Fault manifestations and causes of mechanical seals

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Mechanical seals are widely used in rotating equipment. The sealing performance of mechanical seals directly affects the operation of the entire machine; in severe cases, it can even lead to major safety accidents. From the perspective of the internal and external conditions of mechanical seals, several factors affecting the sealing performance and the appropriate measures to be taken are analyzed. I. Principle and Requirements of Mechanical Seals A mechanical seal, also known as an end-face seal, is a shaft sealing device used in rotating machinery. It consists of at least one pair of end faces 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 liquid pressure and the elastic force (or magnetic force) of a compensating mechanism, along with the assistance of auxiliary seals, thereby preventing fluid leakage. Its main function is to change the axially sealed design, which is prone to leakage, into an end-sealed design that is less likely to leak. It is widely used for sealing the rotating shafts of pumps, kettles, compressors, and other similar equipment. 　　Mechanical seals typically consist of a rotating ring, a stationary ring, compression elements, and sealing elements. Among them, the moving ring rotates together with the pump shaft; the moving ring and the stationary ring fit tightly together to form a sealing surface, thereby preventing leakage of the medium. The rotating ring is pressed against the end face of the stationary ring by the pressure of the liquid in the sealing chamber; this creates an appropriate specific pressure on the end faces of both rings and maintains a very thin layer of liquid film, thereby achieving sealing. The compression element generates pressure, which allows the pump to maintain end-face contact even when it is not in operation, ensuring that the sealing medium does not leak out and preventing contaminants from entering the sealed end faces. The sealing element serves to seal the gap between the rotating ring and the shaft, as well as the gap between the stationary ring and the gland; meanwhile, the elastic element acts as a buffer against vibrations and shocks in the pump. In actual operation, mechanical seals function in conjunction with other components of the pump; their proper operation is greatly influenced by both their own performance and external conditions. But first, we must ensure the performance of our own components, as well as the technical requirements for the auxiliary sealing devices and installation, so that the mechanical seal can fulfill its intended function. II. Fault manifestations and causes of mechanical seals 1. Faults in the components of mechanical seals: During operation, rotating equipment’s sealing surfaces often experience wear, thermal cracking, deformation, and damage. Springs, over time, can also become loose, break, or corrode. The auxiliary sealing ring can also develop cracks, twisting and deformation, as well as ruptures. 　　2. Causes of vibration and heating faults in mechanical seals: During the rotation of the equipment, the mating surfaces of the rotating and stationary rings can become rough; if the gap between these rings and the sealing chamber is too small, collisions caused by vibrations can lead to vibration issues. Sometimes, vibration and heating of mechanical seals can be caused by poor corrosion and temperature resistance of the sealing surfaces, insufficient cooling, or the presence of particulate impurities trapped in the surfaces during installation. 　　3. Causes of mechanical seal medium leakage （1）Leakage during hydrostatic testing. During installation, carelessness with mechanical seals can lead to damage, deformation, or injury of the sealing surfaces; improper cleaning that results in the presence of particulate impurities; or issues such as loose setting screws, inadequate compression of the gland, and insufficient precision of the machinery or equipment, all of which can prevent the sealing surfaces from fitting together properly and thus cause leakage of the medium. If it is a leakage at the sleeve, it is because the sealing ring of the sleeve was not pressed tightly during installation, the compression amount was insufficient, or the ring is damaged. 　　(2) Periodic or paroxysmal leakage. Periodic vibration of the rotor assembly in mechanical seals, as well as excessive axial movement, can both cause leakage. The sealing surfaces of a mechanical seal must have a certain specific pressure in order to function as seals; this requires that the spring in the mechanical seal have a certain degree of compression, thereby applying a force to the sealing surfaces and enabling them to generate the specific pressure needed for sealing when the seal is in operation. To maintain this specific pressure ratio, mechanical seals require that the pump shaft not have excessive play, which should generally be kept within 0.25 mm. However, in actual design, due to unreasonable design, the pump shaft often experiences significant axial movement, which is highly detrimental to the use of mechanical seals. 　　