of a seal is to close the gap between the mating surfaces, isolate or block the leakage path, increase the resistance in that path, or insert small working elements into the path to apply pressure on the leaking fluid, thereby offsetting or completely balancing the pressure difference that causes the leakage and preventing it from occurring. Regarding the sealing of vacuum systems, in addition to leakage through the sealing surfaces caused by the aforementioned sealing materials, two other forms of leakage must also be taken into account: seepage. That is, the leakage of the sealed medium through the capillaries of the sealing material under the effect of a pressure difference is referred to as seepage ； Diffusion. That is, under the effect of a concentration difference, the transfer of the sealed medium through the sealing gap or the capillarity of the sealing material is known as diffusion. 1.2 Classification of seals Seals can be divided into two main categories: static seals, which are used between surfaces that remain relatively stationary, and dynamic seals, which are used between surfaces that move relative to each other. Static seals mainly fall into three categories: point seals, adhesive seals, and contact seals. Based on operating pressure, static seals can be divided into medium and low pressure static seals and high pressure static seals. For medium and low-pressure static seals, softer materials and gaskets with a wider width are commonly used, while for high-pressure static seals, harder materials and metal gaskets with a very narrow contact width are employed. Dynamic seals can be divided into two basic types: rotary seals and reciprocating seals. Seals can be classified into contact seals and non-contact seals, depending on whether the seal contacts the component with which it moves relative to each other. Generally speaking, contact seals offer good sealing performance, but are limited by frictional wear and are suitable for applications where the linear speed of the sealing surfaces is low. Contactless seals have poor sealing performance and are suitable for applications at higher speeds. 1.3 Selection of Seals The basic requirements for seals are good sealing performance, safety, long service life. Additionally, it is desirable that they have a compact structure, a simple system, be easy to manufacture and maintain, and have low costs. Most seals are wear-prone components; interchangeability should be ensured, and standardization and serialization should be achieved. 1.4 Sealing Materials 1.4.1 Types and Applications of Sealing Materials Sealing materials must meet the requirements of the sealing function. Due to the different sealed media and the varying operating conditions of the equipment, seal materials are required to have different levels of adaptability. The general requirements for sealing materials are: 1) The material should have good density to prevent leakage of the medium ； 2) It has appropriate mechanical strength and hardness ； 3) Good compressibility and resilience, with minimal permanent deformation ； 4) Does not soften or decompose at high temperatures, and does not harden or crack at low temperatures ； 5) It has excellent corrosion resistance, enabling long-term operation in media such as acids, alkalis, and oils. Its volume and hardness change little, and it does not adhere to metal surfaces ； 6) Low coefficient of friction and good wear resistance ； 7) Possesses flexibility to bond with the sealing surface ； 8) Good aging resistance, durable over time ； 9) It is easy to process and manufacture, inexpensive, and the materials are readily available. Rubber is the most commonly used sealing material. In addition to rubber, materials suitable for use as sealing materials include graphite, polytetrafluoroethylene, and various sealants. 1.4.2 General rubber sealing material components General rubber sealing components are being used increasingly widely in industries such as national defense, chemicals, coal, petroleum, metallurgy, transportation, and machinery manufacturing; they have become essential components and parts in various sectors. The common materials for rubber sealing products are as follows. 1.4.2.1 Nitrile rubber: Nitrile rubber exhibits excellent resistance to fuel oils and aromatic solvents, but it is not resistant to acids, esters, hydrogen chloride, and similar substances; therefore, oil-resistant sealing products are made primarily from nitrile rubber. 1.4.2.2 Neoprene Neoprene exhibits good oil and solvent resistance. It has good resistance to gear oil and transformer oil, but not to aromatic oils. Neoprene also exhibits excellent resistance to weathering and ozone aging. The cross-linking break temperature of neoprene is above 200°C, and it is commonly used to make door and window seals. Neoprene also exhibits good corrosion resistance to inorganic acids. Furthermore, due to its good flexibility and airtightness, neoprene can be used to manufacture diaphragms and sealing components for vacuum applications. 1.4.2.3 Natural Rubber Compared with most synthetic rubbers, natural rubber possesses excellent overall mechanical properties, cold resistance, high resilience, and wear resistance. Natural rubber is not resistant to mineral oils, but it is more stable in vegetable oils and alcohols. In hydraulic braking systems that use a brake fluid composed of a mixture of n-butanol and refined castor oil, the rubber cups and rings used as seals are all made of natural rubber; generally, sealing gels are also made from natural rubber. 1.4.2.4 Fluororubber: Fluororubber boasts excellent heat resistance (200–250°C) and oil resistance; it can be used to manufacture cylinder liner seals, rubber gaskets, and rotary lip seals, thereby significantly extending their service life. 1.4.2.5 Silicone rubber: Silicone rubber boasts excellent resistance to extreme temperatures, ozone, and weathering. It maintains its unique elasticity as well as its resistance to ozone and weathering within an operating temperature range of -70 to 260°C. Therefore, it is suitable for use in making gaskets required in thermal devices, such as sealing rings for high-intensity light sources and valve gaskets. Due to its oil intolerance, low mechanical strength, and high cost, silicone rubber is not suitable for making oil-resistant sealing products. 1.4.2.6 Ethylene propylene diene monomer: The main chain of ethylene propylene diene monomer is a fully saturated straight-chain structure without double bonds, while its side chains contain diene groups, which allows it to be vulcanized using sulfur. EPDM possesses excellent resistance to aging, ozone, weathering, and heat (it can be used for extended periods in environments at 120°C), as well as chemical resistance (to alcohols, acids, strong bases, oxidizing agents). However, it is not resistant to the attack of aliphatic and aromatic solvents. EPDM has the lowest density among rubbers and features high fill content, but it lacks self-adhesion and inter-adhesion. Furthermore, EPDM possesses excellent steam resistance, making it suitable for producing steam-resistant diaphragms and other sealing components. EPDM has been widely used in washing machines, components in televisions, and door and window sealing products, as well as in the production of rubber strips for various composite profiles. 