Common Leakage of Pump Seal and Countermeasures
Abstract: This article outlines the most common causes of mechanical seal failures. A mechanical seal is a highly precise component that requires careful design, precision machining, and accurate assembly. When using mechanical seals, it's essential to analyze various factors such as pump technical specifications, the properties of the medium being handled, and adequate lubrication conditions to ensure long-term and reliable sealing performance.
Keywords: mechanical seal leakage, seal failure, pump maintenance
Mechanical seals, also known as face seals, consist of two perpendicular surfaces to the axis of rotation. One surface remains stationary while the other rotates, and they are kept in contact by fluid pressure and an external elastic force. The auxiliary seals help maintain this contact and sliding motion to prevent fluid leakage.
First, common leakage phenomena
Over 50% of pump maintenance issues are related to mechanical seal failures. These failures directly impact the normal operation of the pump. Common problems include:
1. Periodic leakage
(1) Excessive axial movement of the pump rotor can cause the auxiliary seal to bind on the shaft, preventing the rotating ring from moving freely. During pump operation, wear on the rotating and stationary rings may prevent proper compensation for displacement.
Solution: During assembly, the axial play of the shaft should be less than 0.1 mm, and the interference between the auxiliary seal and the shaft should be moderate. This ensures radial sealing while allowing the rotating ring to move freely on the shaft, with the spring able to bounce back without resistance.
(2) Insufficient lubrication on the sealing faces can lead to dry friction or incomplete sealing.
Solution: Ensure that the oil chamber contains sufficient lubricating oil to cover both the rotating and stationary ring sealing surfaces.
(3) Rotational vibration of the rotor due to misalignment, imbalance, cavitation, or bearing damage can reduce the life of the seal and cause leaks.
Solution: Follow maintenance standards to correct these issues and ensure stable operation.
2. Seal leakage caused by grinding shaft in small submersible sewage pumps
(1) In small pumps (715 kW and below), seal failure often leads to grinding shafts, typically occurring at the auxiliary seal of the rotating ring, the static ring, or occasionally on the spring.
(2) Main reasons for grinding shafts include poor back pressure in double mechanical seals, where particles and impurities can enter the sealing area. The upper sealing surface may lack lubrication, leading to greater friction between the static and rotating rings than the torque between the rubber bellows and the shaft, causing relative rotation. Additionally, corrosion from weak acids or bases in sewage can degrade the rubber components, reducing their elasticity and function, which contributes to the grinding shaft phenomenon.
Solution: Ensure cleanliness in the lower end cap and oil chamber, avoid using unclean lubricants, and keep the oil chamber level above the sealing surfaces. Use appropriate mechanical seal structures based on the media type. For high-lift pumps, redesign the seal structure. For corrosive media, use fluorinated rubber that resists weak acids and bases. Add anti-rotation features to the seal ring.
3. Leakage due to pressure
(1) High pressure or pressure waves can cause mechanical seal failure. If the spring pressure or total pressure exceeds the design value, or if the pressure inside the sealed chamber exceeds 3 MPa, the end face pressure becomes too high, making it difficult for a liquid film to form. This leads to severe wear and heat generation, resulting in thermal deformation of the sealing surface.
Solution: Follow the specified spring compression during assembly, avoiding excessive or insufficient compression. Under high-pressure conditions, take additional measures to ensure balanced end face forces and minimize deformation. Use materials like cemented carbide or ceramics, and improve cooling and lubrication systems.
(2) Vacuum conditions during pump startup or shutdown can cause dry friction on the sealing surface, especially in built-in mechanical seals, leading to water leakage. The difference between vacuum and positive pressure sealing lies in the direction of the force, and mechanical seals have specific directional adaptability.
Solution: Use dual-face mechanical seals to improve lubrication and sealing performance.
4. Leakage caused by the medium
(1) Most submersible sewage pump mechanical seal failures involve inelastic or degraded static and dynamic ring seals. Corrosion from weak acids and bases in wastewater can damage the rubber seals, leading to significant leakage and even grinding shaft issues. Nitrile rubber (NBR-40) is commonly used but is prone to degradation in acidic or alkaline environments.
Solution: For corrosive media, use high-temperature, acid- and alkali-resistant fluorinated rubber.
(2) Solid particles entering the sealing face can scratch or accelerate wear. Accumulated scale or grease on the shaft can exceed the wear rate of the friction pair, preventing the rotating ring from compensating for wear displacement. Hard-to-hard friction pairs last longer than hard-to-graphite pairs because solid particles are more likely to embed in graphite seals.
Solution: In areas where solid particles are present, use tungsten carbide friction pairs in mechanical seals.
5. Other issues causing mechanical seal leakage
Design, selection, and installation errors can also lead to seal failures.
(1) Spring compression must be within the specified range (±2 mm). Too much compression increases end face pressure and frictional heat, leading to thermal deformation and accelerated wear. Too little compression results in insufficient sealing force.
(2) Chamfer and smooth the shaft (or sleeve) and seal gland (or casing) surfaces where the seal is installed to avoid damaging the static and dynamic ring seals during assembly.
Conclusion
This article has summarized the most common causes of mechanical seal leakage. As a high-precision component, mechanical seals require strict design, machining, and assembly standards. Proper analysis of factors such as pump requirements, media properties, and lubrication conditions is essential to ensure long-term, reliable sealing performance.
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