Modular process plants are small or medium scale plants which consist of separately engineered and automated modules. To speed up the start of production, a modular process plant is composed from several of such modules. Nowadays, safety engineering refers to the whole process plant, is very individual, time consuming, and hardly integrated. For safe modular process plants, the safety engineering has to be performed and implemented on module level to keep the engineering processes of the modules and the modular process plant independent. The safety engineering has to be more integrated to keep the advantage of modular process plants. This paper presents the challenges of such a modular safety engineering and our approach to cope with them. We use a case study of an exemplary modular process plant to show the results of a modular HAZOP study with limited information and how to combine the modular HAZOP studies to fill the gaps.