One of the many novel features on ICI's leading Concept Ammonia Plants is the use of Slam Shut Valves instead of safety relief valves for certain important relief duties. The design intention was to reduce the environmental impact of gas releases to the vent and flare systems, to allow a smaller vent and flare system to be installed, and to contain gas within the plant systems to allow rapid re-start following a plant upset giving considerable savings in flaring and plant re-start time. As the design proceeded throughout 1984–86, various implications of the design were revealed, and these are discussed in this paper. Nearly 15 years of operating and maintenance experience on the 2 plants at ICI's Severnside factory have justified the decision to use Slam Shut Valves.