Abstract: In a system in which compressed ethylene is scheduled to flow through pipes, and in which there is danger of decomposition of ethylene and danger of the spreading of a reaction boundary of the exothermic decomposition reaction, a method and means are provided for quenching the advancement of the decomposition. Ethylene is directed from large diameter pipes into an array of spaced apart small diameter tubes which are not overheated when subjected to the combination of head and tail boundaries of the advancing plug of decomposition products. Such overheating of the tubes by the leading and trailing flames can be prevented by cooling the exterior of the tubes with a fluid normally maintained at about ambient temperature, thereby cooling the compressed ethylene sufficiently to quench the decomposition reaction. The cooling fluid is desirably an aqueous system adapted to be converted to steam under severe conditions.