Dummy Leg Calculations

[Mehr_1139]

Hi everybody;
When a dummy leg is attached to an elbow in a piping system, C2 considers its length in flexibility calculations, so the proper length of dummy support matters. I want to share my understanding of this issue with you and know if you guys agree with me or not.
1- If the dummy direction is perpendicular to the run pipe with larger expansion, we can use dummy legs with longer length. So in this case, by increasing the dummy length, the SIF will be increased as well and the piping stresses will be decreased.

2- If the dummy direction and pipe expansion have the same direction, the dummy leg is supposed to be shorter. In this case the support should be shorter to absorb more expansions. (please have look at the picture enclosed)

Besides, does anyone know how we can calculate the proper length if a dummy support attached to an elbow? any suggestion is highly appreciated.

 

[pennpiper]

Mehr_1139,
Based on the little amount of information and only the two picture I would say that "#2" is the correct method of attachment for a "Dummy Support".
However, having the knowledge of the missing information could make a big difference.

Sometimes its possible to do all the right things and still get bad results

 

[mk3223]

The size and orientation of the dummy leg support design can be varied by many factors, such as piping routing, process P&T, concentrating load, etc., and the actual design can be one of either case as the sketches shown.
However, without the design parameters, the "longer" or "shorter" dummy legs are only a relative terms with no meaning.