Stress Analysis: High Temperature Line Fails Under Wind/Seismic Loading

[simon17]

Hello,

I'm currently doing stress analysis in Caesar II on an existing 8" SS line with a design temperature of 1400F. The client would like to know if the line is overstressed and potentially replace it if so. Since this is an existing line, the analysis uses all of the existing springs and snubbers in their current locations.

After making changes, the line is no longer overstressed in Sustained or Expansion cases, but all the Occasional load cases are overstressed. The occasional load cases involve the Sustained Loads (Weight + Pressure) + Wind/Seismic. If guides and directional anchors are added to the pipe, it will fail in expansion and not be able to grow (displacements are +5"). But without them, in a shutdown state, the pipe is susceptible to overstress during high wind or seismic activity.

What are some ways to resolve an issue like this? Any help would be appreciated.

Thanks,
Dmitriy

 

[davefitz]

not sure, but the anchors and bumpers could be replaced with hydraulic snubbers when addressing transient loads, and the allowable stress used for review of transient cases likely would not be the same as that for steady prrimary loads ( usually governed by creep stress at 1400F)==> check code to confirm.

"Whom the gods would destroy, they first make mad "

 

[Duwe6]

". . . replaced with hydraulic MECHANICAL snubbers"
With that pipe surface temperature, and the huge growth, you will be much happier with mechanical snubbers, and the maintenance folks will be much, much happier. Hydraulic snubbers require a lot of scheduled maintenance. Mechanical ones need a lot less frequent and less messy maintenance. And they can be run a lot hotter than hydraulic ones.

http://en.wikipedia.org/wiki/Mechanical_snubber

 

[simon17]

Thank you for the replies.

Dave, Caesar II automatically applies higher allowable stress for occasional load cases. I verified this in the model after reading your comment. Thank you.

The line already has snubbers in place. But from my understanding of snubbers (limited as I've only recently been introduced to and used them in design), they activate during an earthquake for example but are supposed to allow for thermal expansion and regular operating conditions. This means in the C2 model, I have placed displacements on them for those points at the operating conditions, and therefore they become rigid when the occasional loads displaceme the pipe PAST those operating displacements.

However, I am now realizing that snubbers become rigid for vibrations and impulse forces but are supposed to allow for gradual deflection. Would it be accurate then to design with Sustained Case displacements on the snubbers instead of expansion? And if I did so, I assume that my loads and reactions in the Operating + Occasional load cases would be innacurate because the snubbers would be activating much sooner (ie: become rigid at 1" displacement instead of 5").

Thanks again for the replies and help,
Dmitriy

 

[DSB123]

Hi,
Have a look at B31E for seismic!!!!

 

[DubMac]

There is a product made that incorporates wire rope technology that both addresses steady state vibes as well as transient slugs/wind/seismic. It only depends upon flexion of the wire rope for absorbing/dissipating the vibration. Looks somewhat like a shock absorber with a wire rope unit in between the two ends. There were almost zero moving parts and no fluids.
Don't remember who makes it or what it is called; it solved a severe slug shock problem on a coker line in a refinery. Worked beautifully.

 

[Duwe6]

Try calc'ing with your snubbers shown as rigid during seismic loading, and wind gusts. I'm pretty sure that hydraluic snubbers start 'snubbing' prior to 1" of movement. The mechanical ones go rigid in about 1/4" or less of rapid movement.