Transformers with expansion vessels are same with hermetic transformers in terms of core and coil structures.
Sizes of expansion vessels of transformers are designed in a way that it shall store expansion of oil increased with temperature as the result of the calculations.
Since transformers with expansion vessel are open to atmosphere, oil pressure changing with the thermic effects, air circulation is ensured by dehumidifying the air by silica gel in the vessel of the transformer and oil pressure changing.
As the result of the air circulation arising out of heat differences depending on the load of transformer with expansion vessel operating, it loses its silica gel characteristics in time and causes that oil is humidified and its structure is spoiled. Thus, there are maintenances in certain periods such as exchanging silica gel and measurement of disruptive voltage by taking sample from the oil.