Understanding Secondary Sealing Elements in Mechanical Seals

Mechanical seals are critical components in rotating equipment such as pumps, compressors, mixers, and agitators. Their primary purpose is to prevent fluid leakage from process systems into the environment. While much attention is often directed toward the primary seal faces, the secondary sealing elements play an equally important role in ensuring seal reliability and overall system performance.

The Role of Secondary Sealing Elements

Secondary sealing elements are designed to prevent leakage around the seal faces and other adjacent components. These elements perform all static sealing functions within a mechanical seal and accommodate the dynamic axial motion of the seal faces. They are located at various interfaces within the assembly, such as between the seal sleeve and shaft, between the gland and mounting flange, and between the rotary and stationary components. Their performance directly influences seal integrity, longevity, and operational safety.

Proper selection of secondary sealing elements requires detailed information about the operating environment. Factors such as temperature, pressure, chemical compatibility, and equipment design determine the most suitable type and material. There is no universal solution; each application must be evaluated individually to ensure optimal performance.

Types of Secondary Sealing Elements

There are several types of secondary sealing elements, each with distinct characteristics and applications.

O-rings are the most common type due to their simplicity and versatility. They are available in a range of elastomeric materials with varying hardness levels to suit different temperature and pressure requirements.

Elastomeric or thermoplastic bellows are used where sliding seals are less effective. These flexible components allow movement of the seal faces without relative sliding, making them suitable for dynamic applications.

Wedges, typically made from polytetrafluoroethylene (PTFE) or carbon/graphite, are used in applications where temperature or chemical conditions exceed the limits of standard elastomers. They require external energization but can provide a cost-effective solution for demanding conditions.

Metal bellows replace elastomeric elements in high-temperature, vacuum, or sanitary applications. These components, formed or welded from thin metal plates, provide both sealing and spring action for axial movement.

Flat gaskets are used for static sealing, commonly between the seal gland and mounting flange. They must be replaced during seal refurbishment to maintain performance.

U-cups and V-rings are used in low-temperature or high-pressure environments and are typically made from elastomeric or thermoplastic materials.

Material Selection for Secondary Seals

Material selection is critical to achieving reliable sealing performance. Common materials include:

Elastomers such as nitrile, neoprene, EPDM, fluoroelastomers (FKM), and perfluoroelastomers (FFKM), chosen for flexibility and chemical resistance.

Thermoplastics such as PTFE and polyether ether ketone (PEEK), which provide high-temperature resistance and structural strength.

Metal alloys like stainless steel and nickel-chromium-molybdenum alloys, used for metal bellows in aggressive environments.

Flat gasket materials made from synthetic fibers or carbon/graphite composites.

Carbon/graphite materials used in static high temperature sealing applications.

Conclusion

Secondary sealing elements are vital to the effectiveness of mechanical seals. Understanding their function, type, and material compatibility ensures proper seal performance, minimizes leakage, and extends equipment life. A thoughtful approach to secondary seal selection supports operational efficiency and reliability across diverse industrial applications.

For those interested in reading the full Sealing Sense Article “Understanding Secondary Sealing Elements in Mechanical Seals” click here.