The choice of a seal material for a ball valve is vital to its successful operation. In this post, we are going to look at some of the major characteristics of six commonly used options for polymer seals in ball valves.
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Key Material #1: Virgin PTFE
Virgin PTFE (trade name Teflon) is ideal as a ball valve seal material for pressures less than 5 ksi and temperatures between -20 F and 400 F; however, its temperature performance does depend on pressure. Speaking of pressure, PTFE does not decompress well after being pressurized. Note that PTFE does not perform well when subjected to temperature fluctuations greater than 167 F. One of its greatest strengths is chemical resistance, being close to insoluble; another strength is extremely low friction. It is also fire resistant.
Key Material #2: Glass Reinforced PTFE
Reinforced PTFE as used in ball valve seals is typically 15% glass fiber, increasing the temperature and pressure rating available with virgin PTFE. Like unreinforced PTFE, glass reinforced PTFE still has very good chemical resistance with the exception of hot caustics and hydroflourics. It, too, is fire resistant and has low friction, though not as low as virgin PTFE.
Key Material #3: Stainless Steel Reinforced PTFE
There is an alternate form of reinforced PTFE that is sometimes used in ball valve seals: stainless steel reinforced PTFE. This composite seal material is made of 50% PTFE and 50% powdered 316 stainless steel. Its temperature range is -20 F to 550 F (a bit higher than virgin PTFE) and it has higher pressure capabilities than either virgin or glass fiber reinforced PTFE. It, too, is fire resistant, however its coefficient of friction is higher than PTFE.
Key Material #4: PEEK
PEEK is an option when the requirements lay outside the temperature range of PTFE. PEEK works well in environments with temperatures between -70 F to 600 F, and is unaffected by continuous exposure to steam and hot water. It is tougher than PTFE, but also harder. Its major drawback, besides its rigidity, is its brittle behavior at lower temperatures.
Key Material #5: UMHW Polyethylene
UHMW Polyethylene seems to be choice for more specialized applications, including those where there will be low to medium radiation exposure. Its pressure rating is 1.5 ksi and its temperature range is -70 F to 200 F 1500 psi -57C to 93C. UHMW Polyethylene also has very good abrasion resistance.
Key Material #6: Chlorinated Polyether
Chlorinated polyether is sometimes used as a ball valve seal material, functioning at temperatures up to 257 F. It functions well in the presence of acids and solvents if softening can be tolerated, and is resistant to more than 300 chemicals. It does not creep, and does not tend to absorb water.
Seals Continually Evolving:
Seal materials is a continually evolving field, but these six materials seem to be the leading contenders for thermoplastic ball valve seal choices. Their major characteristics seem to the pressure and temperature performance, low friction, chemical resistance.