Butterfly valves are widely used in numerous industrial sectors such as petroleum, chemical, water treatment, and municipal water supply and drainage due to their simple structure, quick operation, low flow resistance, and cost-effectiveness. However, seal ring leakage is one of the most common failures encountered during their actual operation. Leakage not only leads to media loss and environmental pollution but can also cause serious safety incidents, especially when handling toxic, hazardous, flammable, or explosive media. Therefore, in-depth analysis of the causes of seal ring leakage in butterfly valves and the implementation of effective preventive and corrective measures are crucial for ensuring the safe and stable operation of systems.
I. Primary Causes of Seal Ring Leakage in Butterfly Valves
The sealing of a butterfly valve primarily relies on the tight contact between the disc when closed and the seal ring (seat) inside the valve body. Any factor affecting this tight contact can lead to leakage. The specific causes can be categorized as follows:
1. Improper Selection and Design
- Material Incompatibility: The seal ring material is incompatible with the chemical properties, temperature, or pressure of the conveyed medium. For example, a standard Nitrile Rubber (NBR) seal ring used for high-temperature steam will rapidly age and harden, losing elasticity; EPDM rubber used for strong acids might corrode or swell.
- Incorrect Pressure Rating: Under excessively high working pressure, the medium pressure may exceed the seal ring's rated pressure capacity, causing it to be "blown out" or undergo permanent deformation, rendering it ineffective.
- Incorrect Type Selection: Selecting a unidirectional seal butterfly valve for an application requiring bidirectional sealing; or using a standard concentric soft-sealed butterfly valve in harsh conditions requiring zero leakage, instead of a high-performance double-offset or triple-offset metal-seated valve.
2. Installation and Operational Issues
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Improper Installation: This is a frequent cause of leakage.
- Uneven Bolt Preload: If the flange connection bolts are not tightened sequentially and diagonally during installation, the valve body can warp, preventing uniform contact on the sealing surface.
- Pipe Stress Influence: Excessive installation stress or thermal stress within the piping system imposes external forces on the valve, causing body distortion.
- Insufficient Clearance: Failure to leave adequate space for the 90-degree operation of the disc during installation can prevent the valve from opening or closing fully, leaving it partially open for extended periods and subjecting the seal to media erosion.
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Improper Operation:
- Use as a Control Valve: Using a butterfly valve, designed for fully open or fully closed service, for prolonged throttling. In the partially open position, high-velocity fluid directly impinges on the seal ring, causing localized erosion, wear, and cavitation.
- Forced Operation: Using an overly long lever or actuator to force the valve closed. If debris is present on the disc, excessive closing torque can damage or even crush the seal ring.
3. Aging and Damage of the Seal Ring Itself
- Natural Aging: Elastic materials like rubber naturally harden and lose elasticity and sealing performance over time.
- Media Corrosion and Swelling: Chemical reactions between the medium and the seal material cause it to become brittle, crack, or undergo excessive swelling and deformation.
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Wear and Scoring:
- Media Erosion: The presence of solid particles (e.g., silt, welding slag) in the medium abrades the sealing surface as it flows through the valve.
- Foreign Object Damage: Debris like rust or welding slag inside the pipeline gets trapped between the sealing surfaces when the valve closes, scoring the seal ring.
- Fatigue Wear: Under frequent cycling, the constant compression and friction on the seal ring lead to permanent set or surface wear.
4. Disc and Stem Issues
- Disc Deformation: The disc deforms due to excessive pressure or external impact, preventing uniform contact with the seal ring.
- Stem Wear or Bending: Wear at the connection points between the stem, packing, and disc, or a bent stem itself, prevents the disc from returning to the correct sealing position upon closure, causing misalignment.
II. Solutions and Preventive Measures for Seal Ring Leakage
Addressing the causes above, we can implement targeted solutions and proactive prevention strategies.
1. Scientific Selection and Standardized Installation (Prevention Focused)
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Correct Selection:
- Media Compatibility: Select the appropriate seal ring material (e.g., NBR, EPDM, Viton/FKM, PTFE) based on the corrosiveness and temperature of the medium.
- Pressure and Structure: Choose the appropriate pressure rating and butterfly valve type (concentric, double-offset, triple-offset) based on system working pressure and sealing requirements.
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Standardized Installation:
- Clean Piping: Thoroughly clean all debris from the pipeline before installing the valve.
- Alignment and Bolt Tightening: Ensure the valve aligns correctly with the pipe flanges. Use a torque wrench and tighten bolts diagonally and evenly in multiple steps.
- Check Actuator: Ensure the actuator stroke matches the valve travel to avoid over-opening or over-closing.
2. On-Site Troubleshooting After Leakage Occurs (Solutions)
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Minor Leaks or Temporary Fixes:
- Cycling the Valve: For leaks caused by minor debris, try rapidly opening and closing the valve completely several times to flush the debris away with the medium.
- Adjusting Actuator Limits: Check if loose limit screws are causing incomplete closure and adjust the closed position limit accordingly.
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Disassembly and Repair (Offline Maintenance): If the above methods fail, the valve must be removed from the line for disassembly and inspection.
- Cleaning and Inspection: Thoroughly clean the valve interior, disc, and stem. Carefully inspect all components for wear.
- Replacing the Seal Ring: This is the most direct and effective method. During replacement, ensure the new seal ring is the correct type and material, and install it properly without twisting or damaging it.
- Inspecting and Replacing Other Damaged Parts: Simultaneously check the disc for scratches or deformation and the stem for bending. Repair or replace any damaged parts.
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Online Repair Techniques (For Specific Conditions):
- Sealant Injection: Some specially designed valves have a sealant injection system. When a leak occurs, specialized sealant can be injected via the injection fitting to temporarily form a new seal layer.
- Online Seat Replacement: Certain high-performance butterfly valves are designed for online seat replacement, allowing the seal ring to be changed without removing the valve from the pipeline, significantly reducing downtime.
3. Establishing a Comprehensive Maintenance System (Long-term Mechanism)
- Regular Inspections: Periodically check valves for external leaks and ensure actuators operate normally.
- Periodic Operation: For valves in standby systems that are not frequently operated, perform full open/close cycles regularly (e.g., monthly) to prevent the seal ring from adhering or permanently deforming in one position.
- Maintenance Records: Keep records for each valve, including model, parameters, maintenance history, and replacement of wear parts, to facilitate predictive maintenance.
Conclusion
Seal ring leakage in butterfly valves results from a combination of factors. Negligence at any stage—from initial selection and design, through installation, to daily operation and maintenance—can lead to leakage failure. Therefore, addressing this issue requires a systematic approach: First, prevent problems at the source through scientific selection and standardized installation. Second, when leakage occurs, accurately diagnose the cause and implement correct online or offline repair measures. Finally, establish a long-term maintenance mechanism to extend valve service life and ensure the safety, stability, and efficiency of the entire fluid conveyance system. Only by following this approach can the advantages of butterfly valves be fully realized, minimizing the economic losses and safety risks associated with leakage.
