On trunion design 2

[s1111]

Greetings group,

When designing a trunion, if there is overstress in the trunion near shell (branch at junction), should the pad thickness be increased or should the pipe thickness be increased. From my understanding it should be the pipe whose thickness should be increased, and it should be easier as well. any references on this?

Thanks in advance for your reply.

s1111

 

[TGS4]

It would help if you would describe in more detail this "over-stress" condition you speak of. Are you performing an evaluation to ASME Section VIII, Division 2, Appendix 4? If so, what stress category is being exceeded?

 

[s1111]

Branch at junction.
Primary Bending Load Case
Sect VIII Ref: 4-112(h)(2), Table 4-120.1, Fig.4-130.1, 4-133
Plot Reference: (2) Qb < 3(Smh) (SUS,Bending) Case 1

 

[TGS4]

OK - so you're quoting a primary bending load case, and yet it is the secondary bending (Qb) that is being exceeded. Are you using some black-box type of software for this evaluation? Do you understand what information it is outputting to you?

Start by getting your head out of the software and explain the problem and the overstress situation in plain English.

 

[s1111]

you are right I am using nozzlepro to size a trunion.
the software gives me a larger stress than 90% of yield stress at the branch at junction, which is basically the trunion pipe near the edge of the shell.
Since it is the nozzle which is overstressed, it makes more sense to use a thicker trunion pipe right?
thanks.
s1111

 

[s1111]

I am not sure if I wrote as well, please bear with me... I really appreciate your help TGS4.
Thanks

 

[TGS4]

OK - let's start by forgetting the software (it's merely a tool which you will use to solve the problem - but first you must understand the problem).

Describe for us, in detail, the problem that you are trying to solve, complete with dimensions and loads.

Once we are sure that you understand the problem, then we'll look at how you are modelling the problem in your software...

 

[s1111]

I am looking for stresses in a simplified model for trunions at a vessel.
I am only modeling 1 trunion, in practise there will be 2 trunions 180 degree, for lifting the vessel.
The reactions at trunion sum up to the vessel weight. So I'm dividing the vessel weight by 2, to obtain each reaction.
This is to determine the size of the pipe to be used for the lift, to avoid any overstress at shell or in thr trunion.
I'm using a 120 inch cylinder, and locating the trunion (basically a pipe) at length-midpoint. wall thickness of cylinder 1.125,
pipe is 30 inch dia, 1 inch thick, 7.125 length, pad width 12 inch, 1.125 thick, moment at free end of pipe is 182 kips-feet.

illustration || is shell wall, } is pad, == is pipe

||
||
||
||}.==+
||
||
||

yield=38000, I am shooting for 90% yield=34200.

The . indicates the location of pipe overstress in the trunion. The + is location of moment.

Thanks.
s1111

 

[TGS4]

Two observations - first is that your loading does not include the vertical shear - you'll need to include that.

Second, and most importantly, what is your criteria for determining that 0.9Sy is your limiting stress? Is this a membrane stress intensity, or a membrane-plus-bending stress intensity? Further to that, why would your criteria be based on yield, and not Sm?

Is this trunnion going to be used for anything other than a one-time lift? Is it going to be removed after installation? Will it be used when the vessel is being decommissioned?

Other thoughts - can you guarantee that the load will be no more than 50% on any one trunnion? - what if the vessel shifts under the hook and one trunnion carries 55% of the load, or even 100% of the load? What about some sort of impact factor? How are the trunnions going to be supported themselves - by slings or will they be eventually placed on flat steel? Is the contact stress for the latter acceptable?

Not a simple problem, is it? Glad to help, though...

 

[s1111]

wow, that is pretty insightful. I think I may spend some time with your comments.
The trunions are usually for one time lift only, thay are removed after erection. Shackles are used for the lift, the trunion is designed for shackle thickness + some clearance. The trunions are weld in place on the shell or on shell with padding.
Will post in here more, after I am done pondering.
Thank you so much for your valuable comments.

Regards,
s1111