Nozzle Loads + External Pressure 2

[Lee2006]

Hi,
I have been asked to carry out some Nozzle loads calcs for a Vessel which we are designing.
The customer spec asks for the radial nozzle load to be checked as a positive value and combined with the internal pressure as usual.
However I have also been asked to assess a negative radial load in conjunction with the external pressure (which is a Full Vacuum).

Can anybody give me some advice on how to approach this ?

Thanks
Lee

 

[alanw7272]

Hi Lee,

Yeah this is a common request to check the radial load as a positive and negative value, ie: acting outwards and inwards.

You can check this simply by changing the radial load sign in whichever program you are using, WRC 107 / 297 / App.G, however just watch the directions as they all differ.

So when you check your nozzle for vacuum, run a nozzle load check with the radial load acting inwards.

Hope this helps
cheers

 

[nickelkid]

http://paulin.com/FE107.aspx

The above link expands on alanw7272's reply.

 

[JStephen]

Something that doesn't seem to be covered in either case is that if you apply inward nozzle loading to a vacuum conditon, the nozzle loading should contribute to buckling of the vessel, rather than just having localized high stresses at the nozzle.

 

[Lee2006]

The software I use is called Finglow and you dont seem to have the option of running a nozzle load calculation in conjunction with an external pressure.
I though at first that I could get away with using an equivalent Internal pressure but that doesnt satisfy the buckling analysis as JStephen pointed out.

 

[nickelkid]

Lee2006, we use Nozzle/PRO

http://paulin.com/NozzlePRO.aspx

for this type of analysis.

 

[Lee2006]

Is Nozzle Pro based on FEA or on WRC's.

 

[alanw7272]

JStephen,
I kinda agree with you but am also not sure that a radial load on a nozzle, acting inwards, contributes to buckling in such a simplistic manner. Typically these loads are thermally produced and cause secondary stresses local to the nozzle (as you said). It tends to be requested but I think the reasoning is wrong. Say for example, external pressure governed a shell thickness such that the shell is only just thick enough for vacuum. Are you suggesting that adding a small radial load to the nozzle will buckle the shell? Or am I being naive and it is due to lack of analytical methods that this cannot be checked without the use of FEA?

Lee2006,
Can you not show a -103.4 kPa for your pressure and radial load acting inwards? I am not convinced that this request is correct but we all have to keep our Clients happy!!

 

[JStephen]

What I'm suggesting is that an inward nozzle load could contribute to the buckling of a shell without exceeding allowable bending/membrane stresses at the nozzle.

It would be somewhat similar to checking lateral loads on a column under compression. You could check for bending due to the lateral load and check for buckling due to the compression, but satisfying each criteria separately doesn't mean that the overall column is still stable.

How important this is in practice, I can't say. Nor do I know of a good way to combine external loads with the shell stabilty requirements of ASME. You could do FEA on it if the software was able to model the buckling.