A trunnion ball valve for sour gas service at 38C, 285 psig. What would be the best seal material?
For sour gas, consider Aflas before ethelene propylene rubber (EPDM) or Nitrile (Buna N, HSN, HNBR).  I used to keep lots of elastomer data on my hard drive but don't have it now.  You can download lots of material compatibility charts from the web.  Consider Kalrez when money is no object. the valves suppliers here use James Walker O-Rings. In order to increase the resistance of an o-ring against methanol (which cause O-ring swells or volume increase) you need to increase fluorine content of the O-ring which this has an adverse effect on low temp resistability of the o-ring. A better material to use where methanol exists, is HNBR 958!

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