Abstract: The concept and risks of medium blow-by in hydrogenation unit are introduced. According to the operating pressure, the hydrogenation unit is divided into high pressure, medium pressure and low pressure systems, and medium blow-by occurs in the connection parts of systems with different pressure levels. Typical medium blow-by is divided into five categories, i.e., medium blow-by in high-pressure pumps, compressors, high-pressure vessels/columns, multi-branch systems and special parts. The prevention and control philosophy for medium blow-by is put forward, that is to carry out the prevention and control step by step in the sequence of anomaly discovery and alarm, software prevention and control measures, hardware prevention and control measures, and final prevention and control measures. The multi-level combined prevention and control measures are given against the five categories of medium blow-by risks in hydrogenation unit. These philosophy and measures can be applied in various refining and petrochemical plants for reference.

Key words: hydrogenation, medium blow-by, risks analysis, prevention and control philosophy, types of medium blow-by, prevention and control measures, refining and petrochemical plant