The sealing principle of packing fillers

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I. Sealing principle of packing materials The sealing principle of packing materials depends mainly on the labyrinth effect and the bearing effect. Labyrinth effect: The microsurface of the shaft is highly uneven, allowing the packing to make contact with it only partially; there are areas where no contact occurs. As a result, tiny gaps exist between the packing and the shaft, forming a labyrinth-like structure. The pressurized fluid is throttled multiple times within these gaps, thereby achieving a sealing effect. Bearing effect: A thin layer of liquid film exists between the packing and the shaft, causing the packing and shaft to function like a sliding bearing and providing some level of lubrication, thereby preventing excessive wear on both the packing and the shaft. II. Requirements for packing materials Due to factors such as the temperature, pressure, and pH of the sealing medium, as well as the equipment’s linear velocity, surface roughness, coaxiality, radial runout, and eccentricity, packing materials must possess the following properties: 1. Certain degree of elasticity and plasticity; 2. Chemical stability; 3. Impermeability; 4. Self-lubricating properties; 5. Heat resistance; 6. Ease of installation and removal; 7. Simplicity in manufacturing and low cost. The properties of the aforementioned materials directly affect the sealing performance and service life of gasket fillers. Few materials can meet all of these requirements, which is why the search for high-quality sealing materials and the improvement of their properties has always been a focus of research in the field of sealing. III. Forms and characteristics of packing materials As production processes continue to evolve, the ways in which packing materials are structured also become more diverse. Depending on the operating conditions and environment, different structural formats have a direct impact on the sealing performance and service life of these materials. The main weaving methods used for braid-type packing materials include braid weaving, layered weaving, through-core weaving, and sandwich weaving, etc. The weaving methods and characteristics of packing are as follows: 1. Braid weaving: This method involves using eight spindles that move along two tracks to carry out the weaving process. There is no wadding in the corners or in the center; the cross-section of the resulting product is square. Its advantage is that the packing is loose, but it provides some compensation for shaft vibration and eccentricity. It is used only for packing with small cross-sections. However, when the cross-sectional size is large, the surface of the packing becomes rough, its structure becomes loose, and its density decreases. 2. Layered weaving: In this method, 8, 12, 16, 24, 36, 48, 60, etc., spindles are used to weave along two tracks. The number of layers depends on the specifications of the packing; generally, 1 to 4 layers are used. There is no wadding in the center. Layered packing has good density and strong sealing properties. But since the layers are separate with no fibers connecting them, they tend to separate from each other, so this method is often used for static seals or in low-speed equipment. 3. Sandwich weaving: Sandwich weaving uses rubber or metal as the core, with fibers on the outside; the layers are woven one upon another, with the number of layers determined as needed. It is similar to layered weaving. Sandwich weaving offers good density, high strength, excellent bending properties, and good sealing capabilities. However, just like layered structures, the surface layer tends to wear off once it is damaged. It is generally used in pumps and valves, and rarely in reciprocating equipment. 4. Through-core weaving: This type of weaving is carried out using 8, 12, 16, 24, 36, 48, 60, etc., spindles on three or four tracks; the resulting cross-section is square, the surface is smooth, it possesses good elasticity and wear resistance, high strength, and good density. The contact area with the shaft is larger and more uniform compared to the braid-style structure, and the gaps between fibers are small, which ensures good sealing properties. Even when the surface layer wears out, the entire filler does not become loose, granting it a long service life. It represents a relatively advanced weaving structure. When selecting packing, the weaving pattern of the packing must be chosen based on the specific operating conditions of the equipment, so that the packing can exert its proper sealing performance. IV. Classification, composition, and applications of packing materials: Due to varying operating conditions, there are a great variety of types of packing materials available. To better distinguish and select packing materials, we usually classify them based on the material of the main sealing substrate used in those packing materials: 1. Natural fiber packing – These include packing materials whose sealing substrate is made from natural fibers such as cotton, linen, and wool. 2. Mineral fiber packing – Examples of this type include asbestos-based packing materials. 3. Synthetic fiber packing – This category includes packing materials made from graphite, carbon fibers, polytetrafluoroethylene, Kevlar, and acrylic fibers combined with silicone. 4. Ceramic and metal fiber packing – Examples here include silicon carbide packing, boron carbide packing, and alkali-resistant glass fiber packing. Since each type of fiber has its own shortcomings, using a single type of fiber to create packing materials can lead to leaks due to the gaps between the fibers. Additionally, some fibers have poor self-lubricating properties and high friction coefficients; therefore, lubricants, fillers, and special additives are often used in such packing materials. To improve the density and lubricity of the filler, materials such as mineral oil mixed with graphite powder or molybdenum disulfide grease are used, as well as talc, mica, glycerin, vegetable oils, etc. Additionally, polytetrafluoroethylene dispersion emulsions are employed, with appropriate amounts of surfactants and dispersants added to these emulsions. Special additives typically include zinc particles, barriers, molybdenum-based corrosion inhibitors, and the like, to reduce the corrosion of packing on equipment. With the advancement of technology, the application of nanotechnology is becoming increasingly widespread. Nanotechnology primarily exploits certain inherent properties of materials; some of these properties can reduce or eliminate the drawbacks associated with those materials, while allowing their superior characteristics to be utilized. In the sealing industry, the research and development of sealing materials have always been at the forefront of technological advancement. Through long-term collaboration with renowned nanotechnology research institutions both domestically and internationally, a series of nano-sealing rings has been successfully developed. Patent applications have been filed with the Intellectual Property Office, and new applications for these sealing rings have been identified, thereby enabling a significant improvement in sealing technology using such rings. Their use results in enhanced sealing performance and longer service life for equipment, bringing substantial economic benefits to users.