Failure Analysis and Troubleshooting of Mechanical Seals for Pumps

1. Working Principle and Technical Requirements of Pump Mechanical Seals

Mechanical seals are devices that achieve fluid containment through axial relative sliding. Their core function relies on at least one pair of end faces perpendicular to the rotating shaft. Under the combined action of fluid pressure, elastic elements (or magnetic assemblies), and auxiliary sealing rings, these end faces maintain tight contact. The sealing mechanism operates by forming an ultrathin liquid film between the end faces, which establishes a dynamic balance under specific pressures to ensure effective sealing. Pressure elements apply sustained mechanical loads to guarantee stable contact between the sealing faces. This design prevents media leakage while blocking ingress of external contaminants. It should be noted that the actual performance of mechanical seals depends not solely on their structure but is intrinsically linked to the pump’s overall operating conditions.

2. Causes of Mechanical Seal Leakage in Pumps

(1) Scaling Between the Rotating Ring and Sleeve

Scaling deposits between the dynamic seal ring and shaft sleeve can impede ring movement, leading to leakage. Scaling typically forms at temperatures around 80°C (176°F), this temperature becomes even lower in water with high impurity content. In high-temperature pumps, elevated cooling water temperatures cause calcium/magnesium salts to crystallize between the bellows and baffle sleeve, forming scale that reduces bellows elasticity and induces leakage.

Recommendations: Upgrade cooling media—use low-temperature demineralized water (soft water) for seal cooling. For high-temperature pumps, employ steam cooling to prevent sleeve scaling and improve seal responsiveness. Where process conditions allow, use seal oil as a cooling medium to inhibit scaling.

(2) Notches on Inner/Outer Edges of Friction Pair

Recommendation: Select appropriate seal configurations based on actual operating conditions.

(3) Rotating Seal Ring Detachment

Disassembly often reveals detached rotating rings or seal face deformation in high-temperature pumps.

3. Analysis of Seal Failure Causes

Impact of Pumping Conditions on Mechanical Seals
When the bottom depressurization system operates under negative pressure, the depressurization pump frequently disengages during operation. Following vacuum extraction, pressure on the liquid film surface decreases. However, if the seal face temperature exceeds the seal chamber temperature while the liquid saturation pressure drops below the surface temperature, flash vaporization occurs (phase transition). During this phase transition，sealing face pressure fluctuates abruptly，opening forces destabilize and mechanical seal equilibrium is disrupted. This imbalance triggers, dry friction, thermal shock (rapid temperature surge), excessive face separation and catastrophic leakage.

4. Countermeasures for Resolving Pump Mechanical Seal Failures

(1) Modify the Structure of the Pump's Moving Ring:
Leveraging advanced international technology and pump design principles, the material of the pump's moving ring should be changed from solid tungsten carbide to stainless steel. This modification reduces wear and sublimation of the tungsten carbide during operation. The moving ring is subjected to various forces during shaft positioning and operation, including clamping force, centrifugal force, and friction. While tungsten carbide is used for the seal ring to prevent deflection and misalignment, any gap in the moving ring seat can lead to free-floating under pressure and expansion/cracking under heat. This directly causes oil leakage in the pump. Replacing it with stainless steel significantly reduces the risk of leakage events caused by moving ring fracture. Stainless steel moving rings offer high toughness and hardness, effectively ensuring the pump's normal operation and preventing oil leaks.

(2) Implement Regular Flushing and Cleaning; Select Appropriate Flushing Methods:
Thorough flushing and cleaning procedures must be performed before and after pump operation. After shutdown, promptly flush and clean out internal contaminants. Strictly inspect the operating environment before starting the pump. If the slurry concentration handled by the pump is excessively high, a flushing regimen must be applied to ensure proper dilution. Pay close attention to the accuracy of the flushing fluid composition to guarantee that no powder deposits accumulate on sealing faces, seal rings, or other components. This effectively ensures the safe and reliable operation and use of the cleaned mechanical equipment.

5. Common Misconceptions in Repairing Pump Mechanical Seal Failures

Multiple factors can cause seal leakage, such as shaft faults and misalignment, impurities on the shaft, improper fit or positioning of the shaft sleeve, damage to contact surfaces, excessive clearance between shaft-mounted components, and excessive thread length on the shaft end.

Misconception: Over-tightening the lock nut improves sealing.
Reality: Excessive tightening only leads to premature failure of the shaft packing. Conversely, moderate tightening allows the shaft packing to retain essential compressive elasticity. During operation, the lock nut will self-secure appropriately. While the stationary seat ring remains static long-term, polymer and impurity deposits may temporarily improve sealing but are not a reliable solution.

Mechanical seals offer advantages like minimal leakage, reliable sealing, and low power consumption. They meet diverse operational requirements and are widely used in fluid machinery such as pumps and compressors. High-performance mechanical seals are crucial for the long-term, safe, and reliable operation of motorized pumps. However, in practice, internal and external factors frequently cause mechanical seal failure, potentially leading to motor burnout. Over half of all pump maintenance incidents stem from mechanical seal failures.

To ensure the long-term, safe, and reliable operation of mechanical seals, a systematic analysis of failure causes is essential. Detailed precautions must be established based on design and installation experience, supported by targeted improvement measures.

Enhancing seal effectiveness requires:

1. Proper selection of mechanical seal devices.

2. Thorough inspection before installation.

3. Correct operation and diligent maintenance within routine equipment management.

Only this comprehensive approach can reduce seal leakage and lower failure rates. Furthermore, analyzing the causes of mechanical seal leakage is complex and context-specific. It demands continuous practical experience and careful diagnosis to identify the correct solutions. While pump mechanical seal failures manifest diversely with intricate causes, any failure jeopardizes the safe storage and transport of corrosive substances like sulfuric acid and caustic soda. Implementing systematic and effective maintenance procedures is therefore imperative.