UKCats,
When the PSV Tail Pipe is 'Square Cut' the thrust force is opposite the flow in-line with the axis of the tail pipe. I perfer, if I can, to support directly under the elbow to absorb the force. The reaction can be 5000 lbf or more for high pressure reliefs and API 520 gives equations to calculate this as well as most PSV catalogs.
If the tail pipe is beveled on the outlet then the reaction force AT THE END OF TAIL PIPE is NORMAL to the bevel as illustrated in B31.1, Appendix II.
For a 45 deg. bevel the forces components would be the following for a 5000 lb reaction:
.707 x 500 = 3535 lbsf horizontal and 3535 lbsf vertical down.
For 7 foot long unguided tail pipe the horizontal force will create approximately 24745 ft-lbs of moment at the bottom of the tail pipe and even more at the PSV inlet connection. Depending on your pipe sizes and wall thickness, this may create an over stress situation.
It can be corrected by square cutting the tail pipe or adding steel to guide the tail pipe near the top.
As far as the forces and flows involved, car exhuasts are really not a good comparison.
NozzleTwister
Houston, Texas