At a chlor-akali membrane plant which produces saturated hydrogen, typically 300mbar at 200F, max design of 400mbar. We have had continuing issues with leak through on 12 inch butterfly valves. These are the primary hydrogen isolation valves off the electrolyzers. Hydrogen at system pressure is slowly backflowing into offline equipment to concentrations at or above LEL. Obviously an issue when it comes to performing maintenance, since an inertization purge is not an option. Currently moving towards a double block and bleed butterfly setup. Any other cost effective valve options, whether it be an individual valve or double block combination, for this service? My current thinking is to avoid a double block and bleed using valves we already know are very susceptible to leak through.