Basic knowledge of sealing technology

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Basic knowledge of sealing technology. 1. Sealing technology. 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, when there is a pressure difference on both sides of the seal, the working medium will leak through the gap. Reducing or eliminating gaps is the main way to prevent leaks. The function of a seal is to close the gap between mating surfaces, isolate or block leakage paths, increase the resistance within those paths, or incorporate small actuating elements in the paths to exert pressure on the leaking substance. This pressure partially offsets or completely balances the pressure difference that causes the leakage, thereby preventing it. 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: This refers to the leakage of the medium being sealed through the capillaries in the sealing material, as a result of pressure differences ； Diffusion: That is, under the influence of a concentration difference, the transfer of a substance through a sealing gap or the capillarity of the sealing material by a sealed medium 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. Depending on the operating pressure, static seals can be classified 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. Based on whether the seal comes into contact with the components that move relative to it, seals can be classified into contact seals and non-contact seals. 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 and reliability, 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 Uses of Sealing Materials Sealing materials must meet the requirements of the sealing function. Due to the differences in the sealed media and operating conditions of the equipment, sealing materials are required to have different levels of adaptability. The general requirements for sealing materials are: 1) The material should have good density and be resistant to leakage of media ； 2) 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, allowing it to operate for extended periods in media such as acids, alkalis, and oils. Its volume and hardness change little, and it does not adhere to metal surfaces ； 6) Low friction coefficient and good wear resistance ； 7) Possesses flexibility to fit with the sealing surface ； 8) Good aging resistance; durable and long-lasting ； 9) Easy to process and manufacture, inexpensive, and 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 possesses good oil and solvent resistance. It has good resistance to gear oil and transformer oil, but is not resistant to aromatic oils. Neoprene also possesses excellent resistance to weathering and ozone aging. The crosslink breakdown temperature of neoprene is above 200°C; neoprene 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 products 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 gaskets 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 rings, 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, making it suitable for use in 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 exhibits excellent resistance to aging, ozone, weathering, and heat (it can be used for long periods at 120°C). It also has good chemical resistance (to alcohols, acids, strong alkalis, and oxidizing agents). However, it is not resistant to attack by aliphatic and aromatic solvents. EPDM has the lowest density among rubbers and features high fill content, but it lacks self-adhesion and inter-adhesion. In addition, EPDM exhibits excellent steam resistance, making it suitable for producing sealing products such as steam-resistant membranes. 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 cold 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, as well as other sealing products. 1.4.2.9 Acrylate rubber Acrylate rubber exhibits resistance to hot oils (mineral oil, lubricating oil, and fuel oil); in particular, it demonstrates excellent oil resistance at high temperatures, typically up to 175°C. In intermittent use or for short periods of time, it can withstand temperatures of up to 200°C. Its drawback is poor cold tolerance. Therefore, it is suitable for making oil seals that can withstand high temperatures in non-cold regions; however, it is not suitable for sealing products subjected to tensile or compressive stresses at high temperatures. Rubber type, main characteristics, operating temperature, main applications: Nitrile rubber – good oil resistance, heat resistance, and wear resistance. It is widely used in the manufacture of sealing products, but is not suitable for phosphate ester-based hydraulic fluids and gear oils containing polar additives. -40–120°C is used for manufacturing O-rings and oil seals; suitable for general hydraulic and pneumatic systems. Hydrogenated nitrile rubber has high strength, oil resistance, wear resistance, heat resistance, and aging resistance. -40–150°C is suitable for high-temperature, high-speed reciprocating and rotary seals. Rubber and plastic materials have a high elastic modulus and strength; other properties are the same as above. -30–80°C: Used for manufacturing \"O\"-rings, \"Y\"-rings, dust seals, etc., for sealing in construction machinery and high-pressure hydraulic systems. Fluororubber is heat-resistant, resistant to acids, alkalis, and other chemicals, as well as oil-resistant (including phosphate ester-based hydraulic oils). It is suitable for use with all types of lubricants, gasoline, hydraulic oils, and synthetic oils. -20–200°C: Suitable for seals resistant to high temperatures, chemicals, and flame-resistant hydraulic oils; widely used in industries such as metallurgy and power generation. Polyurethane boasts excellent wear resistance, high strength, and good aging resistance. -20–80°C is suitable for sealing high-pressure, high-speed systems in construction machinery and metallurgical equipment. Silicone rubber has good heat and cold resistance. It has low compression set, but low mechanical strength. It is suitable for high- and low-temperature applications involving high-speed rotary seals and seals in food processing machinery, within the temperature range of -60 to 230°C. Polyacrylate has better heat resistance than NBR. It can be used in various lubricants, hydraulic oils, and petroleum-based hydraulic oils containing polar additives; however, it has poor water resistance. -From 20 to 150°C, it can be used for various automobile oil seals as well as various gearboxes and transmissions; it can withstand medium to high temperatures. Ethylene-propylene rubber has good weather resistance; it is resistant to aging in air, its oil resistance is average, and it can withstand fluorocarbons as well as various refrigerants. -50–150°C, used for sealing in refrigerators and refrigeration equipment. Polytetrafluoroethylene has good chemical stability, as well as excellent heat and cold resistance. It is resistant to various media such as oil, water, steam, and chemicals. It has high mechanical strength, is resistant to high temperatures and wear, has an extremely low friction coefficient, and excellent self-lubricating properties. -At temperatures ranging from 55 to 260°C, it is used to manufacture wear-resistant rings, guide rings, and retaining rings. It is a commonly used sealing material in machinery, and is widely applied in the metallurgy, petrochemical, construction machinery, and light industrial machinery sectors. Nylon has good oil resistance, heat resistance, and wear resistance, as well as high compressive strength and impact resistance; however, it has poor dimensional stability. -40–120°C: Used for manufacturing guide rings, support rings, compression rings, and retaining rings. Polyoxymethylene has good oil resistance, heat resistance, and wear resistance; it features high compressive strength and impact resistance, as well as good self-lubricating properties and dimensional stability. However, its flexibility is poor. -40–140°C, used for manufacturing guide rings and retaining rings. 