For an open relief valve system, where the vent piping is hard piped and not with an umbrella fitting, Ceasar technical manual says that the horizontal reaction force can be ignored "as it (the thrust force) will be balanced by tension loads in the pipe".

Will somebody provide a more detailed and technical explanation of the mechanics behind this "balancing of the tension loads in the pipe"?

I believe you are talking about F1+F4 force generated (see attachment).
In established flow condition, the force F1 generated (Primarily considered as momentum component of API 520 eqn in most companies, see http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=28012&page=2) is balanced especially in short runs as the force quickly cancels each other and the +P*A component acts at exit of vent pipe.
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=21003#Post21003
"...we haven’t a realistic model for the steady state flow, exactly here, in the PSV body, where the flow is not one-dimensional. Probably is not a significant value and exactly for this reason, you cannot see, in API520 part II or B31.1 figures, this force as horizontally applied on the PSV body, but for some safety reasons, the stress tradition asks to count as this force exists, and I would say is more a way to be conservative rather to be realistic…. of course, this is just my opinion!
...."
and C2TR says,
"this is the thrust load that acts on the elbow just before the pipe opening into the atmosphere"
In case of hard piped, the thrust would be at Vent exit point rather than at relief valve piping.
Which is F1+F4 in the attached image.

Please read also at page 12. Note that they are PSV manufaturer.

http://www.parcol.com/docs/3-I_gb.pdf

Thank you stressguy for the links to previous threads. They were most helpful. I think I've almost got it. A couple of questions to help clarify in my own mind.

1. The PSV reaction force, commonly provided by relief valve manufactures, is only applicable, as a horizontal force, when the outlet from the relief valve is completely open - no discharge vent pipe. As that is the point of discharge. Yes? Once a discharge pipe is added, the point of discharge is changed. Yes?
2. The value of your F1 is from the momentum at the elbow : F1 = M*V, in both the vertical and horizontal directions. Yes?
3. Why show a horizontal force F1 at the PSV body on the left side of the image?

Yes, the earlier thread by mariog explains this.


The vertical component does not cause much problem ( it doesn't cause bending moment) if you provide a hold down at PSV i/t, which is a general practice to avoid vibration problems.



Pl. refer, "Momentum Balance" on page 6-6 of the 7th Edition of Perry's Handbook and Example 1 on page 6-8.

Stressguy,
I agree that for conservation of momentum, the horizontal force of the fluid momentum on the bend and the force of bend on the fluid are equal and opposite, thereby canceling each other at that point. But what about the external load of the bend on the "world" due to the fluid momentum through the bend. Thus the need for Thrust Blocks on large piping.

Do I have this right, or should I go back to my text book to get it right?

Thanks,