Metallic flexible joints shall not be permitted in the pipeline for?

Metallic flexible joints are not permitted in the pipeline for dielectric breaks primarily because these joints can create electrical continuity in a system where electrical isolation is necessary. Dielectric breaks are components designed to prevent electrical current from flowing between different sections of a pipeline, ensuring that different parts of a system do not inadvertently create a path for electrical conduction. The presence of metallic flexible joints would compromise this isolation, allowing unwanted electrical flow and potentially leading to safety hazards or damage to the system.

In contrast, the other options relate to functions where metallic flexible joints may be more appropriate or necessary. For instance, they can be used in expansion joints to accommodate thermal expansion or in systems designed for vibration control, where the flexibility helps absorb and mitigate movement or stress.