Effect of gas flow on the bending moment acting on a shaft in a sparged vessel stirred by a Pitched Blade Turbine

•The shaft bending moment in gas–liquid stirred tank was measured.•The trend of shaft bending moment over gas rate presents an “S” shape.•The shift of two loading gas ways causes a low-frequency fluctuation.•The fluctuating characteristics of shaft bending moment are presented. The bending moment acting on the overhung shaft of a gas-sparged vessel stirred by a Pitched Blade Turbine, as one of the results of Fluid–Structure Interactions (FSI) in stirred vessels, was measured using a moment sensor equipped with digital telemetry. The amplitude and Power Spectral Density of the shaft bending moment were analyzed. It shows that the gas flow has a considerable influence on the characteristics of the bending moment, such as the amplitude mean, distribution, Standard Deviation and peak, and the low-frequency and speed frequency contributions to the fluctuation. The relative mean bending moment initially increases with gas rate till the transition from complete dispersion to loading regimes, approaching a peak, then decreases to a valley and again rises gradually, going through the transition from loading to flooding regimes. The “S” trend of the relative mean bending moment over gas flow rate, depending on the flow regime in gas–liquid stirred vessels, results from the competition among the nonuniformity of bubbly flow around the impeller, the formation of gas cavities behind the blades and the gas direct impact on the impeller as gas is introduced.