Sewer lines (pipes) are connected to the city sewer (or septic tank) on one end, and to a toilet flange on the other. The flange is circular, has a hole on the middle for waste to pass through, and is screwed to the toilet floor. The toilet then "sits" on the flange, with a wax ring situated between the two to avoid waste spillage. Replacing the sewer line and flange requires cutting the sewer line at the desired spot, connecting a new sewer line to the existing line, and finally attaching a new flange to the end of the new sewer line.
Choose the exact location where you want to replace the existing sewer line. Cut the line in the chosen place, making sure that the cut is straight. Remove the screws where the flange is connected to the toilet floor. Also cut the sewer line 6 inches away from the 90 degree elbow joint, below the flange.
Pull the flange and elbow joint up through the hole in the floor and place to one side. Also remove the old section of sewer line. Push the new flange down into the hole in the bathroom floor until the flange lip (with screw holes) sits snugly on the floor. Glue the bottom of the flange and male end of the elbow joint. Push the joint up onto the flange, holding it in position for 30 seconds. When dried, turn the elbow joint in the direction of the existing sewer line.
Measure the distance from the inside bend of the new elbow joint to the end of the existing sewer line. Depending on distance, more than one piece of sewer line (usually 8 feet in length) may be needed. If the existing sewer line is lower than the elbow joint, different types of couplings (30, 45, 60 degree) may be required, as well as straight couplings. It's best to study the old sewer line and replicate the types of coupling, as well as where they were located in the old sewer line.
Work your way from the new elbow joint, cutting new sewer line and gluing couplings as required. Always glue both the inside of the coupling and the end of the new sewer line before pushing the glued coupling onto the end of the new sewer line. Hold each new joint for 30 seconds, for the glue to set. Once the new sewer line is connected to the existing line (and all glue is dried), use 1 1/2 inch galvanised screws to screw the flange to the bathroom floor.
Sewer lines, flanges and couplings are generally ABS (Acrylonitrile Butadiene Styrene) black plastic. The glue will be ABS also, and requires no primer. Flanges generally have a 90 degree elbow joint directly connected to them. The elbow joint is then connected to the sewer line. Sewer lines are generally 3 inches in diameter, as are all couplings. However, check your existing sewer line first for the right size before purchasing any new sewer line, couplings, flange, etc.