LNG선용 극저온 버터플라이밸브의 유동특성에 관한 연구

Abstract

Butterfly valves are widely used as on-off valves and control valves for industrial process. The importance of butterfly valves as control valves has been increasing because the pressure loss is smaller than that of other types of valves and compactness is very desirable for installation. These features are desirable for saving energy and high efficiency of instruments.

In modern industrial fields of a water supply plant, cooling plant for nuclear power station, the petrochemical industry, marine industry and so on, butterfly valves have been adopted and the hydrodynamic characteristics of butterfly valves have been actively studied in terms of the pressure loss. On the other hand, there are not so many reports on cryogenic butterfly valves in spite of broad application in LNG storage station and LNG carriers.

This study has focused on the investigation of the detailed hydrodynamic and aerodynamic characteristics of cryogenic butterfly valves to contribute to the operation during valve handling on LNG transportation system and practically utilization in design of butterfly valves and actuators.

In this study, the three-dimensional and two-phase numerical analysis were carried out on incompressible flows in cryogenic butterfly valves after verification of numerical analysis in water flow field because the experimentation of liquid methane is almost impossible on the condition at -162℃. Therefore, the procedure of this study is divided into the 1st stage using working fluid, tap water, by Particle Image Velocity (PIV) together with CFX-ANSYS ver. 10, systematic commercial CFD software, and into the 2nd stage using working fluid, liquid methane, by CFD.

As comparison of both analyses by using water in the 1st stage, vector velocities, pressure distributions, kinetic energy contours, flow coefficient and pressure coefficient indicated the similar characteristics pattern during the course of this investigation.

After verification of the previous stage, the numerical analysis for cryogenic butterfly valves were undertaken with the understanding that they could identify the fluid dynamics mechanisms and the acting force on valve disc related to the hydrodynamic and aerodynamic characteristics of butterfly valves. The flow field was selected as a 16 inches pipe, the upstream region 5D and the downstream region 10D with a working fluid of liquid methane at -162℃ and flow rate 1,700m3/h. ICEM-CFD, reliable gird generation commercial software, was also adopted to secure good quality of grid generation necessary for the reliable CFD simulation as 400,000 nodes grid meshing with the hex-tetra-prism grid. The turbulence model of SST was selected to guarantee cavitation phenomena, one of 3-D separation flow occurring on valve disk. The boundary conditions were inlet velocity 4m/s, no slip wall condition, outlet pressure 4.2kg/cm2 and saturation pressure 103,100Pa at 111.7K.

The hydrodynamic characteristics of complex flows including cavitation phenomena were investigated at different valve disk angles. The recirculation vortex in the down stream region was founded and the cavitation flows were intensively generated on the surface of valve disc as max. volume of fraction(VOF) at 10 deg. angle. The aerodynamic characteristics, lift, drag and torque acting on the valve disc, were calculated and showed a corrected torque coefficient. The pressure distribution and the pressure loss coefficient

showed the similar pattern with general flow-field.

Several important characteristics of hydrodynamic and aerodynamic characteristics of cryogenic butterfly valves were also studied, and they will contribute to the safety operation and the design of butterfly valves.