In this research, in order to predict the dynamic characteristics of a regulating valve, a mathematical model is proposed for a pneumatic control valve using a smart valve positioner (AVP300), and the dynamic characteristics of the control valve were simulated. We modeled the components of the control valve (i.e., nozzle flapper, pilot valve, Auto/Manual (A/M) screw, bleed orifice, pneumatic actuator, gland packing, and pressure reducing valve), and simulated the dynamic characteristics using SimulationX, a one-dimensional analysis software.
For the nozzle flapper, we proposed a model that considers the influence of fluid force due to pressure change as well as the influence of the change in effective area by measuring the displacement, pressure, and flow rate of the nozzle flapper. The diaphragm chamber, which operates the pilot valve of the positioner, was made of transparent acrylic. The displacement of the pilot valve was measured by a laser displacement sensor, and its movement against pressure change was clarified. The sonic speed conductance and critical pressure ratio of the A/M screw and bleed orifice were determined experimentally and reflected in the model. In the pneumatic actuator, the effective cross-section of the diaphragm was obtained from the change in pressure and displacement. The change in volume was calculated from the experiment using a fixed chamber. The friction force of gland packing was modeled using static and dynamic friction forces.
The experiment on the dynamic characteristics of valve displacement was performed with the input signal of the valve displacement set from 20% to 80%. A comparison of the experimental results of the valve displacement and simulation results showed good agreement. The simulation in this study is considered effective in predicting the dynamic characteristics of the control valve.