Dynamic instability of double-seated control valve induced by switching of internal flow patterns

Double-seated control valves are used to a lot of industrial applications due to advantages of reduction in actuator force, a great flow capacity and a deal of slurry fluids. The authors have developed an innovative structure with a compact body carrying out smooth and stable controls. This paper describes a fluid mechanics study to completely suppress unstable behavior possibly induced even in an extremely restricted condition. In the bluffest case of a flow-to-open plug configuration, which is required for large size valves, the unstable behavior was caused within a considerably narrow travel range of 0.5%. By flow visualization using a scale-down model valve, a bi-stable flow pattern of a flow-to-open flow, which sharply turned along the bluff plug configuration, was observed at the particular travel. Based on momentum theory of assumed 1D axisymmetric flow passage internal of a valve body, it was predicted that an abrupt increase in stem thrust was enough to cause the instability of a plug position. Therefore, the unstable behavior was attributed to the bi-stable pattern of the flow-to-open flow. Finally, a plug configuration was improved by understanding valve internal flow. A valve with the improved plug provided smooth characteristics in stem thrust over the entire travel range.