A butterfly valve on LP Nitrogen and Instrument Air (9 barg @ 85ºC) with hard seats (SS 316L).What will stop me on changing it to Soft Seats (PTFE)? I have to assumption from the wording of your question is that this applies to an existing valve leaks. If it is academic, nothing stops you from the change unless it is a company piping specification. Both the air and nitrogen services should be compatible with PTFE unless there is something unusual about the applications. Things that might stop you from making the change include requirements for the line to be in service. Another thing might be cost. Things that stop us tend to be time and money. If small you might consider changing the valve hard seated valve for a soft seated valve. The hard seated valve may be suitable for a hot or errosive fluid where PTFE might not hold up well.

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