Pipe Wall Thickness for Recip Applications

[qwerpoiu]

I am a junior engineer in this field and just want to get more understanding behind piping specifications.

To determine the pipe wall thickness to be used, the minimum wall for internal pressure can be calculated using B31.3, than most books ask to use the next pipe wall schedule. I noticed that for smaller diameter pipe, sometimes heavier wall are specified (e.g. for NPS 2, sch 160 may be used instead of sch 80). Is this because we want to avoid buckling during pipe handling and installation?

I was also advised to use two schedule up (from the min wall calculated) for recip applications. I was reading some books but did not find a direct answer for the reason behind this. Is "2-wall schedule up" an industrial practice for vibration/fatigue consideration? (B31.3 addresses “low cycle, high load” fatigue calculations, but vibration is a “high cycle, low load” situation and guideline was not provided in detail).

I am wondering if someone can advise where I can look for the reasoning behind the above (thanks ahead for the help).

 

[JStephen]

I'm not familiar with your application, but in small-diameter pipe, the thin wall is more susceptible to corrosion issues, and that may be another reason to use a considerably heavier thickness than required by actual loading.

 

[qwerpoiu]

Thank you for giving me some direction.

I was not clear in my original question - The application is for process fluid (natural gas, produced water and hydrocarbon liquids). When I calculate the min WT required per B31.3, corrosion allowance of 1/8" was included in the wall thickness calculation for sour applications (< 2% H2S); and for sweet applications, 1/16" was used.

 

[pipesnpumps]

Well the pipe wall thickness calcs are only for internal pressure.. Even if the stress analysis passes for thermal expansion the pipe can fail in reciprocating applications. The thermal expansion allowables are based on 7000 cycles.

For large piping in recip service a dynamic analysis should be used. The rule of thumb for two sizes up is to stiffen up the pipe so that the dynamic forces don't cause the pipe to flex such that it exceeds the yield stress. If this happens it just a crapshoot on how many cycles it create stress cracks. Like "how many licks does it take to get to the center of a tootsie roll pop"..

Think of it in terms of bending a wire. If you bend it easily back and forth you can break it.. If you make the wire so thick that the applied forces
never cause it to exceed yield then it will last..

The supports are very important. Every thing between the recip equip and the first pressure vessel need to be restrained as much as possible.

 

[BigInch]

Its quite typical to specify Sch 80 for 2" and below in high pressure liquid and gas applications, simply to provide for those unexpected loads from a large wrench, someone using one as a rope tie-off to lift something they shouldn't, liquid slugs when you really shouldn't be having one there, as well as blowdown "thrust" laods.

For recip piping, sometimes thicker walls are specified in an often vain attempt to limit vibration stresses, or to at least move the natural resonant frequencies of the pipe lower down in the spectrum to keep it away from compressor operating range rpms, but a better approach I think is to change the spacing of supports and do not use equal spans between supports, or do both.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[qwerpoiu]

Thank you very much for all the helpful information, it is especially helpful for people with less experience like me (thanks!)