Circumferential weld on tank annular plate

[massad]

Is circumferential welding on outside of tank annular plate is allowed? Means if the outside projection of annular plate is less than specified in API then can a butt welded ring be circumferentially welded to annular plate in order to increase the outside projection.

 

[MATEGRITY2009]

New or in-service tank?

 

[massad]

NEW TANK

 

[Bonswa]

How far is the projection? is it close?

 

[JStephen]

You've asked about this and the 3-way laps near the shell. I think in both cases, those requirements represent good practice, but likely can't be quantified by any calculation. I doubt that either one is going to make the tank fall down, and that leaves it as primarily a contractual issue between fabricator and owner.

The bottom plate near the shell gets stressed to some extent in hoop tension, and presumably, this effect becomes more pronounced on a large tank, so it is desirable to keep 3-way laps out of the highly stressed area. The larger the tank is, the more critical I would view this. The minimum chime dimension helps support the shell on a soil foundation, and a greater extension helps make sure you have room to land repads on it. Also, that area is subject to corrosion, which is usually worse at the edge. Note that the required chime is less on App. J tanks, and may be less on App. A tanks as well.

In either case, it is undesirable to build the tank in that way, but also undesirable to do a lot of re-work to fix it, too, as that re-work is going to induce residual stresses and reduce toughness in that area. It becomes a matter of judgment and negotiation.

 

[massad]

@ Bonswa : Its a new tank 70m , but after welding of bottom plate it shrinked and the OD of annular plate become less , hence the distance between the shell OD and the outer edge of annular plate become less than that is specified in API. The reason of this question was, if its only about the projection of annular plate so can another strip be welded at the outside edge on annular to increase the projection.