Classification and maintenance of mechanical seals--hope this is helpful to everyone

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Classification and maintenance of mechanical seals. Mechanical seals, also known as mechanical end face seals, are dynamic seals used for rotating shafts. Mechanical seals offer reliable performance, low leakage rates, 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 various highly corrosive media. 1 Working principle of mechanical seals: A mechanical seal is a shaft sealing device that prevents leakage by relying on one 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. The common mechanical seal structure is shown in Figure 1-1. It is mainly composed of a stationary ring 1, a rotating ring 2, an elastic element 3, a spring seat 4, a set screw 5, an auxiliary sealing ring 6, etc. The anti-rotation pin 7 is fixed to the gland 9 to prevent the stationary ring 1 from rotating. Rotating rings and stationary rings can also often be referred to as compensating rings or non-compensating rings, depending on whether they possess axial compensation capability. There are 4 possible paths for fluid leakage in mechanical seals, represented by the four channels A, B, C, and D in Figure 1-1. On the C and D leakage paths, there are seals between the stationary ring and the gland, as well as between the gland and the housing; both of these are static seals. Channel B features a seal between the rotating ring and the shaft; when the end face wears out, it can only move slightly axially along with the compensation ring, and in effect it serves as a static seal between the shaft and the rotating ring. Therefore, these leakage channels are relatively easy to seal. The most commonly used static sealing elements are rubber O-rings or PTFE V-rings, while rotating or stationary rings serve as compensating rings to assist in sealing; sometimes, structures that combine the functions of elastic elements such as rubber, PTFE materials, and metal bellows are employed. Channel A forms a dynamically sealing arrangement with relative sliding, achieved by the end faces of the rotating ring and the stationary ring coming into contact with each other. It is the primary seal in mechanical sealing devices, and it is the key factor determining the performance and lifespan of mechanical seals. Therefore, the requirements for machining the sealed end face are very high (planarity of 0.0009 mm, surface roughness Ra ≤ 0.1 μm, and soft ring Ra ≤ 0.2 μm). To maintain a necessary lubricating film between the sealed end faces, it is essential to strictly control the compressive force per unit area on those end faces. Excessive pressure per unit area makes it difficult to form a stable lubricating film, thereby accelerating wear of the end faces. The unit pressure on the end face is too low, resulting in an increased leakage rate. Therefore, to achieve good sealing performance and a long service life, it is essential to ensure that the unit pressure on the end faces is within the most appropriate range when designing and installing mechanical seals. 2 Characteristics of mechanical seals compared to soft packing seals 2.1 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 typically last 1–2 years or even longer, while in chemical media it can function for more than half a year ； • 3) Low friction power consumption; its friction power is only 10% to 50% of that of soft packing seals ； • 4) The shaft or bushing experiences virtually no wear ； • 5) It has a long maintenance cycle; automatic compensation is available after end-face wear, and regular maintenance is generally not required ； • 6) It has good vibration resistance and is insensitive to vibrations of the rotating shaft, deflection, as well as skewing of the shaft relative to the seal ； • 7) Wide range of applications: Mechanical seals can be used in environments with high or low temperatures, high pressures, vacuum conditions, different rotation frequencies, as well as in the presence of various corrosive media. 2.2 Disadvantages: • 1) The structure is relatively complex, requiring high standards for processing and manufacturing ； • 2) Installation and replacement are rather troublesome, requiring workers to have a certain level of skill ； • 3) In the event of accidental accidents, handling them is more difficult ； • 4) High price. 3 Types of mechanical seals Mechanical seals can be classified according to operating parameters and structural types. 3.1 Classification of mechanical seals by operating parameters Mechanical seals are classified according to different operating parameters, as shown in Table 1-1. Classified by seal chamber temperature: t＞150℃ for high-temperature mechanical seals; classified by seal face velocity: v＞100m/s for ultra-high-speed mechanical seals. 