The Viking Model D Pressure Operated Relief Valve (PORV) is used in Viking Deluge, Preaction, Firecycle® Multi-Cycle and Surefire® Systems. Once tripped, it maintains a positive vent to prevent the deluge valve from automatically resetting prematurely. The device is automatically reset when the pressure is removed from the control diaphragm. The device is designed to trip when the trip port is pressurized with water from the intermediate chamber of the control valve, or when the water in the inlet is drained from the prime chamber of the control valve. The Model D-4 PORV is similar to the D-3, except some components are specially plated for additional corrosion resistance.