Water tracking and what other condition typically occur at the header ends of water wall tubes?

Water tracking in the context of water wall tubes refers to the movement of water along the outer surface, which can lead to significant wear and damage over time. The correct answer highlights cratering, which is a condition that typically occurs in this scenario.

Cratering is characterized by the formation of depressions or pitting on the surface of the metal, often due to the combined effects of high temperatures and the continuous impact of water. This leads to localized erosion that can severely degrade the structural integrity of water wall tubes. The combination of heat and water fluctuations creates an environment where cratering is facilitated, especially at the header ends where water flow may not be uniform.

In this case, while other conditions like cavitation, corrosion, and wear may also occur in the vicinity of water wall tubes, cratering specifically captures the nature of surface damage associated with the phenomenon of water tracking. Therefore, understanding that cratering is a consequence of fluid dynamics and thermal stresses is critical for maintaining the longevity and safety of water wall systems.