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