Appendix 13. 13-14 External Pressure 1

[NimChimpsky]

Evening All,

I am working on a rectangular stayed header box for an air-cooled heat exchanger and I am using ASME VIII Appendix 13 to complete the stress calcs on the pressure vessels components. During a recent design review, I was asked to provide proof that the stress calcs adhere to section 13-14: "13-14 VESSELS OF NONCIRCULAR CROSS SECTION SUBJECT TO EXTERNAL PRESSURE."

Does anyone have experience with this section? Bearing in-mind the cooler will experience atmospheric pressure only, I am slightly unsure whether there is any reason to include this clause. Perhaps I am just misunderstanding the purpose of the clause.

In addition, our stress calculations work by inputting the Design Pressure (=MAWP) and inputting the minimum tubesheet, plugsheet, end plate and top/bottom plate thickness and checking the combined stresses are not exceeded, or increasing wall thickness via an iterative process.

The calcs are a proven method but during a design review, there was concern that there is no clearly visible comparisoin proving the minimum wall thickness is exceed for each component. The End plates are subject to UG-34 and the minimum wall thickness is shown to be surpassed. I am having trouble of displaying the minimum wall thickness of the other components (tubesheet, plugsheet and top/bottom plate) as all their thickness are interlinked and dependent on a variety of variables. Does anyone have a clear way of showing the minimum thickness of the components of a stayed header box vessel? Is there away to work back from the combined stresses to obtain minimum wall thickness of each component?

Many Thanks for all your help and I welcome any comments or further questions.


Regards,


Rich

 

[SnTMan]

richward5, I'd say you are going to have to find out what your software is DOING.

Regards,

Mike

 

[MikeG7]

Hi Richward5

You said 'there is no clearly visible comparisoin proving the minimum wall thickness is exceed for each component".

The rules of section 13 give the bending stresses and membrane stresses for the pressure condition for the flat sides and there is a calculation method for the flat end. The stresses must be compared to the allowable stresses of the material in bending.

For external pressure, the situation is the reverse of internal pressure but the same method applies.

 

[NimChimpsky]

Not very helpful SnTman, but thanks anyway.

MikeG7, I appreciate your feedback. I have calculated the flat side minimal wall thickness using UG-34, this is acceptable. I have calculated the bending stresses and membrane stresses of the flat sides and compared this to the allowable material stress at design temperature.

For external pressure, I assume that running the same process using the external pressure (atmospheric) and the external box dimensions will prove the vessel will withstand external pressure. However, this process does not take into account the effective Elastic Modulus or Possion's Ratio associated with perforated sheets(taken from ASME II - Part 2 - Table TM-1) as detailed in clause 13-14. I do not see how calculating the boxes ability to handle external pressure should include the effective young's modulus, when it is not incorporated into the internal bending & membrane stresses calculations?

Once again, I am grateful for any feedback or comments.

 

[SnTMan]

richward5, actually very good advice. Nobody knows what you are using, or what it is doing. Hard to provide anything useful under the circumstances.

Regards,

Mike