During centrifugal pump maintenance, leakage issues are quite common, with mechanical seal (mechanical seal) leakage being the most prevalent. This article discusses the possible causes of mechanical seal leakage and their respective solutions.
1. Leakage Due to Wear on the Moving and Stationary Seal Faces
One of the primary reasons for mechanical seal leakage is wear on the moving and stationary seal faces. The following are six main causes of such wear:
Cause 1: Excessive Tightness During Installation
If the seal faces show severe burn marks, blackened surfaces, deep grooves, and the sealing rubber has hardened and lost elasticity, the issue is likely caused by excessive tightness during installation.
Solution: Adjust the installation height. After installing the impeller, use a screwdriver to push the spring-it should have sufficient tension and return to position after release, with a movement range of 2-4mm.
Cause 2: Loose Installation
If a thin layer of scale is observed on the seal faces but can be easily wiped off, and there is little to no wear, this may be due to a loss of spring tension, improper assembly, or axial movement of the motor shaft.
Cause 3: Poor Water Quality with Particles
If the water contains small particles or has a high salt concentration, it can cause abrasive wear or scratches on the seal faces, leading to grooves or circular marks.
Solution: Improve water pressure or change the medium. Replace the mechanical seal if necessary.
Cause 4: Dry Running Due to Lack of Water
This issue is common in bottom-valve installations where negative pressure at the inlet, air in the suction pipe, or air in the pump chamber prevents proper cooling of the mechanical seal. As a result, friction generates excessive heat, causing the seal faces to burn and turn black, while the rubber hardens and cracks.
Solution: Remove all air from the pipeline and pump chamber, and replace the mechanical seal.
Cause 5: Cavitation (Common in Hot Water Pumps)
Cavitation occurs in hot water pumps when excessive water temperature leads to steam formation. Trapped steam accumulates at the highest point in the pump chamber, preventing proper water circulation. This results in dry running and mechanical seal failure.
Solution: Install an automatic air release valve and replace the mechanical seal.
Cause 6: Assembly Issues
Improper installation of the pump cover may cause misalignment between the pump shaft and the cover, preventing the seal faces from achieving proper contact. This leads to uneven wear and leakage within a short operating period. Additionally, the rubber components or seal faces may be damaged during installation.
Solution: Reassemble the pump, ensuring the pump cover is installed correctly.
2. Leakage Due to Seal Rubber Failure
Another possible cause of mechanical seal leakage is the failure of the rubber sealing components.
Causes of Rubber Failure:
Rubber components may degrade due to aging, deformation, or prolonged exposure to high-temperature media. In hot water pumps, the high water temperature can cause rubber components to dissolve, leading to a loss of elasticity. The rubber surface may appear loose and rough.
Solution: Replace the rubber components with suitable materials that can withstand the operating conditions.
3. Leakage Due to Pump Cover Seal Chamber Issues
If the pump cover's seal chamber has excessive machining tolerances or a rough surface, it may cause axial leakage. This happens when gaps exist between the stationary ring and the seal chamber, leading to water spraying from the axial direction. Additionally, a rough inner surface may cause the stationary ring to rotate, wearing down the rubber seal.
Solution: Replace the pump cover. As a temporary fix, wrap the stationary ring with PTFE tape or apply additional sealing adhesive.
4. Leakage Due to Poor Shaft Surface Quality
If the motor shaft journal (where the moving ring is installed) has poor machining accuracy, is undersized, or has a rough surface, it can lead to leakage. Additionally, if the medium is corrosive, it may cause shaft corrosion over time. Long periods of inactivity may also lead to electrochemical corrosion due to the potential difference between the steel shaft and other metal components such as the impeller and pump cover. This typically appears as pitting on the shaft surface.
Solution: Sand the corroded surface with fine sandpaper and wrap it with PTFE tape for temporary use. However, the best solution is to replace the shaft or install a shaft sleeve and machine it to the correct dimensions.
By understanding these common causes and applying the appropriate solutions, mechanical seal leakage in centrifugal pumps can be effectively managed and prevented, ensuring reliable pump performance and extended service life.
