Abstract: A method of sealing a pipe and a device for sealing the pipe which prevents gas leakage out of a pipe stub plugged by a stopper disposed in a tee fitting, to allow a permanent cap to be welded onto the pipe. The device comprises an expandable plug comprising a pair of plates with a sealing member disposed between the plates. The plates can be squeezed together, for example by tightening nuts onto bolts extending through the plates to compress the sealing member and cause the periphery of the sealing member to expand against the inner wall of the pipe. Stabilizing means, for example a nut having a reverse thread and a complimentary tool, is provided for stabilizing the expandable plug radially within the pipe as the plates are squeezed together. The expandable plug is inserted into the pipe stub after the bypassed section of pipe has been removed, and lodged in the pipe by squeezing the plates together to expand the sealing member against the inner wall of the pipe stub.