The CFD Numerical Simulation And Discharge Capacity Calculation Of Safety Valve On Discharging Process

Safety valve is an important accessory to the most kinds of pressure equipments, while good dynamic characteristics and accurate selection are the premise to play a protective role. However, the accurate calculation of discharge capacity is crucial for the safety valve selections. Nowadays the domestic and foreign regulations and standards define the calculation processes of the discharge capacity of safety valve for the single-phase relief. However, the results calculated by the domestic regulations and standards are quite different from those calculated by foreigns due to the different selections of discharge coefficients. Although the related regulations and standards provide the formulas to sizing for the compressible gas relief at sub-critical discharging condition, accurate discharge capacity calculation of safety valves with discharge piping is still a problem. In addition, the domestic regulations and standards haven’t provided the calculation methods and formulas to sizing for gas-liquid two-phase relief and flashing liquid relief. Moreover, the accuracy of the discharge capacity methods provided by foreign regulations and standards is still need to be verified. In view of the above problems, this paper combines CFD numerical simulation with theoretical calculation to carry out related research. The specific work and conclusions are as follows:Firstly, the discharging processes of safety valve are simulated for ideal gas relief at sub-critical and critical discharging condition by FLUENT. The transient flow field analyses show that when the medium flows through the sealing surface and the channel between disc holder and valve seat, the throttle effect occurs. In addition, a strong jet is formed and its tail upturns gradually along with the lift. Morever, the secondary flow vortex appears on both sides of the jet. Due to the effect of the lift, the critical cross-section may not appear at throat, but at the sealing surface and the channel between disc holder and valve seat.Secondly, the discharge capacities calculated from dynamic simulations for ideal gas relief are compared with those calculated by some domestic and foreign standards. Results from the comparisons reveal that the selection of discharge coefficient has a great influence on the calculation results of discharge capacity. However, the valve size and discharging conditions have an influence on the discharge coefficient. In addition, the certified discharge coefficient on the nameplate of JB standard safety valve may not be applicable to the practical discharging condition. The discharge capacities calculated by API STD 520-2008 are larger than those calculated by domestic standards because of the different selections of discharge coefficient.Thirdly, this paper proposes an accurate discharge capacity calculation method of safety valve based on the back pressure correction by combining the pressure drop calculation formula of isothermal flow with the discharge capacity calculation formula in GB 150-2011. At the same time, the numerical simulation results from FLUENT are used to verify the accuracy of the proposed method. The results show that the relative percentage errors of results calculated by the proposed method are within ±0.5%. So the proposed method can accurately calculate back pressure and discharge capacity of safety valves with discharge piping.Fourthly, the discharging process of safety valve is simulated for air-water two-phase relief. The transient flow field analyses show that when the two-phase mixture flows through the sealing surface and the channel between disc holder and valve seat, the static pressure drops suddenly, while flow velocity and the volume fraction of air increase sharply. In addition, the discharge capacity calculated by two-point omega method provided by API STD 520-2008 is larger than the discharge capacity obtained from dynamic simulation. At the same time, the research on the accuracy of two-point omega method is made for different volume ratio of air-water relief. The results reveal that the composition of air-water is closer to the single-phase state, the calculation results using two-point omega method are more accurate.