1.4.2.7 Polyurethane rubber: Polyurethane rubber possesses excellent abrasion resistance and good airtightness, with a typical operating temperature range of -20 to 80°C. In addition, it also has moderate resistance to oil, oxygen, and ozone aging, but it is not resistant to acids, alkalis, water, steam, and similar substances. Suitable for manufacturing various rubber sealing products, such as oil seals, O-rings, and diaphragms. 1.4.2.8 Chloroether rubber: Chloroether rubber combines the advantages of nitrile rubber, neoprene, and acrylate rubber; it exhibits excellent resistance to oil, heat, ozone, fire, alkalis, water, and organic solvents. It also has good processability, although its resistance to low temperatures is relatively poor. At temperatures that are not too low, chloroether rubber remains an excellent material for manufacturing oil seals, various types of sealing rings, gaskets, diaphragms, and dust covers, among other sealing products. 1.4.2.9 Acrylate rubber: Acrylate rubber exhibits resistance to hot oils (mineral oils, lubricating oils, and fuel oils), particularly in terms of oil stability at high temperatures; this resistance generally reaches 175°C, and it can withstand temperatures of up to 200°C under intermittent use or for short periods of time. Its drawback is poor cold resistance. Therefore, oil seals suitable for producing high-temperature resistant oils are appropriate for use in non-cold regions, but not for sealing products that are subjected to tensile or compressive stresses at high temperatures. 2. Gasket sealing is widely used for static sealing at the joints of pipes, pressure vessels, and various housings. Gasket seals are divided into three categories: non-metallic gaskets, combined non-metallic and metallic gaskets, and metallic gaskets. Common materials include rubber, leather, asbestos, cork, polytetrafluoroethylene, steel, iron, copper, and stainless steel. There are three forms of leakage in gasket seals: interfacial leakage, permeation leakage, and destructive leakage. Among them, the first two are the main forms. 3.1 Overview The function of sealing materials is to fill gaps that are complex in shape and difficult to work on, in order to provide a seal. There are mainly three types of sealing materials: 1) vulcanized rubber gaskets or seals ； 2) Non-vulcanized sealing tape ； 3) Liquid sealant in a paste or putty form with no fixed shape. 3.2 Classification of sealants There are many varieties and types of sealants. To meet the same application requirements, sealants with several different base materials can be used ； And the same base material can be used to produce sealants with different properties and for various applications. There are various classification methods for sealants, considering both their manufacturers and users. It can generally be classified using the following four methods. 3.2.1 Classification by sealant base material 3.2.1.1 Rubber-type These sealants use rubber as their base material. Common rubbers include polysulfide rubber, silicone rubber, polyurethane rubber, neoprene rubber, and butyl rubber, among others. 3.2.1.2 Resin-type: These sealants use resin as their base material. Common resins include epoxy resin, unsaturated polyester resin, phenolic resin, polyacrylic resin, polyvinyl chloride resin, etc. 3.2.1.3 Oil-based type: This type of sealant uses oil as its base material. Common oils include various vegetable oils such as flaxseed oil, castor oil, and tung oil, as well as animal oils (such as fish oil). 3.2.2 Classification by sealant vulcanization method: This type of sealant series utilizes moisture in the air for vulcanization. It mainly includes single-component polyurethanes, silicone rubbers, and polysulfide rubbers, etc. Its polymer matrix contains active groups that can react with water in the air to form crosslinks, thereby curing the sealant into a network structure. Moisture in the atmosphere acts as a catalyst in sulfidation reactions. 3.2.2.2 Chemically vulcanized sealants: Two-component polyurethane, silicone rubber, polysulfide rubber, neoprene, and epoxy resin sealants all fall into this category; vulcanization generally takes place at room temperature. Certain single-component chlorosulfonated polyethylene and neoprene sealants, as well as polyvinyl chloride sol-gel paste sealants (such as spot welding adhesives for automobiles), must undergo vulcanization through a chemical reaction under specific conditions. 3.2.2.3 Thermotropic Sealants Sealants that use a polyvinyl chloride resin dispersed with plasticizers together with rubber containing asphalt represent two different types of thermotropic systems. Vinyl resin plasticizers are liquid suspensions at room temperature, and they harden by turning into a solid when heated ； Meanwhile, the rubber-asphalt composite sealant is hot-meltable. 3.2.2.4 Oxidation-curing sealants: Sealants used for sealing joints or installing glass on dry surfaces belong to this type, and are primarily based on dry and semi-dry vegetable oils. The base oils can be refined and polymerized, blown, or chemically modified. Cobalt naphthenate is used as a drying accelerator to speed up surface drying without hardening the interior ； Lead naphthenate can harden both the surface and the interior ； Manganese naphthenate makes internal hardening more effective. 3.2.2.5 Solvent-vaporized solidifying sealant: This is a sealant that uses a non-adhesive polymer as its base after the solvent has evaporated. This category of sealants mainly includes butyl cross-linked types, high-molecular-weight polyisobutylene, acrylates with a certain degree of polymerization, chlorosulfonated polyethylene, and neoprene-based sealants. 3.2.2.6 Non-drying permanently plastic sealants These sealants typically include those based on polybutylene, polyisobutylene of medium molecular weight, high-viscosity non-oxidizing adhesives such as styrenic oils, non-drying vegetable oils, blown semi-drying oils, or butyl rubber. 3.2.3 Classification by sealant form 3.2.3.1 Paste-type sealants These types of sealants are considered to be of lower quality; they typically consist of three main materials: oil and resin ； Polybutylene ； Asphalt. It is commonly used for fixing glass in small windows; the allowable displacement of its joints is up to +5% or -5%, and its useful life is generally 2 years. 