The importance of seals in chemical pumps

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The importance of seals in chemical pumps; Maintenance strategies for mechanical seals under special operating conditions. In chemical plants and chemical processing units, the mechanical seals of mechanical equipment account for approximately 80%-85% of all dynamic seals in use. Among the failures of chemical pumps, mechanical seal failures account for about half of the total maintenance workload. It can be seen the role of mechanical seals in the maintenance of chemical pumps. Given the wide variety of media transported by chemical pumps and the different process conditions, it is important to be aware of these factors when selecting and maintaining mechanical seals. The following describes the operating characteristics and precautions of mechanical seals under several special operating conditions. I. Mechanical seals in corrosive environments: Some mechanical seals are placed in corrosive environments, and in such cases it is necessary to pay attention not only to the corrosion resistance of each individual component of the seal, but also to prevent electrocoupling effects that may occur when these components are combined together. For mechanical seals in corrosive environments, it is important to choose materials that are resistant to corrosion when selecting them. Furthermore, an external mechanical seal can be used. For those with extremely high corrosivity that can lead to safety accidents, double-end sealing can be used, along with the introduction of an isolation fluid for protection. II. Mechanical seals under high pressure: Mechanical seals that operate at pressures exceeding 4–5 MPa fall within the category of high-pressure seals. Under high-pressure conditions, the excessive specific pressure at the sealing end face causes heating of that end face, which in turn disrupts the formation of the liquid film and leads to abnormal wear. High pressure can also cause the friction pair to deform and crack. High-pressure seals generally adopt a balanced structure to keep the end-face specific pressure within an appropriate range. However, considering the reliability of sealing, the balance coefficient cannot be increased indefinitely; thus, in some cases, even with a balanced structure in place, the end-face specific pressure may still be high. It is difficult to meet the operational requirements relying solely on a balanced structure; in such cases, a multi-face mechanical seal or a controlled-diaphragm mechanical seal can be used. III. Mechanical seals at high speeds: When the rotational speed exceeds 3000 r/min or the linear velocity of the friction pair exceeds 25 m/s, it is generally considered a high-speed seal. At high speeds, due to the high linear velocity of the friction surfaces, more heat is generated by friction; as a result, it is difficult for a liquid film to form on these surfaces, leading to increased wear. High speed also easily causes vibration in the rotating ring, and subjectes it to significant centrifugal forces, which hinders the formation and maintenance of a liquid film between the end faces. Therefore, at high speeds, in order to reduce the effects of centrifugal force and vibration, the number of rotating components should be minimized, and a spring-mounted stationary structure should be used. It is also possible to enhance the cooling and lubrication of the end surfaces of the friction pair. Additionally, high-value friction pair materials can be selected to reduce the contact width of the end faces, or a controlled diaphragm mechanical seal can be used. In recent years, a mechanical seal called a differential ring type has also been applied in high-speed equipment. By adding a differential ring between the original stationary and rotating rings, the linear velocity of the friction pair can be reduced by half, achieving good results. IV. Mechanical seals in media containing particles: Some mechanical seals may operate in media that contain solid particles, and when these particles enter the friction surfaces, they can cause severe wear, leading to rapid failure of the sealing surfaces. Furthermore, solid particles may prevent the rotating ring seal from moving axially, thereby preventing the rotating ring from floating and losing its compensatory function, which leads to the failure of the mechanical seal. A relatively effective approach for this is to add an external flushing device. Since the pressure of the flushing fluid introduced into the sealing chamber is higher than the pressure of the medium in that chamber, it is possible to effectively prevent particulate media from entering the sealing chamber. In addition, a double-end mechanical seal can be used, with a sealing fluid added to the middle of the sealing chamber. An external mechanical seal can also be used. V. Mechanical seals at high temperatures: Generally, when the temperature of the medium exceeds 120°C, it is considered a high-temperature seal. The main problems associated with mechanical seals at this time are as follows: 1. As the temperature at the end faces of the friction pair rises, the liquid film between these end faces vaporizes, which increases the friction coefficient, accelerates wear, further raises the temperature, and causes the sealing ring to undergo thermal deformation or thermal cracking, resulting in failure. 