What Kind of Lubricant Is Used for Mechanical Seals

Mechanical seals use lubricants like synthetic oils, grease, dry lubricants, and barrier or buffer fluids to minimize friction and wear. The selection of the right lubricant depends on factors such as temperature, pressure, speed, fluid compatibility, and viscosity.

When the mechanical seal is installed correctly and the pump is running, the rotating seal face creates a centrifugal force that draws a thin film of fluid between the seal faces. This fluid film lubricates the contact surfaces, minimizing friction and wear. The fluid also helps to dissipate heat generated by the rotating surfaces, preventing overheating and thermal damage to the seal components.

Types of Lubricants for Mechanical Seals

Synthetic Oils

Synthetic oils are widely used lubricants in mechanical seals due to their excellent thermal and oxidative stability, low volatility, and compatibility with various elastomers and seal face materials. They maintain their viscosity over a wide temperature range, ensuring proper lubrication even in challenging environments. Common synthetic oils include polyalphaolefins (PAOs), polyalkylene glycols (PAGs), and perfluoropolyethers (PFPEs).

Synthetic oils are preferred for high-temperature applications, such as in boiler feed pumps or reactors, where mineral oils may break down or form deposits on the seal faces. They also excel in applications involving aggressive chemicals or corrosive fluids that can degrade conventional lubricants.

Grease

Grease is a semi-solid lubricant consisting of a base oil, thickener, and additives. It provides excellent sealing and lubricating properties in mechanical seals, particularly in applications with slow speeds, high loads, or intermittent operation. The thickener forms a protective film on the seal faces, preventing direct contact and reducing wear.

Greases are classified by their base oil type (mineral, synthetic, or vegetable) and thickener (soap or non-soap). Lithium-based greases are common in general-purpose applications, while silicone, fluorinated, or polyurea greases are used in high-temperature or chemically aggressive environments.

Dry Lubricants

Dry lubricants, also known as solid lubricants, are used in mechanical seals where conventional liquid or grease lubricants are not suitable. They are applied as thin coatings on the seal faces and provide lubrication through their low shear strength and high load-carrying capacity. Common dry lubricants include graphite, molybdenum disulfide (MoS2), polytetrafluoroethylene (PTFE), and hexagonal boron nitride (hBN).

Dry lubricants are ideal for high-temperature, vacuum, or radiation environments where liquid lubricants would evaporate, decompose, or outgas. They also excel in applications involving abrasive particles or contaminants that could clog or damage conventional lubricated seals.

Barrier and Buffer Fluids

In some mechanical seal designs, barrier or buffer fluids are used to lubricate and cool the seal faces while isolating them from the process fluid. Barrier fluids are used in double or tandem seals, where they fill the space between the primary and secondary seals, providing a clean and compatible environment for the seal faces. Buffer fluids are used in dual seals, where they are maintained at a higher pressure than the process fluid to prevent leakage.

Barrier and buffer fluids are selected based on their compatibility with the process fluid, seal face materials, and elastomers. They can be water, synthetic oils, or specially formulated fluids with additives to enhance lubrication, prevent corrosion, or resist degradation. In the pharmaceutical or food industries, food-grade lubricants or FDA-approved fluids are used to ensure product purity and safety.

Factors Influencing Lubricant Selection

Temperature

The operating temperature range significantly influences lubricant selection for mechanical seals. High temperatures can cause lubricants to break down, losing their viscosity and protective properties. This degradation can lead to increased wear on seal faces, reducing seal life and potentially causing leaks. Conversely, low temperatures may cause lubricants to thicken, impairing their ability to flow and properly lubricate the seal faces. Selecting a lubricant with a wide temperature range and good thermal stability ensures optimal performance across varying conditions.

Synthetic oils, such as polyalphaolefins (PAOs) and perfluoropolyethers (PFPEs), offer excellent high-temperature stability compared to mineral oils. These synthetic lubricants maintain their viscosity and lubricating properties even at elevated temperatures.

Pressure

The operating pressure of the system also plays a vital role in lubricant selection. High pressures can cause lubricants to thin out, reducing their load-carrying capacity and leading to increased wear on seal faces. In extreme cases, high pressures may even cause lubricants to vaporize, leading to a complete loss of lubrication. Selecting a lubricant with a high viscosity index and good pressure-viscosity coefficient ensures that it maintains its lubricating properties under high-pressure conditions.

For high-pressure applications, lubricants with additives such as extreme pressure (EP) agents and anti-wear additives can provide additional protection. These additives form protective films on the seal faces, reducing wear and preventing surface damage under extreme loads.

Speed

The rotational speed of the shaft also influences lubricant selection. High-speed applications generate more heat due to increased friction, which can lead to lubricant breakdown and reduced viscosity. This can result in inadequate lubrication, increased wear, and premature seal failure.

Low-viscosity lubricants, such as light oils or low-consistency greases, are often preferred for high-speed applications. These lubricants have lower internal friction and can flow more easily, reducing heat generation and providing adequate lubrication to the seal faces. Synthetic oils, such as polyglycols and esters, offer excellent shear stability and are well-suited for high-speed applications.

Fluid Compatibility

Compatibility between the lubricant and the process fluid is another critical factor in lubricant selection. The lubricant must be able to withstand exposure to the process fluid without breaking down or losing its lubricating properties. Incompatible lubricants may degrade, forming deposits on the seal faces or causing swelling of elastomeric components, leading to leaks and premature seal failure.

When selecting a lubricant, consider the chemical composition of the process fluid and its reactivity with various lubricant types. For example, in applications involving harsh chemicals or solvents, fluorinated oils or perfluoropolyether (PFPE) greases may be required due to their excellent chemical resistance. In food and pharmaceutical industries, lubricants must be food-grade and comply with relevant regulations, such as NSF H1 or FDA guidelines.

Viscosity

Viscosity is a critical property that determines a lubricant’s ability to form a stable film between the seal faces and protect against wear. The optimal viscosity depends on factors such as the operating temperature, pressure, speed, and seal design. A lubricant with too low viscosity may not provide adequate film thickness, leading to increased friction and wear. Conversely, a lubricant with too high viscosity may create excessive drag and heat generation, impairing seal performance.

The viscosity should be high enough to maintain a stable lubricating film under operating conditions but low enough to allow for efficient heat dissipation and minimize friction losses. Viscosity index improvers can be added to lubricants to maintain their viscosity over a wide temperature range, ensuring optimal performance in variable conditions.