I am writing a test procedure for water flow testing a 10 inch butterfly valve. The application is 10000 GPM liquid hydrogen at 15 psig at the inlet. In addition to pressure drop, I am trying to model the dynamic load (torque) to the blade/shaft assembly experienced at the application flow condition with a water flow test. In order to have dynamic similitude, the Reynolds Number should be constant, but that results in a test flow rate 5 times that of the application. Instead, I will be keeping the dynamic pressure constant to match pressure drop. Will the torque applied to the blade/shaft assembly during the water flow test equal that of the application flow condition? I remember reading in the literature somewhere that the coefficient of torque w/r/t dynamic pressure (Ctd = Torque / (1/2 Density * Velocity ^2) is function of Reynolds number and viscosity, therefore the torque should be different in these two cases. Is there a way to match torque between these two flow conditions?Dynamic torque is created by a non-uniform pressure distribution across the valve disc. The shape and thus the resultant magnitude of the force on the disc and center of pressure from this pressure ditribution in relation to the shaft centerline will vary with the media density, viscosity, temperature, pressures, and flow rate. It is an unsolvable problem when approached this way.So how is it done? Flow tests are done with water and the torque measured at the valve shaft is measured in both while opening and closing the valve. Mathmatecally subtrascting the two results eliminates the effects of friction. Knowing the valve Cv, flow rate, and differential pressure at each measured position will allow you to calculate and factor known as the dynamic torque coefficient (Cd).

MORE NEWS