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Applications of SiC Mechanical Seals
Mechanical seals, also referred to as end-face seals, are primarily employed for sealing the rotating shafts in devices such as pumps, compressors, hydraulic transmissions, and analogous equipment. The fundamental structure of a mechanical seal encompasses components such as a dynamic ring, a static ring, an auxiliary seal (O-ring), and an elastic element (spring).
A mechanical seal is an axial end-face sealing device that relies on the pre-tensioning of the sealing pair through the combined action of the medium and the elastic element’s pressure. It achieves sealing by tightly compressing the sealing pair. The sealing ring consists of two types: a moving ring and a stationary ring, with the former and the latter being the primary components constituting the mechanical seal. The working condition of the sealing ring determines the operational performance and lifespan of the mechanical seal, and the performance of the sealing ring material directly influences its working condition. The function of a mechanical seal can be described as an existence of dynamic sealing, aiming to minimize the inevitable leakage that occurs across the dynamic sealing surface. Therefore, understanding the requirements of sealing material performance for mechanical seals and selecting suitable sealing materials are crucial for ensuring the safe operation of mechanical seals.
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Performance Advantages of Silicon Carbide Mechanical Seals
The ceramics required for the mechanical seal industry demand materials with high strength, high toughness, low creep, resistance to abrasive wear, corrosion resistance, and excellent oxidation resistance. Silicon carbide ceramics, due to their favorable physical and chemical properties, have attracted considerable attention and experienced rapid development in various industrial sectors. In recent years, silicon carbide seals have found extensive applications in mechanical seals across multiple fields, including machinery, automotive, petroleum, and chemical industries.
- Performance at High Speed and High Pressure:
Silicon carbide’s low density as the moving ring in high-speed rotation results in reduced centrifugal forces, minimizing vibrations and eccentricity. This stabilization contributes to a comparatively stable pressure on the sealing face, with a relatively fixed friction trajectory. Additionally, silicon carbide’s superior thermal performance prevents thermal cracking and ensures minimal thermal deformation, maintaining a good fit on the sealing face. The flexural strength of hot-pressed silicon carbide is strong, and it resists plastic deformation even under high pressure, surpassing the performance of hard alloys.
- Performance at High and Low Temperatures:
Silicon carbide exhibits excellent high-temperature mechanical and thermal properties, making it suitable for end-face sealing in high-temperature fluids such as hot water and hot oil. It can operate under certain conditions without requiring a cooling system. Moreover, silicon carbide has been successfully utilized in mechanical seals for sealing low-temperature and ultra-low-temperature fluids, such as liquefied petroleum gas, liquefied ethylene, liquid ammonia, and liquid nitrogen.
- Application in Strongly Corrosive Media:
Silicon carbide boasts good chemical stability, high hardness, wear resistance, heat resistance, and excellent emergency operation characteristics. Therefore, it is suitable for various corrosive acidic and alkaline media. Pressureless sintered silicon carbide and hot-pressed silicon carbide have a single-phase structure, providing good corrosion resistance. However, reaction-sintered silicon carbide, containing free silicon, cannot be used in strong oxidizing media.
- Application in Media with Solid Particles:
Many fluids in the petrochemical industry contain solid particles, causing severe abrasion on the sealing faces of mechanical seals. Silicon carbide’s self-mating properties in the presence of solid particles offer greater potential for applications in such environments. The unique hardness of silicon carbide helps resist abrasive wear and local heating, reducing the risk of thermal cracking.
- Additionally, silicon carbide seals exhibit a lower coefficient of friction, contributing to reduced energy consumption and increased efficiency of mechanical seals. Its excellent thermal conductivity aids effective heat dissipation at high temperatures, preventing overheating of sealing components.
Common Issues with silicon carbide Mechanical Seals
Mechanical seals may encounter various issues during operation, affecting their performance and leading to leakage, damage, or equipment failure. Here are some common problems associated with mechanical seals:
- Leakage:One of the most common issues with mechanical seals is leakage, which can be caused by uneven sealing surfaces, damage to sealing materials, or loosening of fasteners.
