An inflatable purge dam apparatus for use with pipes (22, 24) of different nominal pipe size, comprising: a first inflatable purge bladder having a first interior gas-receiving chamber and defining a first port on one side thereof and a second port on another side thereof, said first port and said second port being aligned proximate to a central longitudinal axis of said first purge bladder; a second inflatable purge bladder having a second interior gas-receiving chamber and defining a third port on one side thereof, said third port being aligned proximate to a central longitudinal axis of said second purge bladder; a bridge conduit extending between said second port on said first purge bladder and said third port on said second purge bladder, said bridge conduit providing fluid communication between said first gas-receiving chamber and said second gas-receiving chamber; said bridge conduit having a pressure-sensitive gas discharge valve operable to release a purge gas into a weld zone formed between said first purge bladder and said second purge bladder when said first purge bladder and said second purge bladder have been inflated with said purge gas to a predetermined sealing pressure that is effective to seal said weld zone; said inflatable purge dam apparatus being characterized by: said first purge bladder and said second purge bladder each comprising an expandable inner purge gas retaining liner and a heat-resistant stretchable outer protective cover; said protective covers comprising a stretchable material that is more heat-resistant than a material used for said gas retaining liners, said stretchable material being constructed to undergo variable stretching to increase a size of said first purge bladder and said second purge bladder as they are inflated with said purge gas to said sealing pressure; and said variable stretching being sufficient to allow said first purge bladder and said second purge bladder to form an effective weld zone seal at said sealing pressure in pipes of different nominal pipe size. a first inflatable purge bladder having a first interior gas-receiving chamber and defining a first port on one side thereof and a second port on another side thereof, said first port and said second port being aligned proximate to a central longitudinal axis of said first purge bladder; a second inflatable purge bladder having a second interior gas-receiving chamber and defining a third port on one side thereof, said third port being aligned proximate to a central longitudinal axis of said second purge bladder; a bridge conduit extending between said second port on said first purge bladder and said third port on said second purge bladder, said bridge conduit providing fluid communication between said first gas-receiving chamber and said second gas-receiving chamber; said bridge conduit having a pressure-sensitive gas discharge valve operable to release a purge gas into a weld zone formed between said first purge bladder and said second purge bladder when said first purge bladder and said second purge bladder have been inflated with said purge gas to a predetermined sealing pressure that is effective to seal said weld zone; said inflatable purge dam apparatus being characterized by: said first purge bladder and said second purge bladder each comprising an expandable inner purge gas retaining liner and a heat-resistant stretchable outer protective cover; said protective covers comprising a stretchable material that is more heat-resistant than a material used for said gas retaining liners, said stretchable material being constructed to undergo variable stretching to increase a size of said first purge bladder and said second purge bladder as they are inflated with said purge gas to said sealing pressure; and said variable stretching being sufficient to allow said first purge bladder and said second purge bladder to form an effective weld zone seal at said sealing pressure in pipes of different nominal pipe size. said first purge bladder and said second purge bladder each comprising an expandable inner purge gas retaining liner and a heat-resistant stretchable outer protective cover; said protective covers comprising a stretchable material that is more heat-resistant than a material used for said gas retaining liners, said stretchable material being constructed to undergo variable stretching to increase a size of said first purge bladder and said second purge bladder as they are inflated with said purge gas to said sealing pressure; and said variable stretching being sufficient to allow said first purge bladder and said second purge bladder to form an effective weld zone seal at said sealing pressure in pipes of different nominal pipe size.