55

@MadScientist: Actually, that's just a water tower in the background, on the far side of the storage tank.

@Shar: I don't work at this part of the plant and don't know what they call it. The "bubble" is just a water tower behind the storage tank --not part of the tank. And I'm not sure since I don't work this part of the plant, but I think the "55" just designates a storage location in the pole/material yard.