Trying to source a butterfly isolation valve for a hydrostation. Valve size is 800 mm (32") and the head of cold water is 150 m (492 ft). Would it be possible to get away with a cheaper non-offset valve or would I need double offset?
The term isolation leads me to believe that you require repeatable tight shutoff.  The tight shutoff requirement could eliminate the traditional / conventional butterfly valves of the sixties.
there will be no safty margin if you use centerline butterfly valve
se a Double-offset. 
as stated in an earlier response-you will have no margin if you can ecven get a large butterfly valve rated to those pressures.
150M head is 213 psi to an American.and that's a LOT on such a large valve. 

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