2. Gasket Sealing 2.1 Overview 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 catastrophic leakage. Among them, the first two are the main forms. 2.2 Selection of gaskets The principle for selecting gaskets is that in applications with low requirements, they can be chosen based on experience; if they prove unsuitable, they can be replaced. However, for applications with strict requirements, such as equipment involving explosive, highly toxic, and flammable gases as well as highly corrosive liquids, reaction tanks and pipeline systems, the appropriate choice must be made based on the operating pressure, operating temperature, corrosivity of the sealing medium, and the type of sealing surface. Generally, at room temperature and low pressure, non-metallic soft honeycomb gaskets are used ； At medium pressure and high temperature, use a composite metal-nonmetal gasket or a metal gasket ； When there are significant fluctuations in temperature and pressure, it is advisable to use seals with good elasticity or self-tightening seals ； In low-temperature, corrosive medium, or vacuum conditions, gaskets with special properties should be considered. It should be noted here that the flange conditions affect the selection of gaskets. Effect of flange type. Different flange types require different gaskets. Smooth-faced flanges are generally used only for low pressures, with soft, thin gaskets ； Under high pressure, if the flange has sufficient strength, a smooth-faced flange can also be used, but a thick, soft gasket should be employed, or a wound gasket with internal reinforcing rings or reinforcing rings. In such situations, metal gaskets are also unsuitable, as the required clamping force is too great; this causes significant deformation of the bolts, making it difficult to achieve a proper seal on the flanges. If metal gaskets are to be used, the smooth surface should be reduced to decrease the contact area with the gasket. In this way, with the same bolt tension, the compressive stress on the reduced narrow smooth surface increases. Effects of flange surface roughness. The surface roughness of the flange has a significant impact on the sealing performance; especially when non-soft gaskets are used, a high roughness value of the sealing surface is one of the main causes of leakage. For example, the tool marks on a turned flange surface form helical lines. When using metal gaskets, if the roughness value is relatively high, the gasket cannot completely fill this helical groove created by the tool marks; under pressure, the medium will leak out along this groove. Soft gaskets have much lower requirements regarding the surface finish of flange surfaces. This is because they are easily deformable and can fill in any machining marks, thereby preventing leaks. For soft gaskets, an overly smooth flange surface is actually disadvantageous, as it reduces the resistance to interfacial leakage. Therefore, different gaskets require different surface roughness levels for the flanges. 3. Adhesive Sealing 3.1 Overview The function of sealing materials is to fill gaps that are complex in shape and difficult to work with, thereby providing 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 ways to classify 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. Commonly used rubbers include polysulfide rubber, silicone rubber, polyurethane rubber, neoprene, and butyl rubber, etc. 3.2.1.2 Resin-type: These sealants use resin as their base material. Commonly used resins include epoxy resins, unsaturated polyester resins, phenolic resins, polyacrylic resins, polyvinyl chloride resins, etc. 3.2.1.3 Oil-based type: This type of sealant uses oil as its base material. Commonly used oils include various vegetable oils such as linseed oil, castor oil, and tung oil, as well as animal oils (such as fish oil). 3.2.2 Classification by vulcanization method of sealants This category of sealants utilizes moisture in the air for vulcanization. It mainly includes one-component polyurethanes, silicone rubbers, polysulfide rubbers, etc. Its polymer base contains reactive groups that can react with water in the air to form cross-links, causing the sealant to vulcanize into a network structure. Moisture in the atmosphere acts as a catalyst in the sulfidation reaction. 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 solidify and harden upon heating ； Meanwhile, the rubber-asphalt composite sealant is thermoplastic. 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-based sealants, high-molecular-weight polyisobutylene, acrylates with a certain degree of polymerization, chlorosulfonated polyethylene, and neoprene sealants. 3.2.2.6 Non-drying, permanently plastically sealants: These sealants typically include those based on polybutylene, polyisobutylene of medium molecular weight, highly viscous 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-like sealants
These types of sealants are considered low-grade sealants. They typically consist of three main components: oil and resin ； Polybutene ； Asphalt. It is commonly used for fixing glass in small windows; the allowable deformation of its joints is up to +5% or -5%, and its useful life is generally 2 years. 3.2.3.2 Liquid elastomer sealants. This type of sealant consists of liquid polymers that can be vulcanized to form an elastic state. They have the ability to withstand repeated seam deformations. 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, refer to sealants based on a mixture of elastomers and thermoplastic resins. 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 agents are required. 3.2.3.4 Liquid sealants These sealants are primarily used for sealing mechanical joints, serving as a substitute for solid sealing materials (paper, asbestos, cork, and vulcanized rubber). They prevent any leakage of fluids from within the machinery through these joints; therefore, liquid sealants are also known as liquid gaskets. 3.2.4 Classification based on the properties of sealants after application 3.2.4.1 Curing sealants This type of sealant can be further divided into rigid and flexible categories. 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 viscosity enhancers continue to move 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 commonly used sealants. In the classification of sealants mentioned earlier, several methods were outlined; among these, the most widely used approach is to categorize sealants into two main types: vulcanized and non-vulcanized. 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, add the commonly used anaerobic adhesive. 