Abstract: A movable scaffold for use in gaining access to the inside wall of an upright cylindrical tank such as a petroleum refinery reactor tower. The scaffold comprises a central frame having four radially extending, adjustable arm sets each connected to a scaffold platform, or cage, and hydraulically actuated to bring the platforms into working proximity with the inside tank wall. The scaffold platforms are each connected by cables received over sheaves at the top of the tank to winches outside the tank. Said winches are controllable to rotate the platforms within the tank, and/or to adjust the elevation of the platforms within the tank.