Hi Everyone,

I'm hoping someone can help me.

I'm currently investigating some some static force combinations on a PSV vent system.

In this system I have included some standard project guides with a gap of 3mm. These guides are particularly important as it prevents large loads being applied to a small connections attached to vessels. Herein lies my issue. Please refer to the example loadcases to illustrate my normal stress check for occasional forces applied to the system.

L1: W+P1+T1 OPE
L2: W+P1+T1+F1 OPE
L3: W+P1 SUS
L4: L2-L1 OPE
L5: L3+L4 OCC

My problem is that when chekc my guide loads in my Operating Case (L2), I notice that my guide has been engaged, as my displacement has been stopped at 3.0mm. When I look at my occasional load case (L5) to check my system stresses due to the applied force, I see that at the same node where I checked my guide load, the displacement is now 5.9mm and I subsequently run into over stress problems at my vessel connection.

If I remove my gaps, CAESAR will recognise that a 0.0mm gap exists and my problem is solved, but I don't really want to have to go down this path.

Why won't CAESAR recognise my 3mm gap in my guides in my occasional stress check case and halt my lateral displacement at 3.0mm?

Any insight will be helpful.

This is the reality. Here is an example
Assume you have a guide in X direction with 3mm gap.
It is possible that in L1 to have a displacement of -2.9mm due to temperature.
Due to F1 and temperature it is possible to have in L2 a displacement of +3mm.
Therefore you have 5.9 net displacement due to F1 alone.

Hope is clear. In addition there is a Mr. Loren Brown webinar that explain in detail these issues.

Regards,

Hi Dan,

I will have a search for the webinar.

I'm not sure this is making sense to me. You are suggesting a net displacement of 5.9mm takes place. The pipe intilly moves in the negative direction 2.9mm and then the positive direction 3mm. Resulting in an overall movement of 0.1mm positive from its initial position? Why wouldn't CAESAR just tell me the node has displaced 0.1mm and the guide isnt acting? Surely displacements are given as a difference from the starting resting point of the pipe. Or is it taking the initial reference point of the pipe as its position after L1 has been calculated?

(Quick thought) Should L5 be a be and algebraic, or scalar combination?

I have had a look at my output graphics to show what the piping system is doing. It looks like some guides are working (i.e. the pipe appears to flex around the restraint, whereas the guides that I am having a problem with, it looks like the piping is displacing as if there was no guide in the first place. Your hypothesis may still hold true here, so could you please quickly confirm that I have understood you correctly?


Thanks

James



Thanks
James

No, the movement was -2.9mm from initial to L1 (operating condition), then 5.9mm from L1 to L2.

But read this, it explain better:

http://coade.typepad.com/coadeinsider/2010/07/C2_STATIC%20LOAD%20CASE%20EDITOR.ppt

Also read the attached file, written also by Mr.Brown

L5 should be scalar.

Regards,

Thanks Dan,

I can see what is happening now. Thanks for the useful links aswell!

My final point for discussion is, would this be a realistic stress check? As you can see that in the operating case (L10) the force and the temperature components engage the guide. Whereas in the stress check occasional load case, (L24), we are pushing the pipe through pretty much its entire displacement range, from one side of the guide to the other? This suggests that the temperature component acts first (+2.9mm), and then the force pulls the pipe (-5.9mm) the other way.

The temperature and force will most likely be acting concurrently.

I've never really put to much thought into how CAESAR analyses these loadcases, but is it a code requirement to check entire displacement range in these stress checks?


James

Sorry, the forum shrunk my print screen iamge. You may have to download the image to your desktop or something to veiw it properly

I’m not a very good writer and my English does not help me much, but I will try to give you an answer.

The code requirement is that you must check the primary stresses and secondary stresses.

How you can do this? This is the main issue. The same issue is the lift-off problem.
It is very difficult to know this when you deal with non-linear and complex systems.
You must segregate the various stresses types. This is clear for everybody the only problem is that is not really easy to know how.

We can only do this by identifying various operating scenarios (system state) then to isolate components.

There is a time when the force and temperature are coincident, but when force will stop acting, the system will still be hot, so you still have L2 and L1 and you should consider that L2-L1 is the net effect of the F1. If these two scenarios (L1 and L2) are real then is the F that “moves” the system from state L1 to L2 or L2 to L1 so this we must consider.

At least following the L2-L1= (W+P1+T1+F1) – (W+P1+T1) = F1

Take a look of the two images. If you rotate the first and replace F with W it will be about the same with the lift-off problem.

At this point there are a lot of different opinions so I will stop for now.

Regards,