Fluid Flow Force Measurement Under Various Cavitation State on a Globe Valve Model

Abstract

Proper actuator sizing and valve-stem guide selection requires accurate knowledge of the thrust requirements for linear valves. For globe valves, the flow-induced forces acting on the disc can be a substantial part of the total required thrust that the actuator is required to overcome. The flow-induced forces acting on the globe valve disc are decomposed into the rejection (or axial) force and the transverse (or side load) force. The axial force acts along the disc stem axis and the transverse force acts perpendicular to the disc stem axis. The axial force generally lends itself to simple analytical models or experimental measurement, making force predictions straight forward. The nature of the transverse force and globe valve design however, make predicting or experimentally measuring the transverse force nontrivial. An experimental test matrix was developed to better understand the behaviour and dependencies of the axial and transverse force on key factors, including the valve geometry and flow state (cavitating and noncavitating). In addition to the experimental investigation, numerical predictions using a 3-D model were performed to better understand and correlate the flow-induced forces with valve design and operating conditions. To facilitate the experimental study of the axial and transverse forces acting on a globe valve disc under cavitating and noncavitating conditions, a full model of a 2" Plexiglas globe valve was used to perform experimental testing so that visual observations of the formation and location of cavitation could be made. The disc stem was specially machined and instrumented so that the flow-induced forces and moments acting on the disc could be measured. The valve model was installed in a flow loop and the operating conditions and flow state were controlled using flow rate and downstream pressure. The flow loop was designed so that the pressure downstream of the valve could be sub atmospheric to provide accurate control over the level of cavitation. Experimental testing was conducted for a large set of cavitation conditions, fluid flows and disc opening positions. The CFD predictions were performed using ANSYS CFX 11.0 on a SGI ALTIX 330. The CFD predictions were performed using non-cavitating water. The flow field was assumed to be fully turbulent and the turbulence was modelled using the k-ε RNG model. Based on the experimental testing, it is found that the transverse force can reach the intensity of the axial force. Because the transverse force directly affects the guide friction force and other performance factors, the transverse force should be accurately quantified and included to determine the total required actuator thrust. It was also found that the transverse force depends very weakly on the severity of cavitation. The experimental results and visual observations provided a more complete understanding of the behaviour and significance of cavitation with respect to the flow-induced forces. CFD simulations provide insights into the resulting flow field allowing for a greater understanding of the relationship between the valve differential pressure and the flow-induced forces. CFD predictions were generally in good agreement with experimental results.