WRC 107 - EXTERNAL PRESSURE 1

[mikebb]

Hi,

Would appreciate some guidance re WRC 107 if possible !

I am designing lifting lugs and doubler pads to lift a vertical vacuum - insulated pressure vessel, with lugs sited near the top of the cylindrical shell, below juncture with semi-ellipsoidal dished end.

My question is - Can WRC 107 methods for calculating stresses in cylindrical shells at the edges of lugs and doubler pads ,due to External moments, be used in the case of external pressure ? My understanding is that WRC allows for internal pressure only. In my case external pressure of x1 bar applies (atmospheric pressure outside, vacuum inside ).

Any help would be appreciated.

 

[TGS4]

WRc107 will give you a good estimate of the stresses at the lug/cylinder junction - regardless of internal/external pressure. However, the failure modes for external pressure are much more complicated than those for internal pressure - namely buckling. Even if you know the local stresses, hat doesn't help you with buckling failure.

 

[mikebb]

Thanks for that.

Any suggestions as to how I might determine wether imposed stresses will result in buckling failure ? One further question if I may : Using 107, the required doubler and Lug dimensions I have calculated appear to be quite a lot larger than the dimensions of those I have witnessed on vessels in the field. Could it be that other designers are accounting only for shear in determining stresses incurred on lugs when lifting without consideration of the effects on the vessel shell, or perhaps they could be using an internal stiffener ? Any suggestions ?

 

[TGS4]

Simply put, WRC-107 is conservative. Also, there is no provision in WRC-107 for repads (or doubler plates as you call them - prefer the term repad which is short for reinforcing pad). How some implementation of WRC-107 account for a repad is that they basically extend the lug to the dimension of the repad and perform the stress calculation there. This approach is even more conservative, and will likely grossly over-estimate the state of stress at the edge of the repad.

Suggestions - try WRC-297 to start. I prefer to use FEA, but that's because that's what I get paid to do.

For buckling, the only way to check for local buckling is to use an FEA approach. I suppose that you could try to limit the stresses to the B values from II-D, Subpart 3, but that may be even more conservative. Does your vessel see a vacuum while it's being lifted? Or am I misunderstanding your situation?

 

[JohnGP]

Yes, I guess that was my question. mikebb, it reads as though you are designing the lifting lugs for a case where the vessel is under vacuum, and I can't imagine that being the case.

 

[mikebb]

Its a good question, but yes, space between inner and outer vessel will be under vacuum during lifting. I'll try WRC 297, and hopefully repad and lug sizes won't come-out too big, but realistically I expect they will be similar to WRC 107 results (ie large).

Failing that I do have the option of FEA analysis via a professional contact, although am reluctant as prefer hand calcs.

Thanks again for the advice