Nozzle loads review after course

[Ehiman1]

Hello everyone,
I took part in a course a couple of weeks ago about how to consider nozzle loads in calculation and they told me that Pb load is always secondary load when I study nozzle shell junction but I don t understand very well because from Asme code div 2 I understand that’s true only if I am inside the discontinuity length of nozzle neck (R*t)^0.5.
Could you please help me about this point?

 

[DriveMeNuts]

How have you come to "understand" what you do?
Either the code specifies or it doesn't.

 

[TGS4]

You seem to be confusing loads with stresses. The Pb that you reference is a stress categorization. I suggest that you re-review your course notes.

 

[Ehiman1]

You are right, I used the wrong word...I meant Pb as stress...not force obviously.
What they told me is that a primary load produces primary or secondary stresses and in this case when we have a nozzle, so a discontinuity, the external moment produces a Pb stress that is always secondary (I suppose this becomes Qb).
At the end a primary load can produce primary or secondary stress, a secondary load always produce a secondary stress

 

[TGS4]

There is no such thing as a secondary or primary load, only a primary or secondary stress. You can have a load-controlled load or a strain-controlled load.

The bending that occurs at a discontinuity is due to maintaining compatibility in displacement. That makes it strain (displacement) controlled, and hence that bending stress is usually categorized as secondary - Q if you will.

For this type of stuff, you need to go back to first principles and thin-shell theory.

 

[Ehiman1]

Thank you for your answers.
I studied Mindlin and Kirchhoff plate theory and all stuff about finite elementh analysis, but I noted this is different.
Could you please suggest me a book for this?

 

[DriveMeNuts]

At a flat head to cylinder junction under internal pressure only, the load is strain-controlled and the bending stresses in the cylinder are secondary. The same goes for bending stresses local to a nozzle under internal pressure only.
For a nozzle under external load-controlled or strain-controlled loading, defining the stress category seems to be a grey area which is up for interpretation.
If the bending stress is categorised as secondary, then the "excessive plastic deformation" component of Plastic Collapse will be acceptable however some plastic distortion will occur which may affect the operation of the equipment.
If the bending stress is categorised as Primary (as per EN 13445-3) then the bending stress never exceeds yield, therefore there is no plastic distortion due to external loading around the nozzle.

I suppose put simply, setting the bending stress to secondary protects against Collapse and Excessive Plastic Deformation, however does not necessarily protect against plastic distortion which may affect the operation or superficial appearance of the equipment. EE 13445-3 suggests setting bending stress to Primary to prevent plastic distortion.

 

[Ehiman1]

Thank you for you answer! This means that EN 13445 is always much more conservative than Asme code because consider Pb as primary means that I need to use 1.5*S as allowable instead of 3*S.
But after a review of the En 13445 I noted that for nozzle in the vicinity of the junction to shell pressure and global loads produce Qb and not Pb. Pb is for other mechanical loads (I suppose wind and seismic loads?)

 

[DriveMeNuts]

Global Loads are for bending of the entire cylindrical pressure vessel from wind, Seismic, or some other source (e.g. a horizontal vessel has global bending stresses halfway between the support saddles.)
Other Mechanical loads are for externally applied loads on the nozzle, as per your OP.
EN 13445-3 isn't really more conservative, it just sets the default classification to Primary. Note 5 of Table C-2 gives the option to classify the stresses as Secondary if you are prepared to accept permanent deformation.

It is this entry at the top of Table C-2 which is relevant.
"Vicinity of a junction with another shell, an end, a flange or of the point of application of an external local load; vicinity of an opening (with or without nozzle) 5) 6)"
The nozzle section of the table is referring to the nozzle neck only.

 

[TGS4]

The selection of the category of a stress is certainly not straightforward. I like to say that many stresses are primary-ish with some secondary-ness, or vice versa. While the elastic stress analysis method forces a binary choice, the reality is much more nuanced.

Hence my preference for the elastic plastic analysis method.

 

[VlaKoval]

After taking a course I have also been puzzled by the "Bending (other than gross structural discontinuity stresses) averaged through nozzle thickness" qualified as Pm (primary membrane) in BPVC.VIII.2-2019 Table 5.6 under Nozzle->Within the limits of reinforcement.
Bending stress seems to be classified as secondary in most cases. Does anyone know how to classify the bending stress in the nozzle wall within the limits of reinforcement?
The Section 5.6 does not say anything about averaging the bending stress through the thickness.

Thanks all!
TGS4, I'm a big fan of your posts!