Abstract
In this paper, instability of flow due to break of symmetry between sag of the valve body and flow direction was analyzed on the basis of momentum theory in axis symmetric flow field, and characteristics of fluid force acting on the valve body (deviation fluid force to the valve shaft center) are clarified. The analysis result shows that fluid force is proportional to sagging of the valve edge. Till the valve opening is around 60%, the working angle is upward, that means sagging of the valve body is suppressed. When the valve opening is beyond 70%, the working angle is downward, that means sagging of the valve body is increased. Moreover, validity of the analysis result is examined by comparing it with the measurement result in discharge.
