How can I calculate/estimate the air flow through a butterfly valve as a function of the rotation angle/opening. The catalog comes with the Cv as a function of rotation angle. Can I use the sizing equation in terms of flow
I'm not sure I'm understanding the question, but Cv is used to calculate flow. It gives the relationship between flow and pressure drop. Flow is proportional to absolute inlet pressure. Hence if you know inlet pressure and valve rotation angle, the Cv figures you have can be used to calculate flow.
Find an equation that is in the form flow = f(Cv and other variables).  Develop an equation for Cv = f(rotation angle).  Then use algebraic substitution to get flow = f(rotation angle and other variables).

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