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. 

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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.

Want more information on surface safety valve(s) (ssvs)? Feel free to contact us.

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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. 

In today&#;s guide, I will discuss all essentials of selecting the correct valve seat materials.

Ball valves are essential in allowing to cut or pass the flow and pressure of a piping system. Moreover, they are used in industries such as the petrochemical and pharmaceutical where leakage could represent a terrible consequence for the environment or the product itself. Therefore, the ball valve seat is critical in such situations since it is responsible for sealing the fluid inside and distributing the seating stress uniformly.

In the below picture, we have painted the valve seats in red color so you can identify where is the valve seat:

So, selecting the appropriate ball valve seat materials is vital to guarantee the performance of the valve and the safety of the product and the environment. The most common ball valve seat materials include PTFE, PEEK, and POM.

POM, called Acetal, POM-C, or Delrin, is trademarked by 3M. To select the most appropriate one, there are three main factors to consider:

• Fluid compatibility
• Temperature range
• Pressure range

Valve seats fluid compatibility

Fluid compatibility refers to the material being capable of operating in contact with the fluid without having an adverse reaction.

PTFE, also known as Teflon, has very high chemical compatibility, which is why it is commonly used across different applications. Moreover, it is also produced in FDA grade, thus making it compatible with pharmaceutical, food, and drinks products. The only compatibility problem of the PTFE appears when exposed to fluorine or alkalies. We also do not recommend PTFE seats for pressures up to 350 bar.

PEEK is a material with very similar properties to those of the PTFE. Therefore, its fluid compatibility is also similar, including FDA grades. However, it may fail when exposed to sulfuric acid.

Finally, while POM fluid compatibility is extensive and includes radioactive applications, it is essential to highlight that it does not perform well with oxygen flow.

If you want to verify compatibility with the fluid of your application, you can use our compatibility tool here. Our tool allows you to select a material and will tell you how compatible it is with the specific seat.

Valve seats temperature range

Another critical aspect of selecting the appropriate ball valve seat materials is the range of temperatures where they can operate. Many materials react differently to temperature changes, and some lose their properties when the temperatures go above or below their normal operational ranges. If a ball valve seat loses its properties or experiences thermal deformation, the sealing power is lost, and the undesired leakage may occur.

We recommend selecting a high-temperature seat or a three-piece ball valve for Socket Weld valves. In that case, the temperature will increase and may melt the valve seats.

When it comes to PTFE, the operational temperature can be in the range of -45°C to 230°C. Now, if your application exceeds these limits, you will be better off using PEEK seats since this material can perform well within the range of -56°C to 315°C, including steam service up to 260°C. Those figures are theoretical since it needs to be added a safety factor.

POM is a more limited material when it comes to temperature. This material can only work at low temperatures within -56 °C to 100°C. However, POM would be an appropriate ball valve seat material for applications where pressure is more critical than temperature, which takes us to the next point.

Temperature ranges with safety factors:

POM PTFE PEEK -30ºC-40ºC-50ºC+100ºC+190ºC+250ºC

Pressure range

Similar to temperature range, pressure range refers to operating pressure values where the ball valve seat material will not suffer deformation or alter its properties. While some materials may be strong enough to withstand high pressures, others may fail at certain levels.

For example, PTFE ball valve seats will have no problems operating at pressures reaching the 350 bar.

You can refer to this table for maximum pressure ranges according to the valve diameter:

Valve Bore SizePOMPTFEPEEKDN6 DN10 DN13 DN20 DN BAR OR MORE350 BAR500 BAR OR MOREDN 32 &#; DN40 &#; DN BAR OR MORE175 OR 150 BAR APROX.450 BAR OR MORE

As you can see, there are many different aspects to consider. PEEK may be better for some applications because they need valves operating with hot water or steam. For others, the low friction coefficient of POM may be more relevant.

Pressure and temperature chart POM, PEEK, and PTFE

So far, we have only analyzed the maximum working pressure at ambient temperature. Depending on the temperature, the valve pressure is less. But when we arrive at very high or very low temperatures, the pressure that can withstand the valve is different. In the below graph, we see a comparison of the three seats for a 1/2&#; valve (DN13) working pressure and temperature chart:

Also, there are variations of these materials, like the reinforced version of the PTFE called RPTFE that, with its 15%-25% glass fiber reinforcement, provides an improved lifecycle. Another example would be the TFM, a second-generation PTFE with better stress resistance and recovery and higher pressure capability, elasticity, and resilience. We have recently added TFM as an option for some specific valves.

What is the difference between a seat and an Oring?

An Oring or gasket is an elastic ring that prevents the fluid from leaking. This elastomeric material usually is FKM or Viton, NBR or Buna, FFKM or Kalrez, EPDM, etc. The temperature of the o-ring needs to be checked as well since it can melt if the working temperature of the valve is too high. 

If you have any other questions, do not doubt contact us. And if you prefer, you can call us; we love to have a little chat with you.

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