On what size pipe should screwed unions not be used?

Screwed unions are typically used in plumbing and sprinkler fitting applications to allow for easy disconnection of piping systems. However, their use becomes impractical and potentially problematic as pipe sizes increase. For example, pipes larger than 2 inches can generate significant stress and tension due to fluid dynamics and connection tightness, which can lead to leaks and mechanical failures.

Using screwed unions on larger pipes introduces complications in terms of strength and sealing capability. The larger the diameter, the more challenging it is to securely connect and seal the union with conventional methods. For pipes above this size, other types of fittings, such as flanged or welded connections, are preferred because they provide greater strength, integrity, and reliability in high-pressure applications.

Therefore, indicating that screwed unions should not be used on pipes larger than 2 inches takes into account both mechanical safety and performance in piping systems.