I’m relatively new to performing piping stress analyses and AutoPIPE and I'm having trouble understanding how AutoPIPE considers pressure thrust forces.
My understanding is that pressure thrust forces only occur where there the piping "dead ends" (for example capped piping, blind flanges, and closed valves). From what I can tell AutoPIPE only calculates pressure thrust forces at the end of modeled pipe runs and anchors. This doesn't make any sense to me. Pipe anchors are welded to the outside of the pipe, there's nothing for a thrust force to occur against. It's like AutoPIPE thinks pipe anchors go completely through the piping. I guess I kind of understand putting pressure thrust at the end of pipes but just because I stopped modeling the piping doesn't mean the piping hits a dead end.
I have modeled a 30ft long run of 5” schedule 40 steel piping in three 10ft sections, two anchors, and a valve in the middle. I’ve run two load cases, both with the Pressure Extensions option turned on. Case#1 has 100psi steam running through the piping as if the valve is open. Case#2 has 100psi steam up through the valve and 0psi downstream of the valve as if the valve were closed.
I would expect that in the first case there would be no pressure force on the anchors and in the second case there would be a 2000lbf on the valve (100psi x 20sqin internal area of pipe). However, this is not the case. In the Case#1 I’m getting 800lbf on each anchor and 400lbf in Case#2.
With all of this being said:
Can someone explain to me how to model the pressure thrust force on an anchor due to a closed valve?
Do I just need to add a concentrated force (2,00lbf in the example above) at the valve in the direction of steam flow? I guess I'm OK with AutoPIPE adding extra pressure thrust forces at every anchor, but I have some complicated piping runs where valves are out-of-plane with the anchor racks and I'd like to verify that the anchor rack design can handle the forces and moments imposed by the pressure thrust at the closed valves.
A closed pipe does not exert an external pressure force unless the pipe as some type sliding joint. between anchors. A fully connected pipe with closed ends will be in tension but there is no pressure external force.
If the pipe is open at one end the it will discharge it's contents and there will be a jet trust force as it does.
To understand how AutoPIPE calculated the Force due to pressure, see WIKI here.
Regards, Mike Dattilio Design Analysis Engineering Group Senior Analyst Bentley Systems Inc=============================================================================
Answer Verified By: Mike Dattilio