What should not be used for unions or gaskets?

Unions or gaskets are critical components in piping systems, specifically designed to create tight seals and provide connection points that can be disassembled for maintenance or repair. In this context, the answer indicates that using "gaskets or packings" for unions is inappropriate.

Gaskets are typically used to form a seal in static applications where two surfaces are bolted together. However, unions are mechanical fittings that allow for assemblies to be disconnected without cutting pipes. Using gaskets or packings in unions can lead to potential issues such as leakage or misalignment since unions rely on flat surfaces meant to be compressed evenly, while gaskets are designed for static seals. The structure and intended function of unions do not require or benefit from the additional sealing material that gaskets or packings would introduce.

In contrast, flexible seals, rubber fittings, and adhesives might have applications in different contexts, particularly where flexibility and bonding are needed, but they don't fulfill the specific requirement for using gaskets in union applications. Understanding the right applications for each component enhances the efficiency and safety of a piping system.