What type of unions must be used for NH3 piping that is 3/4" and smaller?

The correct choice is ground faced socket welded unions, typically required for ammonia (NH3) piping that is 3/4 inch and smaller due to the specific properties and handling requirements of ammonia. Ground faced socket welded unions provide a strong, leak-tight seal suitable for high-pressure piping systems. In environments where ammonia is present, the integrity of the piping system is critical, and these unions reduce the risk of leaks that could occur with other types of connections.

Socket welded unions also allow for secure connections that can withstand the stresses and potential thermal expansions associated with high-pressure systems. The ground facing helps in achieving precise alignment and greater surface contact, enhancing the overall reliability of the joint.

In contrast, other types of unions may not offer the same level of security or compatibility with high-pressure ammonia applications. For instance, threaded unions may present risks of leaks or corrosion over time, while swivel unions might introduce complexity in the piping layout and also be less reliable under high pressure. Flanged unions, while robust and easier to install, are generally used for larger diameter pipes and can be more cumbersome in smaller piping systems like those under consideration. Thus, ground faced socket welded unions are the most suitable choice for safe and effective ammonia piping connections in this particular scenario.