I'm trying to purchase some low temp valves and I'm having a hard time determining what seats and seals I should get. It seems the main Seat materials are PTFE/RTFE, Devon, Nylon, PEEK, Viton-A, and Metal-Metal. The Seal materials are Buna-N, HNBR, PTFE, and Viton-A.So my problem is that I was told not to use Buna/HNBR/etc because the benzene in our product eats through that too easily, and I need low temp capabilities (-40) so that puts out Viton-A. The valves are higher than 300 ANSI so PTFE is out. And I talked to a few people and they said not to use Nylon because it's too soft and doesn't work well. What do I do? At this point it seems that I either go with Buna-N and hope it lasts, or go with Viton-A and hope the line is never shutdown long enough for the temp to get that low.Not sure I understand your comment about teflon being limited to Class 300. Reinforce Teflon (RTFE) is commonly used well above class 300? When I've checked in the past, ball valves with teflon seats have typically had full flange rating up to Class 600 and it can be used above this rating as well but the manufacturer will derate the valve to something below the flange rating. Since full flange rating is not always required (especially at higher classes) the valve with RTFE seats may still meet your required design pressure. Of course, this depends on the specific valve design so you might want to check with a variety of suppliers. RTFE is always my preferred choice due to its wide temperature range and chemical compatibility.

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