Small Opening Verification according to ASME VIII Div.2 (4.5.5) - Problem 2

[Henzy1977]

I am designing a very simple pressure vessel with ID=88mm / THK=21mm / p=400bar / T=20degC.

I use the ASME VIII Div.2 methodology, acc.to 4.3.3 the thickness is ok with safe margin.
I have also an opening of D=25mm on the side of the vessel (like a drain, there will be a blind flange bolted to the vessel also fully verified for thickness and bolting).

I am using the ASME VIII Div.2 4.5.5 methodology "RADIAL NOZZLE IN A CYLINDRICAL SHELL" for what concern the side on the vessel only of course ( Figure 4.5.1 ).
Because of the mathematic of the model, for my quite small dimensions I have some limitation and seems that the calculation is not satisfied with the diameter and thickness of the vessel even if I apply an opening diameter of D=1mm ONLY !!!

ASME VIII Div.2 4.13.7.1 requires that all opening shall meet the requirements of 4.5 but seems strange to me because this methodology seems appropriate for large opening, for small opening because of some minimum values (e.g. 4.5.5/4.5.15 ) seems not appropriate.

Is my interpretation of the standard wrong?

ASME VIII Div.1 - UG-36 OPENINGS IN PRESSURE VESSELS applies some exceptions for small openings that I normally use for such cases (when I can use Div.1...), seems strange to me that there is nothing in Div.2 about that.
Design is for sure safe, do you know what I can do to skip this verification?

Thanks everybody!

 

[BJI]

Relative to the size of your vessel this is not a small opening. There aren't enough details provided to assess it completely but without any additional reinforcing at the opening it looks like the design would not be acceptable, as indicated by your calculation.

 

[Henzy1977]

Thanks BJI for you help!

Is it in your opinion ok that if I run the calculation with an opening of just 1mm it is still not verified?
The method of 4.5.5/4.5.15 seems valid from a minimum size of diameter, below that value does not matter the input value of the opening the verification is performed with the main vessel geometry only.

Seems strange to me that for small drain opening ( typical drilling NPS 3/8 for example ), I need to add reinforcement for this kind of equipment.
Am I wrong?

 

[BJI]

You should run it with the actual opening size you will be using. There is integral reinforcing remaining in the shell exceeding that required for a cylindrical shell under internal pressure, so you have to use the pressure area method of 4.5.5 to check if it is sufficient for the opening size. Have a read through 4.5.1 through 4.5.3, and take the thread major diameter, or any spot facing, as your opening size in the 4.5.5 method. It will likely be acceptable with a 3/8" NPT or 1/2 NPT without any additional reinforcing. Are you performing hand calculations or is it verified software?

 

[Henzy1977]

The software is custom made but should be ok, it is verified by a thrusted third party and because of this problem I did also an independent xls, the same results.
Honestly speaking I think it is not the software: I think I maybe are doing some error in my interpretation of the standard because I have only the opening and not the nozzle if the verification is mandatory like it seems to be.

In fact, I want to do exactly what you are writing in your last post: reducing the opening diameter to 3/8 or 1/2 to have the calculation verified.
I cannot do that because for small opening the Local membrane stress (4.5.53), Membrane stress at nozzle intersection (4.5.54) and Max local membrane stress (4.5.55) become independent from the opening diameter so even if I apply an opening diameter D=8mm I have the same stress of D=20mm.

It is (surprisingly for me) correct if you look at the method: look for example at the Membrane stress at nozzle intersection (4.5.54): the formula is independent from the opening diameter, it contains only dimensions related to the vessel (diameter and thickness).
Do you think 4.5.53 and 4.5.54 are both applicable if I have only the opening and not the nozzle?
Maybe this is my mistake, I really hope it is. Last thing I want is to apply Part-5 ( I need the CRN Approval….. ).

Thanks again, It is really helpful have a second opinion.

 

[DriveMeNuts]

I question whether the Div 2 4.5.5 nozzle reinforcement method is applicable to your nozzle.

You are correct that Equation 4.5.54 is not applicable. As the shell radius / thickness ratio (R/T) is greater than 4, the elastic membrane stresses are no longer conservative. At this R/T, uniform membrane stresses no longer exist, therefore the elastic design method is no longer applicable. See 5.2.1.3.

