One of the most important factors affecting a valve's shut-off ability is the nature of media being handled. The service life of valves is affected by all of the following factors:
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Industrial Valve Seals and Seats materials are generally manufactured using the following materials:
BUNA-N (HYCAR orNitrile)
Buna-N is a general-purpose polymer which has good resistance to oil, water, solvents and hydraulic 'uids. It also displays good compression, abrasion resistance, and tensile strength. This material performs extremely well in process areas where paraf'n base materials, fatty acids, oils, alcohols or glycerin are present, since it is totally unaffected.
Buna-N should not be used around high polar solvents (acetones, ketones), chlorinated hydrocarbons, ozone or nitro hydrocarbons. Temperature range is 107°C maximum. Hycar is black in color and should not be used where discoloration cannot be tolerated. It is regarded as a comparable replacement neoprene. Major differences are: Buna-N has a higher temperature limit; neoprene is more resistant to oils.
CARBON FILLED TFE
Carbon filled TFE is an excellent seat material for steam applications as well as high effeciency oil-based thermal 'uids. Fillers including graphite enable this seat material to have a better cycle life than other 'lled or reinforced TFE seats. Temperature range is -29°C to 260°C. Chemical resistance is equal to other TFE seats.
DELRIN ®
Delrin is DuPont's trademark for Polyoxy-methylene. It may also be known as Hostaform, Celecon, Kepital, Duracon, Lupital or Ultraform. Delrin is very rigid material and does not undergo cold flow. It has an excellent combination of strength, hardness, stiffness, stability, abrasion resistance and low friction. Delrin has high heat resistance and low water absorption, but is susceptible to acids. When used as a seal material, Delrin is acceptable for pressures up to 5,000 PSI. Its operational temperature envelope is -70 ºF to 180 ºF.
EPDM
EPDM is a terpolymer elastomer made from ethylene-propylene diene monomer. EPDM has good abrasion and tear resistance and offers excellent chemical resistance to a variety of acids and alkaline. It is susceptible to attacks by oils and is not recommended for applications involving petroleum oils, strong acids, or strong alkaline.
This Material should not be used on compressed air lines. It has exceptionally good weather aging and ozone resistance. Temperature rating is from -29°C to 107°C (121°C intermittent service). It is fairly good for ketones and alcohols.
HYPALON®
Hypalon has very good resistance to oxidation, ozone and good 'ame resistance. It is similar to neoprene except with improved acid resistance where it will resist such oxidizing acids as nitric, hydro'ouoric and sulfuric acid. Abrasion resistance of Hypalon is excellent, about the equivalent of the nitriles. Oil and solvent resistance are somewhat between that of neoprene and nitriles. Salts have little if any effect on Hypalon.
Hypalon is not recommended for exposure to concentrated oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons. Not to be used in steam service. Temperature rating is from -18°C to 104°C. Seats are white food grade and may be used on food
NEOPRENE
Neoprene is an all-purpose polymer with many desirable characteristics. Neoprene features high resiliency with low compression set 'ame resistance and animal and vegetable oil resistance. It is principally recommended for use in pulp and paper applications.
Generally, neoprene is not recommended and is attacked by strong oxidizing acids, most chlorinated solvents, esters, ketones, aromatic hydrocarbons and hydraulic 'uids. Neoprene is not generally affected by moderate chemicals, fat, greases and many oils and solvents. Temperature rating is from -29°C to 82°C. Seats are black in color and should not be used on bleached paper lines.
PEEK (Polyetheretherketone)
PEEK is a high performance non-porous semi-rigid thermoplastic with excellent resistance to harsh chemicals. Virgin PEEK is naturally abrasion resistant. It has excellent mechanical strength, dimensional stability and is suitable for use in high temperature / high corrosion environments. PEEK is considered a premium valve seat material due to the fact it is unaffected by continuous exposure to hot water and steam. Its operational temperature envelope is -70 ºF to 600 ºF.
PTFE (TFE of Te'on®)
PTFE is the most chemically resistant of all plastics. It also has excellent thermal and electrical insulation properties. PTFE's mechanical properties are low compared to other engineering plastics, but its properties remain at useful levels over a great temperature range -40°C to 204°C, depending on brand and application.
RTFE (Reinforced TFE)
RTFE is compounded with a selected percentage of 'ber glass 'ller to improve strength and resistance to abrasive wear, cold ' ow, and permeation in molded seats. Reinforcement permits application at higher pressure and temperature than un'lled TFE. Typical temperature range is -40°C to 232°C. RTFE should not be used in applications that attack glass, such as hydro'uoric acid and hot strong caustics.
TFM
TFM appears on our Valve Seals and Seats list because it is a modified version of PTFE that maintains the exceptional chemical and heat resistance properties of PTFE, but has a significantly lower melt viscosity. The result is reduced cold 'ow, porosity, permeability and void content. Surfaces are smoother and reduce torques. The theoretical service range for TFM is -200°C to 260°C.
TFM+20%GF
TFM+20%GF is a 'ber glass reinforced version of TFM. Similar to RTFE, but with the bene't of TFM, the glass 'lled version provides greater abrasion resistance and improves stability at higher pressures.
TFM
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TFM is an elector graphitized carbon 'lled TFM material. The added carbon improves stability for higher pressure and temperature combinations.
UHMWPE (Ultra-High Molecular Weight Polyethylene)
UHMWPE is a durable, long-wearing, abrasion / corrosion resistant material. It is also known as High Modulus Polyethylene (HMPE) or High Performance Polyethylene (HPPE). Also, It meets FDA and USDA acceptance for food and pharmaceutical equipment and is a good performer in applications up to 180°F when periodically cleaned with live steam or boiling water to sterilize.
UHMWPE is ideal for low-radiation service and meets the requirements for the tobacco industry where TFE is prohibited. It is highly resistant to corrosive chemicals, with the exception of oxidizing acids and organic solvents. Its operational temperature envelope is -70 ºF to 180 ºF.
VITON® (Fluorocarbon, FKM, or FPM)
Fluorocarbon elastomers are inherently compatible with a broad spectrum of chemicals. Because of this extensive chemical compatibility which spans considerable concentration and temperature ranges, 'uorocarbon elastomers have gained wide acceptance as a material of construction for knife gate valve seats.
Fluorocarbon can be used in most applications involving mineral acids, salt solutions, chlorinated hydrocarbons and petroleum oils. It is particularly good in hydrocarbon service. Temperature rating is from -29°C to 149°C. Color is gray, black or red and may be used on bleached paper lines.
VITON is not suitable for steam or hot water service, however, in O-ring form, it can be acceptable for hydrocarbon lines mixed with hot water depending on the type/brand. For seat materials FKM can offer more resistance to hot water.
Valve Seals and Seats
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
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.
Conclusion
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.
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