Pressure Vessels Legs

[mafa]

Hi everybody!
Does anybody know whether exists or not a limit for a vertical vessel´s diameter / height to use LEGS as support? (I am talking about pressure vessels under ASME Secc. VIII, Div. 1).
Also, if you have a vessel exposed to seism and wind loads, should you add the thicknesses obtained from both calculations to get the total thickness of the support?

 

[johnp]

Legs for a vertical vessel have to be designed from basic engineering principles. There is no rule of thumb relationship between diameter and height. All these factors have to be taken into account in the structural design- otherwise there are too many variables.

Seismic load is assessed on different design parameters. The issue here has to do with natural frequency or inertia. To some extent this is controlable by the designer.

In all load combinations wind and earthquake loads need not be assumed to occur simultaneously. When a vessel is designed for both wind and earthquake, only the one which produces the greater stress need be considered.

 

[corus]

In asnwer to the second part, in the british design standard for pressur evessels it lists in 3.2 varuious load applications which include wind AND seismic loading together with other operating loads. It would be foolish to think that an earthquake only occurs when there are no winds.

corus

 

[mafa]

thanks guys!

 

[panduru]

@corus:

I do not know about the British Design Standard (?) but
I had always thought it is the practice to consider only the greater of seismic or wind loads.

I would like to confirm if the Code (ASME BPV Code) clearly specifies somewhere if that is the correct design procedure.

 

[johnp]

I'm on a Processing Plant 500 km from the nearest ASME Code. Both the British and Australian codes consider seismic events and storm loads separately.

My take on the issue is as follows:

1 - Load combinations in almost all design codes are based on very crude probabilistic rules-of-thumb rooted in long engineering history.

2 - The more individual load components we add together, particularly in the 50 to 100 year range, the more fuzzy becomes our understanding.

3 - When it comes to forces acting during a tornado the Code wind load coefficients are a crude approximation of what could possibley happen.

4 - If we get into serious earthquake disaster zones and tsunamis there are so many other things happening that to add 10% or so to a design calculation for wind won't add significantly to the precision of the calculation.

5 - When designing to resist wind forces the trend is to add mass and increase stiffness to provide additional safety. When dealing with seismic events one design option is to reduce mass and increase flexibility - which is diametrically opposite to the wind load approach.

6 - When it comes to professional liability the fine print in the codes usually acknowledges that reasonable designers would accept that, providing we cater for the dominant effects, there is no real benefit in making fine adjustments to the real factor of safety to cater for extreme combined effects.