I have a relief valve on top of a run pipe (same as in ASME B31.1 2007, Nonmandatory Appendix II “Rules for the design of safety Valve installations”, Page 244, Fig.b) and there’s a dummy leg under the relief valve discharge elbow down to the run pipe.

My question is; how should I model this dummy leg from the elbow to the run pipe taking into consideration the reaction force that I’m applying at elbow and also considering the movements of the relief valve discharge elbow and the run pipe?!!

FYI; I’m running a static analysis and the relief valve discharges to a stack that goes to the atmosphere.

Thank you

I have never modeled the movement of the relief valve.
Of course your dummy leg will just rest on your structure, guided or not, but you will not weld it to the structure, as always.
Don't forget the DLF, between 1.1 and 2.0

Don't you mean Fig. a? But anyway, you could insert an element - say a rigid one - from the elbow to the header below. I would CNODE the end of the added element that rests on the header to an associated node on the header such that the only active restraint is the +Y direction. Then the issue becomes one of distributing the load imposed by the RV reaction force, transmitted via the added element, so that the local stresses in the header aren't "too high". How that is done I'll leave to others. Peng discusses somewhere in his book insulating the new element to keep all the temps as uniform as possible.

Thank you both SO MUCH for your input, which I’m currently consider modeling my Safety Relief valves (I’m following fig.b ASME B31.1).
Since I’m modeling this rigid element that runs down from the PSV discharge elbow to the run pipe, I can only apply the reaction force at the elbow, or shall I just apply it at the run pipe right below the discharge elbow. Could you please advise?!

I'd apply it at the elbow since the force is applied there. More importantly, if the rigid is CNODED to the run pipe via a +Y relationship, imposing the force on the run pipe may not have much effect on the relief discharge, which is what you're interested in.

thank you so much Goodsalt

I am just trying to remind that you need to consider the thermal expansion of the SV nozzle in the dummy leg design. Most cases, there is a differantial thermal expansion between nozzle (means underside of the elbow) and the dummy leg. If there is, the dummy leg may not be working as anticipated because of the lift up of the elbow.

Ibrahim, that was a good point thank you

If your PSV out let pipe leg is minimum in you case then there will be a left up at elbow, If you consider maximum pipe leg length at outlet of PSV in you case you will find resting.

consider PSV discharge pipe diameter to increase and see if its lefting or not. I hope you will find high loads on supports and PSV outlet Elbow. As per the loads at Elbow calculate trunnion support.