Abstract: Methods and apparatus are disclosed for directing a fluid spray against the interior surfaces of a reaction vessel, without the need for operating personnel to enter the vessel, employing orbiting nozzles connected by a conduit to an annular piston assembly all of which are contained in a cylinder housing mounted externally on the vessel. A hydraulic or pneumatic force is alternately applied to one of the piston surfaces, the rate and direction of movement of the piston, and hence the spray nozzles being controlled by the discharge of fluid from the cylinder housing on the opposite side of the piston. In one embodiment the hydraulic pressure used to drive the piston is provided by the same pressurized fluid which is used in the spraying operation.