3.2.3.2 Liquid elastomeric sealants These sealants include liquid polymers that can be vulcanized to achieve an elastic state. They have the ability to withstand repeated seam deformation. The service life of liquid elastomer sealants is generally 15 to 20 years. This type of sealant possesses high adhesion and shear strength, as well as good flexibility at room temperature. Its disadvantages include high cost, the need for an adhesive primer in most cases, the inconvenience of mixing two-component sealants on-site, and sensitivity to temperature and humidity during curing. 3.2.3.3 Hot-melt sealants Hot-melt sealants, also known as heat-applied sealants, are sealants that use an elastomer and thermoplastic resin blend as their base material. Hot-melt sealants can be formulated to have properties similar to those of liquid elastomeric sealants, but their advantage over liquid elastomeric sealants is that no vulcanizing agent is required. 3.2.3.4 Liquid sealants: These types of sealants are primarily used to seal mechanical joints, serving as a substitute for solid sealing materials such as paper, asbestos, cork, and vulcanized rubber. They prevent fluids inside the machinery from leaking through the joints; hence, liquid sealants are also known as liquid gaskets. 3.2.4 Classification by performance after sealant application 3.2.4.1 Cured sealants These sealants can be further divided into rigid and flexible types. The solid formed after the vulcanization or curing of rigid sealants rarely possesses elasticity ； Flexible sealant possesses elasticity and softness after vulcanization. The characteristic of rigid sealant is that it cannot bend, and the joint usually cannot move. Flexible sealant remains soft after vulcanization. 3.2.4.2 Non-curing sealants: These are soft, settable sealants that remain in a non-dry state after application (as the tackifier continuously moves to the surface). This adhesive is usually in paste form and can be applied to joints using a spatula or brush; it can be used in combination with sealants of different viscosities and properties. 3.3 Types of Common Sealants Various methods for classifying sealants were listed earlier; the most widely used approach is to divide them into two main categories: vulcanized and non-vulcanized sealants. The most widely used type in vulcanized sealants is the room-temperature vulcanizing type, while the heat-vulcanizing type is used less frequently. Non-vulcanized sealants include liquid sealants and putties. In addition, common anaerobic adhesives are also used. 3.4 Selection and Application Techniques of Liquid Sealants The selection of sealants should be determined by taking into account various factors such as operating conditions, the material of the sealing components and the condition of the sealing surfaces, the type and properties of the medium to be sealed, as well as the application techniques. Under normal circumstances, when high forces are applied, as well as impact forces and alternating forces, sealants with higher strength should be used ； When the temperature difference is large, a sealant with good toughness should be selected. 3.4.1 Application method of liquid sealant The application method of liquid sealant can be selected based on the state of the sealant. Paste sealants can be applied using a spatula or injected with a gun ； Liquid sealant is applied by brushing or spraying ； Membrane sealant is applied using a tiling method. 3.5 Domestic Sealants 3.5.1 Room-temperature vulcanizing polysulfide rubber sealants are multi-component materials that vulcanize at room temperature to form elastomers; they are dry-adhesive type sealants, with the XM series of sealants being commonly used. 3.5.2 Silicone rubber sealants at room temperature: Silicone rubber sealants possess various special properties, such as resistance to high and low temperatures, as well as excellent electrical insulation properties. 3.5.3 Anaerobic adhesives 3.5.4 Other vulcanized sealants 3.5.5 Non-vulcanized sealants Most non-vulcanized sealants are non-drying or semi-drying; those in putty form are also known as non-vulcanized putties. These types of adhesives are less sensitive to temperature; within their operating temperature range, they do not deform, crack, or form a crust. They also maintain good storage stability over time. However, their mechanical properties are low. They are suitable for sealing detachable parts or the joints of fastened connections, as well as for filling larger structural gaps. They play a major role in liquid seals and are widely used for sealing detachable components. 4. Packing seal: The packing seal is one of the oldest types of seals, with a history of over a thousand years in China. It was initially used by filling the leakage channels with cotton and linen fibers to prevent fluid leakage, mainly as a seal for water-lifting machinery. Packing seals are mainly used as dynamic seals. It is widely used for shaft sealing in centrifugal pumps, compressors, vacuum pumps, mixers, and ship propellers, as well as for the reciprocating shaft sealing in reciprocating compressors and refrigerators, and for the rotary sealing of various valve stems. To adapt to the operating conditions of the aforementioned equipment, a packing seal must meet the following requirements: 1) It must possess a certain degree of plasticity, so as to generate a sufficient radial force under compression and make tight contact with the shaft. 2) It has sufficient chemical stability, does not contaminate the medium, the filler is not swelled by the medium, the impregnant in the filler is not dissolved by the medium, and the filler itself does not corrode the sealing surface. 3) It has good self-lubricating properties, is wear-resistant, and has a low coefficient of friction. 4) When there is a slight eccentricity in the shaft, the packing should have sufficient floating elasticity. 5) Simple to manufacture and easy to fill. There are many types of packing, which can be classified based on their function, structure, and material. The most commonly used types are: twisted packing, knotted packing, plastic packing, and metal packing. 4.1 Twisted packing and knotted packing: Twisted packing is achieved by twisting several strands of asbestos together, which are then filled into the packing chamber to provide sealing. Woven fillers are made by weaving fibers of cotton, linen, and asbestos, with a lubricant or polytetrafluoroethylene inserted into them. 4.2 Plastic fillers Plastic fillers are fillers that have been shaped by pressing them through a die multiple times. 4.3 Metal fillers Metal fillers are divided into semi-metal fillers and all-metal fillers. So-called semimetallic fillers are composed of a combination of metal and non-metal, while all-metallic fillers contain no non-metals. 