Which of the following is NOT a common method for sealing pipe threads?

Using a flange is not a common method for sealing pipe threads because flanges serve a different purpose in piping systems. Flanges are used to connect two sections of pipe or to connect a pipe to another component, such as a valve or a fitting, and provide a mechanical joint that can be bolted together. They do not seal the threaded connections themselves; rather, they create a connection that may involve a gasket to create a seal between flat surfaces.

In contrast, tapering the threads, applying joint compound, and wrapping with Teflon tape are all established techniques specifically for sealing threaded joints. Tapering the threads allows for a better fit as the threads engage, while joint compound and Teflon tape provide a filler material that helps to fill gaps and create a tighter seal, preventing leaks in the piping system.