Pipes larger than what diameter should not be screwed but instead flanged?

The appropriate threshold diameter for transitioning from screwed connections to flanged connections is typically determined by the need for reliability and ease of assembly. Flanged connections are preferred for larger pipes due to several factors, including ease of assembly, maintenance, and the ability to better manage the stresses and pressures involved in piping systems.

When the diameter of the pipe exceeds a certain size — in this case, 63.5 mm — the increased weight and potential for thermal expansion or contraction can complicate screwed connections, making them potentially less reliable over time. Flanged joints provide better robustness, facilitate easier alignment during installation, and allow for quick disconnection for maintenance or repairs, which is crucial in industrial applications where downtime can be costly.

In contrast, smaller pipes, like those with diameters below this threshold, can still efficiently use threaded connections, as they are generally lighter and easier to handle. Thus, the choice of 63.5 mm as the transition point aligns with industry practices emphasizing safety and efficiency in fluid transport systems.