Mechanical Seals: The Starter Guide from Flexaseal

A mechanical seal is a device designed to contain media (liquids, solids, or gases) within a vessel where a rotating shaft enters a stationary housing, preventing external leakage while ensuring minimal friction between rotating and stationary parts. Most commonly, mechanical seals are used in pumps, mixers, and agitators to contain fluids, prevent contamination, and maintain the integrity of high-value industrial processes.

• Who Uses Mechanical Seals?
  Mechanical seals are vital to operations in industries like oil & gas, chemical processing, pulp & paper, power generation, water & wastewater treatment, pharmaceuticals, and more. Key stakeholders often include engineers, maintenance personnel, operators, and procurement professionals who are responsible for maintaining reliable systems and reducing unplanned downtime.
• Why Are They Critical?
  Effective sealing solutions help companies:
  • Reduce leakage and associated product loss
  • Comply with environmental and safety regulations
  • Extend equipment lifespan and improve equipment and plant reliability metrics
  • Minimize downtime and costly repairs

Key Takeaway: A properly specified and well-maintained mechanical seal can improve reliability, safety, and overall operational performance.

Mechanical seals create a barrier between a rotating shaft and the housing that contains the fluid. In simplest terms, two very flat (lapped) surfaces are pressed together. One surface rotates with the shaft, while the other remains stationary in the equipment housing.

Between them, an ultra-thin fluid film lubricates the faces, keeps them cool, and prevents excessive face contact. This is the heart of the mechanical seal and the entire sealing system is designed to support the stability of the thin film.

1. Primary Seal Faces
   • Typically made from carbon, ceramic, silicon carbide, or tungsten carbide.
   • These faces are lapped to extreme flatness to provide a precise sealing interface.
   • Specialty fillers such as antimony impregnation and engineered resins, or surface treatments like diamond coatings, can be applied to seal faces to improve performance in service.
2. Secondary Sealing Elements
   • O-rings, gaskets, flexible graphite, or other elastomers ensure no fluids escape past metallic or other seal components.
   • Material choice (e.g., Viton®, EPDM, Kalrez®, TFE/P) depends on fluid compatibility and operating temperature.
3. Drive Elements
   • Set-screws, clamp-collars, or a shrink-disks are each mechanical means of locking the rotating elements of the seal to the rotating portion of the equipment, typically the shaft.
   • The choice between various drive elements is generally dependent on the pressure rating and design of the equipment being sealed.
4. Load Elements
   • Multiple springs, a single spring, wave springs, or bellows load the primary seal face with axial force to maintain contact with the mating ring.
   • Metal or other engineered components hold the seal elements in the correct alignment and tension.
5. Adaptive Hardware
   • Two chief components, the gland and sleeve, hold the stationary and rotating elements, respectively, of the seal to the equipment.
   • Usually manufactured in 316 stainless steel, the gland and sleeve can be manufactured from a variety of materials based on exposure to process chemistry.

Mechanical seals come in various designs to meet different application requirements. Below are some of the most commonly used seal types:

Cartridge Seals

• What Are They?
  Pre-assembled units containing seal faces, secondary seals, gland, sleeve, and adaptive hardware.
• Benefits:
  • Simplified installation (reduced chance of installation error)
  • Easier equipment maintenance
  • Quick replacement/retrofit
• Flexaseal Examples:
  • ANSI / General Process
  • API for Oil & Gas and High Duties
  • Slurry
  • Mixer
  • Dry Gas

Split Seals

• What Are They?
  Split seals can be both cartridge style (Flexaseal Style 85) and component style. Both styles are split down the axis of the shaft they are meant to seal around. The two halves come together around the shaft and once assembled there are loaded into a seal chamber like any other cartridge seal.
  
  Split seals are desirable in industry for their ease of replacement: no equipment disassembly required!
• Benefits:
  • Ease of replacement
  • Lower downtime
  • Great for both water service and slurry duties (when paired with an API Plan 32 flush).

Component (Multi-Part) Seals

• What Are They?
  Separate seal components (rotating and stationary seal faces, O-rings, springs) are installed individually.
• Benefits:
  • Potentially lower upfront cost
  • Flexibility in customizing materials
• Challenges:
  • More complex installation
  • Higher risk of errors if not installed properly

Single vs. Double Seals

• Single Seals
  • One pair of seal faces (one rotating, one stationary).
  • Typically used when leakage risk and fluid hazards are moderate.
  • Support systems either flush on the process side or quench on the atmospheric side of the seal.
  
