The nameplate capacity of chemical plants, and in particular ammonia plants, has been steadily increasing over the past 30 years. As the plant capacity grows, so does the cost of an unscheduled production stop. Furthermore, the safety awareness in chemical complexes has been steadily increasing, resulting in more complex trip systems. This article analyzes the complexity of the control system using tools known from safety integrity level (SIL) analysis, and it describes how the plant availability and reliability may be affected by the selection of different control philosophies. This study concludes that a two out of three voting trip system is a sound investment in a modern plant, and that an even more advanced system will add benefit to the plant owners. (C) 2010 American Institute of Chemical Engineers Process Saf Prog 29: 278-282, 2010