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Many critical valve services make use of soft-seated ball valves.  A key design decision to be made about these ball valves is the material used for the seat, which used to be limited to elastomers or metals, but now includes many high-performance polymers.

In this blog post, we are going to look at the types of polymer materials that work well for critical service ball valve seats.

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What Makes a Ball Valve Critical

It can sometimes be challenging to narrow down the definition of “critical service” valves.  Examples of critical service applications include:

  • high pressures or pressure drops over 650 psi
  • flowing media that can be harmful to the environment or to personnel valves for flashing service
  • tight shutoff is necessary
  • there is a potential for cavitation or vibration

Such applications are found in the mining, petrochemical, refining, power generation, and chemical industries.

PTFE

Some of the critical service applications involve the use of caustic or dangerous chemicals.  When such media is involved, one of the first choices for a ball valve material is PTFE.  PTFE is the most chemically compatible polymer on the market today. 


PTFE is the most chemically compatible polymer on the market today.


PTFE also has excellent heat resistance and extremely low friction, and is well adapted to application temperatures ranging from cryogenic at -365°F to 450°F. The fact that it is non-contaminating and has FDA approval are additional characteristics that enable PTFE to work well in a variety of applications. The primary weakness of PTFE is alkalis and fluorine.

Modified PTFE

Modified PTFE is a second-generation PTFE polymer which retains the heat and chemical resistance capabilities of PTFE.  In addition, it has a denser molecular structure which gives is superior stress recovery (which can be important for critical valve services) and very good creep and compression resistance.  It also can handle higher pressures than PTFE.  Seals made from modified PTFE tend to be more elastic and resilient than those made from virgin PTFE.  Like PTFE, however, it exhibits non-stick properties.  Modified PTFE performs well in temperatures ranging from cryogenic at -365°F to 500°F.

PEEK

PEEK does not exhibit quite the same level of chemical compatibility that PTFE does, but it is usually a close second.  It is not to be used in contact with concentrated sulfuric acid, but is especially well adapted to situations that involve hot water steam. PEEK has excellent heat resistance and works well in temperatures up to 600°F.  PEEK also has excellent flame retardant characteristics.  It is often chosen for situations that involve both high temperatures and high pressures.

Conclusion

There are several different materials to choose from for when it comes to ball valve seats for critical applications, including metals, flexible graphite, polymers, and elastomers.  High performance polymers such as PTFE, modified PTFE, and PEEK, however, are popular choices because of their combination of chemical resistance and high pressure/temperature performance.  When selecting a ball valve material for a critical services valve, be sure to consider these materials.

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