80＜t≤150℃ for medium-temperature mechanical seals; 25≤v≤100m/s for high-speed mechanical seals. -20≤t≤80℃ for normal-temperature mechanical seals; v＜25m/s for mechanical seals with normal velocity. t＜-20℃ for low-temperature mechanical seals. Classified by seal chamber pressure: p＞15MPa for ultra-high-pressure mechanical seals. Classified by shaft diameter: d＞120mm for mechanical seals with large shaft diameters; 3＜p≤15MPa for high-pressure mechanical seals; 25≤d≤120mm for mechanical seals with normal shaft diameters; 1＜p≤3MPa for medium-pressure mechanical seals; d＜25mm for mechanical seals with small shaft diameters. Normal pressure ≤p≤1MPa for low-pressure mechanical seals; negative pressure/vacuum mechanical seals. Table 1-13.2 Classification of mechanical seals by structural type. The basic types of mechanical seals based on their structural design are: 1) Balanced and unbalanced mechanical seals. Those that can relieve the pressure exerted by the medium on the seal face are balanced mechanical seals ； Those that cannot be removed are unbalanced mechanical seals. Depending on the degree of unloading, balanced mechanical seals can be further divided into partially balanced (partial unloading) and over-balanced (full unloading) types. The forces acting on the end face of a balanced mechanical seal change little as the medium pressure increases, making it suitable for high-pressure sealing. In contrast, the forces acting on the end face of an unbalanced mechanical seal change significantly with medium pressure, so it is only suitable for low-pressure mechanical seals. Figure 1-2a shows a balanced mechanical seal, while Figure 1-2b shows an unbalanced mechanical seal. A balanced mechanical seal can reduce friction and wear on the end faces, minimize friction, and possess a high load-bearing capacity. However, its structure is relatively complex; generally, steps need to be machined on the shaft or bushing, resulting in higher costs. The unbalanced type has a simple structure and is widely used when the medium pressure is less than 0.7 MPa. 2) Internal and external mechanical seals: Those in which the spring and moving ring are installed inside the seal box and in contact with the medium are internal mechanical seals (Figure 1-3a) ； A mechanical seal in which the spring and moving ring are installed outside the sealing box and do not come into contact with the medium is known as an external mechanical seal (Figure 1-3b). Built-in sealing can utilize the pressure of the medium within the sealing chamber for sealing; the sealing components are all immersed in the fluid medium, and the stress conditions on the sealing surfaces, along with cooling and lubrication, are optimal, making it a commonly used design approach. Most of the components with external sealing do not come into contact with the medium; they are located outside the equipment, which facilitates observation, installation, and maintenance. However, since the medium pressure in the external structure acts in the opposite direction to the elasticity of the elastic elements, fluctuations in the medium pressure, combined with a limited amount of spring compensation, can lead to unstable sealing or even severe leakage. External mechanical seals are used only in applications with highly corrosive, highly viscous, or crystalline media, as well as in situations where the medium pressure is low. 3) Internal flow and external flow mechanical seals: The mechanical seal with the medium leakage direction opposite to that of the centrifugal force is an internal flow mechanical seal ； A mechanical seal in which the medium leakage direction is consistent with the centrifugal direction is an external flow type mechanical seal (Figure 1-4). Since centrifugal force prevents the leakage fluid in internal-flow mechanical seals, their leakage rate is lower than that of external-flow types; they offer reliable sealing and are suitable for high-pressure applications. At very high rotational speeds, an external flow mechanical seal is more suitable for improving end-face lubrication; however, the medium pressure should not be too high, generally ranging from 1 to 2 MPa. 4) Stationary and rotary mechanical seals: A mechanical seal in which the spring does not rotate together with the shaft is considered a stationary mechanical seal (Figure 1-5) ； A mechanical seal in which the spring rotates together with the shaft is a rotary mechanical seal. Since the spring of a static mechanical seal is not affected by centrifugal force, it is commonly used in high-speed mechanical seals. The rotary mechanical seal features a simple elastic element design and small radial dimensions, making it a commonly used structure; however, it is not suitable for high speeds. Because the unbalanced mass at high speeds can easily cause vibration and intense agitation of the medium. Therefore, when the linear velocity is greater than 30 m/s, a stationary mechanical seal is advisable. 