Mechanical seal repair

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With the requirements of production safety and environmental protection, the scope of use of mechanical seals has become more and more extensive. At present, most enterprises' rotating equipment uses mechanical seals to ensure no leakage between dynamic and static sealing surfaces. However, the mechanical seal device is a very precise component, which requires high technical personnel during operation and maintenance. This article mainly discusses the problems of mechanical seal repair from the following aspects. 1. Installation of mechanical seal The operation quality and life span of the mechanical seal largely depend on the conditions of the equipment itself. Therefore, careful inspection of the equipment before installing the mechanical seal will reduce the possibility of seal failure and the cost of maintenance. The equipment must be inspected from the following aspects:: 1. The aperture and depth dimensions of the seal cavity should be consistent with the dimensions on the seal assembly drawing. The general deviation is ±0.13mm. ； The dimensional deviation of the shaft or sleeve is ±0.03mm or ±0.00mm-0.05. Check the axial play of the shaft. The total axial play cannot exceed 0.25mm. ； Check the radial runout of the shaft. The radial runout of the shaft is generally less than 0.05mm. Excessive radial runout may cause: Wear of shaft or bushing ； Increased leakage between sealing surfaces ； The vibration of the equipment increases and reduces the life of the seal. 2. Check the curvature of the shaft. The maximum curvature of the shaft should be less than 0.07mm. Check the runout of the sealing cavity surface. The runout of the sealing cavity surface cannot exceed 0.13mm. If the seal cavity surface is not perpendicular to the shaft, it may cause a series of mechanical seal failures. Because the sealing gland is fixed on the sealing gland by bolts, excessive vibration of the sealing cavity will cause the installation of the gland to tilt, which in turn will cause the seal static ring to tilt, causing the entire seal to shake abnormally. This is the main cause of fretting wear. In addition, the wear of the auxiliary seal between the mechanical seal and the shaft or sleeve will also increase. In addition, abnormal shaking of the seal will also cause wear and fatigue of the metal bellows or transmission pin, leading to premature failure of the seal. 3. Check the alignment between the sealing cavity hole and the shaft. The misalignment should be less than 0.13mm. Misalignment of the seal cavity hole and the shaft will affect the dynamic load between the sealing surfaces, thereby shortening the operating life of the seal. To adjust the alignment, you can obtain better alignment by adjusting the gasket between the pump head and the bearing frame or reprocessing the contact surface. 2. Inspection of the seal before installation. Whether it is a new mechanical seal or a repaired seal, the following items need to be carefully checked before installation.: Check all components. The components must be clean and free of scale, especially the sealing surfaces of the dynamic and static rings. ； Check that the rotating parts of the seal, transmission pins, spring pins, etc. move freely ； Check the positioning screws of the rotating parts. The positioning screws should be loose and tight. The old ones should be replaced. ； Carefully check the thickness of all auxiliary sealing rings according to the seal installation diagram. Improper thickness will affect the correct positioning of the seal and the spring load applied to the sealing surface. ； Check that all rotating parts fit properly with the shaft or bushing ； Check the sealing surface of the dynamic and stationary rings. There should be no nicks or scratches. Any slight damage may cause leakage of the sealing surface. 3. The startup, operation and use of equipment equipped with mechanical seals. Correct operation and use are also crucial to the life of mechanical seals. Generally speaking, the following precautions apply to the correct use of most mechanical seals. 1. Most mechanical seals are designed to work in a liquid environment, so the sealing surface must be immersed in liquid before driving. Dry operation will seriously damage the sealing surface and affect the use and life of the seal. Of course, the inching device confirms that the motor rotation will not damage the seal, but the mechanical seal of the device running at full speed must work in the liquid. 2. The piping system connected to the equipment must also be cleaned. If the debris in the system is not cleaned up in time, debris may enter the seal after long-term operation, thus affecting the life of the seal. Premature failure of seals in new equipment is mostly caused by this reason. Therefore, it is necessary to set up a filter at the inlet before starting the new equipment. 3. Open the cooling or flushing system. Regardless of whether the equipment is running or not, it is best not to cut off the cooling or flushing system, especially when the equipment delivers hot medium or the medium may crystallize after cooling. 4. For newly installed equipment, slight leakage may occur during the first operation. This is a normal phenomenon. As the seal wears in, the amount of leakage will decrease until it no longer leaks. But if the leakage lasts for a long time, it means that the auxiliary sealing system may be damaged or the sealing surface is damaged. 5. The quality of equipment operation is also a key factor affecting seal life. Pump cavitation not only affects the performance of the pump, but also adversely affects the seal. Cavitation will cause abnormal vibration and swing of the shaft, damage the bearing, and overheat the seal cavity, thereby destroying the liquid film formed between the sealing surfaces, eventually leading to seal failure. 4. Maintenance of mechanical seals No special tools and materials are required for the maintenance of mechanical seals. Generally speaking, the following tools and materials are required for maintenance of mechanical seals.: 1. White cloth and soft paper: During the maintenance and installation of mechanical seals, cleanliness is the most basic and critical condition. When maintenance of mechanical seals, gloves are generally not allowed to be worn. 2. Clean lubricating oil: All auxiliary seals of mechanical seals have a thin layer of oil film in contact with them. Therefore, when installing the auxiliary seal, a thin layer of clean lubricating oil must be applied to the auxiliary seal. The selected lubricating oil should be miscible with the medium or sealing liquid, but cannot dissolve the auxiliary sealing material. It is best to apply a small amount of lubricating oil to all parts in contact with the shaft or sleeve during installation. 3. Grinding machine or grinding plate and emery abrasive for grinding. 4. Hexagonal wrench for tightening the set screws. Generally, the positioning screws used for mechanism sealing are made of stainless steel. Unsuitable tools may damage the screws and prevent the moving ring seat from being properly tightened. 5. Common causes of failure of mechanical seals: The mechanical seal begins to leak a lot, indicating that the seal has failed. Common causes of abnormal failures generally include the following:: 1. There is a problem with the sealing components selected during installation. This includes parts that have not been cleaned, parts that are damaged (scratches or cracks) or other reasons. 2. The seal is installed incorrectly, which mainly means that the seal positioning setting is incorrect. 3. Incorrect seal selection, including improper selection of seal body or seal ring materials, and failure to comprehensively consider various factors such as pressure, temperature, rotation speed, and fluid characteristics during design. 4. Operational errors. The pressure of the sealing fluid is insufficient when the equipment is started, the seal runs dry, or the sealing auxiliary system is not put into operation. 5. There is debris in the sealed cavity. It is possible that debris was brought in during installation or that the auxiliary system was not cleaned properly. 6. Poor operation of the equipment itself leads to seal failure. The shaft's displacement, swing or vibration is too large. Once a mechanical seal fails and leaks, it cannot be stopped immediately for maintenance. When the equipment is still running, relevant personnel should immediately go to the site for inspection and carefully observe various factors such as the working conditions of the equipment, the pressure flow of the auxiliary system, the leakage location, and the vibration of the equipment, so as to analyze the cause of the failure. Mechanical seal is a relatively complex and precise component. In addition to the knowledge introduced above, there are also many aspects such as the setting plan of the sealing auxiliary system, the analysis of the causes of seal failure, the correct selection of sealing ring materials, the grinding and inspection methods of the sealing ring during maintenance, and many other aspects are also important to understand and master the seal.