A method of refurbishing a facility for converting hydraulic energy into electrical energy, this refurbished facility comprising: - a water passage, - a turbine with a runner of the axial flow type located within the water passage and moveable around an axis and belonging to a rotatable part of the turbine and comprising a hub and several blades connected to the hub, - a drive shaft connected to the runner, - an inner head cover located adjacent to the hub and surrounding the drive shaft and belonging to the non-rotatable part of the turbine; this method being characterized in that it includes the following steps: - Providing a first cover plate, - Providing a second cover plate, - Providing a first opening located at the inner head cover, - Providing a second opening located at the hub, - Providing a third opening located at the runner, - Connecting the first cover plate to the inner head cover, - Connecting the second cover plate to the hub, whereas the first cover plate and the second cover plate are surrounding the axis, and a part of the outer surface of the inner head cover, a part of the outer surface of the hub, the inner surface of the first cover plate and the inner surface of the second cover plate are confining an annular shaped space in order to allow a gas to flow from inside the inner head cover through the first opening, the annular shaped space and the second opening and to be discharged through the third opening into the water passage during operation of the refurbished facility. - a water passage, - a turbine with a runner of the axial flow type located within the water passage and moveable around an axis and belonging to a rotatable part of the turbine and comprising a hub and several blades connected to the hub, - a drive shaft connected to the runner, - an inner head cover located adjacent to the hub and surrounding the drive shaft and belonging to the non-rotatable part of the turbine; this method being characterized in that it includes the following steps: - Providing a first cover plate, - Providing a second cover plate, - Providing a first opening located at the inner head cover, - Providing a second opening located at the hub, - Providing a third opening located at the runner, - Connecting the first cover plate to the inner head cover, - Connecting the second cover plate to the hub, - Providing a first cover plate, - Providing a second cover plate, - Providing a first opening located at the inner head cover, - Providing a second opening located at the hub, - Providing a third opening located at the runner, - Connecting the first cover plate to the inner head cover, - Connecting the second cover plate to the hub, whereas the first cover plate and the second cover plate are surrounding the axis, and a part of the outer surface of the inner head cover, a part of the outer surface of the hub, the inner surface of the first cover plate and the inner surface of the second cover plate are confining an annular shaped space in order to allow a gas to flow from inside the inner head cover through the first opening, the annular shaped space and the second opening and to be discharged through the third opening into the water passage during operation of the refurbished facility.