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Introduction to Mechanical Seals Mechanical seals are among the precision mechanical components with relatively complex structures; they serve as key parts in various pumps, reaction kettles, turbine compressors, submersible motors, and other equipment. Its sealing performance and service life depend on many factors, such as the choice of model, the precision of the machine, and proper installation and use. Basic Knowledge of Mechanical Seals 1. Basic concept of mechanical seals: A mechanical seal is a device designed to prevent fluid leakage, it consists of at least one pair of end faces that are perpendicular to the axis of rotation; these end faces remain in close contact with each other and slide relative to one another under the action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, as well as with the help of auxiliary sealing elements. A mechanical seal in which the auxiliary seal of the compensation ring is a metal bellows is called a bellows mechanical seal. 2. Components of mechanical seals: There are mainly the following four types of components. a. Main seals: rotating ring and stationary ring. b. Auxiliary seal: sealing ring. c. Compression elements: springs, push rings. d. Transmission components: spring basket seat and keys or fixing screws. Points to note: 1. Precautions during installation: a. Great care must be taken to avoid any installation errors. (1) Tightening the gland should be done after the coupling has been aligned; the bolts should be tightened evenly to prevent the gland surface from becoming skewed. Use a feeler gauge to check each point, ensuring that the error is no more than 0.05 millimeters. (2) Check the fit clearance between the gland and the outer diameter of the shaft or sleeve (i.e., the degree of concentricity); it should be uniform around the perimeter. Use a feeler gauge to ensure that the tolerance at each point is no more than 0.01 millimeters. b、The compression amount of the spring must be in accordance with the specified values; it is not allowed to be too large or too small, with an allowable error of 2.00 millimeters. An excessive value will increase the end-face specific pressure and accelerate end-face wear. If it is too small, the specific pressure will be insufficient to provide a sealing effect. c. After installation, the moving ring must be able to move freely on the shaft; when pressed against the spring, it should bounce back automatically. 2. Precautions during disassembly a. Be careful when removing the mechanical seal; it is strictly prohibited to use hammers or spades to avoid damaging the sealing elements. A pair of wire hooks can be made, which are inserted into the notch in the drive seat in the direction that corresponds to the self-profit-and-loss mechanism, in order to pull out the sealing device. If the scale cannot be removed, it should be cleaned thoroughly before attempting to remove it. b. If mechanical seals are used at both ends of the pump, care should be taken during assembly and disassembly to avoid neglecting one aspect while focusing on another. c. For mechanically sealed devices that have been in operation and where the gland loosening has caused the seal to shift, the stationary and rotating ring components must be replaced; they should not be retightened and used again. Because after such movement of the columns, the original operating path of the friction pair changes, which easily compromises the sealing performance of the contact surface. Normal operation and maintenance of mechanical seals 1. Preparatory work and precautions before startup a. Thoroughly inspect the mechanical seal, as well as the auxiliary devices and pipelines, to ensure that they are all in place and meet the technical requirements. b. Perform a hydrostatic test on the mechanical seal before starting it, to check for any leakage. If there is significant leakage, the cause should be identified and steps taken to eliminate it. If it still doesn’t work, it should be disassembled for inspection and reinstalled. The typical hydrostatic test pressure is 2 to 3 kilograms per square centimeter. c. Rotate the pump according to its rotation direction to check whether it turns smoothly and evenly. If the turntable struggles to rotate or does not move, check whether the assembly dimensions are incorrect and whether the installation is proper. 2. Installation and Shutdown a. The sealing chamber should be kept filled with liquid before startup. When transporting a solidified medium, steam should be used to heat the sealed chamber in order to melt the medium. The shaft must be turned before starting to prevent the soft ring from breaking due to sudden startup. b. For mechanical seals that utilize an external seal oil system, the seal oil system should be started first. After parking, stop the oil sealing system last. c. After the hot oil pump is shut down, the cooling water used to cool the oil seal chamber and the face seal should not be stopped immediately; it should only be ceased once the oil temperature at the face seal drops below 80 degrees, in order to avoid damaging the sealing components. 