A decoking device suitable for use on a reaction vessel for the thermal cracking of heavy petroleum oils, including an assembly mounted on top of the reaction vessel and having a cylinder and a piston slidably and rotatably received in the cylinder, the cylinder and piston defining a scrubbing liquid chamber which is sealed from the outside. A scrubbing liquid is fed through the scrubbing liquid chamber to a main injection pipe to inject the liquid through a multitude of jet nozzles in the main injection pipe against the inner wall surfaces of the reaction vessel, the main injection pipe being connected to the piston for movement therewith.