During the development of the Div 2 nozzle reinforcement method, the R/T ratio's investigated to destruction during laboratory tests were between 17 and 50. Yours is a tiny 2.1. I think that the geometry of your nozzle is way outside the scope of the Div 2 nozzle reinforcement method.

I would advise that you design the nozzle per ASME VIII Part 5 (design by analysis), using limit load or elastic plastic analysis (Elastic analysis is not applicable as explained above). Your pressure is so high that you could study ASME VIII Div 3 and use it as a reference to guide you.

 

[Henzy1977]

Yes, this is interesting you are perfectly right. Thanks.

One more question:
I need to comply with Div.1 for such vessel (mandatory requirement) so because of the 3000psi I need to give additional evidence acc.to ASME VIII Div.1 U-1(d):
I have selected the verification of Div.2 for the thickness (OK) and the opening (???) as additional verification required by U-1 but like you say for the opening it is a bad selection because of the ratio.
What would you do in this case? Would you “skip” the opening verification because the diameter of the opening is below the requirements of UG-36 or would you perform a FEM in any case?
Do we need to perform a FEM because there are no applicable alternative rules even for a very small opening diameter (e.g. D=10mm)?

 

[DriveMeNuts]

That 10mm hole is 12% of the shell diameter. That is quite a big hole.

U-1(d) and U-1(c)(2)(-i) give you all the information you need to know that your application isn't intended to be a Div 1 vessel.
If you insist on giving it a div 1 u-stamp, then higher division rules are where you should focus. UG-36 is not applicable because the vessels diameter doesn't comply with U-1(d) and U-1(c)(2)(-i). Probably none of the Div 1 rules are applicable, other than the mandatory use of a design margin of 3.5x.
For Div 2, failing to comply with Section 1.2.4.2(i) means you need to pay more attention to the rules to see if they are applicable. Even using Div 2 for the shell thickness is brings up questions because of the small diameter causing non-linear through-wall stress distribution. Because of the shell diameter and pressure, the Div 2 nozzle reinforcement method is not applicable. You'd be taking a leap of faith in using it. Design by non-linear analysis is your only option there.
I think your application is most suited to Div 3. Div 3 has no lower limit on diameter and the non-linear through wall stresses of your small R/T make Div 3 non-linear analysis methods the most appropriate choice.

It looks like if you conduct all of the calculations, materials, materials testing and NDE testing to Div 3 you can still certify the vessel as Div 1. Of course, you need to use the Div 1 design margin of 3.5, rather than the Div 3 margin of 1.8x, which means that your U1 stamped vessel is double the thickness of what it would be if you had given it a U3 stamp. From table KD-230.4 you would have to use 3.5x(PD + PS + D), not 1.80x(PD + PS + D). I'm sure there are many more adjustments and considerations that you will need to make to ensure you maintain the design margins of Div 1.

What you have sounds like process piping. Have a look at ASME B31.3. It classes a pressure of less than 400 bar as standard pressure. It has rules for branches in pipes which match your application perfectly. Look at "304.3.4 Reinforcement of Extruded Outlet Headers" and "K304.3 High Pressure Branch Connections". It refers to Div 3 for pressures greater than 400 bar (ASME B16.5 Class 2500 at ambient temperature).

As ASME B31.3 has quick and easy design by rules, perhaps you could investigate making this Div 1 vessel with the use of ASME B31.3 to satisfy U-2(g) and U-1(d), and not bother at all with Div 2 and 3 and the associated complex FEM. Your Materials and testing would have to comply with ASME B31.3, and of course, the allowable stress margins and testing would have to continue to comply with Div 1.

 

[Henzy1977]

Thanks, your analysis is very helpful, makes sense.

FYI: You're right the application is not far from B31.3 (even if it is not applicable) but Div.1 U-Stamp should be a mandatory requirement.
In fact it is a pressure vessel designed at the max of the rating of class 2500 for SS316.

Your idea to use B31.3 for the opening to satisfy U-2(g) and U-1(d) is great, I will perform the verification to see the results.
Thanks a lot, a great post.