3.4 Selection and application procedures of liquid sealants. The selection of sealants should be determined through comprehensive consideration of various factors, such as operating conditions, the material of the sealing components and the condition of the sealing surfaces, the type and characteristics of the sealing medium, as well as the application process. Generally, when subjected to high loads, impact forces, and alternating forces, a sealant with higher strength should be selected ； When the temperature difference is large, a sealant with good toughness should be selected. 3.4.1 Application methods for liquid sealants The application method for liquid sealants can be selected based on the state of the sealant. Paste-like sealant can be applied by scraping with a spatula or injected using a caulking gun ； Liquid sealant is applied by brushing or spraying ； Membrane sealants are applied using a paving 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 sealing grooves and filling large structural gaps. They play a major role in liquid gaskets 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 liquid 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 swollen 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 the shaft has a slight amount of eccentricity, 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 braided packing: Twisted packing is created by twisting several strands of asbestos yarn together; when this is packed into the packing chamber, it serves as a seal. Woven fillers are made by weaving cotton, linen, and asbestos fibers together, 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 metals and non-metals, 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 great 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 fillers. ` 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 particularly important, along with the pH value and operating temperature. 4.6 Proper packing of packing material The proper packing of packing material should be carried out in the following steps: 1) Clean the packing 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 packaged 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 loading, it should be done circle by circle; multiple circles should not be loaded at the same time. 6) Take a wooden half-shaft sleeve of the same size as the packing, fit it over 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 packing in the final circle 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, such as V-shaped, U-shaped, L-shaped, J-shaped, and Y-shaped. 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 area 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, allowing it to function well in situations with alternating pressures ； 5) The dimensions and grooves are standardized, resulting in low costs and ease of use and procurement from external suppliers. 5.2 V-ring seals. V-ring seals are a type of lip seal and are among the earliest and most widely used molded packings. It is mainly used for reciprocating motion, serving as a seal for pistons or piston rods. Rarely used for rotating applications or as a static seal. V-shaped seals have the following characteristics: 1) Good sealing performance ； 2) Allow certain eccentric loads and eccentric movements ； 3) It can be used with multiple overlaps, and the maximum sealing effect can be achieved by adjusting the clamping force ； 4) Impact pressure and vibration pressure resistance ； 5) When the packing cannot be installed axially, it can be used with cuts made on it; as long as the cuts are staggered during installation, the sealing effect will not be affected. Its drawback is that the frictional resistance is greater than that of other molding packings. 5.3 Y-type sealing rings: The U-type and Y-type sealing rings used for piston sealing differ slightly in shape. The lip of a U-type ring is longer, and the thickness of its base is equal to or slightly greater than that of the lip. The Y-ring is short in length and thick at the base; this design is intended to overcome the tendency of U-rings to fail to remain stable in place. It also increases the strength of the lip, thereby preventing it from tearing at its base. 5.4 Structure of drum-shaped 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 both unidirectional and bidirectional manners. The cross-section of the seal 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 ring is used for sealing the piston rod. 6. Oil seals and dust seals 6.1 Oil seals Oil seals are used for sealing lubricating oil. It is commonly used in the bearings of various machinery, especially in rolling bearing applications. Its function is to isolate the oil chamber from the outside environment, sealing in the oil and keeping dust out. Oil seals have the following advantages over other types of seals: 1) Oil seals are lightweight and require less consumable material. 2) The oil seal has a small installation size and axial dimensions, 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) The price is cheap. 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. For dust-proof seals, rubber is commonly used in hydraulic machinery, while felt is preferred in pneumatic machinery. In cylinders used in aircraft and cold-climate environments, metal is employed to prevent ice formation on the outer surface of the piston rod. In the chemical industry, metal is also used to prevent adhesions from forming on the piston rod. Dust seals are very important for protecting critical hydraulic equipment. Dust infiltration not only wears out seals, but also **wears out the guide sleeves and piston rods. In addition, the entry of impurities into the hydraulic medium can also affect the functioning of operating valves and pumps; in the worst-case scenario, 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. Magnetorheological Sealing 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 finely divided highly magnetic particles 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 simultaneously 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 agglomerating with each other ； 3) Carrier fluid (solvent). 7.1.2 Properties of magnetofluids Magnetofluids are a type of colloidal solution. For use as a sealing magnetic fluid, its performance requirements are: good stability, no coagulation, no precipitation, and no decomposition ； High saturation magnetization ； High initial magnetic permeability ； The viscosity and saturated vapor pressure are low; there are also certain requirements for other properties such as freezing point, boiling point, thermal conductivity, specific heat, and surface tension. The main factors affecting the stability of magnetofluids are: particle size, surfactants and carrier fluid, as well as their proper 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 magnet, pole shoes, and rotating shaft, under the influence of the magnetic field generated by the magnet, 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 shaft to form the magnetic circuit, whereas in the latter, the magnetic fluxes do not pass through the shaft; instead, the magnetic circuit is formed by the magnetic fluid in the sealed gap. 7.2.3 Limiting conditions: Magnetohydraulic seals are subject to the following constraints during operation: 1) Evaporation: The magnetohydraulic fluid consists of magnetic particles, surfactants, and a carrier fluid; the evaporation of this carrier fluid is the main factor determining the limit rotational frequency and service life of the seal. Because the seal operates using a limited amount of magnetorheological fluid. 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 demagnetization of the magnets and evaporation of the magnetic fluid. 