(3) Frequent leakage of the mechanical seal. There are many reasons for the frequent leakage of mechanical seals. Firstly, frequent leaks caused by defects in the sealing surfaces. The second aspect is the frequent leaks caused by the auxiliary sealing ring. Thirdly, there is leakage caused by spring defects. Other aspects include leaks caused by rotor vibration, leaks resulting from poor quality or looseness of transmission, retaining, and thrust components, leaks caused by the auxiliary mechanisms of mechanical seals, and frequent leaks due to issues with the medium. 　　(4) The mechanical seal vibration is too high. The vibration of the mechanical seal is too high, which ultimately results in a loss of sealing efficiency. However, the reason for excessive vibration in mechanical seals is often not solely due to the seals themselves; other components of the pump can also be the source of vibration, such as an improperly designed pump shaft, issues related to manufacturing, insufficient bearing precision, poor parallelism of the couplings, and high radial forces. III. Measures taken to address faults If the components of the mechanical seal fail, it is necessary to replace those components or improve their machining precision. Enhancing the machining precision of the mechanical seal itself as well as that of other components in the pump is highly beneficial for the performance of the mechanical seal. To improve the sealing effect, high requirements are placed on the smoothness and flatness of the friction surfaces of the stationary and rotating rings. The width of the friction surfaces of the stationary and rotating rings is not large, generally ranging from 2 to 7 millimeters. 　　1. Handling of vibration and heating in mechanical seals: If the gap between the rotating and stationary rings and the sealing chamber is too small, it is necessary to increase the inner diameter of the sealing chamber or reduce the outer diameter of the rotating component, ensuring a gap of at least 0.75 mm. If the friction pair is not properly matched, the materials of the stationary and rotating rings need to be changed to make them heat-resistant and corrosion-resistant. This will reduce the vibration and heat generation of the mechanical seal. 　　2. Handling of mechanical seal leaks: Leaks in mechanical seals can be caused by various factors; we must address each issue on a case-by-case basis. To minimize leakage as much as possible, mechanical seals must be installed in strict accordance with the technical specifications, and attention should also be paid to the following points. 　　(1) The assembly must be clean and smooth. The components, tools, lubricants, and cleaning materials for mechanical seals must be extremely clean. The sealing surfaces of the stationary and rotating rings should be wiped with soft gauze. 　　(2) Trim, chamfer, and round off. The chamfers on the shaft, sealing end caps, etc. should be polished smoothly, and the relevant rounded corners of the shaft and end caps should be sanded until they are shiny. 　　(3) When installing the auxiliary sealing ring, the rubber auxiliary sealing ring must not be soaked or washed in gasoline or kerosene, to prevent it from expanding and deforming as well as from aging prematurely. After the stationary and rotating rings are assembled, press the compensation ring by hand to check whether it is in place and flexible, as well as whether the elastic split ring is securely positioned. After the rotating ring is installed, it must be ensured that it can move axially on the shaft with ease. 　　3. Treatment for excessive shaft play in pumps: Design an appropriate mechanism for balancing axial forces in order to eliminate axial play. To meet this requirement, for multi-stage centrifugal pumps, the design approach is to use a balance disk in combination with axial thrust bearings; the balance disk is used to balance the axial forces, while the axial thrust bearings provide axial restraint for the pump shaft. 　　4. Add an auxiliary flushing system. The sealing medium in the sealed chamber contains particles and impurities, which must be flushed; otherwise, the formation of crystals and the deposition of such particles and impurities can cause the springs of the mechanical seal to fail. If particles enter the friction pair, it can lead to rapid damage of the mechanical seal. Therefore, the auxiliary flushing system for mechanical seals is very important; it can effectively protect the sealing surface by providing cooling, lubrication, and removing debris. 　　After briefly examining the application of mechanical seals in rotating equipment as well as the faults that can occur, in the future when encountering problems with mechanical seals, one should first consider the factors affecting the seals themselves, and then also take into account some external factors. For example, when analyzing the causes of quality failures in mechanical seals, one must consider the impact of other pump components on the operation of the mechanical seal, and take measures to continuously improve its performance.