4.4 Carbon fiber filler The carbon fiber filler is a new type of filler. Its excellent self-lubricating properties, resistance to high and low temperatures, and chemical resistance have attracted considerable attention. It also possesses very good elasticity and flexibility as a compressive filler; its only drawback is permeation leakage, which can be prevented by impregnating it with polytetrafluoroethylene or other adhesives. At present, its cost is high, but as carbon fiber technology advances, its cost will surely decrease; therefore, carbon fiber fillers are the most ideal and promising type of filler. ` 4.5 Selection of Packing When selecting packing, factors such as the type of machine, the physical and chemical properties of the medium, operating temperature and pressure, as well as the speed of movement, should be taken into consideration. Among these, the corrosivity of the medium (expressed in terms of pH value) is of utmost importance, along with the pH value and operating temperature. 4.6 Proper filling of the filler: The proper filling of the filler should be carried out according to the following steps: 1) Clean the filler chamber and check whether there are any scratches, burrs, or similar defects on the surface of the shaft. 2) Use a dial indicator to check the radial runout of the shaft at the sealing area; its tolerance should be within the allowable range. 3) The filler chamber and the shaft surface should be coated with a sealant or a sealant suitable for the medium. 4) For the filler packed in rolls, when using it, first take a wooden stick of the same diameter as the shaft, wind the filler around it, and then cut it off; the cut should be made at a 45-degree angle. Each section of the cut filler should not be left loose or straightened out, rather, a paper strip of the same width as the filler should be used to wrap each section into a circle, and they should be placed in a clean area. 5) During filling, it should be done circle by circle; multiple circles should not be filled at the same time. 6) Take a wooden half-shaft sleeve of the same size as the packing, fit it onto the shaft, push the packing deep into the cavity, and apply pressure to the wooden sleeve using a gland in order to pre-compress the packing. 7) Load the second and third rings in the same way. 8) After the final layer of packing has been installed, it should be compressed using a gland, but the compression force should not be too great. 5. Molded packing seals: Molded packing seals refer to ring-shaped sealing elements made from rubber, plastic, leather, or metal materials through molding or turning processes. Based on their operating characteristics, molded seals are divided into two categories: extruded seals and lip seals ； Based on materials, they can be classified into rubber, plastic, leather, and metal types. Among the extruded sealing rings made from various materials, rubber extruded sealing rings are the most widely used. Among these, O-rings have the longest history and are the most typical. There are many types of lip seals, including V-shaped, U-shaped, L-shaped, J-shaped, and Y-shaped ones. 5.1 O-ring Seals O-ring seals, also simply referred to as O-rings, first appeared in the mid-19th century, where they were used as sealing elements for steam engine cylinders. O-ring rubber seals have the following advantages: 1) The sealing mechanism is simple, the installation space is compact, and the weight is low ； 2) It has a self-sealing function; often, only one seal is needed to achieve sealing ； 3) It has good sealing performance; when used as a static seal, it can virtually achieve zero leakage ； 4) The kinetic frictional resistance is very low, and it can also adapt to situations with alternating pressures ； 5) The dimensions and grooves are standardized, resulting in low costs and ease of use as well as procurement. 5.2 V-type seal rings: V-type seal rings are a type of lip seal and one of the earliest and most widely used molded packings. It is mainly used for reciprocating motion to seal pistons or piston rods. It is rarely used in rotation or as a static seal. V-shaped sealing rings have the following characteristics: 1) Good sealing performance ； 2) Allow certain eccentric loads and eccentric movements ； 3) It can be used with multiple layers overlapping, and the maximum sealing effect can be achieved by adjusting the clamping force ； 4) Impact pressure and vibration pressure resistance ； 5) When the filler cannot be inserted axially, it can be used with a cut, as long as the cuts are offset from each other during installation so as not to affect the sealing performance. Its disadvantage is that the frictional resistance is higher compared to other shaped fillers. 5.3 Y-type seals: The U-shaped and Y-shaped seals used for piston sealing differ slightly in shape; the lip of the U-shaped seal is longer, and its bottom has the same thickness as the lip or is slightly thicker than it. The Y-ring has a short length and a thick bottom, which is designed to overcome the problem that U-rings often cannot be placed stably; it also increases the strength of the lip to prevent tearing at its base. 5.4 Structure of drum and mountain-shaped seals 5.4.1 Structure of drum-shaped seals Drum-shaped seals, also known as piston seals, are designed for pistons that operate in one direction or in both directions. The cross-section of the sealing ring, as well as the structure of the bushing or retaining ring, are closely related to the design of the piston. Due to various performance requirements, the structure of drum seals cannot be uniform either. 5.5 J-shaped and L-shaped seals: Both J-shaped and L-shaped seals are used for sealing pneumatic or hydraulic mechanical equipment operating under pressures not exceeding 1 MPa. The J-shaped seal is used for sealing the piston rod. 6.1 Oil seals: Oil seals are used to seal lubricating oil. It is commonly used in the bearings of various machines, especially in rolling bearing applications. Its function is to isolate the oil chamber from the outside world, sealing the oil on the inside and dust on the outside. Oil seals have the following advantages over other types of seals: 1) Oil seals are lightweight and require less material. 2) The oil seal has a small installation size and axial dimension, making it easy to manufacture. 3) It has good sealing performance and a long service life, and it can tolerate certain levels of machine vibration and spindle eccentricity. 4) Easy to disassemble and convenient for maintenance. 5) Low price. 6.2 Dust-proof sealing: Oil seals can be used as components for dust-proof sealing. However, in cases of severe dust or to protect other seals, specialized dust-proof seals are often used. Materials for dust-proof sealing: rubber is commonly used in hydraulic machinery, while felt is used in pneumatic machinery. In aircraft and cylinders used in arctic conditions, metal is employed to prevent icing on the outside of the piston rods; the chemical industry also uses metal to avoid deposits forming on piston rods. Dust-sealing is very important for protecting critical hydraulic equipment. Penetrating dust not only wears out the seals but also **wears out the guide sleeve and piston rod. Furthermore, the entry of impurities into the hydraulic fluid can also affect the performance of control valves and pumps; in the worst cases, it may even damage these devices. The dust seal can remove dust and debris from the surface of the piston rod, but it damages the oil film on the piston rod, which also has an impact on the lubrication of the seals. 