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 decrease. Therefore, the temperature of the magnetofluid 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 diester lubricants can meet the requirements of ultra-high vacuum technology at 1.333×10-7 Pa. 4) Peripheral speed: Ordinary magnetohydrodynamic seals are suitable for operations with high peripheral speeds of over 30 m/s, with no limit specified. However, considering temperature and heat dissipation, the peripheral speed should be limited to 60–80 m/s, and at the same time the ultimate pressure resistance must also be taken into account. 8. High-pressure sealing: There are many types of high-pressure seals. Based on their working principles, they can be classified into two categories: forced sealing and self-sealing. Forced sealing relies on the preload force of fasteners (bolts) to ensure that a certain contact pressure exists between the pressure vessel’s head, sealing elements, and the ends of the cylindrical shell, thereby achieving the purpose of 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, thereby providing a seal. The characteristic of self-sealing is that the higher the pressure, the greater the compressive force on the contact surface between the sealing elements, resulting in better sealing performance; moreover, the seal remains reliable even when operating conditions vary. 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, Kazzari sealing, and octagonal gasket sealing ； 2) The semi-self-sealing type features a biconical seal ； 3) Self-tightening seals include wedge seals, Wede seals, hollow metal O-ring seals, C-ring seals, B-ring seals, triangular gasket seals, octagonal gasket seals, flat gasket self-tightening seals, and rubber O-ring seals, etc. 9. Vacuum sealing: The sealing performance of a vacuum online system depends on leaks at the connections and the outgassing of the vacuum materials. For any vacuum system, it is generally expected that the level of leakage and gas release is related to various factors such as the type of sealing, the sealing materials, manufacturing precision, and assembly quality. Therefore, there is always a certain amount of leakage and gas release at the connection points. Accordingly, requirements can be set based on the nature of the vacuum system’s operation, the degree 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. Instead, ultra-high vacuum sealing materials such as gold or copper must be employed as gaskets. Additionally, soft steel cannot be used for vacuum chambers; stainless steel must be used instead. The state of gases within ultra-high vacuum is a dynamic equilibrium. The pressure limit within the system is related, on 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 a material for use inside vacuum systems, it is required to have a low saturated vapor pressure. To minimize chronic desorption and outgassing, it must also be able to withstand high-temperature baking at 450°C without any reduction in mechanical strength or any chemical and physical damage. As a material for vacuum system enclosures, it is required to exhibit negligible gas permeability, withstand atmospheric pressure, be resistant to air erosion during baking, and prevent any leakage. In addition, the selected materials should be easy to process and manufacture, and 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 once a vacuum seal is formed, and they can be reused multiple times. However, since ultra-high vacuum systems require seal ring materials to withstand baking at 250°C, in reality, none of the few available rubber materials can meet this requirement. For ultra-high vacuum, where the degree of vacuum is even higher (i.e., the pressure is lower), 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 J-type rubber seals for vacuum applications must be flat and smooth; defects such as bubbles, impurities, or unevenness are not permitted. 2) O-ring rubber seals 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. Detachable connections sealed with metal seals are a commonly used type of connection in ultra-high vacuum systems. It is a sealing method employed to meet ultra-high vacuum requirements, necessitating high-temperature bake-out at 200–400°C for degassing. 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 exhibit good sealing performance, they undergo significant deformation hardening under the action of clamping force, resulting in increased 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, in connections that require high performance and are not intended to be disassembled, the seal must be replaced when reassembling after disassembly. 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 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 gasket 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 connecting 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 typical structure of a vacuum shaft seal 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 a new type of sealing structure with low friction loss and a long service life has become one of the major problems to be solved in vacuum devices. To address this issue, the technology of using magnetohydraulic fluid for dynamic vacuum shaft sealing has achieved success at home and abroad in recent years. Advantages of using magnetorheological fluid seals in vacuum applications: 1) Magnetorheological fluid seals for vacuum rotary shafts can eliminate frictional losses caused by contact between sealing elements, thereby increasing the shaft’s rotational speed (up to 120,000 r/min) and **reducing leakage**. If a magnetofluid with low vapor pressure is used, the vacuum level within the vacuum chamber can be maintained at above 1.3×10⁻⁷ Pa. 2) The seal structure of magnetorheological 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 magnetofluid is held in place by the magnetic field generated by the magnet within the sealed gap; therefore, starting and stopping the shaft is relatively easy. 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 of connecting bulkheads: The sealing method that utilizes magnetic force to transmit power into a vacuum vessel involves applying a rotating magnetic field 1 outside the vacuum vessel; this magnetic field causes the squirrel-cage rotor inside the vessel to rotate, thereby achieving the purpose of sealing the bulkhead. Characteristics of this sealing device: 1) The magnetic coupling diaphragm seal has no significant effect on the vacuum conditions inside the vacuum container; compared to several other types of dynamic seals, it offers greater vacuum reliability. 2) The moving parts do not come into contact with the walls of the vacuum chamber. During the transmission of motion, apart from bearing the pressure difference, the diaphragm or isolation ring cylinder does not endure any other loads; this ensures the reliability of the seal of the magnetically coupled diaphragm. 3) The “contamination” inside the vacuum vessel depends solely on the structural components of the moving parts themselves, particularly the outgassing from frictional components and the permeability of 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 iron core that transmits motion should match the shape of the magnet; furthermore, the container wall or other components within the vacuum chamber must ensure the direction of the iron core’s movement ； 4) To reduce outgassing and friction, it is recommended to use a glass-coated iron core ； 5) The magnetic field strength and the distance between the magnet and the core should be selected so that their movement causes little impact on the container walls or the mercury, indium, etc., inside the container. 10. Centrifugal sealing, shutdown, and fully enclosed sealing
