by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PCTFE Ball Valve Seats for Low Permeation Applications

Ball valve seats that show signs of swelling, blistering, or “popcorning” have been permeated at a molecular level. Needless to say, this can cause some serious issues such as leaks and catastrophic failure. The solution is to find a ball valve seat material that is highly resistant to permeation and an excellent choice would be PCTFE. In this week’s blog post, we will talk about PCTFE Ball Valve Seats and how they are used in Low Permeation Applications.

Introduction

Certain types of media may permeate the ball valve seat, leading to swelling, blistering, and leakage. Applications such as chemical processing and petrochemical transport may require a seat material that is resistant to permeation but still exhibits key properties such as low friction, compressive strength, and resistance to deformation is still needed.

How Permeation Works

Permeation refers to the molecular level penetration of gases, vapors, and liquids through a solid material via diffusion. In diffusion, molecules pass from an area of high concentration to an area of low concentration. This can be extremely problematic when a ball valve is being used because of the potential distortion and leaking of the ball valve seat.

Keep in mind that permeation can take place through a surprising variety of materials, including metals and polymers. In addition, some materials are only semipermeable, which means that only ions or molecules with certain properties can pass through the material. 

The rate of permeation is directly related to crystal structure and porosity, which is why factors such as density and molecular structure are important when selecting materials for applications where low permeation is important. 

Why Permeation is a Problem for Ball Valve Seats

Gas permeation can not only compromise gas stream purity but also result in dimensional changes of the ball valve seat. One form of these dimensional changes is swelling, which can occur if the permeating media becomes a part of the molecular structure of the material. In reinforced polymers, such as glass-reinforced PTFE, swelling can cause separation between the glass fibers and the PTFE matrix. 

Another common manifestation of permeation is referred to as “popcorning” or “popcorn polymerization” which occurs due to a polymeric chemical reaction. And among the most notorious source of problems with popcorning and swelling are monomers with extremely small molecular sizes such as Butadiene and Styrene.

Both popcorning and swelling will lead to leakage, and over time popcorning will completely destroy the ball valve seat. This makes the choice of ball valve seat materials extremely important for applications where this is a problem.

PCTFE for Low Permeability Ball Valve Seat Applications

One of the best materials for a ball valve seat application where permeability is a problem would be PCTFE (Polychlorotrifluoroethylene), a thermoplastic chlorofluoropolymer. PCTFE is sometimes referred to as Modified PTFE or PCTFE, as well as by trade names Kel-F, Voltalef, and Neoflon. PCTFE is often thought of as a second-generation PTFE material that maintains the chemical and thermal resistance of PTFE along with its low friction. It is also similar to other fluoropolymers such as PFA or FEP.

One of the defining characteristics of PCTFE is that it has a much more dense molecular structure and a low void and micro-porosity content when compared to similar ball valve seat materials. This gives it a very low permeability coefficient, which means that the likelihood of it swelling or popcorning is far lower than other materials. For example, its permeability for O2, N2, CO2, and H2 are 1.5 x 10-10, 0.18 x 10-10, 2.9 x 10-10, and 56.4 x 10-10 darcy, respectively.

PCTFE also provides improved toughness and strength along with good deformation recovery and excellent creep and cold-flow resistance. In addition, it has a wide operating temperature range of -100°F to 500°F. In fact, it performs extremely well at cryogenic temperatures. Because of its low friction, it also results in a very low ball valve operating torque. PCTFE also exhibits zero moisture absorption and is non-wetting. 

PCTFE works well in operating environments where other polymers may fail. For example, it is well adapted to nuclear service that may involve high radiation exposure, is non-flammable (D 635), and is resistant to attack by the vast majority of chemicals and oxidizing agents. The only chemicals that might lead to slight swelling are ethers, esters, aromatic solvents, and halocarbon compounds.

In addition to its use in applications requiring low permeability, PCTFE is also considered an excellent choice for applications that need a low-outgassing material and is commonly used in semiconductor applications. Also note that there are PCTFE grades that are FDA approved, such as Fluorolon PCTFE 2800. 

Conclusion

Fuel processing and transport, chemical processing, petrochemical systems, and emissions control are just a few of the applications where low permeation materials may be necessary. For such applications, PCTFE is an excellent option for ball valve seat materials because it combines the basic properties necessary for a seat with an extremely low rate of permeation.

If you need a solution to blistering, swelling, or popcorning of a ball valve seat, contact the experts at Advanced EMC. Our sealing team will work with you to find the right ball valve seat material for your application.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Why PTFE and PEEK Are Used for Ball Valve 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 -20°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 (-20°F  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. 

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

The Basics of Ball Valve Seat Materials

Ball valves play a critical role in controlling the flow of fluid and pressure within a pipeline, but their effectiveness and safety is only as good as the seat material used. In this blog post, we are going to review the basics of five commonly used ball valve seat materials.