2. The material of the auxiliary sealing ring in mechanical seals is generally rubber or polytetrafluoroethylene; due to exposure to high temperatures over extended periods, it tends to age and decompose, leading to seal failure. 3. At high temperatures, the elastic elements of mechanical seals are prone to fatigue and creep, leading to seal failure. 4. High temperatures accelerate the corrosion of metal materials and shorten the service life of mechanical seals. To ensure the proper operation of mechanical seals in high-temperature environments, the following measures can be taken: 1. Install a cooling and flushing system for the mechanical seal. 2. Choose materials resistant to high temperatures, selecting different materials based on the various operating temperatures. The maximum temperature for nitrile rubber is 80°C, for silicone and fluororubber it is 200°C, and for polytetrafluoroethylene it is 250°C. Furthermore, the non-metallic friction pairs in mechanical seals are often made of graphite-impregnated materials; therefore, different types of impregnated graphite must be selected depending on the operating temperature. Generally, graphite impregnated with babbitt alloy is suitable for temperatures below 150°C, while graphite impregnated with resin can be used at temperatures ranging from 170 to 200°C. Graphite impregnated with copper, aluminum, or lead can be utilized in operating conditions below 400°C. For the rotating ring assembly, materials with similar expansion coefficients should be used as much as possible to prevent the connection between the rotating ring and its seat from loosening at high temperatures. 3. Choose a mechanical seal with a metal bellows. Metal bellows seals have been widely used in high-temperature sealing in recent years, achieving excellent results. In this type of mechanical seal, a metal bellows replaces the spring found in conventional mechanical seals; there is no need for an auxiliary sealing ring on the rotating ring, and it is not required to overcome the friction and wear that occur when the rotating ring moves relative to the shaft. As a result, it can perform well when used in high-temperature environments. VI. Mechanical seals at low temperatures: In chemical plants and chemical processing units, media transported at low temperatures generally have low viscosity, poor lubricity, and tend to vaporize easily. Moreover, when these media vaporize, they absorb a large amount of heat, causing the surrounding temperature to drop sharply. This leads to the condensation of water vapor in the air, resulting in freezing on the sealing surfaces and their surroundings. Such freezing prevents proper sealing between the surfaces, thereby causing leaks. At the same time, due to the poor lubricity, a significant amount of heat is generated by friction at the end faces; if this heat is not removed in time, it will inevitably exacerbate vaporization at those end faces, creating a vicious cycle that accelerates the premature failure of the sealing surfaces. Furthermore, the auxiliary sealing ring of low-temperature sealing can also become hard and lose its elasticity at low temperatures, which easily leads to leaks. For low-temperature sealing, in addition to selecting an appropriate material for the auxiliary seal ring, the following points should also be taken into account: 1. Improve the sealing structure and choose the materials for the friction pair correctly. Practice has shown that creating arc-shaped grooves on the surface of the stationary ring can achieve a hydrodynamic seal, increase the load-bearing capacity of the fluid film, improve the lubrication conditions at the end faces, and effectively extend the service life of the mechanical seal. In terms of end-face pairing, the tungsten carbide-M106K graphite combination (i.e., the materials for the moving and stationary rings) is generally superior to other materials, owing to its good lubricity, low friction coefficient, and fairly good thermal conductivity. 2. Select an appropriate end face specific pressure. Due to the poor lubricity of the end faces in low-temperature sealing, vaporization and icing are likely to occur; therefore, while ensuring reliable sealing, the end-face specific pressure can be appropriately reduced. The recommended specific pressure value for general mechanical seals is 0.29–0.59 MPa; for low-temperature seals, the lower limit can be used. For metal bellows mechanical seals, since there is no axial friction in the moving ring, the end-face specific pressure can even be below the lower limit. 3. Take corresponding measures in terms of process. Such as minimizing the water content in the medium ； Maintain relative stability of the pressure in the sealed chamber; where possible, install flushing and cooling devices, and properly preheat the seal area before starting the equipment.