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Leakage is one of the common failures that occur in mechanical equipment. There are mainly two reasons for 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 the mating surfaces, isolate or block the leakage path, increase the resistance in that path, or insert small working elements into the path to exert pressure on the leaking fluid, thereby offsetting or completely balancing the pressure difference that causes the leakage and preventing it from occurring. Sealing can be divided into two main categories: static sealing, which occurs between relatively stationary mating surfaces, and dynamic sealing, which occurs between mating surfaces that are in relative motion. A static seal usually refers to a seal between two stationary surfaces. Static seals mainly fall into three categories: gasket seals, sealant seals, and direct contact seals. Dynamic sealing refers to the sealing between moving parts within a machine (or equipment). 1 Packing seal: Packing seals are primarily used for dynamic sealing of the moving parts of process machines and equipment in the machinery industry. Examples include the shaft sealing of centrifugal pumps, compressors, vacuum pumps, mixers, and reaction vessels; as well as the sealing between the plunger or piston rod of reciprocating pumps and reciprocating compressors. They are also used for sealing the valve stem against the fixed housing in valves that move in a linear or spiral manner. The basic forms of stuffing boxes are: (1) Asbestos packing: It has good resistance to both heat and corrosion, but its sealing performance is poor when used alone; therefore, it is always impregnated or combined with other materials. Oil-impregnated asbestos packing: It comes in two basic structural forms, one being twisted and the other being braided. They can also be divided into round and square types. (2) Polytetrafluoroethylene braided gasket: Polytetrafluoroethylene strips are woven into a gasket; it has excellent corrosion resistance and can also be used with cryogenic media. (3) Rubber O-ring: Provides good sealing performance at low pressures. The operating temperature is limited; for example, natural rubber can only be used at 60°C. (4) Plastic molding fillers: Generally made in a three-piece design, but can also be made in other shapes. Polytetrafluoroethylene is the most commonly used plastic, with nylon 66 and nylon 1010 also being used. (5) Soft packing seal. Soft packing, also known as paste-like packing, is a high-grade “packing” made from a new generation of high-performance synthetic fibers combined with special barrier agents and lubricants. Soft fillers come in any size and shape, are extremely easy to install – they can be injected directly using a specialized high-pressure gun. Unlike traditional packing, which requires pre-cutting, they can be used for sealing filler chambers of any size. Soft packing seals have a wide range of applications, such as in high-pressure reciprocating pumps or pumps with low rotational speeds, as well as for sealing high-temperature valve stems, valves, or packing boxes. It has advantages such as a simple structure, reliable operation, and low cost, and is still widely used in applications with modest requirements. 2 Expansion ring sealing: An expansion ring is a metal elastic ring with cuts, commonly known as a piston ring. In its free state, the expansion ring has its cut open; once it is inserted into the cylinder body, the cut closes, and the outer surface of the expansion ring presses tightly against the inner surface of the cylinder body due to elastic force. During operation, the expansion ring is pressed against one side of the expansion ring groove by the pressure of the working medium; the outer cylindrical surface of the expansion ring together with the abutting end face serve to provide sealing. Expansion rings are commonly made of cast iron or tin bronze, and are primarily used for sealing the pistons in reciprocating compressors and internal combustion engines that operate at high temperatures, as well as for sealing the shafts in steam turbines. 3 Contact-type dynamic sealing: Gasket sealing. Sealing gaskets are sealing components used in machinery, equipment, and pipelines wherever there is fluid flow; they consist of materials that provide a sealing function on both the inner and outer sides. Sealing gaskets are made from metallic or non-metallic sheet materials, through processes such as cutting, stamping, or trimming, and are used to achieve a sealed connection between pipes, as well as between various components of machinery and equipment. Based on material, they can be divided into metal gaskets and non-metal gaskets. Metals include copper gaskets, stainless steel gaskets, iron gaskets, aluminum gaskets, and so on. Non-metallic gaskets include asbestos gaskets, non-asbestos gaskets, paper gaskets, rubber gaskets, etc. The latest addition is graphite gaskets. Thread sealing: There are generally two forms of thread sealing – one is sealing using threaded connection gaskets, and the other is sealing by using threads in combination with a filler material. Both are used for sealing small-diameter threaded connections. In a threaded connection, the sealing element is the gasket; threads serve only to apply compressive force. The sealing effect depends not only on the performance of the gasket itself. The roughness of the sealing surface, as well as the precision of its relative geometric position with respect to the threaded hole, also have a significant impact on the sealing performance. Since the sealing gasket is subjected not only to compressive forces but also to torque when the threads are tightened, it may become deformed or even damaged. Therefore, when the gasket is made of non-metallic materials, it is generally suitable only for applications with low pressure levels. In contrast, if the gasket is made of metal, it can be used in applications where the pressure exceeds 30 MPa. Felt ring sealing: The felt ring is installed in the trapezoidal groove of the bearing cover; the pressure from the sides of this trapezoidal groove compresses the felt against the shaft, thereby providing a sealing effect. It has good stretchability and heat retention properties, as well as moisturizing and elastic qualities. In primary and secondary gear reducers, felt ring oil seals are commonly used. In the bearing end caps of the input shaft and output shaft, at the exact center where they come into contact with the shaft, there are ring grooves with a trapezoidal cross-section; felt rings are placed in