Which method is not used for joining plastic pipe, tubing, and fittings in gas piping installation?

In gas piping installations, the methods used for joining plastic pipe, tubing, and fittings primarily involve techniques that create a strong, reliable bond suitable for handling gas. The heat-fusion method involves heating the pipe and fitting ends until they become soft and then fusing them together, creating a seamless joint. The solvent cement method uses a special adhesive that chemically bonds the plastic surfaces, effectively merging them into a single piece. The adhesive method is often used interchangeably with solvent cement, specifically for materials compatible with such adhesives.

Welding, however, is not a method applicable to plastic materials used in gas piping. Typically associated with metal pipes, welding involves melting edges to join metals together. Since plastics cannot be welded in the same way as metals, this method is not suitable or employed for gas piping installations involving plastic components. Therefore, the correct answer identifies welding as the method that is not used for joining plastic pipe, tubing, and fittings in this context.