D/t ratio limit for thin walled pressure vessels

[stephenebrady]

I have a vertical vessel which is 8500mm in diameter, and general wall thickness of 25mm. Pressure is approx 5barg, but there is wind and siesmic acting as external loads. The calcs show the thickness to be more than adequate but client wishes to reduce the plate thickness. I can reduce some of the upper tiers to 20mm but i am wary of reducing plate thicknesses where I will have a D/t ratio approaching 500!!

Is there any kind of limits placed on this D/t ratio for stability/buckling/secondary effects??

 

[davefitz]

I am not qualified to provide a definitive answer, but other effects during fab, shipping, erecting, hydrotest may prove to be a limting factor, in addition to buckling and secondary effects.

In the general case, a min wall thckness is needed for reliable shop welding. Special temp handling support structures may be required for loading the tank onto , and off of , the shipping vessel, and erecting it onto the final support pad. These temp supports may also cause unusual lods onto the tank during shipping , due to vibration, inertia, etc. And finally, field pressure testing with a fluid that may be denser than the design fluidmay cause unusual loading issues.

The problem you discussed hass probalbb been dealt with by NASA for space fuel tanks/ rocket tanks. Check their jobsite for tech papers on the subject.

 

[MJCronin]

stephen.....

As I recall, there are a few industry references that discuss limiting D/t ratios in process column like applications. (tall - thin walled vertical columns)

To the best of my recollection, the localized affects of column buckling are not significant (local stiffening not required) until d/t exceeds about 100. Then, there is a region where local affects must be considered (but a design is possible). Above a certain D/t limit, all bets are off. (around 250 ?)

The Bednar text discusses this in the section on column design.

http://www.amazon.com/Pressure-Vessel-Design-Handbook-Bednar/dp/089464503X

Another text that discusses this effect is one on tubular structures (silos, bins, conveyors) by M. S Troitsky "Tubular steel structures: Theory and design"

Contact a structural engineer well versed in mechanics....he may have better references.

Please tell us your final decision and your best source of information...

With a D/t that is approximately 340, I believe you may need intermediary stiffeners

Regards

-MJC