• Double Seals
  • Two sets of sealing faces with a barrier or buffer fluid in between the face pairs.
  • Ideal for more hazardous or abrasive fluids where zero leakage is required.
  • Face pairs can be arranged in Tandem (Mating Ring/Primary Ring | Mating Ring/Primary Ring) or Dual/Back-to-Back (Primary Ring/Mating Ring | Mating Ring/Primary Ring)
  

Pusher vs. Non-Pusher Seals

• Pusher Seals
  • Use springs that compress axially to compensate for face wear.
  • Dynamic O-ring pushes along the axis of the shaft.
  • Can handle moderate misalignment.
• Non-Pusher (Bellows) Seals
  • Usually have a flexible metal bellows instead of a spring. Rubber bellows are also available.
  • Often chosen for high-temperature or hazardous chemical applications, as there are fewer secondary seals.

Specialty Seals (Gas Seals, High-Temp Metal Bellows, High-Pressure Seals)

• Designed for specific challenges like extreme temperatures, high pressures, corrosive environments, or dry-running conditions.
• Examples include Flexaseal Style HPPS, HPPD, HPPTL, HPCS, FGSA, 60/62/64/65 and others.

Selecting the right materials is essential to ensure mechanical seal reliability and longevity.

1. Seal Face Materials
   • Carbon: Good all-around material, lower friction, can handle many fluids. Great heat dissipation.
   • Silicon Carbide: Extremely hard and wear-resistant, often used in abrasive or high-speed applications. Certain fillers can improve lubricity and heat dissipation.
   • Tungsten Carbide: Another hard, robust material suitable for demanding environments. Not as brittle as Silicon Carbide
   • Ceramics: Cost-effective, chemical-resistant option, though typically less robust than carbides.
   • Diamond Coatings: chemical vapor deposition (CVD) diamond coatings on seal faces have been around for some time, but only recently have advances been made in their surface adhesion and long-term dry running ability. This technology continues to evolve and can be applied to monolithic seal faces currently.
2. Elastomer Selections
   • Viton® (FKM): A fluoroelastomer with good chemical resistance, widely used. Superb resistance to many oils and hydrocarbons. Generally rated to 400°F (204°C) or lower.
   • Aflas® (TFE/P): Particularly strong resistance to bases, amines, and ammonia-containing environments. Performs well in steam. Generally rated to 400°F (204°C) or lower.
   • EPDM: Commonly used for water-based and certain chemical applications.
   • Kalrez® (FFKM): Perfluoroelastomer with excellent chemical and heat resistance for aggressive media. Generally rated to 600°F (204°C) or lower. Variants known as Chemraz® and other non-proprietary compounds exist within this family.
3. Design Criteria
   • Temperature & Pressure: High pressures or temperatures often require specialized spring materials and face combinations.
   • Chemical Compatibility: Seal materials must withstand process fluids (including CIP/SIP in food and pharma).
   • Regulatory Compliance: NSF 61, FDA, USP Class VI, or other industry standards may apply.

Flexaseal’s mechanical seals are trusted across a broad spectrum of industries. Below are some key use cases:

1. Oil & Gas
   • High-pressure, sour service, and abrasive media are common challenges.
   • Solutions: Double mechanical seals, specialty alloy materials, high-duty seals.
2. Chemical & Petrochemical
   • Aggressive chemicals, volatile organics, and wide temperature ranges.
   • Solutions: Non-pusher seals or fully encased seals with corrosion-resistant metals.
3. Pulp & Paper
   • Slurry pumping with fibrous media requires durable, clog-resistant designs.
   • Solutions: Cartridge seals with silicon carbide faces for abrasion resistance.
     1. Split cartridge seals on stock.
     2. Double seals in liquor and other hazardous applications.
     3. Water and general service seals are often standard ANSI single seals.
4. Power Generation
   • Boiler feed pumps, condensate systems, and cooling water circuits demand robust seals.
   • Solutions: High-temperature capable seals with carefully chosen secondary seals.
5. Pharmaceutical & Food Processing
   • Clean/sterile operations with CIP (Cleaning in Place) or SIP (Sterilization in Place).
   • Solutions: Hygienic seal designs with FDA-approved materials.
6. Municipal Water and Wastewater Treatment
   • Split cartridge seals on vertical turbine and split-case pumps.
   • Ease of maintenance and life in service are key goals.

