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The campus where I work has a long covered walkway (~.5 mile) which has several labeled pipes running under the roof (chilled water, fuel oil, air...). All of the pipes run dead straight except for the natural gas lines, which have little loops spaced about every 250ft, as seen in the attached image (the lowermost, yellow line. There's another natural gas line hidden above all the others, which also does the same thing.)

The line isn't branching at these points, and there doesn't seem to be any need to divert the pipe in order to support it. I've looked at some building codes to see if I could find a reason (or even a requirement) to insert these.

Any ideas as to what these are? It's driving me batty!

enter image description here

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    $\begingroup$ Nice question. How many bends are there? $\endgroup$
    – HDE 226868
    Commented Feb 9, 2015 at 22:55
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    $\begingroup$ HDE: About 10, over the whole length of the walkway (I haven't counted exactly) $\endgroup$
    – Gretchen
    Commented Feb 9, 2015 at 22:59
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    $\begingroup$ Interesting question. My guess is that it is to allow for thermal expansion, but I don't have any idea why only the natural gas lines would need it. Perhaps they are made of a material that has a larger dl/dT. $\endgroup$
    – Chris Mueller
    Commented Feb 9, 2015 at 23:10
  • $\begingroup$ Sometimes thermal expansion loops are the answer, sometimes expansion joints are the answer. I've noticed that the natural gas line is the smallest diameter, and containing a potentially corrosive chemical against rubber (methane), the decision was likely to use expansion loops instead of expansion joints. Believe me, schools are full of red tape to get into - someone looked at this thing. $\endgroup$
    – Mark
    Commented Jun 24, 2015 at 16:44

2 Answers 2

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The loops are known as expansion loops. They need to be placed in pipelines to enable the pipelines to contend with thermal expansion and contraction and other forces that can affect the pipeline.

They are typically placed in gas pipelines, irrespective of when the gas is hot or cold - natural gas or steam.

The following quote is from Pipeline Design. It's near the end of the page under Pipe Expansion and Supports

Steel piping systems are subject to movement because of thermal expansion/contraction and mechanical forces. Piping systems subjected to temperature changes greater than 50°F or temperature changes greater than 75°F, where the distance between piping turns is greater than 12 times the pipe diameter, may require expansion loops.

Such loops are not only used in gas pipelines and are used in pipelines that convey other fluids such as oil. Pipes transporting liquid fluids can use other expansion control devices such as single slip expansion joints or bellow type expansion joints.

Accommodating Thermal Expansion in Pipes

Pipeline Safety

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  • $\begingroup$ Perhaps the gas lines have the expanders because they are more critical, a gas leak is more problematic than a leak from some of the other fluids/gases. $\endgroup$
    – Jiminion
    Commented Feb 10, 2015 at 6:40
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    $\begingroup$ @Fred: There are some less optimistic explanations: The gas pipeline is made to the code, requiring these turns, while the rest is was built in the most straightforward, 'naive' way either because rules of making these lines were unregulated at the time, or someone managed to sneak it through the red tape. Considering the gas pipe runs below the duct, I believe it might have been added later and made to the code, as opposed to the rest. $\endgroup$
    – SF.
    Commented Feb 10, 2015 at 11:37
  • $\begingroup$ @SF. If you can provide evidence for your "less optimistic explanations" & your "belief" please do so. (1) Expansion loops have been put into pipes for decades - they are safety features. (2) Codes are established by experts, if expansion loops are in the code it is so that the pipe can expand & contract, it's not for decoration. Such loops increase the length of pipelines & their cost. (3) If if wasn't safe to have all the pipes running under the roof as they do they wouldn't be there, the buildings approval inspector wouldn't allow it. $\endgroup$
    – Fred
    Commented Feb 10, 2015 at 12:07
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    $\begingroup$ @SF. I fail the see what an inadequately or incorrectly installed smoke removal system in a stadium in Poland & potentially corrupt officials in Poland has to do with a piping system in the USA & in particular one gas pipeline with a loop in it. The picture shown in the question is nothing unusual; I have seen similar arrangements of pipes over the past 3-4 decades. I first learnt about pipe expansion loops when I was studying engineering over 30 years ago. There is an engineering reason why that loop is there. $\endgroup$
    – Fred
    Commented Feb 10, 2015 at 13:47
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    $\begingroup$ Just to clarify - there is another natural gas pipeline above all the other pipes, obviously put in with the rest. I'm assuming the secondary pipeline was put in due to increased demand. That older pipe also has the thermal expansion loops, so I don't think the fact that the other pipes are all straight is due to a difference in code, so I imagine it's the different materials/contents. $\endgroup$
    – Gretchen
    Commented Feb 10, 2015 at 17:21
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If the other pipes are carrying liquids with a greater specific heat coefficient then they would not be expected to experience the same temperature ranges, i.e. the water pipes will be at some average of the outside temp and the water temp. A low-pressure gas line would be made of lighter material and the gas inside would equalize with the external temp pretty quickly. So the code for a gas line and a liquid line (or a light pipe and a heavy pipe) might be different.

Expansion joints are often used for exposed liquids pipelines, like the Alaska pipeline, that are subject to temperature extremes and cover long distances. Buried pipelines don't experience the same range of temperature and don't normally employ expansion loops.

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