7.1 Magnetorheological Fluids 7.1.1 Composition of Magnetorheological Fluids Developed in 1995 by Papell in the United States, magnetorheological fluids are colloidal liquids in which strongly magnetic fine powders such as magnetite (about 100Å in size) are stably dispersed in liquids such as water, oils, esters, and ethers. This liquid has the property of neither settling nor aggregating under normal centrifugal force and magnetic fields, while still being capable of acquiring magnetism and being attracted by magnets. Magnetofluid consists of three main components: 1) solid ferromagnetic particles (Fe3O4) ； 2) Surfactants (stabilizers) that coat the particles and prevent them from agglomering together ； 3) Carrier fluid (solvent). 7.1.2 Properties of magnetofluids Magnetofluids are a type of colloidal solution. As a magnetic fluid for sealing, its performance requirements are: good stability, without aggregation, precipitation, or decomposition ； High saturation magnetization ； High initial permeability ； The viscosity and saturated vapor pressure are low; other properties such as freezing point, boiling point, thermal conductivity, specific heat, and surface tension also have certain requirements. The main factors affecting the stability of magnetofluids include: the strength of the particles, surfactants and the carrier fluid, as well as their appropriate proportions. Stability is a prerequisite for the existence of various properties of magnetofluids. 7.2 Working principle of magnetohydraulic sealing: The magnetic circuit formed by the annular permanent magnets, pole shoes, and rotating shaft, under the influence of the magnetic field generated by the magnets, concentrates the magnetohydraulic fluid located in the gap between the shaft and the top of the pole shoes, forming what is known as an “O”-shaped ring that seals off the gap and thus achieves sealing. This sealing method can be used in both cases where the rotating shaft is magnetic and where it is non-magnetic. In the former, magnetic fluxes concentrate in the gap and pass through the rotating shaft to form the magnetic circuit, whereas in the latter, the magnetic fluxes do not pass through the rotating shaft; instead, the magnetic circuit is formed by the magnetic fluid in the sealed gap. 7.2.3 Limiting conditions: Magnetorheological seals are subject to the following constraints during operation: 1) Evaporation. Magnetic fluid consists of three components: magnetic particles, surfactants, and a carrier fluid. The evaporation of the carrier fluid is the main factor determining the limiting rotational frequency for sealing and the service life of the magnetic fluid. Because sealing works with *limited magnetohydrids. To this end, a carrier fluid with a low vapor pressure should be selected to minimize evaporation losses. 2) Temperature rise. An increase in temperature can cause the magnet to demagnetize and the magnetic fluid to evaporate. Since the temperature increases, the viscosity decreases, and thus power consumption also decreases, which is an advantage. However, as the temperature rises and the magnetic saturation intensity decreases, the pressure resistance of the seal may also decline. Therefore, the temperature of the magnetohydraulic fluid should generally not exceed 105°C; otherwise, cooling measures should be employed. 3) Ultimate vacuum level. The ultimate vacuum level achievable with magnetic fluid sealing depends on the volatility of the carrier fluid; a carrier fluid made from a diglyceride lubricant can meet the requirements of ultra-high vacuum technology at 1.333×10-7 Pa. 4) Peripheral speed. General magnetohydrodynamic seals are suitable for operation at high circumferential speeds of over 30 m/s, with no speed limit. However, considering temperature and heat dissipation, the peripheral speed should be limited to 60–80 m/s, at which point the ultimate pressure resistance also needs to be taken into account. 8. High-pressure sealing: There are many types of high-pressure sealing, which can be divided into forced sealing and self-sealing categories based on their working principles. Forced sealing relies on the pre-tightening force of the connecting elements (bolts) to ensure a certain contact pressure between the top cover of the pressure vessel, the sealing elements, and the ends of the cylinder body, thereby achieving sealing. Self-sealing is achieved as the operating pressure inside the pressure vessel increases, which in turn raises the contact pressure between the sealing element and the top cover as well as the ends of the cylinder body. The characteristic of self-sealing is that the higher the pressure, the greater the compressive force on the contact surface between the sealing elements, which results in better sealing performance; even when operating conditions change, the seal remains effective. However, its structure is relatively complex, making it difficult to manufacture. Self-sealing types can also be classified into axial self-sealing and radial self-sealing based on the deformation mode of the sealing element. Based on the properties of the sealing material, high-pressure sealing can be further divided into plastic sealing, which causes plastic deformation of the sealing element, and elastic sealing, which causes elastic deformation of the sealing element. Currently, the common sealing types used for pressure vessels are as follows: 1) Forced sealing includes flat gasket sealing, cartridge sealing, and octagonal gasket sealing ； 2) Semi-self-sealing type with double-cone seal ； 3) Self-sealing types include wedge seals, PTFE seals, hollow metal O-ring seals, C-ring seals, B-ring seals, triangular gasket seals, octagonal gasket seals, flat gasket self-sealing, and rubber O-ring seals, among others. 