I'm a beginner in pipe stress as in using CII likewise. I need to confirm that to include the thrust load induced by PSV during relief, the load calculated as per B31.3 should be added to the input spreadsheet and refer to the tee. With the PSV modeled like an angle valve, say the branch line is going to atmosphere or could be to a silencer, and the base of PSV is connected to a tee on pipe, is the thrust load be referred to the tee in the pipe or the node (kind of a tee too) where PSV branched to vent? Thanks to this forum.

Art,

Your post is a couple of week’s old. I suspect it hasn’t been answered because it’s not clear what your question is. If I understand your system correctly, you have a PSV that branches from a header at a tee. The PSV discharges to atmosphere through a long, continuous run of pipe (i.e more than a couple of pipe diameters) and does not discharge into a separate vent pipe system. I also assume that you have calculated the reaction forces using B31.1 rules or some other calculation and that you want to input the loads into a Caesar II static analysis to determine whether the branch connection is overstressed. Let us know if these assumptions are correct.

P.S. I am currently on site and do not have my reference materials with me, so it may be two weeks or more before I can get back to you. Someone else on the forum may be able to answer this in the meantime.

Regards,

Thanks for your concern Ricardo. I have shifted to a new job site and only now that I was able to view this forum again. Anyway I think you are right and so let me describe my confusion again. Imagine a 10” horizontal steam line with a tee and a PSV assembly directly above it. The PSV is branching sideways and finally rise vertically to a silencer. On the model I once saw, the analyst was including the thrust load of PSV into the model. The analyst has plug-in this load (input as Force in CII) in a node located in the valve exactly where it start branching sideways for relief. I thought this Force should be inserted at the tee of the horizontal pipe where the PSV is connected. Please correct me or please advice .on proper modeling. By the way the model was analyze in static mode. Thanks again and to you and the for anybody’s help/tip.

Art

What the modeler did was correct. The force needs to be treated as an occasional force and combined accordingly.
One method of calculating the pop-up force is given in B31.1, Appendix-II.

Not quite. Typically, when you have a PSV discharge open to atmosphere through a vertical tailpipe, you need to apply your calculated thrust load in the vertical section. I think most of us would model it at the end to the tailpipe, but you could put it at the bottom elbow if you really wanted. The force is typically going to be acting in the negative Y (assuming your tailpipe is open in the positive Y direction, as usual).

The reason we consider the thrust load in these cases is becuase there is a discontinuity in the pipe (i.e. the open tail end) such that the pressure thrust does not have something to balance it out as it would in a closed system. That thrust generates a force and, more importantly, will generate bending moments going back to the branch at the header. Typically, you would put a support under the elbow below the open vertical run to resist the force/moment of the thrust and direct it to a structural element designed to take the load.

Edward, I like you explanation. I'm just having a hard time picturing how the pressure thrust balances in a closed system. I've heard this many times before and just take it as just what happens. If you could explain it to me, I'd appreciate it very much thanks.