In which type of tubes are thick-lipped ruptures most commonly found?

Thick-lipped ruptures are most commonly found in superheater tubes due to the extreme operating conditions they are subjected to within a steam generating system. Superheater tubes are designed to heat steam above its saturation point, which creates high temperatures and pressures. Over time, these factors can lead to metal fatigue, overheating, and localized failure.

The term "thick-lipped rupture" refers to a specific failure mode where the metal at the rupture site loses its ductility, resulting in a characteristic lip or lip-like formation at the rupture edge. This type of failure typically occurs in areas where there are sharp thermal gradients or stresses that exceed the material's yield strength.

In contrast, water wall tubes, economizer tubes, and evaporator tubes do not experience the same combination of conditions that contribute to thick-lipped ruptures. Water wall tubes are generally less subjected to extreme temperature differentials, economizer tubes primarily handle lower-temperature feedwater, and evaporator tubes also work under different operational stresses compared to superheater tubes. Therefore, superheater tubes are indeed the most susceptible to thick-lipped ruptures among the options provided.