Ball Valves

Whether found in a petrochemical application where a leak could be environmentally devastating, or in a pharmaceutical laboratory where cleanliness and sanitation are critical, ball valve seats must be reliable and robust. A ball valve consists of the body of the valve, the body cap, the stem, the hollow ball, and the round ball valve seat. 

The ball valve seat is responsible for sealing the fluid inside and uniformly distributing the seating stress. In soft seat ball valve designs, either an elastomer or polymer is used as the seal and are inserted into a metallic seat ring. This approach, as opposed to hard seat ball valves, is popular because it provides good sealing action, is lighter weight, and more cost effective. 

Key Properties of Ball Valve Seat Materials

When choosing a polymer material for a ball valve seat, there are numerous factors that are involved. Key material properties include …

  • Sufficient ductility to provide a reliable seal
  • Dimensional stability to ensure the ball valve seat retains its shape for reliable sealing and performance
  • Very low friction to keep stem torque at a minimum
  • Low coefficient of thermal expansion so that the ball valve seat retains its shape when temperature changes occur
  • Excellent wear resistance for a long service life
  • Chemical compatibility with all media involved 

In some operating environments, it is also important that ball valve seat materials exhibit these properties:

  • Low moisture absorption to prevent dimensional changes in the presence of water or high humidity
  • Maintain performance with repeated sterilization that can include hot water, steam, and harsh cleaning chemicals
  • Good performance in the presence of sudden decompression (i.e., pressure drops over 650 psi)

Recommended Materials for Ball Valve Seats

There are several materials that work well as ball valve seats, including acetal, PEEK, PTFE, TFM, and UHMW-PE.

Acetal Ball Valve Seats

When aggressive environments are involved, Acetal (also known as Delrin) is often used. Acetal provides excellent wear resistance, is very rigid, has good toughness, and is resistant to cold flow. Although its operating temperature range is not very wide (-70°F to 180°F), it can handle pressures up to 5,000 psi. Acetal also works well in radioactive environments  but should not be used with oxygen flow.

PEEK Ball Valve Seats

PEEK offers excellent chemical resistance, very low friction, self-lubrication, and is flame retardant while also possessing a wide operating temperature range (from -70°F to 550°F). It can handle very aggressive applications and works well when there is a need for hot water and steam exposure–but does not do well in the presence of sulfuric acid.

In addition, PEEK is very well adapted to nuclear applications and is available in FDA-approved grades as well as filled grades with improved wear properties and better thermal conductivity. Note that PEEK is usually chosen for ball valve seats when the operating temperature range is outside that of PTFE.

PTFE Ball Valve Seats

PTFE (also known by its trade name, Teflon) has many of the same properties as PEEK, but involves even lower friction, dry running capabilities, and more extensive chemical compatibility. Like PEEK, it is available in FDA-approved grades and can handle cryogenic temperatures down to -50°F and high temperatures up to 550°F as well as pressures up to 5,000 psi.  \

Also like PEEK, PTFE can continue to perform even when repeatedly exposed to hot water and steam. Keep in mind, however, that PTFE does not perform well in the presence of fluorine or alkalies. PTFE is also very easy to clean and available in glass or carbon-reinforced grades that can provide improved wear characteristics, less propensity to cold creep, and lower thermal conductivity. 

TFM Ball Valve Seats

TFM (sometimes referred to by the brand name Dyneon) is a second-generation PTFE material that combines the best properties of PTFE (low friction, chemical resistance, high-temperature performance) with better stress recovery and the ability to handle higher pressures. It is also more elastic and resilient than PTFE. The operating temperature of TFM ranges from -100°F to 450°F and it is well adapted to applications involving steam and thermal fluids.

UHMW-PE Ball Valve Seals

UHMW-PE, which stands for Ultra-High Molecular Weight Polyethylene, has a low coefficient of friction, an operating temperature ranging from -70° F to 200°F, good chemical resistance, good dimensional stability, and good abrasion resistance. In general ball valve seats made from UHMW-PE can handle pressures up to 1,500 psi and can handle low to medium levels of radiation exposure.

Conclusion

Ball valve seals are used in many different industries, including chemical processing plants, oil and gas operations, manufacturing facilities, food preparation, and even residential use. As a leak-proof means of pressure and flow control, their smooth and reliable operation is critical–which is why polymer materials work extremely well for ball valve seats. If you are in the market for a ball valve seat material, contact the experts at Advanced EMC. We can put our years of experience to work for you, helping you select the right material for your project.

 

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Ball Valve Seats – 9 Significant Purchasing Options


 

ball-valve-seat-diagramMost ball valve seats are made of a polymer material primarily because of their elasticity, which it vital for achieving a good seal. They perform well at low pressure differentials, provide low operating torques, and usually have low coefficients of friction.  Let’s look at some materials that are being used for ball valve seats.

 

9 Significant Purchasing Options for Ball Valve Seats

 

Option 1:

One of the most popular choices for a ball valve seat material is PTFE because of its…

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