EFFECT OF NOZZLE LOADS ON LEG SUPPORTS OF A VERICAL VESSEL

[brkmech1234]

The radial force and circumferential force of nozzle induce bending moment on legs. The longitudinal force induce compressive or tensile effect on leg.

I am not sure about the effect on leg of the longitudinal moment,Circumferential moment and torsional moment.

I have some where read or heard that since the vessel is flexible, moments will not be transmitted to supports. Am I right?

 

[4Pipes]

Shhhhh, You are exposing a trade secret - that nozzle loads disappear as soon as they reach the shell. Nozzle loads never reach the vessel supports as you suggest. It is probably specified in a little known reference book - Vessel and Pipework Design by Harry Potter.

 

[TGS4]

Ugh. Flexible or rigid, it doesn't matter - all loads applied to the nozzles go through the supports (or through the other supports...). The trick is to know the actual directions of the nozzle loads - many will cancel each other out. However, if your customer insists on paying for the "worst case", then "hello, Mr. Change Order."

 

[jtseng123]

ooops, there is no easy asnwer to this, and it confuses a lot of people reading Div. 2 when loads are classified as primanry and secondary, and all kinds of combinations which are not taught in school. It is somehow against the traditional concept of "Statics".
Since shell and nozzle junction is not really regid, it will absord loads or redistribute, relax, or self-limiting, self-constraint, unrelenting, you name it. The load is not actially transfer to the support due to the effect, especailly thermal load which is considered as secondary. Sustained load, pressure thrust or so called dead load, which may be the load you want to take a serious look.
For a vessel with skirt support, we hardly consider to transfer any nozzle load except the overhead piping and other attachments,which dead load can create eccentric moment you have to consider.
Many loads can cancel each other as TGS4 says.
I have several cases which I do consider to transfer nozzle loads and pressure thrust given by Piping group, especailly the nozzle is not small and come with big rigid re-pad.
In your case since it is leg support, I assume it is a small vessel, and nozzle load can have an impact on leg to pull,push or twist it if the nozzle is not small compared to the vessel. If I were you, I will take all 6 forces transfered to leg, purly based on conventional "statics", forgert about primany/secondary. Structual steel won't cost that much. Be conservative and you can sleep well, and no one can say you are wrong.

 

[1gibson]

Without moving this to the language/grammar section, he said the "moment" might not be transmitted through the a flexible shell. No claim that a related force isn't transferred, I think the question is really "how do you translate a moment on a nozzle to a force on the legs?"

 

[brkmech1234]

1gibson is has understood my question. I appreciate the answer for this question

Jetseng123's comments are valuable and reconfirmed my vague understanding. Thank you.

 

[TGS4]

A few things to clear up here:

1) Loads are not primary or secondary. The stresses that a load causes can be classified as primary or secondary. Does nobody even read the Code these days?!? Furthermore, the local membrane stresses in the shell of a vessel caused by piping restrained free thermal displacement (piping thermal loads, if you will) are to be classified as PRIMARY - see Table 5.6, Vessel Component = Any shell..., 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.

2) Draw a free-body diagram. Remember 1st year statics stuff? A moment at a nozzle transfers down to the support as a moment. And a force transfers down to the support as a force PLUS a moment. IT DOESN'T MATTER if the shell is flexible or rigid.

The load is not actially transfer to the support due to the effect, especailly thermal load which is considered as secondary.

No, no, no, no. Again, draw the FBD. Elementary statics, people! Plus, see my item 1) above. Where do you get notions like this?