3. Operation a. If there is slight leakage after the pump starts, observe it for some time. If the leakage does not decrease after 4 hours of continuous operation, the pump should be stopped for inspection. b. The operating pressure of the pump should be stable, with pressure fluctuations not exceeding 1 kilogram per square centimeter. c. During operation, the pump should be prevented from experiencing vacuum conditions, to avoid dry friction at the sealing surfaces and seal failure. d. The sealing condition should be checked regularly. During operation, if the leakage exceeds the specified limits – no more than 5 drops per minute for heavy oil and no more than 10 drops per minute for light oil – and there is no improvement within 2–3 days, the pump should be stopped to inspect the sealing mechanism. The development of \"sealing\" in our country has a long history; ancient people used fibers such as cotton and linen to create seals for water-lifting machinery, while abroad, packing was not used until 1782. The importance of sealing need not be elaborated here. Nowadays, abroad, there exists a discipline known as \"sealing science\" that studies the laws governing sealing, the design techniques for sealing devices, and the scientific principles behind their application. Research institutions and universities also offer specialized courses in this field. In China, as far as I know, universities offer courses such as fluid mechanics and hydraulic transmission, but there is yet no department dedicated to the study of sealing. Therefore, our level of research lags behind that of foreign countries. There are many specialized fields involved in seal design; in addition to materials and mechanics, these include mechanics (including fluid dynamics, boundary layer theory, etc.), tribology, automatic control, and more. Therefore, the research difficulty for sealing is also relatively high. In my opinion, the level of the domestic sealing industry lags behind that of foreign countries by at least 50 years. Regarding the principle of sealing: If you want to learn about sealing, it is essential to first understand leakage. Once the principle of leakage is understood, the mechanism behind sealing becomes clear as well. There are essentially three types of leakage: the first is breakthrough leakage, which occurs due to gaps in the sealing surfaces; the second is seepage, which happens when the fluid being sealed passes through the capillaries in the material of the seal; the third is diffusion, which refers to the transfer of substances from one side to another through gaps or the capillaries in the material, driven by concentration differences. Regarding sealing methods: There are roughly several types of sealing methods—1. Minimizing the number of areas that need to be sealed; 2. Blocking and isolating; 3. Guiding fluid in or out; 4. Increasing the resistance to leakage; 5. Adding working elements within the channels; 6. Combining multiple sealing methods. Common sealing forms include gasket sealing, packing sealing, mechanical sealing, contactless sealing, as well as pressure-based sealing methods for stopping leaks. Among them, packing seals are considered the most common; they further include soft packing seals, hard packing seals, and molded packing seals. Molded packing seals include the common O-rings, Y-rings, oil seals, and so on. Non-contact seals include gap seals, labyrinth seals, floating seals, dynamic seals, magnetorheological seals, and fully enclosed seals. Properties of Common Washers —— and New Materials and Technologies I) Properties of Common Washers When using valves, it is often necessary to replace the original washers depending on the specific circumstances. Common washers include: rubber flat washers, rubber O-rings, plastic flat washers, PTFE-lined washers, asbestos-rubber washers, metal flat washers, metal custom-shaped washers, metal-clad washers, wave washers, spiral wound washers, and more. (1) Rubber flat gaskets: They deform easily and can be compressed without much effort, but they have poor pressure and temperature resistance; they are only suitable for use in applications with low pressure and moderate temperatures. Natural rubber has a certain degree of resistance to acids and alkalis, and its operating temperature should not exceed 60 degrees℃ ； Neoprene can also resist certain acids and bases, with a working temperature of 80℃ ； Nitrile rubber is oil-resistant and can be used up to 80℃ ； Fluororubber has excellent corrosion resistance, and its temperature resistance is also higher than that of ordinary rubbers; it can be used in media at 150°C. (2) Rubber O-ring: Its cross-sectional shape is circular, and it has a certain self-compressing effect; its sealing performance is better than that of flat gaskets, with less compressive force required. (3) Plastic flat washers: The greatest advantage of plastics is their good corrosion resistance; however, most plastics have poor temperature resistance. Polytetrafluoroethylene is the king of plastics; it not only boasts excellent corrosion resistance but also has a wide temperature range, allowing it to be used over long periods at temperatures between -180°C and +200°C. (4) PTFE-coated washers: To fully utilize the advantages of PTFE while compensating for its weakness in terms of elasticity, washers are made with PTFE covering rubber or asbestos rubber. In this way, it possesses the same corrosion resistance as polytetrafluoroethylene flat gaskets, along with good elasticity, which enhances the sealing effect and reduces the required compressing force. Its cross-sectional shape is shown in Figure 4-20. (5) Asbestos rubber gasket: Cut from asbestos rubber sheet. Its components are 60–80% asbestos and 10–20% rubber, along with fillers, vulcanizing agents, etc. It has excellent heat resistance, cold resistance, and chemical stability, and it is available in large quantities at low prices. When in use, the pressing force does not need to be very strong. Since it can adhere to metal, it is