Common causes of leakage in water pumps and solutions

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Common causes of leakage in water pumps and solutions: Mechanical seals are the main cause of leakage in water pumps. However, mechanical seals are precision components that require high standards; they demand excellence in terms of design, machining, and assembly quality. When using mechanical seals, it is necessary to analyze various factors related to their use, so that the seals can meet the technical requirements of different pumps as well as the requirements of the fluids being handled, and have adequate lubrication conditions; only in this way can reliable long-term operation of the seals be ensured. 　 　　A mechanical seal, also known as a face seal, features a pair of faces that are perpendicular to the axis of rotation. Under the influence of fluid pressure and forces generated by mechanical components, these faces remain in contact with each other thanks to the assistance of auxiliary seals, while sliding relative to one another, thereby preventing fluid leakage. I. Common leakage phenomena Mechanical seal leaks account for over 50% of all pumps that require repair. The proper functioning of mechanical seals has a direct impact on the normal operation of water pumps; the following is a summary and analysis thereof. 　　1. Periodic leakage 　　(1) The pump rotor experiences significant axial movement; the interference between the auxiliary seal and the shaft is large, preventing the rotating ring from moving freely on the shaft. After the pump is reversed and the rotating and stationary rings wear out, no compensatory displacement is available. 　　Solution: When installing mechanical seals, the axial movement of the shaft should be less than 0.1 mm. The interference between the auxiliary seal and the shaft should be appropriate; while ensuring radial sealing, the rotating ring should be able to move freely on the shaft after installation (it should be able to spring back freely once pressed against the spring). 　　(2) Insufficient lubrication on the sealing surface causes dry friction or scuffing of the sealed end face. 　　Solution: The level of the lubricating oil in the oil chamber should be raised above the sealing surfaces of the rotating and stationary rings. 　　(3) Periodic vibration of the rotor. The reason is that the stator is not aligned with the upper and lower end covers, or the impeller and main shaft are unbalanced; cavitation or bearing damage (wear) also occurs, which reduces the lifespan of the seal and leads to leaks. 　　Solution: The above issues can be corrected in accordance with the maintenance standards. 　　2. Shaft wear caused by leakage in the mechanical seal of small submersible sewage pumps (1) Failure of the mechanical seal in small pumps with a power of 715 kW or less often leads to shaft wear. The main locations where shaft wear occurs are as follows: at the auxiliary sealing ring of the rotating ring, at the position of the stationary ring, and in some cases, the springs also cause shaft wear. 　　(2) Main reasons for shaft wear: ① In BIA-type double-face mechanical seals, a reverse pressure condition represents an unfavorable operating state; particles and impurities in the medium can easily enter the sealing surfaces, leading to seal failure. ②The main component of the shaft grinder is the rubber bellows; and because the upper sealing surface is not properly lubricated, the frictional torque between the rotating and stationary rings exceeds the torque transmitted between the rubber bellows and the shaft, resulting in relative rotation. ③Due to corrosion by weak acids and bases in the wastewater, the rubber components of the dynamic and static ring auxiliary seals have lost their elasticity. Some have rotted, lost their proper function, and caused shaft wear. 　　(3) To address the above issues, the following measures are now taken: ① Ensure the cleanliness of the lower cover and oil chamber; the use of unclean lubricating oil is prohibited. ②The oil level in the mechanical seal oil chamber should be above the sealing surfaces of the stationary and rotating rings. ③Select machine seals with different structures based on the medium used. For high-head pumps, the mechanical seal design should be revised; for rubber used with corrosive media, fluororubber resistant to weak acids and bases should be selected. A anti-rotation pin should be installed on the static ring of the mechanical seal. II. Leaks Caused by Pressure (1) Mechanical seal leaks due to high pressure and pressure waves: When the spring ratio pressure and total ratio pressure are designed to be too high, or when the pressure inside the sealing chamber exceeds 3 MPa, the pressure on the sealing surfaces becomes excessive, making it difficult for a liquid film to form. This leads to severe wear of the sealing surfaces, increased heat generation, and thermal deformation of those surfaces. 　　Solution: When installing the mechanical seal, the compression amount of the spring must be carried out in accordance with the specifications; it is not allowed for this amount to be too large or too small. Measures should be taken for mechanical seals under high-pressure conditions. To ensure proper stress distribution on the end face and minimize deformation, materials with high compressive strength such as cemented carbide and ceramics can be used, along with enhanced cooling and lubrication measures. Appropriate transmission methods such as keys and pins should also be employed. 　　(2) Leakage of mechanical seals caused by operation under vacuum conditions: During the startup and shutdown of the pump, factors such as blockage at the pump inlet or the presence of gases in the medium being pumped can result in a negative pressure in the sealing chamber. A negative pressure in this chamber leads to dry friction between the sealing surfaces, and internal mechanical seals will experience air (water) leakage. The difference between vacuum seals and positive-pressure seals lies in the directional nature of the sealing process, and mechanical seals also have their own suitability for certain directions. 　　Solution: Use a double-end mechanical seal, which helps improve the lubrication conditions and enhance the sealing performance. III. Leaks caused by the medium 　　(1) After disassembling most submersible sewage pumps, it is found that the auxiliary seals of the stationary and rotating rings lack elasticity; some of them have even rotted, resulting in severe leaks from the mechanical seal and even shaft wear. Due to the corrosive effect of high temperatures, as well as weak acids and bases present in wastewater, on the auxiliary rubber seals of the stationary and rotating rings, excessive mechanical leakage occurs. The rubber sealing materials used for the rotating and stationary rings are NBR-40, which are not resistant to high temperatures or acids and bases; therefore, they are prone to corrosion when the wastewater is acidic or alkaline. 　　Solution: For corrosive media, rubber components should be made of fluororubber, which is resistant to high temperatures as well as weak acids and bases. 　　(2) Mechanical seal leakage caused by solid particle impurities: If solid particles enter the sealing surface, they can scratch it or accelerate its wear. The accumulation of scale and oil on the surface of the shaft (hub) occurs at a faster rate than the wear of the friction pair, preventing the moving ring from compensating for the resulting wear. The service life of a hard-on-hard friction pair is longer than that of a hard-on-graphite friction pair, as solid particles can get embedded in the sealing surface of the graphite seal ring. 　　Solution: A mechanical seal using a tungsten carbide-tungsten carbide friction pair should be used in areas where solid particles are likely to enter. IV. Leakage of mechanical seals caused by other issues There are also issues related to the design, selection, and installation of mechanical seals that are not optimal. 　　(1) The compression amount of the spring must be carried out in accordance with the specified requirements; it is not allowed for it to be too large or too small. The allowable error is ±2 mm. If the compression amount is too large, the pressure on the end faces increases, resulting in excessive frictional heat, which causes thermal deformation of the sealing surfaces and accelerates wear at those end faces. If the compression amount is too small, the pressure on the end faces of the stationary and moving rings is insufficient, and thus sealing cannot be achieved. 　　　 　　 　　(2) The end faces of the shaft (or sleeve) on which the rotating ring seal is installed, as well as the end faces of the sealing gland (or housing) on which the stationary ring seal is installed, should be chamfered and smoothed to prevent damage to the rotating and stationary ring seals during assembly. V. Conclusion The above summarizes the more common causes of leakage in mechanical seals. A mechanical seal is itself a precision component that requires high standards, with strict demands on its design, machining, and assembly quality. When using mechanical seals, it is necessary to analyze various factors related to their use, so that the seals can meet the technical requirements of different pumps as well as the requirements of the fluids being handled, and have adequate lubrication conditions; only in this way can reliable long-term operation of the seals be ensured.