Required temperature capabilities are also a foregone conclusion, but one that is likely to narrow valve selection possibilities. The considerations include the strength or ductility of
the body material, as well as relative thermal expansion of various parts.
Temperature limits also may be imposed due to disintegration of soft parts at high temperatures or loss of resiliency at low temperatures. The soft materials under consideration include various elastomers, plastics, and PTFE. They may be found in parts such as seat rings, seal or piston rings, packing, rotary shaft bearings and butterfly valve liners. Typical upper temperature limits for elastomers are in the 200 – 350°F range, and the general limit for PTFE is 450°F.
Temperature affects valve selection by excluding certain valves that do not have high or low
temperature options. It also may have some affect on the valve’s performance. For instance, going from PTFE to metal seals for high temperatures generally increases the shutoff leakage flow.Similarly, high temperature metal bearing sleeves in rotary valves impose more friction upon the shaft than do PTFE bearings, so that the shaft cannot withstand as high a pressure-drop load at shutoff. Selection of the valve packing is also based largely upon service temperature.