5) Single-spring and multi-spring mechanical seals: A mechanical seal that has only one spring in its compensation mechanism is called a single-spring mechanical seal or a large-spring mechanical seal (Figure 1-6a), while one whose compensation mechanism contains multiple springs is called a multi-spring mechanical seal or a small-spring mechanical seal (Figure 1-6b). The spring pressure on the face of a single-spring mechanical seal is unevenly distributed, especially when the shaft diameter is large. The spring pressure distribution in multi-spring mechanical seals is relatively uniform. Therefore, single-spring mechanical seals are commonly used for smaller shaft diameters (d≤80～150mm). The springs in multi-spring types have a small diameter; therefore, when corrosion or the accumulation of crystalline particles leads to the failure of the springs, a single-spring type mechanical seal is a better choice. 6) Single-face and double-face mechanical seals: Those composed of a pair of sealing faces are single-face mechanical seals ； A mechanical seal with two pairs of sealed end faces is known as a double-end face mechanical seal. When two pairs of end faces are arranged axially, it is an axial double-end face mechanical seal (Figure 1-7) ； When two pairs of end faces are arranged radially, it is a radial double-end face mechanical seal (Figure 1-8). The single-face mechanical seal has a simple structure and is easy to manufacture and install. It is generally used in situations where the medium itself has good lubricity and minor leakage is acceptable; it is the most commonly used type of mechanical seal. However, it is not suitable for use when the medium is toxic, flammable, explosive, and strict requirements exist regarding leakage amounts. Double-end mechanical seals are suitable for media that have poor lubricity, are toxic, flammable, explosive, volatile, contain abrasive particles, or are gases. The radial face mechanical seal structure is more compact than the axial double-face mechanical seal. Axial double-end face mechanical seals have configurations that are face-to-face or back-to-back. When this type of seal is in operation, if a sealing fluid with a pressure of 0.05–0.15 MPa higher than that of the medium is introduced between the two pairs of end faces, it improves the lubrication and cooling conditions between those end faces and isolates the medium from the outside environment, thereby making it possible to achieve \"zero leakage\" of the medium. 7) Single-stage and double-stage mechanical seals keep the sealing medium under a pressure condition; this is the case for single-stage mechanical seals ； A double-stage mechanical seal is used when under two types of pressure conditions (Figure 1-9). The former is the same as a single-face mechanical seal, while the latter features two seals arranged in series, with the medium pressure decreasing sequentially, making it suitable for high-pressure applications. If the medium pressure is very high, multiple seals can be connected in series to form a multi-stage mechanical seal. 8) Bellows-type mechanical seal: Depending on the material of the bellows used, bellows-type mechanical seals can be divided into metal bellows mechanical seals, polytetrafluoroethylene bellows mechanical seals, and rubber bellows mechanical seals. The bellows-type seal experiences no relative sliding on the shaft, causes no wear to the shaft, has good followability, and offers a wide range of applications. Figures 1-10 show the bellows-type mechanical seal. The metal bellows can itself replace elastic elements; it has good corrosion resistance and can be used in high and low temperatures. The PTFE bellows mechanical seal can be used in various corrosive media due to PTFE’s excellent corrosion resistance. Rubber bellows mechanical seals are widely used due to the low cost of rubber, but their operating temperature is limited by the properties of different rubber materials. 4 Maintenance of Mechanical Seals 4.1 Analysis of the Causes of Failure and Leakage in Mechanical Seals 1) Regardless of the type, the most prominent feature of mechanical seals is that their sealing surfaces are end faces perpendicular to the axis of rotation; this transforms what would otherwise be an axially oriented seal prone to leakage into an end-face seal that is less likely to leak. Therefore, the main form of failure in mechanical seals is wear failure between the stationary and rotating rings. The friction pairs at the end faces of the rotating and stationary rings are usually pressed together by spring force to prevent leakage. The tighter the compression between the moving and stationary rings, the less likely leakage is to occur; however, the friction between them also increases. Under such high friction, the contact surfaces of the moving and stationary rings wear out quickly, eventually leading to leakage. 