A 300° arc of skirt by formula

[ElCidCampeador]

Hi, how is it possible to calculate a skirt of a vertical pressure vessel (designed acc. to ASME VIII Div.1), where the skirt is not completely welded to the vessel for 360° but only for 300°?
Skirt is a rolled plate but hasn't longitudinal seam because edges are not welded to each other...it's not a circular opening it's a complete vertical access to the bottom of the vessel. In other words, skirt is welded to the head with a circular joint of 300° arc.

I checked different softwares but I still haven't found one that allows this calculation.

Now I'm designing this kind of vessel and I'm trying to find a way by formula to check if the calculation for 360° skirt is still valid for my case.

Do you have any reference? Do you have suggestions? I would create something like an excel file (with right formulas) that I could attach to main calcs to validate my skirt...thanks

 

[JStephen]

You can integrate and find the area, centroid, moment of inertia, and section modulus of a section like that without too much problem, and that will let you get a good estimate of the change in stresses involved due to eccentricity, etc.
The free edges will need to be stiffened in some way to retain the same buckling strength that they would have as a complete cylinder.

 

[ElCidCampeador]

You can integrate and find the area, centroid, moment of inertia, and section modulus of a section like that without too much problem, and that will let you get a good estimate of the change in stresses involved due to eccentricity, etc.

From your words it seems easy...but I have 10 pages full of formulas of skirt's calcs and I don't know where to start!

The free edges will need to be stiffened in some way to retain the same buckling strength that they would have as a complete cylinder.

Do you mean something like a strip welded perpendicular to the edge, like for classic circular openings for skirt?

 

[ElCidCampeador]

Looking at calcs...it seems that the only force acting on skirt is vertical weight of vessel.

There aren't external loads on nozzles, no earthquake or wind...

So the only stress is sigma=P/A where A area of section of skirt.
No tau because moment is zero.

I suppose the only reasonable check is that sigma found is lower than K*S.allowable

So simple? mmm naaa...someone suggested me to use FEM

 

[JStephen]

The centroid of the skirt area is offset from the center of the vessel, so you get a moment in the skirt due to eccentricity of the load.

 

[Some Curious Guy]

You are not in an enviable situation. My sympathy is with you.
As others have stated, the solution revolves around adding some kind of stiffeners/repads around the opening so that cross section, sectional modulus, buckling strength is adequately compensated.
You would not find this option in softwares meant for pressure vessels. You could find one in softwares specially meant for designing flares and stacks.
I had this old reference in my computer. I have attached the link. I could be of some help to you hopefully in developing an excel sheet.
All the best

 

[Geoff13]

A little difficult to comment without some notion of the vessel size.  3 feet (1 m) in diameter is much less concerning than 20 feet (6 m) in diameter.  And then there's height.  However I'm imagining your vessel is quite small as you said there's no wind which means it fits inside a building.

The reference posted by Some Curious Guy looks excellent.  The vertical stiffeners in Figure 5.27 are what jumped to my mind immediately.

Personally, I wouldn't waste time looking at the unstiffened opening calcs in 5.6, regardless of the vessel size.  However I'm not convinced you need to replace all the stiffening lost by the opening, as 5.7 does, but that would depend on how big your vessel is.  Once you're spending money to add stiffeners, making them a bit larger so you can sleep easily is essentially free. If not fully replaced then JStephen's comment about moment from the difference between the vessel weight CG and the skirt centroid comes back into play.

You may also need to think about the foundation at the point loads from the two posts and how these point loads affect your vessel where they attach at the top of the skirt.  If your vessel is as small as I'm imagining these checks may be unnecessary.

I'm too old to jump to FEM for every slightly odd problem.  I think this very solvable (hand / Excel calcs) and FEM should be saved for problems where it's really needed.

Good luck.

 

[ElCidCampeador]

You are not in an enviable situation. My sympathy is with you.
As others have stated, the solution revolves around adding some kind of stiffeners/repads around the opening so that cross section, sectional modulus, buckling strength is adequately compensated.
You would not find this option in softwares meant for pressure vessels. You could find one in softwares specially meant for designing flares and stacks.
I had this old reference in my computer. I have attached the link. I could be of some help to you hopefully in developing an excel sheet.
All the best

https://files.engineering.com/getfile.aspx?folder=4ea92ad1-ec97-4bc9-b309-f

It looks very intersting, I will read it even if it seems a bit "exaggerated" for my case...

My vessel is 2 meters tall, weight 600kg and O.D.600mm...I think that what JStephen suggests could be enough.

A FEM analysis would be a more precise study but might also redundant, considering the size of the opening and no relevant external loads applicable...I don't know if could be really usefull (Moreover I can't do this analysis by myself, so it will cost) .

But I can't add any kind of reinforcement: this vessel is a copy of a 20 years ago project and I need to follow exactly the drawing without deviation (I don't have calcs, only dwg :/).

Lastly, consider that baseplate and skirt have a required minimum thickness very low (2mm vs 10mm adopted)...so I hope nothing bad will happen!

 

[saplanti]

ElCidCompeador,

Why do not want to use leg columns instead of skirt¿ Legs will give you better access and you do not have a complicated calculation.

Use of skirt is better in case the vessel is under lateral and external loads.

 

[Some Curious Guy]

In this case since your vessel is so small you can follow what JStephen suggested. The paper which I attached also has the same approach.
Do not go for an FEA. It is unwarrented for such a small vessel.

 

[r6155]

“But I can't add any kind of reinforcement: this vessel is a copy of a 20 years ago project and I need to follow exactly the drawing without deviation (I don't have calcs, only dwg :/). “

No calculation is required if you manufacture according to the mentioned drawing.

Regards

 

[david339933]

No calculation required? At a minimum old calculations must be reviewed to the current Code Edition and signed as such. No old calcs available....you need new calcs.