Small opening

[mechengineer]

Hi Dear All,

I am confused that UG-36(c)(-a)does not give the restriction of r/R. It only limits the size of the hole (r).
I think the basis of UG-36(c)(-a) to exempt from UG-37 opening reinforcement calculation is that even if the peak stress exceeds the yield stress at the small hole, it will still be far away from the yield strength of the entire shell.
For example, if shell ID=248 mm and opening size = 60 mm, assume shell thickness > 10 mm)], the rario of r/R=0.24. I think it should not be considered as a small opening to exempt from UG-37. Where is wrong in my thinking?

Thanks.

 

[SnTMan]

mechengineer, UG-36(b)(1) not doing it for you?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[mechengineer]

Mike, I amend the shell diameter from 150 (6”) mm to 273 (10”) mm, r/R=0.22 to meet UG-36(b) (1).
I just want to know what is the theoretical basis in behind of UG-36 (c) (-a).
Because the small opening shall perform UG-37 reinforcement calculation requested by some client's specification. For example, shell ID=1350 mm, DP= 70 barG, flanges rating is CL600. 3" nozzle shall use heavy barrel type flange with the nozzles thickness 31.5 mm with reinforcement requirement, but for 2" nozzle you can use sch.40 nozzle wall thickness 3.91 mm per table UG-45 without reinforcement requirement. It seems hard to understand so big difference in the nozzle wall thickness between 3" nozzle & 2" nozzle.
Thanks,

 

[goutam_freelance]

but for 2" nozzle you can use sch.40 nozzle wall thickness 3.91 mm per table UG-45 without reinforcement

UG-45 gives minimum nozzle thickness. You need to calculate the actual required thickness based on code formula.

is that even if the peak stress exceeds the yield stress at the small hole, it will still be far away from the yield strength of the entire shell.

Failure of nozzle is also failure of the vessel. For less than 10 mm shell thickness, the shell design pressure is comparatively low, so the allow higher nozzle size(89 mm) without reinforcement . Also in case of comparatively higher nozzle thermal load, the thin shell can elastically deform locally to reduce the nozzle load.
It is expected that if all the rules of the code are followed correctly including welding details, the nozzle will not fail.

 

[SnTMan]

mechengineer, sorry to have to say, but I don't know the theoretical basis for Code rules, except to say that they are based in sound engineering principles.

Yes, it is true that in many cases for an opening that can be exempted under UG-36, the UG-37 calculation will show that opening is inadequately reinforced. This does show some shortcomings with the relatively simple rules-based design philosophy under Sec VIII, Div 1.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[mechengineer]

Hi Mike, thanks for shearing your opinion.

 

[mechengineer]

goutam, thanks for your input.

It is expected that if all the rules of the code are followed correctly including welding details, the nozzle will not fail.

I agree and I think nobody disagree.

UG-45 gives minimum nozzle thickness. You need to calculate the actual required thickness based on code formula.

Oberviously, for so small nozzle the required thickness will be very shall (actual required thickness is around 1 mm) that shall limit by UG-45.

Failure of nozzle is also failure of the vessel, ......

1. I have mentioned 'shell thickness > 10 mm' and high pressure (70 barG)
2. The strength 'failure' criterion and the peek stresses are different concept.
3. The Code formula for the wall thickness of the cylinder is only based on the principal membrane stress, and does not consider the local stress and peak stress due to geometric discontinuities.
4. For the combimed stress where geometry is discontinuities, shall be analyzed by ASME-VIII-2 part 5. The allowable stress shall be Sps=Max.(3S, 2Sy) which is larger than Sy, but the vessel is not failed.

 

[goutam_freelance]

3. The Code formula for the wall thickness of the cylinder is only based on the principle of membrane stress, and does not consider the local stress and peak stress due to geometric discontinuities.

Regarding this please note excerpts from UG 22
'UG-22 LOADINGS
The loadings to be considered in designing a vessel
shall include those from:
(a) internal or external design pressure (as defined in
UG-21);
(b) weight of the vessel and normal contents under operating or test conditions;
(c) superimposed static reactions from weight of attached equipment, such as motors, machinery, other vessels, piping, linings, and insulation;'

So Sec-VIII div 1 considers all loads including membrane stress.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[mechengineer]

goutam,

So Sec-VIII div 1 considers all loads including membrane stress.

Yes, you are correct. But it is far away from the point what i am talking in my post here.

Regards.