10.1 Centrifugal sealing
10.1.1 Structural types of centrifugal seals
A centrifugal seal is a device that utilizes a rotating body to impart centrifugal force on a fluid, thereby preventing leakage. Its sealing capability stems from the work done by the rotation of the machine shaft on the sealing elements; therefore, it falls under the category of dynamic seals. Characteristics of centrifugal seals: They have no directly contacting friction pairs, and a larger seal clearance can be used. Therefore, they can seal media containing solid impurities. They exhibit low wear and a long service life; with proper design, virtually zero leakage can be achieved. However, the pressure difference that this seal can overcome is small, meaning its pressure-reducing capacity is low. Centrifugal sealing consumes a great deal of power; it can even account for up to one-third 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. 10.1.2 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 disk, then the liquid in the gap cavity 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 time, the pressure within the clearance cavity is distributed radially in a parabolic manner. As shown in Figure 10-5, the pressure is 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 dynamic seal loses its sealing capability, and it is only the stop-seal that prevents fluid leakage. Some liquid-sealed and gas-sealed fluids are equipped with a shutdown seal, allowing the sealing fluid and gas sealing systems to be turned off after shutdown. There are various structural types of shutdown seals, among which the most widely used is the centrifugal shutdown seal. In addition, there are also pressure-regulating shutdown seals, expansion-type shutdown seals, and others. 10.2.1 Centrifugal shutdown seal: A typical centrifugal shutdown seal structure relies on the centrifugal force generated by the back blades to provide sealing while the pump is in operation. When the pump is stopped, 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 shutdown seal: A pressure-regulating seal that combines a spiral seal; during shutdown, the spiral sleeve capable of moving on the shaft, driven by spring force, presses its stepped end face against the end face of the housing to achieve sealing. During operation, the two counter-rotating spirals cause the viscous fluid in the gap to form pressure peaks at the end faces, which act on the stepped end face of the spiral shaft and cause it to lose contact with the end face of the housing. Parking and sealing with slide valve. When the pressure difference cylinder releases pressure, the spool pushed by the leaf spring presses against the shaft shoulder to achieve a shutdown seal. 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 substances, 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, due to the poorer heat dissipation and lubrication conditions of gases 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. The multi-layer floating ring also consists only of a combination of these simple parts, which are fewer in number than those of mechanical seals. 2) It is not sensitive to the operating conditions of the machine and has stable sealing performance. 3) The seal does not wear out, provides reliable sealing, and is simple to maintain and 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 operating at 10,000–20,000 r/min; it is particularly suitable for gas compressors. Its allowable speed exceeds 100 m/s, a figure unmatched by any other seals. 7) As long as corrosion-resistant metal materials or corrosion-resistant non-metal linings (such as graphite) are used for the floating ring, it can be used for sealing in strongly 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: They require high manufacturing precision; eccentricity of the rings, non-perpendicularity of the 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, contactless dynamic seal that relies on the fluid resistance effect within the sealing gap to prevent leakage. Due to the presence of a gap, solid friction is avoided; it is suitable for high-speed applications. It can seal both liquids and gases. However, the leakage rate is relatively high, and in some cases, a relatively complex auxiliary sealing system must be employed. 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 split rings, as well as liquid-film and dry floating rings. 11.2.1 Wide rings and narrow rings. Wide rings have a relatively large width compared to their diameter; the ratio l/D is 0.4–0.6. The characteristic of this ring is that the hydrodynamic forces acting on it during operation are greater than those on a narrower ring, and it does not require accessories aligned with its 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 a narrow ring is relatively small compared to its diameter, with a ratio of l/D = 0.1–0.2. 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 maintain concentricity. Rubber O-rings are typically used to help align them properly. Thanks to this auxiliary measure, the eccentricity is low and the downtime is also minimal; thus, even though the ring is narrow, the amount of leakage remains small. For narrow rings, a spring can be used instead of an O-ring for positioning. Under the action of spring force, the ring is pressed against the end face of the isolation ring. When the pressure of the sealing fluid decreases, the ring can still maintain its centered 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 split 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, a relatively large amount of leakage occurs during operation. 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, its leakage rate is lower than that of a smooth ring; especially at high rotational speeds, it can achieve complete leak-free operation, and the liquid film formed is also 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 result in less leakage at high speeds; hence, 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 speed of movement, 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 be in tight contact with one end face of each retaining ring. This arrangement prevents gas from leaking radially. The frictional force between the end faces also prevents the rings from rotating. Any small amount of gas that leaks axially through the floating ring seal is discharged via a vent hole or directed back to the gas inlet of the main machine. The working clearance of graphite floating ring seals is not a fixed value; rather, it adjusts itself according to the amount of frictional heat generated. Hence, it is also known as a “thermally self-adjusting clearance seal”. Graphite is both corrosion- and heat-resistant, but it is too brittle and prone to cracking under radial loads. In centrifugal compressors, graphite is used as the floating ring, and metal rings are often fitted around the outer periphery of the graphite ring to prevent breakage. The graphite ring is shrunk onto a metal ring using a cold-shrinking method; thereafter, the inner hole of the graphite ring is machined to achieve the specified dimensions. When the temperature of the shaft seal rises, if the materials of the insert ring and the shaft are the same or similar, their amounts of expansion will be the same or very similar. without affecting the sealing performance. This structure has been successfully applied to gas sealing at temperatures up to 400°C. 12. Labyrinth seal: A labyrinth seal consists of several annular sealing teeth arranged in sequence around the rotating shaft; gaps and expansion chambers are formed between these teeth. As the fluid to be sealed passes through these intricate gaps, a throttling effect occurs, thereby preventing leakage. Due to the gap between the rotor and the casing in a labyrinth seal, there is no solid contact, thus lubrication is not required, and thermal expansion is allowed. This makes it suitable for applications involving high temperatures, high pressures, and high rotational speeds. This type of seal is widely used for sealing the shaft ends and stages in turbines, gas turbines, compressors, and blowers, as well as as a pre-seal for other dynamic seals. 