Commonly the plant area is divided into the ESD Zone. Each ESD zone may contain one or more equipments. ESDV or SDV valves are provided in each ESD Zone to isolate the system zone. In case of fire, a system will be isolated by those SDV valves. Then the inventory fluid (commonly gas phase only) in the system will be released to flare through BDV valve. Commonly one BDV is provided for one system zone, but in some cases, it is possible to provide more than one BDVs in one system zone.