- Friction and Wear:The friction surfaces of mechanical seals may suffer damage due to friction, wear, or scraping, leading to a decline in sealing effectiveness. This typically occurs during high-speed operations or under adverse conditions. Additionally, when there is no fluid in the equipment seal cavity, dry friction can occur, causing severe wear on the moving ring and thermal cracking on the stationary ring.
- Overheating or Corrosion:Overheating of the sealing faces can cause hardening, deformation, or fracture of sealing materials. This may result from incorrect lubrication, inappropriate sealing materials, or excessive friction. Sealing components may also corrode, leading to damage to the sealing materials or erosion of the sealing faces.
- Friction Pair Engagement Failure:Low-power centrifugal pumps may experience difficulties starting due to engagement of sealing components, preventing the mechanical system from starting. Seals using a combination of graphite and ceramic in the friction pair may encounter issues where graphite adheres to the ceramic during operation, causing engagement failures. Various factors contribute to this problem, primarily stemming from the improper selection of sealing components and mismatched friction pair materials.
- Improvement Measures: Choose sealing components based on power and environmental conditions; use specialized installation tools for mechanical seal assembly to avoid excessive compression affecting the dynamic ring seal; prefer materials with different compositions for the stationary and moving rings, such as pairing silicon carbide with graphite for lower power applications. If using the same material, such as silicon carbide for both components, employ pressureless sintering, reaction sintering, or carbonization processes for preparation and apply atomization treatment to the surfaces of the dynamic and static rings.
- Seal Damage and Improper Installation:Lack of guidance at the installation contact points for the sealing rings can lead to scratches on the sealing rings, resulting in seal failure. Uneven installation of the stationary ring can cause vibration between the dynamic and static rings during operation, leading to seal failure.
- Frequent Start-Stop Operations:Frequent start-stop operations can induce fatigue in mechanical seals, causing damage or leakage.
- Impurities and Particulate Matter:During high-speed equipment operation, solid particles can impact the end faces of sealing components, causing wear over time and leading to seal failure. Adding sand guards or auxiliary blades behind the impeller can reduce particle-induced wear on the sealing components.
Considerations for the application of SiC Mechanical Seals
- Reasonable Material Selection:
Currently, there are options for selecting materials for silicon carbide seals, including reaction-sintered silicon carbide, non-pressure sintered silicon carbide, and hot-pressed silicon carbide sealing rings. The choice of silicon carbide should be based on factors such as quantity, structure, operating conditions, capacity, and cost-effectiveness. Non-pressure sintered or reaction-sintered silicon carbide is suitable for large-scale production; for products with complex shapes, easily formable non-pressure sintered silicon carbide is recommended; non-pressure sintered and hot-pressed silicon carbide are suitable for strong oxidizing media; hot-pressed silicon carbide is preferred for applications with high PV values; and silicon carbide self-matching is recommended for media containing solid particles.
- Uniformity in Assembly Structure:
The assembly components of the overall structure of silicon carbide seals are restricted by the usage range of the metal base material and the O-ring material. If this assembly structure is adopted, it is essential to choose a metal base with an expansion coefficient close to that of silicon carbide and specify reasonable mating tolerances to prevent loosening, detachment, cracking, or deformation of the silicon carbide ring due to temperature and pressure changes.
- Rationality in Design and Manufacturing:
Silicon carbide is a brittle material, so the design of silicon carbide rings should consider keeping the shape simple. It is not advisable to groove or drill holes in the ring to avoid stress concentration. The connection points of the shape should be designed with taper and transitional rounded corners to reduce impact forces.
- Careful Operation and Installation:
During use, it is imperative to strictly follow the sealing assembly and disassembly processes and adhere to usage specifications. Depending on operating conditions, install necessary auxiliary devices such as cooling, flushing, and filtration to ensure the normal operation of the mechanical seal and enhance its service life.
Real-life Cases of Damage to SiC Mechanical Seals issues
1.Specific Damage Description of SiC Mechanical Seals issues
In this case, the silicon carbide sealing ring and sealing sleeve exhibited varying degrees of issues. Some showed scratches, pores, and damage on the sealing surface, while others exhibited problems such as pores and edge damage on the non-sealing surface.
2.Cause analysis of SiC Mechanical Seals issues
In the case of silicon carbide seal issues, some were identified during factory inspection, while the majority were discovered after product usage. Therefore, the analysis of causes is presented from two perspectives: product manufacturing and processing, and assembly and usage.