12.1 Sealing mechanism of labyrinth seals: The mechanism by which fluid encounters resistance as it passes through a labyrinth, resulting in a reduction in its flow rate, is known as the “labyrinth effect”. For liquids, there are fluid dynamics effects, including hydrodynamic friction effects and flow-contraction effects ； For gases, there are also thermodynamic effects, namely the heat conversion that occurs in the labyrinth as a result of compression or expansion ； In addition, there is also the “ventilation effect,” among others. The labyrinth effect is a composite response of these effects; therefore, the mechanism of labyrinth seals is quite complex. 12.1.1 Frictional effect: When the leaking fluid flows through the labyrinth, friction caused by the fluid’s viscosity slows down its flow velocity and reduces the flow rate (leakage volume). In simple terms, the friction along the flow path and the local resistance of the fluid constitute the drag effect; the former is related to the length and cross-sectional shape of the channel, while the latter is related to the number of bends and the geometric shape of the labyrinth. Generally, when the flow channel is long, has sharp bends, and sharp corners, the resistance is high, the pressure drop loss is significant, and the leakage amount decreases. 12.1.2 Beam Contraction Effect: As the fluid passes through the labyrinthine openings, it contracts due to inertia, resulting in a reduction of the cross-section of the flow beam. Let the area of the orifice be A; then the minimum area of the converged flow jet is Cc A, where Cc is the contraction coefficient. At the same time, the velocity of the gas after passing through the orifice also changes. Under ideal conditions, the flow velocity is u1; however, the actual flow velocity is less than u1. Let Cd be the velocity coefficient; then the actual flow velocity can be expressed as u1 = Cd · u1. Consequently, the flow rate through the orifice is given by q = Cc·Cd·A·u1, where Cc·Cd = α (the flow coefficient). The flow coefficient at the labyrinth seal opening is related to the shape of the gap, the shape of the tooth tips, and the roughness of the wall surfaces. For incompressible fluids, it is also related to the Reynolds number ； For compressible fluids, it also depends on the pressure ratio and Mach number. At the same time, it also affects the flow state before the seam. Therefore, in labyrinths with complex configurations, the flow coefficient of one slot cannot be taken as that of all slots. According to the tests, the flow coefficient is lower for the first stage, while it is higher for the slits in stages subsequent to the second stage; generally, the flow coefficient is taken as 1. However, the flow coefficient of sharp teeth is less than 1, around 0.7, while that of round teeth is close to 1; typically α is set to 1, which results in an overestimated leakage amount. 12.1.3 Thermodynamic effects The ideal labyrinth flow channel model consists of a series of annular tooth gaps and inter-tooth cavities. The flow of gas as it passes through each tooth gap and inter-tooth cavity can be described as follows: at the entrance to the gap, the gas has conditions of p0, T0, and zero velocity; the closer the gas gets to the entrance, the more the flow contracts and accelerates, and shortly after reaching the narrowest part of the gap, the flow attains its maximum velocity ； Upon entering the cavity, the flow velocity cross-section suddenly expands, resulting in strong vortices forming within the cavity. From an energy perspective, before and after the gap, the pressure energy of the airflow is converted into kinetic energy. At the same time, as the temperature drops (the heat enthalpy value h decreases), when the gas enters the annular chamber between the two teeth at high speed, its volume expands suddenly, resulting in intense swirling motions. As a result of eddy friction, the vast majority of the kinetic energy of the airflow is converted into thermal energy, which is absorbed by the airflow within the chamber and raises its temperature. The enthalpy then returns to a value close to that before entering the gap; only a small portion of the kinetic energy continues to enter the next gap at residual speed, and this process repeats step by step. 12.1.4 Ventilation effect: In an ideal labyrinth, it is assumed that the kinetic energy of the airflow passing through the gaps in the expansion chamber is entirely converted into heat energy. That is to say, it is assumed that the asymptotic velocity up to the next slit is equal to zero; however, this holds true only when the expansion chamber is particularly wide and long. In a typical straight-through maze, since the airflow passing through the slit can only spread in one direction, the conversion of this kinetic energy into thermal energy cannot take place effectively within the expansion chamber. On the side with smooth walls, the velocity of some of the gas does not decrease or decreases only slightly, and it flows directly over the tops of the teeth toward the low-pressure side. This phenomenon of flowing past such obstacles is known as the \"permeation effect\". 12.2 Structural types of labyrinth seals. Based on the structure of the sealing teeth, labyrinth seals are divided into two main types: sealing strips and sealing rings. The sealing sheet has a compact structure; during operation, when it comes into contact with the casing, it can bend to both sides, reducing friction, and it is also easy to replace. The sealing ring consists of 6 to 8 sector-shaped pieces, which are placed between the housing and the rotating shaft. Each piece is pressed against the housing by a spring strip; the pressing force exerted by these spring strips is approximately 60–100 N. When the shaft comes into contact with the toothed ring, the latter automatically springs away, thereby preventing friction. This structure has relatively large dimensions and complex processing; after the teeth wear out, the entire sealing ring must be replaced. Therefore, its application is not as widespread as that of seal ring structures. 