Pro Tip: Matching the right seal to the specific application is critical for safety, reliability, and cost-effectiveness. Always consult with a seal engineer when dealing with complex or hazardous fluids.

Proper installation and maintenance of mechanical seals can drastically improve seal life and reduce total cost of ownership.

1. Pre-Installation Checklist
   • Verify equipment dimensions, alignment, and surface finishes.
   • Check for shaft runout or misalignment.
   • Use clean lubricants and inspect for burrs or damage.
2. Installation Guidelines
   • Follow manufacturer instructions (e.g., correct torque on all bolts, follow order of assembly, remove clips last).
   • Ensure any flushing, quenching, or barrier/buffer fluid systems are set up correctly.
3. Maintenance Planning
   • Regularly monitor operating conditions (temperature, pressure, vibration).
   • Inspect seals at scheduled intervals for face wear or elastomer degradation.
   • Maintain optimal lubrication or cooling to prevent overheating.
4. Cost-of-Ownership Considerations
   • While high-performance seals may cost more upfront, improved reliability and extended life typically offset these costs with reduced downtime and fewer replacements.

Even the best mechanical seals can fail prematurely due to various factors. A structured approach to troubleshooting helps minimize downtime.

1. Common Failure Modes
   • Dry Running: Inadequate fluid film between faces leads to excessive heat.
   • Thermal Shock: Rapid temperature changes can crack brittle seal faces.
   • Chemical Attack: Elastomers swell or degrade if incompatible with fluids.
   • Abrasion/Erosion: Face wear and hangup from solids attack or particulate build-up.
2. Symptoms & Likely Causes

   Symptom	Possible Cause	Recommended Action
   Excessive Leakage	Worn seal faces, misalignment	Inspect faces, check shaft runout, equipment vibration
   Overheating or Smoking	Dry running, lack of flush	Ensure adequate cooling/lubrication
   Cracked Seal Faces	Thermal shock or cavitation	Gradually ramp up temperature, ensure proper startup procedures; ensure optimal NPSHa for both pump and seal life

3. Professional Support
   • If problems persist, consult with Flexaseal’s engineering team for a detailed failure analysis or seal repair services.

At Flexaseal, we pride ourselves on delivering customer-centric sealing solutions that address your specific application challenges. Our approach involves:

• Customized Engineering
  • Each application is evaluated for operating conditions, fluid properties, and cost constraints.
  • We tailor seal designs to match or exceed performance requirements.
  • Seals. Systems. Solutions. At Your Speed™
• Exceptional Customer Service
  • Dedicated support from inquiry through installation and beyond.
  • Fast turnaround on quotes and product delivery to minimize downtime.
  • Seals. Systems. Solutions. At Your Speed™
• Full Range of Products
  • From single cartridge seals to double mechanical seals, metal bellows designs, and specialty solutions for extreme environments.
  • We also offer seal support systems and lubrication systems to extend seal life and improve reliability.
  • Seals. Systems. Solutions. At Your Speed™
• Global Reach, Local Support
  • Our engineering and service teams serve clients worldwide across diverse industries.

• Real-World Expertise: Decades of hands-on experience designing, manufacturing, and maintaining mechanical seals for challenging industrial applications.
• Proven Performance: Field-tested solutions backed by references from satisfied customers in oil & gas, pulp & paper, chemical processing, and more.
• Innovative Approach: We constantly invest in R&D to develop new seal technologies that reduce downtime and lower total costs.
• Customer-First Philosophy: From custom designs to post-installation support, our priority is solving your most pressing sealing challenges.

Mechanical seals are critical components in any industrial operation that requires reliable containment of fluids—whether that’s oil in a refinery, slurry in a pulp mill, or high-temperature chemicals in a petrochemical plant. By understanding seal fundamentals and best practices, you can significantly reduce leakage, prevent failures, and keep your processes running smoothly.

Ready to learn more or get expert guidance?

• Contact Flexaseal for tailored recommendations or a quote.
• Request a Consultation with our engineering team.
• Explore our Product Catalog to see our full range of mechanical seals and support systems.

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