ASME VIII Div 2 - Tower/Column - Long. Stress Effect on Nozzle Reinf 1

[stanm]

I would be grateful for any opinions/experience on this issue.

In the design of tall towers/columns it is possible that either wind or seismic loading will induce longitudinal stress levels in the shell which can control the shell thickness. Within ASME VIII Div 2 this is covered in section AD-201. Other national codes have similar methods for including the effects of loadings other than pressure. For the instances where longitudinal stress governs, the calculated shell thickness will be greater than that required for pressure alone.

My question/scenario is as follows - In the design of tall towers/columns to ASME VIII Div 2 two shell thicknesses are calculated, (1) for internal pressure only t = 1/4 inch and (2) for internal pressure + wind loading t = 1/2 inch. Which thickness should be used for nozzle reinforcement?

By comparison ASME VIII Div 1 uses tr based on the circumferential stress, PD 5500 nozzle reinforcement rules are pressure based as is AD-Merkblatt. AD-Merkblatt does point out that additional external forces are not considered in the nozzle reinforcement rules and "are therefore to be considered separately".

Again, please offer any of your experiences in this issue or your code interpretations.

Many Thanks

 

[prex]

My interpretation for all codes is as follows:
1) It is true that all the codes you cited (and also a few others I know of) base their reinforcement calculations on pressure only. The reason is that some parts of the calculation method rely on the distribution of pressure stresses: an example if chart UG-37 (or AD-520.1), that would not be meaningful under your conditions.
2) It is also true that all codes, often in a general introductory paragraph, claim that, though rules are supplied mainly for pressure calculations, any other significant loading must be accounted for in the design: so the reinforcement of your opening, after having been checked for pressure only, must also be checked for wind or seismic loads.
3) I think that if, in your reinforcement calculation, you use for t_r the thickness required with wind included and you provide the same reinforcement all around the opening (forgetting about factor F) you should be OK with the code

prex

http://www.xcalcs.com

Online tools for structural design

 

[warrenw]

It has been my experience that the wall thickness of tall self supporting stacks is governed by longitudinal compressive (buckling) stresses. The longitudinal stress from internal pressure are tensile stresses(1/2 the circumferential stress) and counter the compressive stresses but will be additive to the tension side. Mostly I work with internal pressure of up to 50 psi.
Are the full design wind loads and the pressure loads to act concurrently?
As for the nozzles, I always check the stack section assuming the nozzle is a hole in the stack wall (reduced cross-section) to determine if the cross-section needs reinforcement. If no reinforcement is needed for the wind loads, then the nozzle reinforcement is only needed for the pressure requirements.
If you require t = 1/4" for pressure and t = 1/2" for wind, does the 1/2" requirement come from allowable tension stress or allowable compression stress?
It is my understanding that nozzle reinforcement is based on the circumerential stesses.

 

[stanm]

Thanks for the quick responses,

prex - we are in agreement.

warrenw - as a design consideration, yes, wind and pressure would be taken as concurrent loadings. This thread is a code interpretation question more than anything else. The 1/2" requirement comes from the formula t=(0.5PR+F/S-0.5P) where F is positive. Things get more interesting if F is negative! The main code question is whether this calculated t value should be used in the nozzle reinforcement formula. Your comment about nozzle reinforcement and circumferential stresses is similar to my own experience of codes(with the exception of ASME VIII Division 2). However, after "going round the houses" with this question it has been suggested to me, via an AIA, that the ASME VIII Division 1 code committee has this issue marked as an "action item" and it is expected that Division 1 will be brought in line with Division 2. This would harmonise ASME in this respect.

 

[arto]

How about the thickness based on allowable longitudinal stress? Gilbert & Polani had a good article on combining external pressure & longitudinal loading in May 79 ASME Trans. Jnl PVP. pp178-81. Might it be of help?

 

[stanm]

Thanks arto, this seems to be relevant - will see if we can acquire a copy of the article.

 

[unclesyd]

We have 2 very tall >200' columns, vacuum, and looking at the calculations both have nozzle reinforcements and support based on pressure conditions created by fully flooded conditions though the possibility of this condition being achieved is remote due to supported rupture disks in the base.
They were built under section VIII Div 2. in the 70's and 80's.

Anecdotal:
The original process design for this column was done in early 60's on an our first main frame, an IBM 360. I still have some of the input data sheets with all their zeros.



stanm

You can get a copy of the paper here.

http://www.lindahall.org/

 

[stanm]

Thanks for that link unclesyd.

The 20/30 year old designs you mentioned are interesting. I believe that wind or seismic loading and its effect on the nozzle reinforcing calculation is easier to consider today due to developments in available software.