9. Vacuum sealing: The sealing performance of a vacuum online system depends on leaks at the connections as well as the outgassing of the vacuum materials. For any vacuum system, it is generally expected that the amount of leakage and gas release is related to various factors such as the type of sealing, the sealing materials, the precision of manufacturing, and the quality of assembly; therefore, there is always a certain level of leakage and gas release at the connections. Thus, requirements can be set based on the nature of the vacuum system’s operation, the level of stress experienced by the vacuum chamber, and the speed of gas extraction at its outlet. Synthetic rubber, epoxy resin, and plastics are widely used in vacuum systems where the pressure is in the range above 10-5 Pa. When the vacuum level reaches the range of 10-7 Pa, these sealing materials can no longer be used; ultra-high vacuum sealing materials such as gold or copper must be employed as gaskets, and the vacuum chamber cannot be made of soft materials – it must instead be constructed from stainless steel. The gas state in an ultra-high vacuum is in a state of dynamic equilibrium. The pressure limit within the system is related, on the one hand, to the effective pumping speed of the pump, and on the other hand, to the gas flow rate coming from the vacuum chamber and its internal components. Although there is an effective pumping speed provided by the system, practical limitations always exist due to the structural dimensions and cost of the pump. Therefore, reducing the gas flow rate becomes the fundamental design goal for achieving an ultra-high vacuum state, and it serves as the main criterion for selecting ultra-high vacuum materials. As materials for use inside vacuum systems, a low saturated vapor pressure is required. To reduce chronic desorption and outgassing, they must be able to withstand baking at high temperatures of 450°C without a loss in mechanical strength or suffering chemical and physical damage. As a material for vacuum system housings, it is required to have negligible gas permeability, be able to withstand atmospheric pressure, resist air erosion during baking, and not experience leaks. Furthermore, it is required that the materials chosen be easy to process and manufacture, and be inexpensive and readily available. For ultra-high vacuums with a vacuum level below 10-7 Pa, natural and synthetic rubbers are ideal materials for seals: they have good elasticity, the flange bolts experience little stress after being used to create a vacuum seal, and they can be reused multiple times. However, since ultra-high vacuum systems require seal ring materials to withstand baking at 250°C, none of the several rubber materials available actually meet this requirement. For ultra-high vacuum, which has a higher degree of vacuum (i.e., lower pressure), metal seals must be used. 9.1 Rubber seals for vacuum – For contact-type vacuum dynamic seals, the most commonly used types are as follows: 1) J-type rubber seals for vacuum. The working surface of the J-type rubber seal for vacuum applications should be flat and smooth, with no bubbles, impurities, or irregularities. 2) O-ring rubber seal for vacuum use. 3) Skeleton-type rubber seals for vacuum use 4) O-ring rubber seals for vacuum use 9.2 Metal seals for vacuum use – The detachable connections sealed by metal seals are a common type of connection in ultra-high vacuum systems. It is a sealing method that requires high-temperature baking at 200–400°C for degassing in order to meet ultra-high vacuum requirements. The common materials for metal sealing rings are gold wire and oxygen-free copper. They possess the following properties: Gold (Au) has high chemical stability, does not oxidize at high temperatures, has good plasticity, and its yield strength is half that of copper or aluminum; it can undergo plastic deformation under relatively low clamping forces. Its coefficient of expansion is αg = 14×10-6 cm/cm•°C, which is slightly lower than that of stainless steel, αs = 18×10-6 cm/cm•°C. Although gold sealing rings have good sealing properties, they undergo significant deformation and hardening under clamping force, resulting in an increase in strength. To ensure the sealing of the seal ring, it is necessary to increase the clamping force; however, an excessive clamping force can cause pressure marks on the flange surface, thereby affecting the sealing performance. Therefore, it is used for connections with high requirements that do not require disassembly, and the sealing ring needs to be replaced when disassembling and reassembling. Due to the high price of gold, its applications are greatly limited. The coefficient of thermal expansion of copper (Cu) is αs=16.4×10-6 cm/cm•℃. Copper has a relatively high hardness; therefore, copper seals must be annealed in a vacuum or hydrogen atmosphere before use to eliminate internal stresses. Oxygen-free copper is currently a commonly used sealing ring material in ultra-high vacuum sealing connections. Its drawback is that the parts in contact with the atmosphere during high-temperature baking will oxidize; therefore, in applications with high requirements, a layer of gold is plated on the surface of the oxygen-free copper seal ring to improve its sealing performance. The material used for the coupling flanges must also be able to withstand high-temperature baking, oxidation, and maintain good mechanical properties at high temperatures. The most commonly used material is stainless steel. Both the roughness and dimensions of the flange sealing surface must meet the requirements for ultra-high vacuum sealing in terms of precision. 9.3 Dynamic joint seals using soft deformation 9.3.1 Dynamic joint seals with non-metallic soft deformation 9.3.2 Dynamic joint seals with metallic soft deformation 9.4 Other seals for vacuum applications 9.4.1 Magnetohydrodynamic seals for vacuum use. The representative typical structure for vacuum shaft seals is the contact-type Wilson seal. To prevent the shaft from rotating at high speeds and gas leakage, it is necessary to increase the pressure at the sealing contact surface. However, the frictional heating problem that arises as a result is difficult to solve. Therefore, developing new sealing structures with low friction loss and long service life has become one of the major issues to be addressed in vacuum devices. To address this issue, the technology of using magnetohydraulic fluids for dynamic vacuum shaft sealing has achieved success at home and abroad in recent years. Advantages of using magnetohydraulic sealing in a vacuum: 1) Magnetohydraulic sealing of vacuum shafts eliminates the frictional losses resulting from contact between seals, allowing for higher shaft speeds (up to 120,000 r/min) and **reducing leaks**. By using a magnetofluid with a low vapor pressure, the vacuum level in the vacuum chamber can be maintained at above 1.3×10-7 Pa. 2) The seal structure of magnetic fluid is simple and easy to maintain; the gap between the shaft and the pole shoes is large, so high manufacturing precision is not required. 3) The magnetohydraulic fluid is held in the sealed gap by the magnetic field generated by the magnets, thus making it easier to start and stop the shaft. Its disadvantage is that the magnetofluid is difficult to stabilize at high temperatures, with operating temperatures generally ranging from -30 to 120°C. When the shaft operates at excessively high or low temperatures, cooling or heating measures are required, which complicates the sealing structure. 9.4.2 Sealing using a connecting partition: Power is transmitted to the vacuum vessel by means of magnetism. The sealing is achieved by applying a rotating magnetic field outside the vacuum vessel; this magnetic field causes the squirrel-cage rotor 2 inside the vacuum vessel to rotate, thereby achieving partition sealing. The characteristics of this sealing device are as follows: 1) The magnetic coupling diaphragm seal has no significant impact on the vacuum conditions inside the vacuum vessel; compared with other types of dynamic seals, it offers better vacuum retention. 