I have a curiosity about ASME external pressure charts.

I'm checking a vessel with jacket. The jacket pressure is 4 bar, no pressure inside the vessel, T=75°C and I.D. 1800, in A240-316

I will proceed with ASME VIII div1 calculation, which gives me 6.4 mm minimum for the vessel torispherical head.

But I have made a quick comparison with FEM, and if I consider 5mm for the head, I get a ratio between critical pressure and actual pressure of 2.6
This means that the critical pressure corresponding to the head buckling would be more or less 1.5*2.6=3.9 bar on 5mm thick heads.

So, ASME is really conservative, if compared to FEM.
This is reasonable, and safe too, so I'm not discussing the "philosophy" behind the code.

Now, here is the question: WHERE is ASME conservative?
I mean, are the charts themselves conservative? Where did ASME hide the safety factors?
Just a curiosity, because the div.1 that deals with external pressure is using charts and not formulas, so it's difficult to understand what's the theory (or model) behind the result.

Thank you!

Hello fedeghi

You hit the nail on the head. ASME is very (I mean very) conservative. I remember when I reviewed the design of a railway air receiver a number of years ago, the air receiver had an intermediate head. If only one compartment were pressurised, the head would be subject to an external pressure. I told the railway engineer that the head would collapse. He told me not to worry about it, because the head would not collapse. I witnessed the hydrotest expecting the head to collapse, it did not. The pressure was about 3 times the design pressure. I have had a number of cases like that.

In the case of cylindrical shells, the most important thing is the ovality of the cylinder. That is critical.

Dear All,
I am attaching a page from "Jawad & Farr". It is related to the question originator.
I thought just to share with you all.

Hello M

Yes, we have that book. You can also find similar information in the book: Design of Modern Pressure Vessels by John Harvey.

If I were to order a refinery (I am not a multi-millionaire), I would have the plant designed to ASME Section VIII, Division 1 with spot radiography. Then I would know, even if the plant were abused, it would stand up the the rigours of many years of service. I would not choose any European code, or Division 2 of ASME.