Vacuum

[parrot77]

I am currently competing a report on a roofed thin walled steel tank, whose walls have structurally failed. The designers calculations only designed for the load condition whereby the tank was full, ie hydrostatic presssure. The tank was being emptied for servicing however, through a scour valve at the bottom. Negative pressures or a vacuum built up over the head of liquid as the liquid drained away. As the level of liquid in the tank reduced the negatve pressure caused compression buckling in the tank walls. Can anyone indicate to me how i would calculate the vacuum presssure in the sealed tank? How would this vacuum pressure manifest is as loading on the wall over the liquid?

 

[Bobber1]

It's a waste water tank. They probably closed the vent to eliminate odors.