Internal Design Pressure Affecting Overturning Moment 2

[Leinster]

Probably a simple one....

As per API 650 why does the internal design pressure of a tank have any bearing on the overturning moment? Why is only the uplift taken into account whereas the pressure would act equally in all directions so the downward pressure would cancel out the uplift.

 

[IFRs]

Internal pressure tends to lift the tank off the ground because the bottom has no strength in that direction. Pressure on the circular tank shell is resisted without movement.

 

[Leinster]

Thanks for the reply...

Sorry please could you explain "because the bottom has no strength in that direction".

If the pressure is acting in all directions it would surely be pushing the bottom plates down just as much as creating uplift on the roof.

 

[IFRs]

If the pressure on the underside of the roof exceeds the weight of the roof and the tank shell, the tank shell will lift off the foundation. The bottom is basically flat thin steel. It can not hold the tank down. It will bend and the corner weld will be stressed. This needs to be avoided. So, anchors are used.

 

[adamuk]

Flat, thin plates generate high membrane stresses under pressure which will cause them to deform as IFRs states, the net effect of this is that the edge of the plate will be raised upward.

Adam Potter MEng CEng MIMechE

http://www.ax-ea.co.uk

 

[IFRs]

Put a paper cup around your mouth, sealed against your cheeks. Blow into it. See the bottom become convex? Same thing.