A water-degradable welding purge dam apparatus for purging a weld zone of a pipe assembly comprising first and second pipes having respective first and second ends to be welded together at a root gap, characterized in that: a first purge dam subassembly has a hollow first conduit defining a first air flow pathway therethrough and having one or more air flow blocking plates (24, 26, 28) fixedly mounted on said first conduit; a second purge dam subassembly has a hollow second conduit defining a second air flow pathway therethrough, and one or more air flow blocking plates (36, 38, 40) fixedly mounted on said second conduit; a hollow third conduit defines a third air flow pathway therethrough, said third conduit being interconnectable to said first and second conduits members; said purge dam apparatus being installable in said pipe assembly such that: said first purge dam subassembly is arranged inside said first pipe end, said second purge dam subassembly is arranged inside said second pipe end, and said third conduit is interconnected to said first and second conduit and spans said root gap; said one or more blocking plates of said first and second purge dam assemblies respectively engage inside walls of said first and second pipe ends in order to retain a purge gas in said weld zone and to prevent air from entering said weld zone from said first and second pipes; and said first, second and third air flow pathways allow pressurized air within said pipes to pass through said purge dam apparatus and said weld zone in a sequestered by-pass zone defined by said first, second and third conduits; and said purge dam apparatus comprises one or more water degradable materials so that it can be flushed from said pipe assembly following welding. a first purge dam subassembly has a hollow first conduit defining a first air flow pathway therethrough and having one or more air flow blocking plates (24, 26, 28) fixedly mounted on said first conduit; a second purge dam subassembly has a hollow second conduit defining a second air flow pathway therethrough, and one or more air flow blocking plates (36, 38, 40) fixedly mounted on said second conduit; a hollow third conduit defines a third air flow pathway therethrough, said third conduit being interconnectable to said first and second conduits members; said purge dam apparatus being installable in said pipe assembly such that: said first purge dam subassembly is arranged inside said first pipe end, said second purge dam subassembly is arranged inside said second pipe end, and said third conduit is interconnected to said first and second conduit and spans said root gap; said one or more blocking plates of said first and second purge dam assemblies respectively engage inside walls of said first and second pipe ends in order to retain a purge gas in said weld zone and to prevent air from entering said weld zone from said first and second pipes; and said first, second and third air flow pathways allow pressurized air within said pipes to pass through said purge dam apparatus and said weld zone in a sequestered by-pass zone defined by said first, second and third conduits; and said first purge dam subassembly is arranged inside said first pipe end, said second purge dam subassembly is arranged inside said second pipe end, and said third conduit is interconnected to said first and second conduit and spans said root gap; said one or more blocking plates of said first and second purge dam assemblies respectively engage inside walls of said first and second pipe ends in order to retain a purge gas in said weld zone and to prevent air from entering said weld zone from said first and second pipes; and said first, second and third air flow pathways allow pressurized air within said pipes to pass through said purge dam apparatus and said weld zone in a sequestered by-pass zone defined by said first, second and third conduits; and said purge dam apparatus comprises one or more water degradable materials so that it can be flushed from said pipe assembly following welding.