these grooves to prevent dust from entering and to stop lubricant from leaking. Oil seal sealing: Oil seal sealing refers to the sealing of lubricating oil. It isolates the parts of the transmission that require lubrication from the output components, preventing lubricant leakage. Any part of a rotating housing that contains liquid lubricant and is connected to the outside world requires an oil seal. Some are made of rubber, some are metal, and most are of the steel-reinforced rubber type, such as the oil seals behind the crankshaft, the oil seals at the front and back of the gearbox, the oil seals for the left and right half-shafts, the oil seal in front of the main reducer, and the oil seal for the air compressor crankshaft. Fully enclosed sealing: Fully enclosed sealing involves completely blocking any leakage paths between the inside and outside of the system, or placing the working machine (pump) and the prime mover within the same sealed system. This prevents any leakage of the medium. Therefore, it is of great importance in production processes involving highly toxic, radioactive substances, and rare and valuable materials, as well as in precision experiments. Common forms include diaphragm sealing. 4 Non-contact dynamic sealing – Floating ring seal. The floating ring seal is a type of high-speed seal that can be used to seal gases and liquids. When sealing with gas, the floating ring device should be filled with lubricating oil to facilitate sealing and lubrication. Floating ring seal is also known as oil film seal. A contactless dynamic seal that prevents fluid leakage by relying on a minimal gap between the floating metal ring and the shaft or housing. The floating ring can move freely within the casing, so high-speed rotation of the shaft and resulting vibrations have little impact on the seal, and friction and wear are also minimal. Spiral seal: A spiral seal is a type of dynamic seal in which spiral grooves are created on a rotating shaft or on the sleeve that surrounds the shaft; a sealing medium is filled between the shaft and the sleeve. The rotation of the shaft creates a pumping action similar to that of a pump in the spiral groove, thereby preventing the leakage of the sealing fluid. Its sealing capacity depends on the helix angle, pitch, tooth width, tooth height, effective length of the tooth, as well as the gap between the shaft and the sleeve. Since there is no friction between the seals, their lifespan is long; however, due to limitations in structural space, their spiral length is generally short, which limits their sealing capacity as well. When the pump operates at reduced speed, its sealing performance is significantly reduced. Centrifugal seal: A centrifugal seal is a device that utilizes the centrifugal force generated by the rotation of an additional element that does work, in order to prevent leaks. Its sealing capability stems from the work done by the rotating machine shaft on the sealing element. There are mainly two types of centrifugal seals: back vane seals and auxiliary impeller seals. Back vane sealing and auxiliary impeller sealing are common shaft sealing devices used in centrifugal pumps. These two types of centrifugal seals can accommodate relatively large seal gaps; they exhibit low wear and a long service life, and can achieve nearly zero leakage. They are commonly used in impurity pumps and slurry pumps that transport media containing solid particles. However, they involve high sealing power consumption, and a standby seal must be provided. Dry gas seal: A dry gas seal is a new type of shaft end seal that utilizes slotted sealing technology for gas sealing. When the rotating dynamic ring, which has fluid dynamic pressure grooves on its outer surface, rotates, these grooves pump the high-pressure gas on the outer diameter side (referred to as the upstream side) into the space between the sealing surfaces. The pressure of the gas film increases from the outer diameter toward the groove diameter, while it decreases from the groove diameter toward the inner diameter. The increased pressure on the sealing surface results in an opening force that is greater than the closing force acting on the sealing ring, thereby creating a very thin layer of gas film between the frictioning surfaces and allowing the seal to function in a non-contact manner. The formed gas film completely blocks the leakage path for the sealed medium under relatively low pressure, achieving zero leakage or zero escape of the sealed medium. It has minimal leakage in its gas film seal, enabling zero emission of the medium; thus, it is an environmentally friendly sealing solution ； Gas film seals experience minimal wear, have a long service life, and operate in a stable and reliable manner ； The sealing power consumption is low, accounting for only about 5% of that of contact-type mechanical seals; the sealed fluid remains free from oil contamination ； The sealing auxiliary system is simple and reliable, and requires no maintenance during use. 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, which helps to prevent leakage. It has the advantages of good sealing performance at high speeds, no need for lubrication, no friction, simple maintenance, a long service life, and no requirement for other sealing materials ； However, it has high processing precision and is difficult to assemble. Magnetic fluid sealing: Magnetic fluid sealing technology is developed based on magnetic fluids. When these magnetic fluids are introduced into a gap within a magnetic field, they can fill the entire gap, creating a kind of \"liquid O-ring\". The function of a magnetohydraulic sealing device is to transmit rotational motion into the sealed container, and it is commonly used for vacuum sealing. End face seal (mechanical seal) – An “end face seal” is a seal in which the sealing element is located at the end face, and sealing is achieved through compression by another surface. A mechanical seal is a device designed to prevent fluid leakage, consisting of at least one pair of end faces perpendicular to the axis of rotation, which remain in close contact and slide relative to each other under the action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, as well as with the assistance of auxiliary seals. The elastic loading mechanism and auxiliary seal are the mechanical seals for metallic bellows, which we refer to as metal bellows seals. In light-sealing applications, rubber bellows are also used as an auxiliary seal; however, their elastic strength is limited, so springs are generally required to provide the necessary elastic force.