Critical Zone in Floating Roof Tank

[PindiKing]

Hi,
Can somebody explain what is the critical zone in a floating roof tank. Why it is Critical.

Thanks,

PKing

 

[JStephen]

Generally, the corner weld and bottom plate just inside the corner weld- should be defined in API-653. (Same for fixed roof or floating roof tanks). Higher stresses in that area than in the bottom out away from the shell.

 

[jeyantank]

The portion of the tank bottom or annular plate within 3 in. of the inside edge of the shell, measured radially inward

 

[PindiKing]

Stephen , Jeyant,
Thanks for your responses. However, in the mean time I got a clue on critical zone that when floating roof lands on its legs then in this zone the tank volume calibration is very difficult. Due to this reason this area is termed "Critical Zone" ??

 

[DSB123]

PindiKing,
Surely you mean the product volume since the volume under the roof is easily calculated (Height of legs times the tank plan area)

 

[jeyantank]

Critical zone (tank calibration)

In tank calibration reports, volume will be given for each mm of gauge height (liquid level is measured by Radar or floating level gauge or manual dip).

When the floating roof starts floating, the volume of the liquid inside the tank cannot be measured accurately. Because the volume of displaced liquid by the floating roof for an assumed specific gravity of liquid may change when the roof just starts to float. So this zone is called as critical zone.

Jeyan