best to coat its surface with a layer of graphite powder to avoid difficulty during removal. Asbestos rubber sheets are available in four colors: gray, which is used for low-pressure applications (grade XB-200, with a pressure resistance of ≤16 kilograms per square centimeter and a temperature resistance of 200°C) ； Red, used for medium voltage (grade XB-350; can withstand a pressure of 40 kilograms per square centimeter and temperatures up to 350°C) ； Magenta red, used for high pressure (grade XB-450, with a pressure resistance of 100 kg/cm² and a temperature tolerance of 450°C) ； Green, used for oils, and it also has excellent pressure resistance. (6) Metal heating ring: lead, temperature resistance of 100℃ ； Aluminum 430℃ ； Copper 315℃ ； Low-carbon steel 550℃ ； Silver 650℃ ； Nickel 810℃ ； Monel (nickel-copper) alloy at 810°C, stainless steel at 870°C. Among them, lead has a poor voltage resistance; aluminum can withstand 64 kilograms per square centimeter, while other materials can handle high pressures. (7) Metal non-conformal washers: Lens washers: They have a self-tightening effect and are used in high-pressure valves. Elliptical washer: Also belongs to high-pressure self-tightening washers. Tapered double washer: used for high-pressure internal self-sealing. In addition, there are also square, diamond, triangular, toothed, dovetail, B-shaped, C-shaped, etc., which are generally used only in medium and high-pressure valves. (8) Metal sheathed washer: Metals possess excellent temperature and pressure resistance as well as good elasticity. The materials for the prepuce include aluminum, copper, low-carbon steel, stainless steel, Monel alloy, etc. The filling materials inside include asbestos, polytetrafluoroethylene, glass fiber, etc. (9) Wave washer: It features low compressive force and good sealing performance. A combination of metals and non-metals is commonly used. (10) Coiled washer: It consists of very thin metal strips and non-metallic strips that are pressed together and wound into multiple layers in a circular shape; its cross-section is wave-shaped, giving it excellent elasticity and sealing properties. Metal strips can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel alloy, etc. Non-metallic strip materials include asbestos, polytetrafluoroethylene, etc. Above, when discussing the performance of sealing gaskets, some figures were listed. It must be noted that these figures are closely related to factors such as the type of flange, the nature of the medium, and the techniques used for installation and maintenance; sometimes they may be exceeded, and other times they may not be reached. Moreover, pressure resistance and temperature resistance are interrelated – for example, as temperature rises, pressure resistance tends to decrease. Such subtle issues can only be understood through practical experience. II) New Materials and New Technologies The sealing gaskets mentioned above are far from comprehensive, and moreover, sealing technology is in rapid development. The following are examples of several new materials and technologies. (1) Liquid sealing: With the rapid development of the polymer organic synthesis industry, liquid sealants have emerged and are used for static sealing ； This new technology is commonly referred to as liquid sealing. The principle of liquid sealing relies on the adhesiveness, fluidity, and monolayer effect of liquid sealant (the thinner the film, the greater its tendency to return to its original state), allowing it to function like a gasket under appropriate pressure. Therefore, the sealant in use is also known as a liquid gasket. (2) Polytetrafluoroethylene green seal: Polytetrafluoroethylene is also a high-molecular organic compound; before it is sintered into a finished product, it is in its green state, is soft in texture, and exhibits a single-molecule film effect. A tape made from raw material is called raw material tape, and it can be rolled up for long-term storage. When used, it can be freely shaped and joined at any point; upon application of pressure, it forms a ring-shaped membrane that uniformly serves as a seal. As a gasket between the valve body and the valve cover in valves, it is possible to create a gap without removing the valve disc or gate, and then simply insert tape there. It has low compressive force, does not stick to the hands nor to the flange surface, making it very easy to replace. It is most suitable for mortise-and-tenon flanges. PTFE raw material can also be made into tubular and rod shapes for sealing purposes. (3) Metal hollow O-rings: They have good elasticity, require less compressive force, and possess a self-tightening effect. Various metal materials can be used, enabling them to function in low-temperature, high-temperature, and highly corrosive environments. (4) Graphite plate sealing ring: In people’s minds, graphite is a brittle material lacking elasticity and toughness. However, graphite that has undergone special treatment is soft in texture and highly elastic. In this way, the heat resistance and chemical stability of graphite can be utilized in gasket materials ； Moreover, this type of washer has low compressive force, resulting in an exceptionally excellent sealing effect. This type of graphite can also be made into strips that, when combined with metal strips, form winding washers with excellent performance. The advent of graphite plate seals and graphite-metal spiral wound gaskets represents a major breakthrough in high-temperature corrosion-resistant sealing. Such washers are already produced and used in large quantities abroad.