2) The moving parts do not come into contact with the walls of the vacuum vessel; during the transmission of motion, the partition or isolation sleeve bears no other loads apart from the pressure difference, thereby ensuring the reliability of the sealing of the magnetically coupled partition. 3) The “contamination” inside the vacuum vessel depends solely on the structural elements of the moving parts, particularly the gas release from the friction components and the permeability of the partitions. Issues to consider in the design of the magnetic coupling diaphragm sealing structure: 1) The external magnet should be as close as possible to the inner wall of the vacuum device ； 2) The isolation plate or isolation cylinder should be made of non-magnetic materials ； 3) The shape of the core that transmits motion should be adapted to the shape of the magnet, and the container walls or other components within the vacuum chamber must ensure the direction of the core’s movement ； 4) To reduce gas release and friction, it is recommended to use an iron core wrapped in glass ； 5) The magnetic field strength and the distance between the magnet and the core should be chosen such that their movement results in minimal impact on the container walls or the mercury, indium, etc., contained within the container. 10.1 Centrifugal sealing 10.1.1 Structural types of centrifugal seals. Centrifugal sealing is a mechanism that uses a rotating element to generate centrifugal force on a fluid in order to prevent leaks; its sealing capability arises from the work done by the rotation of the machine shaft on the sealing elements, and therefore it belongs to the category of dynamic seals. Features of centrifugal seals: They lack friction pairs in direct contact, allowing for larger sealing gaps; as a result, they can seal media containing solid impurities. They experience low wear and have a long service life, and with proper design, they can achieve nearly zero leakage. However, the pressure difference that this seal can overcome is small, meaning its pressure-reducing capacity is low. Centrifugal seals have high power consumption, which can even reach 1/3 of the pump’s effective power. Furthermore, since it is a dynamic seal, it loses its sealing function as soon as the machine stops, which is why a stop-seal must be used as a supplement. 10.2.2 Pressure reduction capacity of centrifugal seals 10.2.1 Back vane seals If there are no vanes on the rear cover plate of the working wheel, that is, if it is a smooth disc, then the liquid in the gap chamber between the rear cover plate and the pump casing will rotate at a speed of ω/2 of the angular velocity of the working wheel. At this point, the pressure in the clearance cavity is distributed radially in a parabolic pattern; as shown in Figure 10-5, the pressure will be distributed along ABEKG. In other words, the pressure at the shaft seal decreases. 10.2 Parking Sealing: Parking sealing is an important component of dynamic sealing. When the rotation frequency of the component decreases or it stops, the power seal loses its sealing capability, and only the stop seal can prevent fluid leakage. Some liquid and gas seals are equipped with shutdown seals to allow the sealing liquid and gas systems to be shut off after shutdown. There are various structural types of shutdown seals, among which the centrifugal shutdown seal is the most widely used. In addition, there are also pressure-regulating shutdown seals, expansion-type shutdown seals, and others. 10.2.1 Centrifugal shutdown seal: Figure 10-10 shows a typical structure for a centrifugal shutdown seal, which utilizes the centrifugal force generated by the back blades to provide sealing while the pump is in operation. When parked, the spring force causes the pump to slide axially to the left, pressing the cone sleeve packing tightly to prevent leakage. After startup, the centrifuge separates, and its gripping part pulls on the shaft shoulder to move the shaft to the left, thereby disconnecting the cone sleeve from the packing seal and preventing wear on the sealing surface. 10.2.2 Pressure-regulating stop seal: A pressure-regulating seal that combines a stop seal with a spiral seal; during shutdown, the spiral sleeve, which can move on the shaft, is pushed by spring force to press its stepped end face against the end face of the housing, thereby creating a seal. During operation, the two counter-rotating helices cause the viscous fluid in the gap to form pressure peaks at the end faces, which act on the stepped end face of the helix shaft and cause it to lose contact with the end face of the housing. Shutting down and sealing with a slide valve. When the pressure difference cylinder loses pressure, the spool pushed by the leaf spring presses against the shaft shoulder to achieve a stop-sealing effect. 10.3 Fully Enclosed Sealing 10.3.1 Fully Enclosed Sealing Fully enclosed sealing involves blocking all pathways for leakage between the inside and outside of the system, or placing both the working machine and the motor within the same sealed system, thereby completely preventing any leakage of the medium outside. A fully enclosed seal eliminates the friction, wear, lubrication, and fluid leakage associated with conventional dynamic seals; it is a special type of seal. Fully enclosed sealing plays an important role in sealing highly toxic, radioactive, and rare valuable materials, as well as in their experimentation and production. 11. Floating ring seal: The floating ring seal, also simply referred to as a floating ring seal, is used for shaft sealing in centrifugal compressors, hydrogen-cooled gas turbine generators, centrifugal pumps, etc. The available sealing methods for medium and high-pressure centrifugal compressors include mechanical seals, labyrinth seals, and packing seals. However, since gases have poorer heat dissipation and lubrication conditions compared to liquids, packing seals are only used in small-scale, low-speed applications. Mechanical seals also struggle to function when the peripheral speed exceeds 40 m/s or the temperature is above 200°C; thus, labyrinth seals and floating ring seals are the two most commonly used types of seals. The floating ring seal has the following advantages: 1) The sealing structure is simple, consisting of only a few components such as rings, pins, and springs with simple shapes. Multi-layer floating rings also consist only of these simple components, with fewer parts than mechanical seal components. 2) It is not sensitive to the operating conditions of the machine and possesses stable sealing performance. 3) The seal does not wear out, ensuring reliable sealing; it is simple to maintain and easy to service. 4) Since the seal material is metal, it can withstand high temperatures. 