The standard way to calculate dynamic torque is to run a flow test and measure the torque required to rotate the valve at each position. Steps are taken to account for direction of rotation and you then mathmatically determine a dynamic torque coefficient. From this, you can use this coefficient on geometrically similar valves to estimate the torque at various positions. You need to know the differential pressure for each open. To get the DP, you would need to know the Cv, which considering you have a new design, you may not know that either. Dynamic torque is a basically a form of fluid drag. The incomming flow hits the leading edge of the disc and generates a local high pressure area. The net force from this area times the distance to the centerline of the shaft gives you dynamic torque. Fluid drag is linearly proportional to density, which is why dynamic torque in butterfly valves is not a problem with gas. It sometimes (very rarely) is a problem with liquid. Most of the time, the seating / unseating torque is several times greater. If you need an estimate, you could use a manufacturers Cv data, calculate DP assuming constant upstream pressure. Then concervatively use the 1/2 the disc area (upstream side) to calculate a force. Determine the center of pressure for that area and multiply it by the force. You will get a very conservative dynamic torque value. To get overall valve torque, you will need to include bearing friction and seal friction. However, considering the size of your valve, the low density media, and low DP, dynamic torque will be far less than the torque needed to seat the valve.

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