Mechanical Seals
Mechanical seals are essential components in various types of pumps, making them a top priority in maintenance. They are designed to prevent fluid from leaking out of the pump while keeping contaminants from entering. Typically, mechanical seals are located in the pump’s stuffing box or seal chamber, which is the area where the pump shaft connects to the drive, usually an electric motor.
- Lower Maintenance
- Minimised Downtime
- Increased Pump Efficiency
Typical Mechanical Seals
- Stationary face: The stationary face, or seat, is fixed to the pump housing. It often incorporates secondary seals, such as O-rings, bellows, or gaskets, to prevent leakage along the shaft or past the gland
- Rotary face: This is attached to the shaft but is free to rotate. It may also include an O-ring to block potential leakage paths.
A primary seal is formed between these two faces. Since they don’t seal effectively when dry, lubrication is required to create a hydrodynamic liquid film that ensures a proper seal.
Spring or set of springs: Designed to keep the two seal faces pressed together. They also automatically adjust the compression as the seal faces gradually wear down.
Double Seal
Two mechanical seals are used with a liquid barrier between them to prevent the working liquid from coming into contact with the environment. A double mechanical seal is ideal for handling toxic, flammable, or challenging liquids.
Balanced & Unbalanced Seals
Balanced seals distribute pressure evenly, which helps reduce wear in high-speed applications. In contrast, unbalanced seals are more cost-effective and are better suited for lower-speed applications.
Pusher & Non-Pusher Seals
Pusher seals utilise a spring mechanism for high-pressure applications, whereas non-pusher seals are designed for low-pressure conditions.
Elastomer Bellows Seals
These seals are both flexible and resistant to a variety of chemicals, making them perfect for dynamic applications.
Metal Bellows Seals
They are favored in high-pressure and corrosive environments due to their exceptional durability.
Split Seals
They enable component replacement without the need to disassemble the equipment, simplifying maintenance.
Gas Seals
These seals are designed to contain gas and are categorised into dynamic and static types based on the speed of the application.
Single Seals
The single seal features two sealing surfaces and is commonly used in high-pressure and high-temperature conditions.
Cartridge seal
All components come preassembled, which makes installing the seal much easier.
Important Notes
Mechanical seals feature a metal body and an elastomer bellows, making them suitable for demanding applications like high-pressure environments and chemically corrosive media.
All mechanical seal components should be made from high-quality materials to ensure reliable, long-lasting performance. The spring and body should be made of SS 304 or 316 grade stainless steel, while the seal faces can be constructed from carbon graphite, silicon carbide, or ceramic, with tungsten carbide also available as an option.
Seal faces are lapped and polished to an exceptionally smooth finish, allowing them to rotate against each other with minimal friction.
ELASTOMERS FOR MECHANICAL SEALS
Mechanical seals depend significantly on elastomers, often referred to as rubber materials. These elastomers act as secondary sealants, ensuring proper compression between the primary sealing surfaces and compensating for any misalignment or wear in the system. Their ability to fill gaps and flex under pressure while maintaining elasticity makes them crucial for achieving effective sealing.
Types of Elastomers for Mechanical Seals
Selecting the right elastomer material is essential, as it impacts the seal’s performance, durability, and compatibility with different fluids and environmental conditions. Elastomers vary in chemical resistance, temperature tolerance, and mechanical strength, which affects their suitability for various applications.
NITRILE (N)
NBR is commonly used in mechanical seals due to its excellent balance of qualities –oil and abrasion resistance and low-temperature flexibility.
- Abbreviation: NBR/Buna-N
- Chemical name: acrylonitrile butadiene
Applications:
- Hydraulic fluids, fats, animal and vegetable oils, flame retardant liquids, grease, water and air.
- Not recommended for applications exposed to ozone, sunlight or external weather conditions; ketones, esters (including hydraulic oils); and chlorinated hydrocarbons
VITON™ (V)
Viton or FKM—a fluorocarbon elastomer that is a copolymer of vinylidene and hexafluoropropylene—is a high-performance elastomer with excellent chemical resistance. It is perfect for aggressive applications involving fuels, lubricants, and other chemicals. It can withstand temperatures ranging from -20°C to 200°C, making it appropriate for various applications.
- Abbreviation: FKM
- Chemical name: fluorocarbon elastomer
Applications:
- Hydrocarbon services, especially at higher temperatures; lubricants, halogenated hydrocarbons, silicone-based fluids, water, and acids; and are resistant to ozone and sunlight degradation.
- Not recommended for applications involving ketones, amines, low molecular weight ethers, esters, hot water, steam, and temperatures below -26°C.
EP (EPDM)
EPDM is a type of synthetic elastomer produced through the polymerisation of ethylene, propylene, and diene. This material offers several appealing properties, including high elasticity of up to 400%, a broad temperature range, and excellent resistance to many bases, acids (including fatty acids), salts, ozone, and UV rays. Additionally, it boasts a long service life of at least 50 years.
- Abbreviation: EPDM
- Chemical name: ethylene propylene diene monomer
Applications:
- Gasoline, petroleum oil and grease, and hydrocarbon environments; heat, water, steam, alcohols, ketones, engine coolants, alkali, mild organic and inorganic acids and bases, oxygenated solvents, ozone and UV sunlight.
- Not recommended for gasoline, petroleum oil, grease, and hydrocarbon environments.
TEFLON (PTFE)
PTFE, commonly known by its trade name Teflon, is a high-performance plastic and the most widely used fluoropolymer. It boasts a remarkable range of operating temperatures, excellent resistance to highly aggressive chemicals, low friction, and non-stick properties, making it an effective sealing material. However, Teflon’s primary drawback is its relatively low wear resistance—it can be easily scratched and damaged.
- Abbreviation: PTFE
- Chemical name: polytetrafluoroethylene
Applications:
- Non-filled PTFE (virgin PTFE) is used where mechanical properties are not the most important requirement. It is ideal for highly aggressive applications where elastomeric compounds are not suitable. It is also used in batch processes where the changing chemical or process conditions make it impossible to use a single elastomer grade.
- Filled PTFE (encapsulated PTFE) are o-rings with an inner elastomer core, commonly Viton or EPDM and an outside coating of PTFE.
KALREZ® (K)
FFKM provides exceptional chemical resistance and can withstand extremely high temperatures and harsh chemicals. It is considered the top choice for highly demanding applications. However, because of its use in challenging conditions, extra care is needed when handling FFKM elastomers.
- Abbreviation: FFKM
- Chemical name: perfluoro elastomer
Applications:
- FFKM resists thousands of chemicals, including concentrated nitric acid, sodium hydroxide and ethylene diamine.
- Demanding applications such as semiconductor production, chemical processing, and aerospace.
Aflas® (A)
AFLAS fluoroelastomer materials are ideal for parts and components that need to perform reliably in the world’s harshest environments. They are used to fabricate o-rings, gaskets, seals, and packings.
- Abbreviation: TFE/P
- Chemical name: tetrafluoroethylene-and propylene
Applications:
- TFE has excellent oil resistance, making them a suitable choice for oil and gas applications.
- TFE resists strong bases and acids and withstands high continuous service temperatures, making them ideal for various chemical and food processing applications.