5) Multiple floating rings can be used in parallel to form multi-layer floating ring systems, which can effectively seal high pressures of over 10 MPa. 6) It can be used in high-speed rotating fluid machinery with speeds of 10,000–20,000 r/min, especially in gas compressors, where the allowable speed is as high as over 100 m/s – a performance that no other seal can match. 7) By using corrosion-resistant metallic materials or non-metallic materials lined with corrosion resistance (such as graphite) for the floating ring, it can be used for sealing in highly corrosive media. 8) Since there is a liquid film in the sealing gap, the frictional power is extremely low, resulting in higher efficiency for the machine. Disadvantages of floating ring seals: High precision is required in the manufacturing of the sealing element, and the eccentricity of the ring, as well as the lack of perpendicularity of its end faces and surface roughness, have a significant impact on the sealing performance. Furthermore, this seal cannot prevent liquids from leaking. Although the gas can be sealed tightly, a complex and expensive automated fuel supply system is required. 11.1 Mechanism of floating ring sealing: Floating ring sealing is a type of flow-resistance-based, non-contact dynamic seal that achieves leak prevention by utilizing the fluid resistance effect within the sealing gap. Due to the presence of gaps, solid friction is avoided, making it suitable for high-speed applications; it can seal both liquids and gases, but the leakage rate is relatively high, and in some cases a more complex sealing assistance system is required. 11.2 Floating Rings There are various types of structures for floating ring sealing devices; the main types include wide rings and narrow rings, smooth rings and open rings, as well as liquid film and dry-type floating rings. 11.2.1 Wide rings and narrow rings: Wide rings have a width that is relatively large compared to their diameter, with a ratio of l/D ranging from 0.4 to 0.6. The characteristic of this ring is that the hydrodynamic forces acting on it during operation are greater than those on a narrower ring, and there is no need for attachments aligned with the center. Under certain pressure differences and leakage rates, their number can be fewer than that of narrow rings; this allows the structure of the sealing device to be simplified, making it easier to install, remove, and maintain. The disadvantage of a wide ring is that there is a large pressure difference on both sides of the ring; as a result, the pressure acting on the end faces of the ring is also high. This leads to greater friction at the end faces that must be overcome during free floating, making floating more difficult. The width of the narrow ring is small relative to its diameter, with a ratio of l/D ranging from 0.1 to 0.2. The gap between the narrow ring and the shaft is small; during operation, the hydrodynamic forces generated in this gap are low, resulting in a poor ability to achieve automatic centring. Rubber O-rings are usually used to help align the centers. Thanks to the use of this auxiliary measure, the degree of eccentricity is low and the downtime is short; as a result, even though the ring is narrow, the leakage amount is not significant. Narrow rings can also be positioned without an O-ring, using a spring instead. The ring is pressed against the end face of the isolation ring under the force of the spring. When the pressure of the sealing fluid decreases, the ring can still maintain its alignment with the center position. Since the pressure difference acting on each narrow ring is smaller than that on the wide rings, the pressure exerted by the rings on the end faces of the partition rings is also smaller; as a result, the narrow rings tend to float. 11.2.2 Smooth rings and open rings The inner hole of a smooth ring is smooth ； The inner hole of the slotted ring is fully slotted or partially slotted. Due to the low hydraulic friction in the gap between the smooth ring and the shaft surface, significant leakage occurs during use. The inner hole of the slotted ring is provided with multiple annular grooves, which result in greater hydrodynamic friction in the gap with the shaft. Under the same pressure difference and width, the leakage rate is lower than that of a smooth ring; especially at high rotational speeds, complete leakage can be avoided, and the liquid film formed is stable, enabling effective sealing. Therefore, for high-speed rotating shafts, slotted rings are better than smooth rings. When comparing smooth floating rings with mechanical seals, mechanical seals have less leakage at low speeds, while smooth rings have less leakage at high speeds; thus, smooth rings are more suitable for high-speed rotating seals. However, when the rotation frequency is too high, the sealing oil heats up due to the viscous resistance of the oil. To dissipate heat, a certain level of leakage is often intentionally maintained. The leakage rate depends not only on the shape of the ring but also on the movement speed, the properties of the oil, the inlet oil temperature, and the ambient temperature. 11.2.3 Liquid films and dry floating rings Floating seals can be used to seal both liquids and gases. The type used to prevent liquid leakage is called a liquid film floating ring seal ； The type used to prevent gas leakage is called a dry floating ring seal; since the floating ring is usually made of solid self-lubricating materials such as graphite, it is also known as a graphite floating ring seal. Graphite floating ring seal: The elastic force of the wave-shaped leaf springs and the gas pressure cause one end face of each floating ring to make close contact with one end face of each isolation ring, thereby preventing gas from leaking radially. The friction between the end faces prevents the rings from rotating. The small amount of gas that leaks axially through the floating ring seal is discharged via the exhaust vent or led to the gas inlet of the main unit. The working clearance of the graphite floating ring seal is not a fixed value; rather, it adjusts itself according to the frictional heat generation, which is why it is known as a \"thermally self-adjusting clearance seal\". Graphite is both corrosion-resistant and heat-resistant, but it is too brittle and prone to fracture under radial loads. In centrifugal compressors, graphite is used as the floating ring, and to prevent breakage, a metal ring is often fitted around the outer periphery of the graphite ring. The graphite ring is fitted into the metal ring using a cold shrinkage method, and then the inner hole of the graphite ring is machined to achieve the specified dimensions. When the temperature of the shaft seal rises, if the material of the ring and the shaft is the same or similar, their rates of expansion will be the same or only slightly different. without affecting the sealing performance. This structure has been successfully applied to gas sealing at temperatures up to 400°C.