Stress Analysis vs. Code Formula 2

[Latexman]

When sizing relief devices, I frequently come across old tanks and vessels that do not state a design pressure/MAWP. One of our plants has this issue now. They contracted with a local Engineering firm (Italy) to guide us on what the design pressure/MAWP is.

Their report says, “Analysis was conducted by stress analysis and not by formula. Doing it in this way the design pressure is obtained considering the weakest point. The design pressure applicable is equal to pressure yield stress with a safety coefficient equal to 1.5."

So, the plant is saying, the yield stress of 1.5 barg came from the flat, I- or C- beam reinforced top, which is the weakest point. Therefore, design pressure/MAWP is 1 barg.

Does this meet Code? Why or why not?

Good Luck,
Latexman

 

[Vikko]

Which codes ?

From what said it seems you are basing the analysis on elastic limit theory, because you consider the yielding stress as the limit factor.

Also you said that at 1.5barg there is yielding in the materials, but that does not mean that, by reducing the pressure of 1.5 time, you will have lowered to 1.5 the resulting stress. I mean that you are implying that the relation between internal pressure and resulting stress is proportional! Which is not correct. You should instead take the yielding stress of the carbon steel and reduce to 1.5 times (or take only 66.667% of it) as the maximum allowable stress 'S' (or nominal design stress 'f' for EN 13445).

Usually in most codes there are 2 options
- Design By Formula
- Design By Analysis

For simple components, like tube or cylinder, disc, plate, the direct formula application is straightforward, and by simply reversing the formula, you get the maximum allowable pressure. Consider for example ASME VIII div. 1 UG-27.
- Direct formula: t=PR/(SE-0.6P)
- Reverse formula: P=SEt/(R+0.6t) which is the MAWP for the tube

The design by formula is usually based on elastic theory, so considering the yield strength is correct. But not only, for example the ASME VIII div. 2 for cylinders is based on plastic limit theory.

For complex components, just like flanges, tubesheet and so on, there is no direct simple formula, but many iterative methods, so reversing them is not possible and calculation of MAWP would not be simple.

The FEM analysis, or DBA, is another kind of strange beast, it can be based on elastic theory but also on plastic. It is much dependent also on 3D model geometry meshing. To me it is a kind of magic, it is better you go with consultants you trust to be competent in doing their job, because it is very easy to trick the results into what you want.

 

[Latexman]

Which codes ?

Each vessel's report has the following:

NORMATIVA
La normativa adottata è la seguente:
- EN 13445 annex C “Design by analysis – Method based on stress categories”
normativa PED
- ISPESL (Istituto Superiore per la Prevenzione e la Sicurezza del Lavoro)
RACCOLTA VSR – Specificazioni tecniche applicative del Decreto Ministeriale
21/11/1972 e successive modifiche per la verifica della stabilità dei recipienti a
pressione. Revisione ottobre 1995 – bozza definitiva di norma EDIZIONE 1999

Google Translate above to English (for me):

REGULATION
The legislation adopted is the following:
- EN 13445 annex C "Design by analysis - Method based on stress categories"
PED legislation
- ISPESL (Higher Institute for Prevention and Safety at Work)
VSR COLLECTION - Technical application specifications of the Ministerial Decree
21/11/1972 and subsequent amendments for the verification of the stability of the containers a
pressure. Revision October 1995 - final draft of standard EDITION 1999

Each vessel has a report where it underwent a Design by Analysis per the above Code to calculate the yield stress of the equipment components (limit of elasticity). The lowest yield stress was used and down rated by S.F. = 1.5 to arrive at the design pressure (somewhat similar to MAWP in ASME that I am familiar with). I have to trust that our plant has contracted with competent consultants as they are in the local market, and have been in the chemical business for years and years.

Thank you for your help. I have concluded this meets, or is equivalent to, local Code and I can proceed to size the relief device.

Good Luck,
Latexman

 

[XL83NL]

EN 13445-3 annex C is the DBA route for PV analysis, 'similar' to ASME VIII-2 (the 2 codes are obviously different but bear the same principles). There are 2 DBA-options within annex C. I'm not familiar with DBA in general (such as VIII-2 or EN 13445 annex C). However, if the engineering firm is competent, this should suffice. DBA is also covered under the PED via Annex I, 2.2.3 a (2nd indent). See EN 13445-3 Annex ZA, Table ZA.1.

Huub
- You never get what you expect, you only get what you inspect.

 

[Latexman]

Thanks, Huub!

Good Luck,
Latexman

 

[TGS4]

Latexman - I am not convinced that the analysis performed for you, though, is actually compliant with EN 13445-3 Annex C. I would poke around a little bit more.

 

[JStephen]

"Does this meet Code"?- are you asking if the analysis meets a code or if the vessel meets a code?
Typically, if you have a constructed vessel with no other records, there's no way to go back and certify it as a code vessel. You can measure thickness and things like that. But the codes also specify welding procedures, certification of welders, inspections required during fabrication, material requirements, etc., and much of that can't be confirmed after the fact.
Also, note that under ASME the 1 bar/ 15 psi is the cutoff to be exempt from the code, but no idea how other codes work that.

 

[Latexman]

are you asking if the analysis meets a code or if the vessel meets a code?

Just the analysis. On the vessels, as you mentioned, that ship has already sailed. They are not code vessels. Basically, our plant asked the consultant to back-calculate the design pressure of six vessels as if they were code. We understand the pitfalls of this. None of the benefits of the code - materials, methods, welding certification, inspection, testing, etc. We are at the mercy of the company and plant's own vessel procurement practices of many years ago. The vessels are functioning in the service perfectly fine, the consultants have inspected the vessels and interviewed the maintenance folks on their history, and no one really wants to buy new code vessels to replace them. I'm looking for a safe design pressure/MAWP to size a rupture disk for all 6. They never had a relief before. This was about the best way we could come up with a basis to size the reliefs. This should improve the safety of the plant, especially if ever the SHTF, i.e. fire. That seems to be the worst case for most of them.

Good Luck,
Latexman

 

[Latexman]

TGS4,

Understood. I will be taking a deeper dive into the reports.

Good Luck,
Latexman

 

[GD2]

Latexman,
May I ask you what yield strength you will consider to calculate the design pressure?

GDD
Canada

 

[Latexman]

GD2,

After translation, here is the materials discussion:

MATERIALS
The materials used are the following:
For sheets, plates and rounds in the steel part: ASTM SA 240 Tp 304
Mechanical characteristics:
It is considered:
breaking = 517 N / mm² (MPa)
stress = 206 N / mm² (MPa)
permissible = 137 N / mm² (MPa)
E = 200'000 N / mm² (MPa)
Execant = 1,400 N / mm² (MPa)
At the design temperature T = 190 ° C the values become:
stress = 146 N / mm² (MPa)
permissible = 96 N / mm² (MPa)
E = 190'000 N / mm² (MPa)
Execant = 1,400 N / mm² (MPa)

The calculation considered the permissible at design temperature. The design pressure will down rate that with S.F. = 1.5.

Good Luck,
Latexman