I am trying to determine the resistance coefficient, K, of a single-offset, butterfly style damper. Since, there is no readily available data for dampers I was going to utilize the K values of butterfly valves. Unfortunately, the formulas available are limited to sizes below that which I am looking for.On page A-29 of Crane Technical Paper No. 410, they list formulas for determining the K value for butterfly valves of sizes 2 to 8", 10 to 14" and 16 to 24". The formulas are K = 45 ft, 35 ft and 25 ft respectively, where ft is the friction factor in a zone of complete turbulence.The dampers that I need to determine the K values for for 8 sizes up to and including 84".That is true. However, I'm a using the K value to determine my damper Cv. For preliminary calculations I have assumed it to be 0.25 for all damper sizes, but I need to verify and possibly correct this assumption.The most extensive work on hydraulic resistance that I have seen was done by a Russian named I. E. Idel'chik. His work was published by the National Technical Information Service of the U.S. Department of Commerce in 1966. The title of the publication is "Handbook of Hydraulic Resistance - Coefficients of Local Resistance and of Friction". I suggest that you search (Google or other) for this document.One section (Section 9) in this publication gives resistance coefficients ("K" values) for various openings of dampers in pipe and rectangular ducts. Another section (Section 10) treats obstructions of various shapes in pipe or rectanular ducts. You might have to use both sections of the publication to estimate the resistance for wide open (obstruction) versus partial opening.