Units and result are correct, according to your calculation.
You multiply 'Pascal' with 'm' and 'm^2', which is M=p*L*A.
This results as 'Pa*m^3', then you multiply it with 6 and divide to 'm^2', which is sigma=6*M/t^2.
'Pa*m^3/m^2' results 'm*Pa' as unit, this is correct. Nothing wrong here.
If you can provide the tutorial page it would be more helpful.

I don't think this is right thing to do but if it will work for you,
you can get rid of 'm' by dividing calculation with 'm' unit.



Regards

Thanks for reply. The formulas before sigma are from one book and the one for sigma is from another source. It seems that the last equation should use M [(N*m)/m] while previous equations results in M [N*m]. Interestingly, the value of sigma is correct (I verified it with numerical simulation).

By the way, these formulas are for stress in rectangular shell (pressure vessel) subjected to internal pressure.

Take Overlord suggestion seriously, since a mistake in defining a pressure vessel geometry and thickness can have serious "consequences".

I have to underline that also the formula in your file, that determines area "A", seems quite odd: you get as a result a negative area, this should be carefully checked because it isn't definitely what one would expect from an area

I think that this thread is giving you the chance to dig deeper into the proper equations to be used and their meaning, since by mixing equations from two different references there is some risk of using them in an unproper way.

ASME VIII div 1 or EN13445-3 section 15 will guide you through a more complex but safe design, where both corners and shell midspan will be checked against membrane and bending overstresses.
Concepts as corrosion allowance and joint efficiency (to correlate between adopted thickness and Non Destructive Test after fabrication) are also included there.
And you get requirements for allowable stresses as well, which is an important topic in the pressure vessel industry.

Adopting a proven design and construction code will be required in case your vessel needs to comply to some pressure vessel safety regulation (based on the country where the equipment will be installed, or any other contractual agreement); you might be required to CE stamp (EU), or U Stamp (US and many other countries too..), or have compliance with EAC regulations (Russia, KZ, Belarus), etc...
If this is the case, again, you better stick to a coherent pressure vessel construction code (as the main ones mentioned above by Overlord).

In case you want to dig into any other source\book for rectangular vessel design, you may want to check:
- Dennis Moss "pressure vessel design manual" (check every equation again... remarkable book in terms of tons informations, methods, and logic behind, but there is some mistake here and there)
- Bednar (cant' remember the exact title), with more details about the derivation of a formula, while Moss is mainly a design handbook
... and many others

In the end, I found some time ago an old book which is titled "Tubular Structures", including a very clear and clever way to determine wall thickness and stiffener dimensions for rectangular powder silos, bins and bunkers. It was a very interesting reading (besides typos, some equations needed some adjustment..) and, even if powder is not going to behave as hydrostatic pressure at all, the book gave me a useful insight on how to model properly the structure with a "by formula" approach.. finally it was a good way to understand better how rectangle section silos are checked inside Eurocode (EN 1991 for structure, i.e silos).

Bye

Can't comment any further with the available input informations.
For sure equations for thin pipes should not be used on rectangular vessels.

Beware of FEM, you can get awesome results or awesome colored pictures, and that's not the same
Boundary conditions, choice for modeling and meshing (brick elements, tetrahedral, surface elements) will make the difference too.
Take care about classifying stresses properly (I would suggest ASME VIII div.2 app.5) in case you need to produce any official calculation report that makes use of FEM.


Finally, sorry for misleading informations about the mentioned books: I just checked both MOSS and BEDNAR, apparently I remembered wrong info... those two doesn't cover rectangular vessels.
Stick to ASME VIII div.1 or EN-13445 for now.
Tomorrow I check other books and in case I will send you a PM with a temporary link to download.
I can't send you the ASME Code itself, it is assigned to a company user ID and I cannot share that.

BYe

I don't have that particular book, it would be great if you could PM to me too.

Regards

Probably coincidence or wrong mesh, or FEM calculation is selected as circular.


The equation you used doesn't consider the corners.
I won't disagree if you will continue this approach but it is nontraditional.

Here you can use this sheet if no stiffener shall be used.
Otherwise you have to alter formulas according to standard you will use.

Regards

rectangular.sm (59kb) downloaded 23 time(s).
rectangular.pdf (144kb) downloaded 22 time(s).



PS: Never take weld factor as 1, 0.85-0.9 for perfect shop weld or 0.7 for field weld.

Take a look at this example of Ansys for 50mm thickness;
it will give you an idea how much 0.1MPa(1 bar) can effect on rectangular shell.
https://www.irjet.net/archives/V6/i6/IRJET-V6I6747.pdf

Edited by user 24 January 2021 17:53:27(UTC)  | Reason: Not specified

There is welding coefficient table in both standard if I remember correct.
But to ensure safety I usually stick with 0.85, 0.7 if it is field weld. Even with NDT.
Materials I had designed were usually huge (4-5m diameter) so full X-Ray was not possible.
There is no obligation to lower the safety stress except material cost. Increment should be not allowed however.

Regards