Gas Pipeline Branch Reinforcement

[GasRookie]

Howdy all,
I am trying to put together a worst case scenario reference table for our construction crews for the natural gas pipeline we have in the ground for when they branch off of (perpendicular to main) the existing main. We know the minimum wall thickness, maximum pressure, minimum yield strength, and range of sizes for the distribution pipeline we have. For the pipe that I am referencing the MAOP is 99 psig, the minimum wall thickness is .188” for both the main and branch. The minimum yield strength is 35,000 psig for both, and the nominal sizes for the main are 4”, 6”, 8” and 12”, and the nominal sizes of the branches are 3”, 4”, 6” and 8”. I am using the ASME code for pressure piping (B31.8) for the correct equations and exceptions, but so far all of my calculations show that no reinforcement is needed for any of the combinations of main sizes and branch sizes. My question is: are any of you all familiar with this enough to tell me if this sounds about right, or should I go back and check my calculations and equations again?
Thanks
~GasRookie

 

[pmover]

this question should be posted in piping and fluid mechanics forum.

while it has been many years since i've done saddle calcs, there are occasions where a reinforcement is not needed. i simply do not recall the conditions where this is applicable. however, re-verifying the analysis is prudent. i would think there are in-house people that can verify your analysis.

good luck!
-pmover

 

[Zapster]

Lots of considerations you have not mentioned. What is the design factor? Will the design factor be the same for the entire line? Have you reviewed the pipeline’s integrity plan? Are there considerations for limiting the percentage of SMYS that you need to use such as pipeline inspection intervals? Are there any external loads on the branch (ground settling, seismic faults, etc.)? What is the design corrosion allowance?

 

[BigInch]

Your 99 psig pressure is very low for those wall thicknesses, so you won't need any reinforcements.

 

[vzeos]

GasRookie:
I agree that you may not need any reinforcement on a 99 psig main. But there are other considerations. If your gas system is in a high consequence area with a lot of third party excavations that may impose some external loads on you pipe, you may want to install some reinforcement. Consider using Weld-O-Lets. See the attached thread.

thread378-138853

 

[BigInch]

No. Use XXH schedule pipe. Sch 180 or so for 2". Much easier and faster to install than reinforcement and much less of a corrosion problem, no worries about shotty welds, etc. I would only use a weld-o-let on a pipe not subject to any excessive load.

 

[BigInch]

Besides a weldolet, a reinforcement pad, or XH or XXH ain't gonna' stop no backhoe anyway. Your best bet to stop a backhoe is a sign above the pipeline or a fence around the lateral connection, while you hope that guy can read.

 

[Hookem]

Having done numerous calculations, it is no wonder you show that no added repad is required. This is because there is more wall thickness than required for pressure alone; the difference is credited to reinforcement. The driver of course is the 99 psig. The answer would be entirely different with a pressure that corresponded to the maximum allowed factor times the minimum specified yield stress. The other posts are also accurate - for practical reasons, use S/160 for 2" and less buried piping, use S/80 for up to 12", use XH above that - all for mechanical strength including backholes, shovels, earth movements, changing water tables, thermal and hydraulic forces, etc.

 

[Zapster]

What I find interesting/confusing is that GasRookie’s profile shows he became a member of the forum on June 27, 2006 and has not logged on since asking the question that same day. Why would anyone post a question and then not be interested enough to look at the replies?

 

[BigInch]

His question had a pretty simple answer. If the spec calcs show you don't need it, you don't have to use it, unless you have the experience to say you need it anyway ..... but even a sch 160 ain't gonna stop a backhoe, so .... in his case, does it even matter anyway?

 

[GasRookie]

Thanks for your help all. I have been looking at the replys but haven't logged in, on account of forgetting my password. I have it now, and I have compared my calculations to some that have been done before me and you are all right.
Thank again.
GasRookie