There are two main categories of polymers used for ball valve seats: PTFE (both virgin and filled and PEEK. These are popular materials for several reasons, but each one has applications to which they are better adapted.
Ball valves are used to control the flow of water, oil, steam, air, slurries, and corrosive fluids. They can be found in HVAC systems, petrochemical processing, food processing, water distribution systems, automatic combustion systems, and instrumentation control.
While there are various parts within a ball valve (e.g., stem, stem nut, ball, body), one of the most crucial parts is the ball valve seat. Ball valve seats have two main jobs: distribute seating stress uniformly and achieve a solid seal — and to accomplish this they must be made from the right material.
Key Properties of Ball Valve Seat Materials
There are six critical properties that any material for ball valve seats must possess:
- Low friction to reduce stem torque
- Excellent wear resistance
- Good stress recovery
- Sufficient elasticity to maintain a solid seal
- Dimensional stability
- Chemical compatibility with the media involved
If the ball valves are used in connection with food, dairy, or pharmaceutical applications, they may also require materials that are FDA approved. And while there are a variety of polymer materials that can be used, PTFE, filled-PTFE, and PEEK are the most commonly used.
Material Properties for Harsh Environments
There are other considerations involved as well, with many depending on the operating environment. These can include dimensional stability and reliable performance at extreme temperatures (which may include cryogenic) as well as the ability to withstand sterilization routines that involve hot water, steam, and/or caustic cleaning materials. And if there is extended exposure to water or humidity, a ball valve seat must have a low coefficient of hygroscopic expansion. In addition, there may be a need for materials to be flame retardant, fire-resistant, or suitable for use in environments with radiation.
PTFE Ball Valve Seats
PTFE (trade name Teflon) has long been a popular choice for ball valve seat materials and is available in FDA-approved grades. Virgin PTFE has the lowest coefficient of friction of any thermoplastic in existence (even geckos cannot maintain their grip on Teflon) and filled-PTFE grades are available with very low friction as well. This polymer is also capable of dry running (and thus requires no lubricants) and exhibits no stick-slip behavior.
PTFE provides excellent wear resistance and good stress recovery, which can be enhanced by the right choice of additives. It possesses enough ductility to provide a good seal, even in the presence of extreme temperatures and highly corrosive materials. It has both a low coefficient of thermal expansion and a good coefficient of hygroscopic expansions, making it stable dimensionally. It is also fire resistant, hydrophobic, and non-wetting
The chemical compatibility and high-temperature performance of PTFE mean it works well with applications that involve sanitation and sterilization. In addition, its chemical compatibility works well with a wide range of media with the exception of fluorine and liquid alkalis. Its operating temperature range includes cryogenic temperatures between -429°F and 400°F. Note, however, that the temperature performance of virgin PTFE is highly dependent on the operating pressure.
Among the drawbacks of PTFE is its susceptibility to cold creep, is best for temperatures no greater than 5 ksi, and has limited performance in the presence of radiation due to a maximum lifetime radiation dose of 1×104 rads. It is also subject to issues with decompression after it has been highly pressurized and should not be exposed to temperature fluctuations greater than 167°F.
Filled-PTFE Ball Valve Seats
In terms of filled-PTFE, the most well-adapted combinations are carbon graphite and glass filled. Carbon graphite reinforced PTFE is chosen over virgin PTFE when high temperatures and pressures are part of the normal operating environment. It offers better wear characteristics and is less likely to cold creep than virgin PTFE while maintaining a fairly low coefficient of friction.
Glass-filled PTFE also retains many of the positive aspects of virgin PTFE while offering better extrusion resistance and better wear characteristics. It is often the ball valve seat material of choice for the food, dairy, and pharmaceutical industry and still maintains a fairly low coefficient of friction. Because of the glass fibers, however, it is more likely to be abrasive and is also incompatible with chemicals known to attack glass, such as hydrofluoric acid and strong caustics. Note that glass-filled PTFE is fire-resistant.
Another option is stainless steel-reinforced PTFE composed of 50% PTFE and 50% powdered 316 SS. This particular grade of filled PTFE offers a slightly larger operating temperature range than virgin PTFE (up to 550°F) and can handle much higher pressures. Its primary drawback is that the coefficient of friction is significantly higher, which can lead to faster wear and a high stem torque. It is, however, fire-resistant.
PEEK Ball Valve Seats
PEEK has much in common with PTFE, including an extensive temperature range from -70°F to 600°F, and good chemical compatibility. Its coefficient of friction is not as low as PTFE but it does perform extremely well in applications that involve high temperature and pressure. It is also more abrasion-resistant and tougher than PTFE.
Unlike PTFE, it does offer excellent performance when exposed to radiation, making it well-adapted to nuclear applications, and works better in extremely high temperatures that PTFE cannot handle, making it an excellent choice for the oil and gas industry. It is also flame retardant.
PEEK does not have quite the range of chemical compatibility as PTFE, and should never be exposed to sulfuric acid or used in corrosive environments. However, it does perform well in situations that involve continuous exposure to hot water and steam as well as ultra-high vacuum pressures. In addition to being significantly harder than PTFE, PEEK does exhibit brittle behavior at lower temperatures.
PTFE, filled PTFE, and PEEK are excellent high-performance polymers for ball valve seats in a wide range of applications. From check valves for pharmaceutical processing to needle valves for automatic combustion systems, Advanced EMC can help you find the right polymer and fillers to achieve the results you need.