Off-stream decoking of a minor portion of on-stream thermal cracking tubes

In a process for thermally cracking hydrocarbons by passing them in admixture with steam through a multiplicity of tubes arranged in parallel in a cracking furnace, the tubes being subjected to radiant heat sufficient to cause the reactants within the tube to have a temperature of 1200-1600 DEG F., a minor proportion of the tubes are taken offstream by cutting off hydrocarbon feed and passing steam and/or water through said tubes in amount sufficient to maintain the tube temperature at essentially the same level as in the onstream tubes and to effect removal of coke deposit on the interior of the tubes, and thereafter returning said tubes to normal onstream operation. The tubes may pass through a convection-heated section prior to the radiant section, in which case the steam temperature at the cross-over point is at least 700 DEG F. One tube at a time may be decoked in this way, or more than one in which case the change-over may proceed successively.