Aerodynamic Torque of a Butterfly Valve—Influence of an Elbow on the Time-Mean and Instantaneous Aerodynamic Torque

Abstract

Many technological devices use butterfly valves to control the flow of the process or as safety unit. The principal advantages of this type of valve are their simplicity, their low cost, their speed of closing and the weak pressure drop which they produce when they are completely open. For installations of large size, the actuator of the valve can be very expensive; thus it is essential to know well the fluid forces and the resulting torque exerted on the valve. Consequently, the variation of the shaft torque of the butterfly valves according to the opening is of great interest to calculate the power of the actuator. Initially the flow around the valve is characterized by means of hot wire anemometry. It is noted that the disturbances induced by the elbow and/or the valve are felt until a distance from approximately 8 times the pipe diameter. A method of direct measurement by torquemeter and an indirect method by integration of the pressure forces on the faces of the valve give access to the time-mean and instantaneous torque on the valve shaft. Comparisons between the direct and indirect measurement of the torque are made before engaging the analysis of the results. Close to the full opening, the torque presents fluctuations harmful to suitable lifespan of the valve. Compared to the straight pipe case, the temporal and spectral analyses of the instantaneous torque prove that the elbow induces important fluctuations when the valve is completely open. Several tests carried out according to the valve/elbow spacing show that these effects disappear beyond a distance from 8 to 10 times the diameter of the pipe. [S0098-2202(00)02902-3]