I am new to Geothermal plant and working for a company as a
purchase engineer. I received quotation Expanding conduit Gate Valves for the well section and some vendors quoted metal seats some quoted r-ptfe seats.
R-PTFE seats are a bit cheaper and I know that they can operate till 450 F.
To select the right valve I would like to receive your comments. When you compare both seat choices what can you sau? Thanks in advanced to guys sharing ideas. 

For soft sealing valves you have to be more certain of all fluid conditions and temperature influence (not only temperature) on a long term-basis.

It would be sensible to plan for a change of valves and/or sealing after an estimated lifetime. Supplier or producer will have an opinion on lifetime and cost, given operational and fluid data.

For metal seated valves you normally do not have to worry that much for sealing life, but operationol data, fluid properties, sand and grit (if any?), cavitation and corrosion might give influence of sealing and valve lifetime. Again ask supplier.

Best possible (cost)/(required from enduser lifetime) including maintenance cost should be your first choice of valve type.

You should also have a look at cost consequences if unexpected leakage over valve or bad sealing (eg process dependability cost or failure cost)

There are pros and cons for both types of valves, depending among other things on some of the factors above.

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