I recommend you download a vendor program . Use the FL and Xt factors for your particular make of butterfly valve  and you can calculate the valve openings for different flowrates. It sounds like your upstream pressure is constant and you wish to know the pressure drop across the valve at different openings. Note that depending on the system hydraulics the downstream pressure will change based on flowrate which will affect valve DP. You need to understand the system hydraulics to know what process conditions the valve will encounter. possibly Neles software has built-in custom equations, don't know other vendors but I presume most do use the ISA correlations, these require to specify parameters as XT, FL CV/aperture etc. these are not easy to define as they are related to the specific internal geometry of the valve, as suggested by Scotsinst you can use software from Masoneilan, Fisher etc. which include these parameters for their models, when the vendor doesn't provide specific software I use glode Valves and I adopt the values suggestd in ISA standards.

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