12.3 Calculation of leakage in an ideal labyrinth Given the following conditions: 1) The leaking gas is an ideal gas, and the Joule-Thomson effect is neglected; that is, the enthalpy of the gas depends only on temperature ； 2) Assume that the maze consists of a series of continuous slits, with expansion chambers between any two slits being large enough ； 3) Adiabatic cyclic expansion through the slit flow, here a flow coefficient α is cited ； 4) The flow velocity energy after passing through the slit is completely restored to a constant temperature in the expansion chamber due to isobaric conditions; therefore, the velocity just before each slit approaches 0, meaning no air leakage occurs. 12.4 Characteristics of straight-through labyrinths Since it is easier to machine grooves or teeth of various shapes on the surface of a shaft than inside a hole, holes are often made with a smooth surface, which then forms a labyrinth together with the grooved or toothed shaft. This is known as a straight-through labyrinth. Due to its ease of fabrication, the straight-through labyrinth is the most widely used type. However, straight-through labyrinth seals exhibit air leakage, and the amount of leakage is greater than that of an ideal labyrinth seal. 12.4.1 Factors influencing maze characteristics: 1) Effect of teeth. Experiments conducted abroad have shown that, when the pitch remains constant, the greater the number of teeth, the less the leakage. When the tooth pitch is changed, the larger the tooth pitch, the more sharply the leakage rate decreases, and it also helps to reduce the impact of air leakage. 2) Effect of the expansion chamber. Foreign studies have been conducted on the effect of expansion chamber depth; the conclusion is that a shallow expansion chamber is beneficial for reducing leakage. Based on the observation of the flow conditions in the expansion chamber, it is considered that the vortices in the shallow expansion chamber are unstable. Since vortices can quickly deplete energy, the asymptotic velocity in the expansion chamber decreases, thereby reducing leakage. 3) Influence of the auxiliary chamber. The so-called \"secondary chamber\" refers to an auxiliary groove formed on the smooth surface of a straight-through labyrinth; once such a groove is created, the flow pattern within the labyrinth changes significantly immediately. Tests have shown that, as long as the position of the auxiliary chamber is appropriate, the rate of reduction in leakage is quite significant. 12.5 Gaps in labyrinth gas seals: Except in special cases, labyrinth gas seals are generally used in turbomachines such as gas turbines and steam turbines. Its radial clearance should be selected based on the following factors: bearing clearance, manufacturing tolerances and assembly errors, deformation of the components (such as casting shrinkage and out-of-roundness), rotor deflection, as well as the amplitude at the critical rotational frequency, thermal expansion and the resulting deformations. In many cases, the effects of thermal expansion are most pronounced. Therefore, the changes in the dimensions of individual components during start-up and shutdown, as well as their relative displacements, must be estimated in advance. Static and dynamic finite element algorithms can be used to determine the time-dependent thermal expansion patterns, thereby helping to identify which are the critical conditions and what size the gap should actually be. 12.5.1 Points to Note in the Design of Labyrinth Seals Summarizing the experience gained in labyrinth seal design, the following key points can be identified: 1) Try to convert the kinetic energy of the airflow into heat energy, so that the residual velocity does not enter the next gap. An appropriate distance should be maintained between the teeth, or high-low teeth should be used to forcibly change the direction of the airflow. The tooth spacing is generally 5–9 mm. 2) The sealing teeth should be made as thin as possible and have sharp corners. The tip thickness should be less than 0.5 mm; when it occasionally comes into contact with the shaft during operation, the tip wears out first and loses contact, preventing local overheating of the shaft due to friction and thus avoiding accidents. 3) Due to the large amount of leakage in labyrinth seals, when sealing flammable, explosive, or toxic gases, care must be taken to prevent environmental contamination. An inflatable labyrinth seal is used, with an inert gas introduced into the gap; the pressure of this gas is slightly higher than that of the gas to be sealed ； If the medium does not allow air inclusion, a vacuum-type labyrinth seal can be used. 13. Spiral seals are used in many advanced technology fields, such as the sealing of gas-cooled reactor compressors and the sealing of sodium pumps in breeder reactors. It is sometimes also used for the sealing of the high-speed shaft in reducers. Its greatest advantage is that it can provide effective sealing even when there is a large gap between the sealing components. If properly designed, its service life can be infinite. Due to the wide range of materials available for selection and the extreme simplicity of manufacturing, when the pressure difference is not high, the power consumption and heat generation of spiral seals are very low, and cooling with a water jacket is sufficient. Spiral seals often require auxiliary shutdown seals, which complicates the structure and increases its size, thus often limiting their application. Spiral seals can be used for liquids under high temperatures, extreme cold, in corrosive environments, or those containing particles; they perform well even in harsh sealing conditions. 13.1 Sealing mechanism of spiral seals: The shaft surface of a spiral seal features spiral grooves, while the bore has a smooth surface; this is similar to the grooved structure of labyrinth seals. Therefore, a spiral seal can be regarded as a special type of labyrinth seal, known as a spiral labyrinth. However, the teeth of the spiral labyrinth are continuous, unlike the continuous teeth of the various labyrinths mentioned earlier. Due to the continuity of the teeth, the flow state of the medium through the teeth changes. The helical groove no longer functions as an expansion chamber to generate vortices that consume flow energy; instead, it acts as a propulsion device that exchanges energy with the medium, resulting in what is known as a \"pumping effect\" and the creation of a pumping head. This pumping head balances the pressure of the sealed medium, meaning the pressure difference p=0, thereby preventing leakage. Therefore, its sealing mechanism is slightly different from that of a labyrinth seal. However, as the medium passes through the gap, a portion of it remains above the tooth tips and does not flow along the groove, resulting in a ventilation effect, which is the same as in labyrinth seals. Depending on the helical structure, the sealing mechanism of helical seals varies slightly. It is a single-screw type; it utilizes the principle of screw pump operation, employing the pumping action of the screw to push the medium along the leakage gap back, thereby achieving sealing. It is suitable for sealing liquids or gas-liquid mixtures without the need for an external sealant, and is commonly used for bearing lubrication. It should be noted that the oil-repelling direction of the helix must be opposite to the direction of oil leakage; otherwise, not only will a seal not be achieved, but the amount of leakage will also increase significantly. Two helices with opposite rotation directions push the sealing fluid toward the center, creating a liquid seal. A pressure in the liquid seal that is slightly greater than or equal