Allowable Bending Stress Near Support/Shell Interface

[nandviks]

I am working on a small vessel which is basically supported (cantilevered) using (1) 4" Sc 40 pipe support . I want to calculate the stresses near the support/shell interface.

Do you know of any method which could be used to calculate the stresses near the interface and on the shell ? Is WRC-107 too conservative for this case ?

Also, what would be the allowable limit on bending stress close to interface(i.e., should it be considered as secondary bending stress or primary bending stress ? The loading is dead weight + seismic load.

Thanks
Viks
OH

 

[nandviks]

Just a clarification :

It is not the same as Nozzle/Shell interface since there is no opening at support/shell joint.

 

[jte]

It is not the same as Nozzle/Shell interface since there is no opening at support/shell joint.

So... this is one of those rare cases where the physical situation matches up with WRC-107 / -297...

As for primary or secondary... A dead weight load is secondary after the vessel has fallen down to the ground and the load on the support has therefore self limited. If you plan to evaluate the support load with the vessel substantially undamaged, then the load is not self limiting and thus would be considered primary. Likewise with seismic.

jt

 

[nandviks]

I am trying to understand, why the local bending stress can be treated as secondary at nozzle/shell intersection, while at support/shell interface they have to be treated as primary.

NandViks

 

[jte]

the local bending stress can be treated as secondary at nozzle/shell intersection

This statement is not necesserely true. You as the engineer performing the evaluation need to determine whether the loading which is causing the stress is self limiting. Some studies have shown that piping loads caused by thermal expansion of piping are self limiting - but only after substantial damage has been done to the vessel. It is a common misconception that all thermally induced loads are secondary. When you read this in the Code, pay very close attention to the wording. They do not include external loads such as those due to attached piping in the "thermal loading is secondary" part.

jt

 

[nandviks]

I fully agree with you when you say that piping loads due to thermal expansion of pipe should be considered as primary. That's what I am doing too. But what I am currently calculating is Stresses near the intersection by using WRC-107.

Now, per ASME Sec VIII Div. 2, Table 5.6, under,

Vessel Component - Any shell including cylinders, cones, spheres and formed heads

Location - Near nozzle or other opening

Origin of Stress - Net-section axial force and/or bending
moment applied to the nozzle, and/or internal pressure

The bending stress can be considered as Q, no matter what kind of loads (self limiting or not) are applied on nozzles.

Would you agree with interpretation ??

NandViks

 

[jte]

The Code is not a handbook and cannot replace education, experience, and the use of engineering judgment. The phrase engineering judgment refers to technical judgments made by knowledgeable designers experienced in the application of the Code.

In your jugement, do your loads physically behave in a self limiting manner? Then secondary. If not, then can you feel that you have acted in good faith if you do not consider them to be of a primary nature?

Once again, we have a situation where... "Just go with the nonlinear analysis and avoid this primary vs secondary discussion" is definitely applicable.

jt

 

[TGS4]

Just go with the nonlinear analysis and avoid this primary vs secondary discussion

Agreed.

 

[nandviks]

I wish I could ....but we don't have non linear license....and the only option available to me is to perform a linear analysis.

Given this constraint...could you plz explain why the local bending stress can be treated as secondary at nozzle/shell intersection(per ASME), while at support/shell interface they have to be treated as primary (per jte's response)while performing a linear analysis ?

 

[Duwe6]

nandviks: The assumption in ASME is that the Vessel engineer will not allow the Piping Designer to place large loads form the pipe into the vessel nozz's. Thus the secondary nature of the loading [supposed to be only minor loads] The support holds the entire vessel, the entire contents, and any downward nozz forces; thus a Major load. Also if the support fails, the results *will* be catastrophic.

 

[jte]

The quantity of the load is irrelevant ("minor loads"). It is the nature of the loads - self limiting or not - which is important to establish.

jt