Why is bolting together ASME/ANSI flanges to API flanges not recommended?

Bolting together ASME/ANSI flanges to API flanges is not recommended primarily because although they may have similar dimensions, their pressure ratings can differ significantly. ASME/ANSI flanges are designed with specific pressure-temperature ratings that are usually based on the material properties and the intended service conditions. On the other hand, API flanges are tailored for specific applications in the petroleum and natural gas industries, which can involve different service conditions and design methodologies.

When these two types of flanges are combined, the disparity in pressure ratings can lead to potential safety hazards, such as leaks or flange failure under pressure. The inability to match the pressure ratings means that even if the flanges fit together physically, they may not perform reliably under the same operating conditions, leading to operational risks.

The other considerations, such as material differences or alignment issues, may also play a role in flange compatibility, but the critical factor in this context is the mismatch of pressure ratings, which impacts the system's overall integrity and safety. This highlights the importance of using flanges that are designed to be compatible with each other in terms of their pressure ratings to ensure safe and effective operation in piping systems.