2) Vibrations caused by unstable process conditions and poor installation, air entrainment in the centrifugal pump, and sudden flow interruptions can all lead to the destruction of the liquid film between the mating surfaces of the mechanical seal. This results in the mechanical seal operating in a \"dry\" condition without lubrication; the temperature of the sealing surfaces rises rapidly, with some seals being completely destroyed while others crack due to thermal shock when the pump returns to normal operation and is cooled down suddenly. Poor flushing fluid and flushing conditions can also cause thermal shock, leading to radial cracks in the sealing ring and exacerbating the wear failure of the mating ring. When the graphite ring exceeds its operating temperature, crystals form on its surface; carbonization occurs near the friction surfaces at higher temperatures, and these particles entering the friction interface cause rapid wear of the mating parts, leading to their failure. 3) The failure of the sealing ring in the mechanical seal is also a major cause of seal leakage. The dynamic ring seal is installed at an incorrect angle, or the static ring is installed at an incorrect angle ； The surface finish of the shaft or sleeve that fits with the sealing ring is insufficient, or the fit dimensions have become too small ； Physical or chemical reactions between the sealing ring and the sealing medium can lead to corrosion, deformation, aging, and thus leakage. 4) Inadequate cleaning before assembling the mechanical seal, resulting in scratches on the components ； Improper assembly ； The spring is misaligned and the fastening screw is not tightened properly ； Damage during disassembly, etc., are all causes of premature failure of mechanical seals. 5) When a mechanical seal is used with different sealing media, the optimal material combinations for its various components must be selected in order to extend its service life. 4.2 Solutions Based on the above analysis, the following measures and maintenance methods should be adopted to extend the service life of mechanical seals. 1) When assembling a mechanical seal, the compression amount of the spring must be adjusted properly in order to extend its service life. 2) Strictly control the process operations and parameters to prevent phenomena such as vibration, pump cavitation, and interruption of the flushing fluid flow during flow regulation and startup/shutdown. Filter out impurities from the sealing medium, and adopt either a self-balancing flushing method or an external forced-circulation flushing method to improve the flushing and cooling conditions. Alternatively, increase the aperture of the flow-control plates in the flushing system to raise the amount of flushing fluid, ensuring a balanced and sufficient supply of flushing fluid. This helps to improve the lubrication and heat dissipation of the mechanical seal, prevents it from being damaged due to overheating or from crystal formation, and thus extends its service life. 3) When installing the \"O\"-ring of the mechanical seal’s rotating ring, apply a small amount of lubricant to the shaft or sleeve to facilitate installation and prevent the \"O\"-ring from curling sideways. The static ring must be assembled perpendicular to the axis of rotation. The roughness of the shaft or bushing must be below 6.3; those with an outer diameter that is below the minimum allowable value need to be replaced. 4) A meticulous assembly process is also very important. Before assembling a mechanical seal, all components must be thoroughly cleaned. The sealing elements should be wiped with clean, soft cotton yarn or cloth; care must be taken to avoid any scratches when wiping the end faces of the rotating and stationary rings. Check all components for any damage ； Thoroughly inspect the friction end faces of the moving and stationary rings; there must be no minor dents or scratches. During assembly, a layer of clean engine oil should be applied to the end faces of the rotating and stationary rings. Each component is installed in sequence to determine the correct installation position and optimal parameters. When tightening the screws, apply even tension diagonally to avoid skewing, which could cause the seal to fail. After the repairs are complete, open all valves. Do not start the motor right away; instead, turn the shaft by hand to check the tightness of the mechanical seal, whether it rotates smoothly, listen for any abnormal noises, and observe for any leaks. Only after confirming everything is in order should you start the machine.