Tank Clip Pads without Rounded Corners 3

[CobusvanJ]

Hi,

A contractor known worldwide is building 3 tanks for us in a foreign country, and they had to weld some pads to the shell for platforms and stairs clips. Drawings were only submitted after the pads were already welded to the shell, and it turns out the pads do not have rounded corners but 90 degree ones. Now we all know that that is a very easily avoided no-no, but it is what it is...

Question is, since these are not high pressure vessels but API 620/650 tanks, am I unnecessarily worried about this issue as the clips would at most see about 750 kg loading each? Or would the stress concentrations at the corners lead to problems later on?

 

[jtseng123]

You have legitimate concern.
Ask the contractor to prove that is their common practice without problem.
If the contractor can not prove that is common practice, I believe you have the right to ask for a finite element analysis at their own cost.

 

[jharris3]

I would check API 650 for some requirements for the corners. I think they still need to be rounded to some extent. If there isn't anything there then you could perform a fairly quick hand calculation using Blodgett's design of weldments. I think chapter 5 talks about clips and repads.

Your question really depends on the thickness of the tank wall, the size of the repad, and the eccentricity of the load (for moments). Are there nozzles (with loadings) near enough to have local effects become a problem? How tall is the tank? what is the strain on the outer surface of the tank? How good was the welding?

Otherwise, no its not necessarily a problem. Most of the time you will get to rebuild the tank before this sort of thing causes cracks and that will depend the most on the quality of the construction (welding).

 

[JStephen]

I've seen it done both ways. I'm not aware of any requirement that these kinds of pads have rounded corners. API-653 requires lap-welded patches to have rounded corners, but there, you're also assuming that part of the hoop force of the tank is carried by the pad plate.

Note that if you go to analyze shell stresses due to a plate like that per WRC 107 or similar methods, you'll assume that the pad is either rectangular or round- but rectangular with rounded corners is not even one of the options considered there.

 

[MJCronin]

Uuuumm..... It was always my understanding that the "square corner" vs. "round corner" weld pad issue was related to the overall quality of the weld.

As I recall, rounded corners on a pad meant fewer "starts and stops" which promoted a better welded pad.

Anyone ?

 

[doct9960]

CobusvanJ,

API Std 650 requires rounded corners on all non-circular pads.

From API Standard 650, Twelfth Edition...

5.8.11.4 All non-circular miscellaneous pads shall have rounded corners with a minimum radius of 50 mm (2 in.).

 

[dhengr]

CobusvanJ:
I don’t use the API Codes very often, so I couldn’t find a specific paragraph off the top of my head, but Doct9960 seems to have found one. Maybe slightly prescriptive when you don’t know the pad size and thickness or the shell size and thickness, or the loads and how they are actually applied. But, that sounds like a nice clean detail, even if the corner radius is somewhat less than 50mm. 1.6 (750kg) to 2k doesn’t sound like a very big load from the shear and weld standpoint, but it could produce are fair sized moment perpendicular to the plane of the shell.

Some thoughts on the matter....
1.) You can weld continuous around a round corner, which is nice and clean, with fewer starts and stops, etc.
2.) From the Theory of Elasticity and weld design standpoint we all know that corners with weld lines and stresses and residual stresses all coming together at a point, from different directions causes a high localized three dimensional stress situation. The radiused corner softens or eliminates this.
3.) When you try to weld around a sharp corner, as welders are want to do, they invariably leave a notch in the standing sharp corner, at the top toe of the fillet on the pad. This is a stress raiser which could be a problem under some loading conditions. This happens because the weld puddle can’t hold its shape, and sloughs down, at the sharp corner. The weld puddle surface tension goes to zero at the sharp corner and the molten puddle sloughs. This problem can be eliminated with a fairly small radius on the corner. But, the multi directional stress condition would still exist.
4.) What stress levels are you calculating in these pads and the shell around them? Is the stress level really an issue, even if you do take a look at the combined stress picture?

 

[CobusvanJ]

Thank you for all the responses.

dhengr, due to some other 'contractual' factors I cannot calculate these for myself, but I (and seemingly correctly based on the responses) had a concern about these pads having 90 deg corners.

 

[JStephen]

Thanks, doct. I was just checking, and that's not a new requirement, either.

Note that attachments can be welded directly to the shell- so you can weld a rectangular item to the shell, but if there's a repad between it and the shell, the repad needs the rounded corners.

 

[Duwe6]

Gotta disagree with JStephen. "Misc pads" in my mind encompasses all non-linear attachments; i.e. a flat-bar ladder clip that is welded with the end of the bar onto the tank shell is 'linear' and not a pad. Any other shape of attachment requires rounded corners. So if that flat-bar clip needs a pad under it to spread out the load, the corners have to be rounded off. Tanks are a lot thinner than pressure vessels, thus the need to keep weld stresses very low.

 

[JStephen]

A flat bar has a rectangular cross-section welded to the shell, but doesn't require rounded corners, is my point.