Fine chemicals and products of the pharmaceutical industry are produced predominantly in multi-purpose plants. Characteristically, they comprise of many recipes and very different processes. The challenge is often to adapt a certain process to a given plant. At first the sizing scenarios must be identified by means of a systematic risk analysis. Secondly, the process operation must be bounded to fulfill the requirements of the existing protection concept of the plant. This includes proper limits of the process control interlocks and the proper operation of safety devices and following disposal systems. A general concept is presented for the safe operation of multi-purpose plants. Starting with the identification of hazardous potentials an evaluation by means of critically classes is demonstrated. A sizing procedure is given for recalculating an existing safety device according to ISO 4126 part 10. It is based on a concept of energy specific valve seat areas. About 60 typical solvents are applied to the concept.