The most common method for repairing leaking joints is hot gas welding at the fillet formed by the fitting socket entrance and the pipe. Hot gas welding of thermoplastics piping system joints is quite similar to the acetylene welding or brazing process used with metals. The fundamental differences are that the plastic rod must always be the same material as the pieces to be joined and heated gas, rather than burning gas, is used to melt the rod and adjacent surfaces. Welding with plastics involves only surface melting because plastics, unlike metals, must never be “puddled”. Therefore, the resulting weld is not as strong as the pipe and fitting material, or the solvent welded joint. This being the case, hot gas welding as a repair technique is recommended for minor leaks only.
Hot gas welding is not recommended as a primary joining technique for pressure rated systems and may only be used to repair DWV systems or low pressure systems operating at less than 20 psi.
The procedures for making good thermoplastic welds can be summarized into five basic essentials:
- Correct Welding Rod Material
- The welding rod should be the same material as the pipe and fitting. For example, if welding CPVC 24448 then use CPVC 24448 rod. In addition, the welding rod should have the same mechanical properties as the pipe and fitting. Color alone does not confirm that the weld rod is compatible with the pipe and fitting material.
- Correct Heating
- Excessive heating will char or overmelt. Insufficient heating will result in incomplete melting.
- Correct Pressure
- Excessive pressure can result in stress cracking when the weld cools. Insufficient pressure will result in incomplete fusion of the rod material with the base material.
- Correct Angle
- Incorrect rod angle during welding will stretch the rod and the rod material with the base material.
- Correct Speed
- Excessive welding speed will stretch the weld bead and the finished weld will crack upon cooling.
The quality of any welded joint is highly dependant on the knowledge and skill of the operator. The operator needs to be knowledgeable in both the setup of the equipment and the proper execution of the weld.