API 650 Figure F.1 Detail i weld size

[IFRs]

API 650 Figure F.1 Detail i does not specify the size of the weld joining the compression bar to the tank shell. Any wisdom out there? How about for a shell 0.875 inches thick and a compression bar 1.25 inches thick? Would a 0.25 inch fillet weld be OK?

 

[fegenbush]

Per 5.1.3.3 of API 650, the minimum fillet weld size is 1/3 of the thinner member being joined.

 

[IFRs]

Excellent. Thanks! Do you know of any rational or calculation to size this weld?

 

[fegenbush]

Since I have only seen Detail i used when significant internal or external pressure or is present, I would calculate it based on the following loads:

(Internal pressure * Horizontal projected area - Dead Load of Roof) / Circumference = lb/ft of circumference

(External pressure * Horizontal projected area + Dead Load of Roof) / [COS (Roof Angle) * Circumference] = lb/ft of circumference

The second case assumes no center support. This is just one possible approach, and I am sure there are some factors that I have missed.

 

[JStephen]

I think the calculated size based on load will generally come out very small.

 

[IFRs]

JStephen - you are probably correct but the thick shell and massive inch compression bar joined by a 0.25 fillet weld jumped out at me, begging for some review. The one-third thickness minimum increases this weld and leaves me feeling a little better.

 

[IFRs]

I'm also concerned because a small weld may be an issue because the thicker plate acts as a heat sink, and we want to use a large enough weld so the cool thicker plate does not quench the weld as it cools. This quenching can lead to weaker, crack susceptible welds unless the thicker plate is preheated. I am probably over-thinking this...

 

[JStephen]

My point wasn't that the small weld was okay, but rather that attempting to size it by calculated load alone will usually not give a helpful result.