Abstract: In Goupitan Hydropower Station, the layout type of middle discharging orifice was classified into flatbottomed type and upbended type, the maximum operational water head was 95.36 m, and the emergency maintenance gate used an upstream slanted slot，the velocity in the slot region was over 30 m/s. From the result of vacuum tank experiment, the cavitation intensity steam type in the slot region of the upbended type middle orifice did not surpass initial stage. The structural configuration of this middle orifice basically belongs to cavitation free type. The cavitation intensity in the slot zone of the flatbottomed type middle orifice was severe. The steam type cavitation reached developing stage. According to the comparison of structure optimal schemes, adopting a measure of reducing the area of pressure section outlet or enlarging the dimension of pressure section could solve the above problem effectively. Therefore, when a slanted slot was laid in pressure section of discharging structure, the mean velocity of slot section should be controlled below a feasible value, otherwise, the rather serious cavitation would happen more easily in the slanted slot, compared to the straight slot.

Key words: middle discharging orifice ,   inclined slot ,   steam cavitation ,  gas cavitation , Goupitan Hydropower Station