to the pressure of the medium being sealed will achieve sealing. It is commonly used to seal gases or vacuums. Two counter-rotating screws expel gas to both sides at high rotational frequencies, creating a high-vacuum trap in the middle to achieve sealing. This seal can be used as a vacuum seal. Theoretically, a smaller gap in a spiral seal is more conducive to ensuring a good seal. If the gap is large, the liquid medium cannot adhere to the surface of the shaft at the same time. Assuming that the liquid medium adheres only to the pore walls and separates from the axis, the helical seal fails to expel the medium; in other words, the seal becomes ineffective. However, the clearance is too small; there’s a risk of the shaft touching the hole wall. To prevent friction and wear between sealed metal mating parts, a layer of graphite can be applied to the surface of the hole wall. 13.3 Labyrinth helical seals. The use of labyrinth helical seals in industry is relatively recent. They differ from ordinary helical seals in that helical grooves are machined on the surface of the shaft, and a threaded sleeve is also machined onto the sealing hole; this sleeve has a thread direction opposite to that of the shaft, thereby causing the flow between the shaft and the sleeve to become highly turbulent. In addition, the helical motion speed of the labyrinth helical seal is higher than that of the screw seal; it is used for low-viscosity liquids under turbulent flow conditions. Helical seals are generally used for liquids with high viscosity (such as liquids whose viscosity is greater than that of water) under laminar flow conditions. Working principle: Within the working space between the screw and the nut, the liquid is contained in several honeycomb-like compartments formed by the two tooth surfaces of the nut and those of the screw. The gap between the surface of the screw and that of the nut is a grooved toroidal cylindrical surface. When the liquid passes through these threads, vortices are formed in a direction opposite to the flow direction. As a result of the momentum exchange between the screw and the flowing liquid, the screw transfers energy to the liquid. The screw and the nut interact with the liquid, resulting in frictional forces at the nominal interface passing through the gap between the screw and the nut. The friction generated in the liquid creates pressure between the screw and the nut. 14. Mechanical Seals 14.1 Working Principle of Mechanical Seals A mechanical seal, also known as an end-face seal, is a dynamic seal for rotating shafts. Mechanical seals offer reliable performance, low leakage, a long service life, low power consumption, and require little maintenance. They are also capable of meeting the sealing requirements in harsh operating conditions such as automation in production processes, as well as high/low temperatures, high pressure, vacuum, high speeds, and environments with highly corrosive media or media containing solid particles. A mechanical seal is a shaft sealing device that prevents leakage by relying on a pair or several pairs of end faces that slide relative to each other perpendicular to the axis; these end faces remain in contact under the action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, along with the help of additional sealing elements. A comparison between mechanical seals and soft packing seals is as follows: Advantages: 1) Reliable sealing – the sealing condition remains stable during long-term operation, with very low leakage levels; the leakage is approximately 1% of that in soft packing seals ； 2) It has a long service life; in oil and water media, it can generally last 1–2 years or even longer, while in chemical media it can operate for more than half a year. 3) It has low friction power consumption; its friction power is only 10% to 50% of that of soft packing seals ； 4) The shaft or bushing suffers virtually no wear ； 5) Long maintenance cycle; automatic compensation occurs after end-face wear. Under normal circumstances, frequent maintenance is not required ； 6) Good vibration resistance; insensitive to vibrations of the rotating shaft and any misalignment of the shaft relative to the seal chamber ； 7) Wide range of applications: Mechanical seals can be used for sealing in high-temperature, low-temperature, high-pressure, and vacuum environments, at various rotational frequencies, as well as in media containing corrosive substances or abrasive particles. Disadvantages: 1) Relatively complex; requires high processing standards ； 2) Installation and replacement are rather troublesome, requiring workers to have a certain level of skill ； 3) When accidental incidents occur, they are difficult to handle ； 4) High price. Preparatory work before installing the mechanical seal: 1) Check whether the model and specifications of the mechanical seal meet the requirements of the design drawings, and verify that all components (especially the sealing surfaces and auxiliary sealing rings) are free from damage, deformation, cracks, etc. If any defects are found, they must be replaced or repaired. 2) Check whether the fitting dimensions, roughness, and parallelism of all components of the mechanical seal meet the design requirements. 3) When using a small spring mechanical seal, it is necessary to check whether the lengths and rigidity of the small springs are identical. 4) Check whether the shaft’s displacement, swing, and deflection meet the technical requirements, whether the sealing chamber conforms to the installation dimensions, and whether the sealing end cover is perpendicular to the shaft. The general requirement is that the shaft’s displacement should not exceed ±0.5 mm ； The shaft wobble (at the rotary ring seal) is no more than 0.06 mm ； The maximum deflection of the shaft shall not exceed 0.05 mm ； The allowable deviation from perpendicularity between the contact plane of the sealed end cap and the gasket and the centerline is 0.03–0.05 mm. 5) It should be kept clean; in particular, the sealing surfaces of the rotating ring and the stationary ring, as well as the surfaces of the auxiliary seals, must be free from impurities and dust. It is not allowed to wipe the sealing surface with unclean cloths. 6) It is permitted to use tools to tap on the sealing elements to prevent them from being damaged. 14.2 Materials for mechanical seals – Materials for friction pairs: Statistics show that approximately 80%–95% of leaks in mechanical seals are caused by the sealing surfaces and friction pairs. Apart from maintaining the parallelism of the sealing surfaces, the main issue is the material of the friction pair. Friction materials should meet the following requirements: 1) High mechanical strength, capable of withstanding pressure and pressure-induced deformation ； 2) It has resistance to dry friction, high load capacity, and good self-lubricating properties ； 3) The mating materials have good wear compatibility, with no excessive wear or galling corrosion ； 4) Good wear resistance and long service life ； 5) Good thermal conductivity and heat dissipation performance ； 6) Good high-temperature resistance ； 7) Good resistance to thermal cracking ； 8) Strong corrosion resistance ； 9) Low linear expansion coefficient, capable of withstanding thermal deformation and exhibiting good dimensional stability ； 10) Good machinability and good formability ； 11) Good airtightness ； 12) Low density.