Nozzles Allowable Forces & Moments

[Mohyieldean]

Hello All...
I have an old tank (built more than 20 years ago) connected to a loop of piping through two nozzles.Each nozzle has isolation valve and then expansion bellow. These loops now are bended and a repair job has been started, but the client insisting to avoid any using of expansion bellows.Stress engineer asked client to provide 3 allowable moments and 3 allowable forces includes torsion.
The problem is that the client has no any documents for this tank or other similar tanks.
I made a brief tank model by COADE Tank, output report provided: Radial Stiffness, Long. Bending Stiff, Circ. Bending Stiff, Radial Deflection, Long. Rotation, Radial Force Multp, Long. Moment Multp, Circ. Moment Multp, Radial Force Limit, Long. Moment Limit, Circ. Moment Limit.
But still stress engineer reflected the stresses provided by COADE Tank are not enough and he needs more info.
The question is from where/how to obtain these stresses...
Thanks in advance

 

[JStephen]

Have you read through Appendix P (which I assume is the basis for the COADE output)? It sounds like you need to communicate more with the piping designer.

Sometimes, it is assumed that the tank nozzle is a rigid immovable object and that it then needs to resist the independently applied moments and forces calculated to act upon it. But in fact, the nozzle moves with no loads applied to it, and applying loads in one direction reduces the allowable load in another direction, etc. The most common issue seems to be that regarding the nozzle as rigid leads to very high forces being calculated, and recognizing the flexibility of it removes much of those forces.

 

[Mohyieldean]

Thanks JStephen for reply but the problem with App. P that its giving only one force and two moments (radial force, long. moment and circ. moment) what about the others??...another point regarding those mements and force is that App.P method of calculation is based on giving the max. of those three values by assuming 2 equal 0 when calculation the third one, and I need the reasonalbe values since Iam going to remove expansion bellows which will going to increase stresses transfered to nozzles (even with line stops in the loop).
Any advice?

 

[mariog123]

You may try FEA- for example Paulin Research Group FE/Pipe family.

 

[LittleInch]

It sounds like you're being put in a very poor position by your client - "but the client insisting to avoid any using of expansion bellows" whilst "client has no any documents for this tank or other similar tanks". This is unreasonable behaviour by the client, but I guess you need to do something which is safe.

You provide no other details (size of tank, product, temperature, piping configuration / layout, size, etc) so we can only answer in the most general sense. How have the "loops now are bended " occur?? This is not usual behaviour and indicates to me something is seriously wrong.

To my mind I would just replace the piping with some form of flexible piping and when the client asks why it costs so much point him back to the phrases at the top. Clearly the bellows were deemed to be required in the first place. Unless things have changed a lot, and you're replacing like with like, how exactly are you supposed to come up with a different answer to the original designer?? Tank nozzles as I'm sure you know are fairly notorious for having very low limits on force, moments etc so if you get it wrong and the tank nozzle ruptures, whose A** is on the line - yours or the clients??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[fegenbush]

Mohyieldean,

The interaction diagrams which Appendix P (now Annex P) directs you to generate can be used to determine acceptability for a known combination of loads. These loads are given by the pipe stress engineers since they can depend on a great many factors over which the tank designer has little to no control.

The reason Appendix P deals with the moments as it does is due to the nature of the failure modes. The most likely failure modes are covered (longitudinal moment, circumferential moment, and radial force). The longitudinal force (on a sufficiently large diameter tank) is treated as a flat plate in tension/compression. This is a strong arrangement and the failure is likely to occur due to the bending moment caused by said force. Similar can be said for the circumferential force. Finally the torsion (turning the nozzle like a steering wheel of a vehicle) will stress the weld around the nozzle which (using the API fillet weld sizes) is quite strong.

If you require a more rigorous analysis than that provided in Appendix P, it is possible to use one of many FEA packages available or WRC Bulletin 107/537.

 

[mariog123]

Mohyieldean,

As LittleInch said, it is difficult to give you an advice having no idea about the piping layout.
For example I would suppose that the original expansion bellows where in place just to avoid the settlement troubles rather than to solve thermal expansion. At least, your client must know how the expansion bellows had the displacements during the service.

Indeed, your statement "These loops now are bended and a repair job has been started, but the client insisting to avoid any using of expansion bellows" is very, very confusing. Maybe you try to say that you are going to erect a piping layout with loops with bends instead the old configuration, less flexible but having expansion bellows? Or really you had an incident in field?

In case your question is focused only on the allowable on nozzles, this is also a sensitive point. You may perform a FEA and remain responsible about qualifying the nozzle or you may stay protected by the low values provided by Appendix P; it depends on what you think it is the best strategy in front of such client...

As JStephen said, it is important also how you model the nozzle in stress analysis. IMO, the best is to extend the stress model up to the junction nozzle-shell and to input there the local flexibility of the junction, calculated by FEA. After analysis you must evaluate the results and decide about the necessity of an accurate FEA analysis to check stress in nozzle or Appendix P is enough for you.