SOLVING BEARING AND SEAL CHALLENGES

PTFE POLYMERS CAN BE USED IN DEMANDING APPLICATIONS TO ENHANCE TURBOMACHINERY PERFORMANCE

Choosing the correct seal or bearing material for a rotary or reciprocating system is critical to the smooth operation and longevity of equipment. Rotating and reciprocating seal and bearing integrity failures can be caused by aggressive chemical exposure, inadequate lubrication, or thermal overload of the component material.

The resulting leakage leads to contamination of the systems from dirt, metal wear debris and corrosive media and scoring of rod and cylinder bore surfaces. Applications designers employ a number of seal material selections that best fit the need to contain harsh media and prevent contamination. An understanding of the application and attention to the proper material selection can improve reliability and performance, enabling reduced operating costs and a safer work environment.

Demanding applications

A material often used in demanding sealing and bearing conditions is a polymer known as polytetrafluoroethylene (PTFE) which exhibits a wide operating temperature range, a low coefficient of friction and good chemical resistance properties. PTFE is often used to enhance equipment performance at temperature and environmental extremes.

First introduced by the DuPont Company as Teflon, PTFE was originally used as a chemically resistant component of nuclear valves and seals (Figure). Readily machined, filled PTFE components are preferred in areas where popular elastomeric (i.e., rubber) components alone cannot meet the frictional, temperature or chemical resistance requirements of the application. Typical turbomachinery components in which specific PTFEbased solutions are employed include:

• Rotary lip seals — to ensure sealing performance under high pressures, temperature extremes and high surface velocities

• Rotary shaft seals — to seal a shaft in a rotating bore

• Oil seals –– to ensure retention of bearing lubricant as well as prevent ingress of dust, dirt, oil, or water

• Bearings — to reduce friction, most often on a rotating shaft which rests on the bearing

Fluid compatibility is one of the features of PTFE. It is resistant to almost all chemical media and has the ability to shrug off corrosive environments. The resulting seal components, therefore, can withstand regular immersion in water, oil-based or synthetic fluids, acids, bases, solvents, chemical cleaners or fuels, while retaining their mechanical properties and remaining dimensionally stable.

As nothing sticks on the surface, PTFE-based seals offer the lowest dynamic or static coefficient of friction (≤ 0.1) of any plastic, rubber or metal. Its self-lubricity lends itself to applications in which external lubricants, such as oils and greases cannot be used.

Temperature extremes are another reason PTFEs are deployed. Heat degradation and extreme cold exposure, of course, can lead to embrittlement and a loss of mechanical integrity. PTFE can operate from -418°F to 480°F. Within these extremes, PTFE-machined components remain flexible and can also withstand stretch and pressure. Further, these components can tolerate years of sunlight exposure.

In addition, the properties of PTFE can be modified by the incorporation of certain fillers to create what is referred to as PTFE compound. Typical additives include chopped fiberglass, carbon powder, nickel, bronze and polyphenylene sulfide (PPS) for dimensional stability, resistance to cold flow and improved wear resistance. Through judicious use of such fillers, surface hardness, mechanical strength and wear resistance is increased and dimensional distortion is reduced. For example, a bronze-filled PTFE can be used at pressures up to 6,000 psi. Either in its natural or filled form, PTFE is used in seal designs either by itself or in combination with other popular sealing materials such as elastomers.

To minimize down time from sealing components, material factors such as physical strength, thermal stability, wear, and corrosion resistance should be considered. Unfortunately, seal failures caused by lack of understanding of the application and improper material selection are commonplace.A material such as PTFE, that provides for operation in thermal, chemical and mechanical extremes, is a popular choice for critical rotating and reciprocating seal applications.

Author

Al Wiggins is a Business Development Specialist and John Kelly is Engineering Manager at Technetics Group, a source for engineered components, seals, assemblies, and subsystems that are custom-designed for high performance and extreme applications in the semiconductor, aerospace, power generation, oil and gas,medical and other industries. TechneticsGroup is not a manufacturer of PTFE but incorporates PTFE manufactured by various vendors into its products. For more information visit www.techneticsgroup.com