Types of Lubricants for Mechanical Seals

Improper lubrication is a leading cause of mechanical seal failure in industrial machinery. This can result in costly downtime, repairs, and reduced equipment lifespan.

This guide explores the types of lubricants best suited for various mechanical seal applications, from high-temperature and high-pressure environments to water, oil, and chemical settings. Learn how to optimize seal performance and minimize failures through proper lubrication selection.

Process Fluid Lubrication

In many mechanical sealing applications, the sealed process fluid itself acts as the lubricant for the seal faces. This approach, known as process fluid lubrication, is commonly used when the process fluid has sufficient lubricating properties and is compatible with the seal materials.

The effectiveness of process fluid lubrication depends on factors such as the viscosity, chemical composition, and operating conditions of the fluid. Low viscosity fluids like water or light hydrocarbons may not provide adequate lubrication, leading to increased friction and wear. Fluids with solid particles or abrasive contaminants can also damage the seal faces over time.

Barrier Fluids

Barrier fluids are used in dual mechanical seals to provide lubrication and cooling while isolating the process fluid from the atmosphere. The barrier fluid is pressurized to a level higher than the process fluid, creating a positive flow into the seal chamber that prevents process fluid leakage.

Flush Fluids

Flush fluids are used to cool, clean, and lubricate mechanical seals in challenging applications where the process fluid is not suitable for direct seal lubrication. Flush fluids are introduced into the seal chamber from an external source, typically at a lower pressure than the process fluid.

Grease

Grease is a semi-solid lubricant consisting of a base oil thickened with a soap or non-soap thickener. It is used in mechanical seals for applications where a continuous supply of liquid lubricant is not practical or desirable.

Lubricating Fluids or Gases

Lubricating fluids or gases are introduced into the seal chamber from an external source to provide enhanced lubrication, cooling, and protection for mechanical seals in challenging applications. These lubricants are used when the process fluid is not suitable for direct seal lubrication or when additional lubrication is required to extend seal life and improve performance.

Lubricants for Specific Applications

High-Temperature Applications

In high-temperature environments, mechanical seals require lubricants that maintain their stability and performance at elevated temperatures. Several lubricant types are well-suited for these applications:

Polyalphaolefins (PAOs): PAOs are synthetic base oils that offer excellent thermal stability, oxidation resistance, and low volatility. They are commonly used in high-temperature applications up to 150°C (302°F).
Diesters and Polyolesters: These synthetic lubricants provide good thermal stability and lubricity, making them suitable for temperatures up to 200°C (392°F). They also exhibit good compatibility with elastomers and metals.
Phosphate Esters: With exceptional thermal stability and fire resistance, phosphate esters are ideal for extreme high-temperature applications exceeding 200°C (392°F). However, they may have compatibility issues with certain seal materials.
Polyalkylene Glycols (PAGs): PAGs offer excellent thermal stability and low volatility, making them suitable for high-temperature applications. They also provide good lubricity and heat transfer properties.
Silicones and Perfluoropolyethers (PFPEs): Silicone-based lubricants and PFPEs are known for their exceptional thermal stability, with some formulations capable of withstanding temperatures up to 300°C (572°F). They also exhibit low volatility and good chemical inertness.
High-temperature greases: Specialized greases formulated with synthetic base oils and thermally stable thickeners can be used for high-temperature applications. These greases provide lubrication and protect against wear and corrosion at elevated temperatures.

High-Pressure Applications

Mechanical seals operating under high-pressure conditions require lubricants that can withstand extreme loads and maintain their film strength. Two primary lubricant options for high-pressure applications are:

Barrier fluids: In dual-seal arrangements, barrier fluids are used to lubricate and cool the seals. These fluids are typically oil-based or synthetic and are selected based on their viscosity, thermal stability, and compatibility with the process fluid. Barrier fluids help to equalize pressure across the seal faces and prevent process fluid from entering the seal chamber.
Synthetic oils: High-performance synthetic oils, such as PAOs, esters, and PAGs, offer excellent pressure-viscosity coefficients, which enable them to maintain adequate film thickness under high-pressure conditions. These oils also provide good thermal stability and oxidation resistance.

Water Applications

When mechanical seals are exposed to water or aqueous solutions, the lubricants must be compatible with these environments:

Water-based lubricants: For applications involving clean water or water-based fluids, water-soluble lubricants can be used. These lubricants typically contain corrosion inhibitors and additives to improve lubricity and protect against wear.
Temporary lubricants: In some cases, temporary lubricants, such as molybdenum disulfide or graphite, can be applied to the seal faces before installation. These lubricants provide initial lubrication during the run-in period and help prevent damage to the seal faces.

Oil Applications

For mechanical seals operating in oil environments, lubricants must be compatible with the process fluid and provide adequate lubrication:

Oil-based lubricants: Mineral oils or synthetic oils compatible with the process fluid can be used to lubricate mechanical seals. These lubricants should have appropriate viscosity, good thermal stability, and oxidation resistance.
Synthetic oils: High-performance synthetic oils, such as PAOs, esters, and PAGs, offer superior lubrication properties compared to mineral oils. They provide excellent thermal stability, oxidation resistance, and compatibility with seal materials.

Chemical Applications

In chemical processing applications, mechanical seals are exposed to aggressive media, requiring lubricants that can withstand these harsh conditions:

Fluorinated lubricants: Perfluoropolyethers (PFPEs) and other fluorinated lubricants offer excellent chemical inertness and thermal stability. They are resistant to a wide range of chemicals, including acids, bases, and solvents.
PTFE-based lubricants: Lubricants containing polytetrafluoroethylene (PTFE) particles can provide lubrication and reduce friction in chemical environments. PTFE is chemically inert and resistant to most chemicals, making it suitable for use in aggressive media.