Quick closing valve (QCV) method is a common means of calibrating gas holdup in gas-liquid two-phase flow test. Unreasonable selections of spacing between valves and trapping times in QCV could bring about large error in gas holdup measurement

particularly since the gas slug and liquid slug in slug flow present flow characteristics with random variability. In this study

an optimal design was proposed for measuring gas holdup using QCV method. The cross-correlation velocity of gas-liquid two-phase slug flow was measured based on the signals between axially upstream and downstream probes. Then

the lengths of gas slug and liquid slug were extracted from the upstream probe signals under different flow conditions. Also

the gas holdup in liquid slugs was calculated by using Maxwell equation. On that basis

the gas holdup series was simulated at different spacing between valves based on the gas slug ratio in pipe. By analyzing the fluctuation of gas holdup series

the floor level of trapping times was indicated under the condition of 95% confidence coefficient and 5% permissible error at different spacing between valves. Finally

an experiment was conducted to assess the trapping times in QCV with the spacing setting at 1.55 m length. The measuring errors of gas holdup using QCV method was statistically analyzed

and it was proved that the design guideline provided a sufficient condition for setting up the spacing between valves.