a method and apparatus for expanding a replacement thermoplastic pipe installed within an existing conduit is provided . a pipe expansion apparatus having a flexible substantially cylindrical bladder and a relief valve to inflate the bladder to a predetermined gauge pressure and allow fluid to exit the expansion apparatus and fill and pressurize the area behind the apparatus is inserted within the collapsed pipe . fluid , such as cold water , is introduced into the apparatus to inflate the inflatable section to conform substantially to the interior diameter and configuration of the existing conduit . as the area behind the expander fills with pressurized fluid , the expansion apparatus is pushed forward , thereby pushing and expanding the replacement pipe up against the interior of the existing conduit . steam is injected ahead of the apparatus to maintain the replacement pipe in a heated flexible condition . when the fluid introduced into the expander is cold water , the replacement pipe will cool and become rigid almost immediately after it is expanded and significantly reduce stress formation .