- Processing of Silicon Carbide Seals:
In terms of processing silicon carbide seals, they exhibit higher grinding efficiency compared to hard alloys due to the absence of metals, preventing sticking to diamond grinding wheels. However, if the feed rate is too fast during grinding or if there is uneven rotation speed, it may lead to chipping. Improper packaging, including rough handling or inadequate packaging, during transportation can result in collisions and damage to silicon carbide seal products. Therefore, the probability of damage occurring during the grinding and packaging processes is higher, as indicated by the extent of damage upon unpacking.
- Damage to Sealing Surface Flatness:
Damage to the flatness of the sealing surface is often characterized by uneven sliding marks. During the thermal assembly of sliding components, temperature changes and differential thermal expansion can cause deformation of the sealing surface. Specifically, surface wear and damage may occur when fasteners undergo deformation, looseness, excessive structural thermal stress, or significant thermal deformation. Additionally, dry friction in mechanical seals can lead to roughening and rapid wear of the sealing surface, even causing cracks in hard, wear-resistant materials like silicon carbide.
- Formation of “Carbon Scars” during Operations:
In cases where high-viscosity fluids are involved and there are repeated start-stop operations, mechanical systems are prone to developing “carbon scars.” These scars, serving as initiation points, can lead to various defects such as radial crack growth, peeling, and detachment. Therefore, in this case, it is plausible that the silicon carbide seal ring encountered adhesion with high-viscosity fluids during usage, resulting in repeated start-stop operations causing “carbon scars” and subsequent defects like detachment.
- Presence of Impurities or Crystals in Conveyed Media:
If the conveyed medium contains impurities or crystalline substances, the high-speed rotation can introduce them into the frictional interface between the dynamic and static rings, causing damage to the contact surfaces of both rings.
- Incorrect Installation Parameters:
Additionally, if installation personnel miscalculate and exceed the recommended compression requirements, it can lead to damage, scratches, or other issues on the frictional surfaces between the dynamic and static rings.
3.Improvement Measures for SiC Mechanical Seals issues
In response to the defects observed in silicon carbide seals in this case, the following improvement measures are recommended:
- Silicon Carbide Grinding:
During silicon carbide grinding, avoid excessive feed rates, choose an appropriate grit size for the grinding wheel, and maintain high rotation speed to prevent chipping and achieve the required smoothness.During assembly, strictly prohibit striking the packaging and implement shockproof measures.
- Heat Dissipation and Cooling:
Mechanical seals are susceptible to material deformation, part distortion, and changes in load capacity due to heating, leading to reduced sealing performance or even burnout.Depending on the heat generated, employ methods such as natural cooling, water jacket cooling, internal circulation cooling, external flushing cooling, shaft internal cooling, and external circulation cooling.Enhance flushing for improved cooling effects, control the seal temperature, and prevent the accumulation of impurities to ensure the integrity of the seal.
- Installation and Lubrication:
To prevent localized pressure rise, thermal cycling, and fatigue leading to the breakdown of the liquid film, emphasize correct installation, ensure mechanical seal cleanliness, and maintain an intact liquid film.
Regularly inspect and prevent blockages in the supply lines for both sealing liquid and cooling water.Use static mechanical seals with excellent slurry resistance in the cooling water pipeline to effectively prevent dry sliding.
- Preventing “Carbon Scars”:
The formation of “carbon scars” is complex. Implement joint measures such as steam flushing to suppress high-viscosity leaks, repair flatness, and prevent deformation of the sealing surface.
- Risk Reduction and Maintenance:
To minimize the risk of silicon carbide seal damage, emphasize proper cooling, correct installation, and regular maintenance checks to ensure mechanical seal cleanliness and intact lubrication.Rigorously train operators and maintenance personnel to avoid incorrect equipment operation.
Conclusion
In conclusion, if Silicon Carbide Mechanical Seals encounter various issues such as breakage during use, it is influenced by multiple factors. Specific investigations and determinations need to be made based on the on-site conditions and the characteristics of the product itself. If you are looking for Silicon carbide Mechanical Seals or related products, feel free to browse our related products or contact us directly.
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