Abstract: A process vessel cleaning tool comprises a metal tube which can be introduced through an insertion port and sealing assembly into a high temperature process vessel. The metal tube supplies liquid under pressure to a cleaning tool head located at the leading end of the metal tube. The tool head is of generally cylindrical form and has a rolling contact element such as a roller, wheel or ball caster; which is axially displaced a short distance from a rotary jet nozzle head. The rolling contact element permits the tool head to pass readily over rough surfaces in the cyclone passages e.g. over Hexmesh™ and refractory or weld seams and around changes in direction in the snouts and diplegs of the cyclones within the process vessel. The metal tube with its unique tool head design is capable of being introduced into the cyclone while it is in operation, thus enabling considerable economies to be effected.