Pipe rack Design - Thermal+Seismic on pipe anchors

[Kom3]

Hello all,

Should the seismic forces on directional anchors be combined with thermal forces if there are loops that allows the pipes to expand in the direction away from the anchor?

Per PIP STC01015 "Friction loads shall be considered temporary and shall not be combined with wind or earthquake loads. However, anchor and guide loads (excluding their friction component) shall be combined with wind or earthquake loads"

Based on my interpretation of the sentence above, the answer is no. The way I see it, all thermal forces on a directional anchor disappears once the pipe on the supports without anchors overcome friction and slides away from the directional anchor. Hence the "excluding their friction component" refers to this scenario. Am I interpreting this correctly? If this is the case, I would then only be combining the non-thermal operating load with the seismic load at directional anchors.

Thanks.

 

[dauwerda]

I agree with your interpretation, keeping in mind the anchor(s) will have to support the entire pipe's longitudinal seismic load as it is free to slide at the other gravity supports.

 

[MJCronin]

Kom3...

This does not directly answer you question, but I believe will shed light on the design process ...

It has been my experience that piping design departments will "load up" both existing and new pipe racks leaving the structural analysis to others at the end of the job.

On projects in seismically active areas, or with large hot pipe this is a recipe for disaster

The Client, of course, has not paid any attention to his existing racks for years ...

I recommend a cursory evaluation of both new and existing rack designs AT THE BEGINNING OF THE PROJECT to establish how many additional lines that the rack can accept.

Additionaly, certain selected large bore systems should be run underground alongside of the rack (Plant Cooling Water, Waste water, Drains) and some other thermally active systems (Main Steam) should not be run at the top of the rack (run hot systems mid-height on rack)

These measures will help .....

MJCronin
Sr. Process Engineer

 

[RPMG]

Anchor-loop-anchor results in thermal forces at the anchors that should be combined with seismic activity.

On the guide, the pipe slides and is idealized as a roller. The statement says not to include that friction on that slide. Anchors are pins, and they develop self-straining forces that force thermal expansion into the loops, while in service. That force is determined through pipe stress analysis.

 

[humanengr]

I disagree with this statement:
"The way I see it, all thermal forces on a directional anchor disappears once the pipe on the supports without anchors overcome friction and slides away from the directional anchor."

Temporary Friction forces typically occur during start-up and shut-down, as pipe expands / contracts and slides against supporting members. Once the pipe expansion terminates, the friction
forces dissipate. This is the reason for not combining the temporary friction forces with wind and seismic (due to the short duration).

What are normally referred to as "Thermal Forces", on the other hand are resisted permanently or continuously by directional anchors and guides. As the pipe expands, the dimensional change
is restrained by anchor or guide, and the expansion / thermal force will remain during operation.

Therefore, "Thermal Forces" should be combined with wind and seismic.

 

[dauwerda]

It depends what the overall piping and support configuration looks like. If there are multiple anchors with expansion loops in between (as mentioned by RPMG, then yes, there will be a constant thermal force. However if there is only one anchor and the pipe is free to thermally move towards and away from the anchor, the thermal/anchor force is no different than the friction force - once the pipe moves, it goes to zero.