Which of the following defects is not acceptable for metal lines?

The identification of cracked flares as an unacceptable defect highlights the critical nature of structural integrity in metal lines. A cracked flare compromises the component's ability to withstand operational pressures and stresses, which can lead to failure under typical service conditions. Flare connections are vital for maintaining a seal, especially in applications involving fluid transport; any crack can result in leaks, which may have safety, environmental, and operational implications.

In contrast, minor surface scratches, seams, and dents in the heel of a bend that are less than 20% of the diameter might be acceptable under certain industry standards. Minor scratches typically do not affect the functionality of the metal lines, as they do not compromise the material's overall structural integrity. Seams may be acceptable depending on their depth and the type of metal processing involved, while small dents that fall within specified tolerances likely do not significantly impact performance or safety. Thus, the distinction lies in how each defect affects the fundamental properties and safety of the metal lines.