Info double annular plates API 620 - LNG Tank

[TANKO]

Good afternoon,

In the drawings of our client,we are realizing that there are 2 rings of annular plates (for the bottom vapour barrier).

Where can i see this construction detail in the API 620?I would like to know the technical reason.

We are building a LNG tank.

Regards

 

[Duwe6]

Thermal expansion and contraction. The lower ring is a sliding surface for the shell-to-upper ring.

 

[TANKO]

For this reason do we use 2 rings & not 1 ring??????

 

[Roohol]

if this tank is double wall, 2 annular plate are needed too.
and if there is an annular plate that is built with 2 layer of plate it may be because of needed thickness.