Hi respected Engineers;
i was trying to model a horizontal dummy leg i gave offset to element. but it didnot gave the result. i could not understand how to model it from application guid can any body here guide me? i am attaching picture of what i modeled wrong

I think i have asked a childish question to very senior persons here. Can any body give me any reference.

Hi Muhammad Sami
I think you are trying to put dummy pipe at elbow to put support.
Dummy is also a pipe model in Caesar.
Just few things to consider
1) Fluid density , insulation density (if not there) zero.
2) Temperature ambient, pressure zero.
3) Dummy reduce flexibility of elbow hence model bend type “double flange”
4) If pipe diameter is large model from centre of elbow to dummy weld point with zero rigid body. ( for more accuracy rigid angle 41.4 deg)
5) Put SIF manually at dummy weld point.

i tried to delet design temp and pressure for dummy leg pipe but they are automatically inserted again.

you mean to say that in between starting node of dummy pipe and centre of elbow i should model a rigid body.?

i have modeled it as per your advice. but still i am confused that what type of sif will be at weld point? sif type welding?

Hi Muhammad Sami
1)I do not know why you can not change pressure, temperature.
Node 10 to 20 feed T1=60 deg, Node 20 to 30 you can change it to 21 deg,again node 30 to node 40 can change to 60 deg.

2)Yes you got rigid body concept correctly. Regarding SIF it is quite tricky one. You have to calculate it BY Finite element method.If your system is not so much critical may be you can use SIF of that elbow with some additional safety margin. B31.3 to not specify of the same.

hi

for dummy leg u can not introduce any temp and pressure value.

and then u must connect the dummy leg node properly.after that u introduce SIF as per our SHR Advice.

hi thanks priyan and sif
i got my mistake actually i was deleting temp and pressure value but priyan showed me that i should enter zero instead of deleting it thanks

Temperature put ambient value not zero, pressure zero.

yeah i correct temperature to ambient. thnx

Hi All,

I have a dummy leg modelling technique which we use in our company. I think this gives fairly accurate results for stresses/loads. see the attachment.

It's good to see that companies develop in-house modeling techniques. Makes for consistent models between users and projects.
I have a few comments. Not that I consider anything wrong here; mainly just talking points...

- Keep an eye on your dummy leg size. You do not show the reduced section properties.
- Since there is no hole in the run pipe, I see no reason to use an SIF for an unreinforced fabricated tee. No problem as long as this stress does not control design.
- The CAESAR II "Offset" command can be used to replace that short weightless rigid element from pipe center to pipe wall.
- I wouldn't set this dummy element (pipe center to pipe wall) to ambient. I would keep it at the line temperature.
- That Node/CNode pair (120/130)is not required input but it will display the internal forces & moments at this point in the restraint report. For the elbow I might move that "relative anchor" up to Node 110 as that is the point where the leg meets the pipe wall.

Dear SRI and Dave,

Mr. Dave can u be more specific on your comments on Sri dummy leg model.

Also Please clarify the doubts below,

1. Is that is necessary to put an element of node 100 to 110 at the end of elbow point (instead of from mid point of elbow) and in case of pipe, from the center of pipe (instead of putting a single element from center of pipe to the base).

2. Is the anchor with cnode is required at node 120-130. Since in general these type of supports will be welded with pipe without hole in the main run, it is similar to Tee fitting and where the boundary conditions will be defined at the end of base by putting (restraint y, if required x or z, or even both).

There are many valid models for these pipe supports. While the most elegant, most complete model should perform well under all stiffness and load combinations, such a model is usually a waste of time. Simplifications are usually the answer.

A technician can model; an engineer knows why one is better than another.

To answer your questions:

1. Yes, the midpoint MAY be a valid location to start your dummy leg for the elbow.

2. No, the "relative anchor" (Node/CNode) is not required.

Mr. Dave

I just want to have a suggestion from u, is that whether Mr.Sri modeling of dummy support is more accurate as compared to the modeling provided in Caesar application guide. As compared to a caesar application guide, Sri model seems to be time consuming work (if it is accurate) and is that can be used for critical service piping... while the caesar guide model shall be used for non-critical piping ???

Like I said earlier, a more detailed model, in some instances, may produce more accurate or at least more realistic results. But in many cases (your non-critical piping?) such detail is not warranted. Your job is to evaluate the safety of piping systems, not to build beautiful CAESAR II models.

There are several dummy leg models in the CAESAR II Application Guide. They are ranked by ease of input and accuracy.