Why do gas pipelines run straight for a few hundred metres/yards then do a bit of a U-bend, straight run for a bit, then another U-bend etc. Why the bend? No valves or anything where the bend is.
In: 7658
Expansion and contraction of the pipe due to temperature changes.
A 100m piece of pipe will expand or contract about 5cm with a 40C temperature change. Depending where you are, the coldest winter temp to the hottest summer can easily see that.
The bends allow enough flex in the lines to let them move freely and not buckle.
https://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html – calculator for expansion at different temps.
They’re called expansion loops. They make it so that the pipes can contract and expand to adapt to the temperatures.
Pipes get longer when hot, and colder when cold.
It’s the same for rail tracks, but you can insert a gap in the tracks to allow individual pieces to expand and retract.
On a pipe, of course, you can’t just make a cut every 100 meters. What you can do is to make the pipe run zigzag, or have those S sections, where basically the pipe is free to move.
the extra length will become a side force in the zig zag, and the pipe will shift to the side, making wider Z.
In the other option, the straight then S then straight, the S acts like a flexible joint.
It’s done everywhere. Even in your house, you don’t run hot water pipes straight from A to B. You lay them in a way they don’t make a straight line all across the place.
If you don’t do this, basically when the temperature changes, the pipe fails because It wants to get longer/shorter but there’s nowhere to go.
Metal pipes expand when heated up and contract when cooled down. If you have a long right pipe there will be a lot of force on it and it can result in crack pipes from stretching or bending.
Look up sun kink and you see what can happen to the railroad for the same reason. Historically the way it was solved with a small gap between each piece of track
Bridges have expansion joins where they can expand and contract without cracking.
For a gas pipe, you can just have a gap, the gas would escape. The way it is solved for gas pipes is the U bend. The people there can bend in a safe way 90 degrees to the direction it points. Just take a drinking straw or any pipe and compare how it behaves if you bend it compared to stretch it out or compress it. It is quite clear why you like the begin at 90 degrees in the direction of the pipe.
Journeyman pipefitter here and as the top comment states, it’s a thermal expansion loop. I’d like to add a story to the thread. I sometimes work in a refinery and one year they replaced a 24″ steam line that was about 400′ long complete with expansion loops and gate valves with other pipes leading off of it to deliver steam to different units.
After completion a new operator decided that rather than let the steam into it slowly to allow the piping system to warm up over time, he simply opened the valve rather quickly. The screaming and cracking of the steel was something I’ll never forget. It sounded like a train was derailing. The pipes were bouncing and ripping off they’re supports (some were welded) It caused one of the expansion loops to basically explode.
Needless to say the whole system needed replacing because the pipe had multiple fractures and cracks. Some of the welds blew apart. It’s something I never hope to hear again but we got more work out of it tho.
Thermal expansion bend. Another way to do it without a bend is to use a metal bellows expansion joint: [http://www.gopsi.com/site/1905gops/FH_Thermal_Exp_Piping_Sys-Part1.pdf](http://www.gopsi.com/site/1905gops/FH_Thermal_Exp_Piping_Sys-Part1.pdf)
(This is one of those less-smart points people try to score against Hyperloop… that thermal expansion is unsolvable, when in reality you can just use a metal bellows…. the more-smart point is it’ll be insanely expensive for much the same reason that High Speed Rail is, and you’d get much of the benefit but with far less cost at lower speeds and therefore without a need for vacuum.)
Any one got an image?
There is a phenomenon in fluid systems called “water hammer”. It is named because it was studied in water systems, but it exists in all fluid systems. When water is moving, it has momentum, and that momentum is based on velocity. The higher the velocity, the higher the momentum, and the more severe the water hammer if a pressure transient were to occur.
Additionally, these piping systems are subjected to a wide range of temperatures, and will expand and contract with the temperature. If the piping were completely straight, and it expanded, it would have to buckle somewhere to accommodate the added length, and if it contracted it would put a lot of stress on the flanged connections.
To mitigate these effects, engineers put the bends in there. The zig zag shape gives the piping some room to move for thermal expansion and contraction so it can flex instead of break, and breaking up the long runs of straight pipe reduce the flow velocity and shorten each individual run of pipe to minimize the severity of water hammer transients.
I’m sure you’ve mostly got your answer from above but, I’ll explain what my thought process was for someone who worked Frac before I got Pipeline. For the Frac crew itself the site is mostly a massive jumble of very very heavy pipes going from your pumps to the wellhead. We were taught very early that we could only go about 50 feet of straight joints before we had to put what we called a chiksan swivel essentially in between your two straight pipe structures. Now this helped for a number of things, if you had to aim or slightly move where the pipe was headed you had to use the chiksan to aim it but, it was also required because of the massive pressure going through the pipe.
Now I know pipeline has way way way less pressure than we used in Frac, talking like 500 for pipeline and up to 9k, 14k psi in Frac. But pressure is pressure, and the vibrations caused by the pressure would actually put too much strain on a very very long straight join of pipe, hence the chiksan swivels.
I have no idea if this has any bearing on why they use the u-bend’s in pipeline (all I did was haul pipe) but, it’s what i always assumed was the reason for it.
For another example of engineered expansion, read up on the SR-71.
That sucker was engineered to expand several inches (IIRC) in flight because it would heat up so much.
The tour guide mentioned that when on the ground with a full fuel tank, it would leak, then stop when the bird got to flying and up to operating temps.
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