This study is to provide new experimental results for the critical pressure ratio and the critical flow characteristics in a safety valve. To do this, the test facility is newly designed to perform the steam, the hot water, and the two-component mixture tests focused on the visualization internal flow fields in a safety valve.
Subcooled water critical flow phenomena in a safety valve are investigated experimentally at various subcoolings between 10 K and 125 K, and about 1 Mpa of the inlet pressure with 3 different disk lifts, 1, 2, and 3mm. All of the experiments show the critical characteristics such as constant throat pressure and constant flow rate when the backpressure is sufficiently decreased. Several correlations for critical pressure ratio and critical flow rate are developed using the present experimental data expressed in terms of non-dimensional disk lift, subcooling, and pressure. Critical pressure ratios are considerably affected by different subcoolings while the effect of disk lifts on them is relatively small. A critical flow correlation for the safety valve is also developed using the critical pressure ratio correlation, , and the valve coefficient correlation, $(P_{cr1})_{corr}$, and the valve coefficient correlation, $K_{d.corr}$. The correlation shows good agreement with the experimental data with the root-mean-square error of 5.2%
In order to evaluate the effect of the non-condensable gas on the critical pressure ratio and critical flow rate, another group tests are performed when the ratios of outlet pressure to inlet pressure, the subcooling to inlet temperature, and the gas volumetric flow to water volumetric flow are 0.15-0.23, 0.07-0.12, and 0-0.8, respectively. The effect of nitrogen gas flow on the subcooled critical flow rate during the subcooled water critical pressure conditions show that the critical pressure ratio is also mainly dependent on the subcooling and the dependency on the gas fraction and the pressure drop is relatively small. When the nitrogen gas is included into the subcooled water, the subcooled water critical flow rate is decreased about 10 % compared to the critical flow rate without non-condensable gas, but it maintains a constant value for the ratio of gas volumetric flow to water volumetric flow greater than 20%. The developed two-component mixture critical flow correlation estimates the critical flow very well with the root mean square error 1.75% when the backpressure to inlet pressure ratios are 0.15-0.23, the subcooling to inlet temperature ratios are 0.07-0.12, and the gas volumetric flow to water volumetric flow ratios are 0-0.8.
The subcooled water critical flow correlation, which considers subcooling, disc lift, backpressure, and non-condensable gas, shows good agreement with the total present experimental data with the root mean square error of 8.17%.
