Removal of butterfly valve from line requires removal of nuts and all studs from valve body and result in delays, one of the options is to change tapped holes on valve body to through hole by drilling, this will require replacing all studs with longer studs which wil pass through the drilled holes. This will eliminate the need of removing studs from valve body as studs can be removed after loosening the nuts. Technically you could probably do it, and everything could probaly work well. In my opinion there is, however, two important counterpoints to be considered. a) From a QA point you would make a mechanical change in an already fabricated (factory guaranteed, construction checked), and tested (body pressure, seat leakage) mechanical component. You or the factory would then be juridical responsible for all possible internal or external damage resulting from this change. This would be, I belive, a general and worldwide rule, covered by local laws. b) There is a theorethical possibillity that full borings will alter torque, position and give uneven flange sealing tightening. What type of flange sealing and service do you have? Borings could also uncover or cause otherwise hidden/not present defects in the material structure. Check that you are covered or comfortable for a and b, or else (as I personally would have done) change to new valve or pipeline (with dismantling joint?) construction.

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