A process and apparatus are provided for generating at least two types of steam having different purities in steam reforming processes in which at least two steam reforming units (steam reformers) (are operated in parallel. The first steam stream having a higher purity than that of the second steam stream. The steam reformers operated in parallel are combined to form groups of at least two steam reformers (groups) within which the high purity steam is generated in one of the steam reformers by vaporization of degassed and demineralized water and low purity steam is generated in at least one of the other steam reformers by vaporization of degassed impurity-containing water (dirty water). The dirty water is formed at least in part from the condensate (process condensate) produced within a group, and is predominantly made up of water.