Pipe Stress Analysis - Translating moment around pipe inlet

[gfdoug]

All -

I am currently in the process of designing a piping system that offloads LNG from a railcar. The simplified version of this is that I have an 8" pipe with a swivel-arm setup that can attach to the railcars as they approach to offload. I am currently creating a stress analyis model within Bentley Autopipe and I'm running in to problems of how to properly represent this system.

The problem is that the swivel-arm can rotate which makes the moment caused by the weight of the swivel arm at the pipe to go from a bending moment (when the swivel arm is fully in-line with the pipe) to a torsional moment (when the swivel arm is fully perpindicular to the pipe).

The approximate moment that the swivel arm creates has been decided to be 1900 ft-lbs at the inlet connection when acting in-line.

It has been suggested to input a 1900 ft-lb bending and 1900 ft-lb torsional moment, but this will not be an accurate representation of what will be occuring - most likely it will by some resultant of 1900 ft-lbs acting in a non-uniform direction. Can anyone with experience in stress analysis or in AutoPIPE guide me on how to correctly model this?

Thank you.

 

[dhengr]

Gfdoug:
Show us a sketch with good proportions, dimensions, sizes, thicknesses, support points, etc. so we can see what you are dealing with. How is the main pipe oriented and supported? It must be horizontal. How did you arrive at the 1900 ft.lb. moment, did you include the LNG weight? How is this transmitted through the swivel connection? How do you align the car connection end up with the car unloading fitting on the car if the car isn’t located perfectly, or is there a 10' hose section at the end of the pivoting arm? This is a dangerous enough design problem that you probably shouldn’t be asking this kind of question here. And, you shouldn’t be counting exclusively on computer software to solve this design problem, or doing your thinking for you. You really should be talking with your boss, so he knows what you do and don’t know, and can help guide you through this problem. Note, that if the main pipe came up out of the ground vertically or was properly supported and vert. at its end, at the swivel joint, the torsion problem (orientation) would go away.

The moment through the swivel joint will be the same in any case, 1900 ft.lbs. And, you rightly express the two extremes, moment continuing into the main pipe when the swivel arm aligns with the main pipe; and then some moment plus a 1900 ft.lb. torsion when the swivel arm is at 90̊ to the main pipe axis. Remember, the bending moment in the main pipe may continue to grow depending upon how and where it is supported. At 15̊ increments from 0 - 90̊ what are the bending moment and torsional moment components from the swivel arm. Draw a plan (free body diag.), to scale, and figure it out or scale it, and tabulate the results. Run the software for each of these seven conditions.