pressure vessel stiffening rings 1

[boze]

I have had a request for designing longitudinal stiffening rings versus circumferential ring design in the ASME VIII code, for vacuum considerations on a vertical cylindrical tank.

My response was that I am not aware of any code that indicates design for the longitudinal design calculations.

I would appreciate a source (if available) for this type of design. This is the second request I have had in the last several months.

Thanks

 

[brkmech1234]

I never heard of this. My opinion is that it is uselesss; I am not sure though.

 

[prex]

I also never heard of this and don't know of any bibliographic reference.
However the longitudinal stiffeners would not necessarily be useless (though I wonder what their usefulness would be).
In fact the elastic buckling of a long cylinder occurs with a characteristic number of waves (or lobes), where the actual number of waves depends on the geometry (ratii L/D, D/t). The number of waves is two (the cylinder becomes an oval) for long and/or relatively thick cylinders, but may be higher.
Now the longitudinal stiffeners, if suitably supported at the ends by the heads, acting as beams, will prevent the buckling modes with low number of waves, thus increasing the critical pressure.
I guess that the lower modes prevented would be up to less than half the number of stiffeners, as four stiffeners would allow the mode with two waves (an oval). So at least 6 stiffeners would be required to prevent the lowest buckling modes, but a higher number are needed for thick and short cylinders having a characteristic number of waves greater than 2.
Sorry, cannot contribute with anything more practical.


prex
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[jtseng123]

welll, I never heard of and it is not in the code, but that does not mean it can not be done, just not "preferred" for many reasons. For one, you may need many longitudinal stiffening rings, that cost much more than simple circumferential rings. But fabrication can be simple since you don't need to fit to the round shape of wall.
If you do insist of the creation, FEA is the only tool to justify your design.

 

[jamesl]

My understanding is longitudinal stiffeners does not have much use for external pressure (vacuum). Circumferencial stiffener rings are very efficient for external pressure, but not very good for bending or axial compression. Longitudinal stiffeners are mostly used in marine structure. You can search for a DNV report #DNV-RP-C202. It deals with different ways to stiffen a shell structure.

 

[brkmech1234]

I agree with Jamsel

 

[boze]

Thanks to all for your response regarding the lateral stiffening for a cylindrical tank.
I doubt that I will pursue this.

This is such a helpful source for information.

Boze

 

[JStephen]

I think the problem would be more in showing the effect, as opposed to actually having an effect.

If nothing else, a reasonable approximation would be to treat the walls as flat walls supported by the stiffeners.

 

[chicopee]

For what it's worth, you may be better off to make the vessel shell thicker than welding stiffners. It is less expensive; on new bridge construction, designers are moving away from welding stiffners in preference to thicker webs.

 

[AWDMIKE]

For information, API Bulletin 2U-3 - Bulletin on Stability Design of Cylindrical Shells (Third Edition, June 2004) contains the following scope:

This Bulletin provides stability criteria for determining the structural adequacy against buckling of large diameter circular cylindrical members when subjected to axial load, bending, shear and external pressure acting independently or in combination. The cylinders may be unstiffened, longitudinally stiffened, ring stiffened or stiffened with both longitudinal and ring stiffeners.

Some of the limitations are as follows:

The criteria given are for stiffened cylinders with uniform thickness between ring stiffeners or for unstiffened cylinders of uniform thickness. All shell penetrations must be properly reinforced.

The stability criteria are applicable to shells with diameter-to-thickness (D/t) ratios equal to or greater than 300 but less than 1200 and shell thicknesses of 5 mm (3/16 in.) or greater.

There is also a long commentary section as well as an example section. There is a list of references for the main body of the document as well as for the commentary.

 

[401laurent]

I think it's not a good idea to use long. stiffenering vs circ. for vacuum design, because when you design for vacuum you have to eliminate internal collapse , and longinudinal stiffners don't help enough.
So in my opinon forget this type of design.
Thanks.
Laurent.