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Can anyone advise how do i calculate the sizes of holes or slots to be cut to make a 12 inch butterfly valve equivalent to 175 mm RO (orifice) in 12" line? We want to introduce this valve as variable orifice to create variable pressure drop. The opening will be set once and only adjusted for summer or cold weather. We do not want the valve to be fully shut even by mistake. There is upstream/downstream gate valves for our isolation of heat exchanger.