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I recently noticed that the cylindrical shape of a railroad tank car is not completely straight but has a bend in the middle. The entire tank is a bit lower there. What's the reason for this bend?

This is also visible on the drawing of a DOT 117 tank car on Wikipedia (the red nearly-horizontal lines were added by myself).

I first thought it had something to do with pressure containment, but the model 117 is apparently used for non-pressured goods only.

drawing of a dot 117 railroad car

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    $\begingroup$ It will drain liquids ; It also put more than half of the wall thickness in tension Think of a simple straight beam ; neutral axis is at the center , half in tension , half in compression. The bend in the car puts over half of the wall in tension and tensile stress limits the load.. $\endgroup$
    – blacksmith37
    Commented May 2, 2021 at 16:57
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    $\begingroup$ @blacksmith37 No it doesn't. That's not the way the neutral axis works. It is only a property of the section shape, not how the beam kinks. The neutral axis is in the middle of the round section in the middle of the car. $\endgroup$
    – Phil Sweet
    Commented May 2, 2021 at 19:24
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    $\begingroup$ It's worth noting that it's not really a bend, rather a kink. $\endgroup$
    – leftaroundabout
    Commented May 3, 2021 at 9:47
  • $\begingroup$ It looks like the first very small step toward a catenary . $\endgroup$
    – blacksmith37
    Commented May 3, 2021 at 14:22
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    $\begingroup$ @blacksmith37 You should never answer questions in the comment section, especially if you're wrong. $\endgroup$
    – pipe
    Commented May 3, 2021 at 15:34

3 Answers 3

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As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point.

The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in:

  • the edge, the angle would be $\arctan\left(\frac{0.15}{15}\right)=0.57^\circ$ or 1%.
  • the middle , the angle would be $\arctan\left(\frac{0.15}{7.5}\right)=1.14^\circ$ or 2%.

The steeper angle helps gravity assisted cleaning.

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    $\begingroup$ Apart from the angle, note that the ends rest right on top of the bogies, which have a big pin right in the middle. Not a convenient place where to put a big cleanout. $\endgroup$
    – TooTea
    Commented May 3, 2021 at 14:19
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    $\begingroup$ Okay, that explains why the bottom is lower in the middle than at the ends, but not the top. Is it designed for a specific maximum height and the hardware on top needs to fit within that? Or is it just cheaper to use a tube with a constant cross-section and weld 2 of them together at that angle? $\endgroup$
    – AaronD
    Commented May 3, 2021 at 21:43
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    $\begingroup$ 2% is also an extremely steep rail grade, avoided whenever possible, and so, unusual. Whereas 1% is common. That would make it easier to get all the contents out, either at an improvised usage point or at a wreck site. $\endgroup$
    – Harper - Reinstate Monica
    Commented May 3, 2021 at 23:06
  • $\begingroup$ @AaronD, have a look at Harper's answer, it covers exactly your question. More specifically. a) it creates a safe working space at the top, b) it creates at the two end pockets where the transferred liquid can expand if required to without increasing the internal pressure. Regarding your comment about shape, the cylindrical cross-section just lends itself to more manufacturing processes, and it might benefit the construction cost slightly, however, the real driver here is safety. $\endgroup$
    – NMech
    Commented May 4, 2021 at 2:51
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    $\begingroup$ Having a kink/bend at the top of the tank car is no justification for requiring a sloping bottom for drainage; keep the top of the tank level while adding slopes to the bottom of the tank. $\endgroup$
    – RonJohn
    Commented May 4, 2021 at 5:08
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Same reason the Corsair fighter has bent wings. Makes clearances easier.

Look at the lumpy, sticky-outy things on a tank car. You have the trucks/bogeys at the end bottom... and the massive fill valve and work safety area at center top. Lowering the center is advantageous to the car's clearance envelope, allowing it to have more tank volume in the same clearance plate (loading gauge for Brits, not that it would fit in any British tunnel save for the Eurostar).

For the last 100 years, railroads have been careening toward heavier and heavier cars, while many older system tunnels do not have high clearances. (Lines commercially viable for double-stack container service are being notched for double-stacks).

Assures head space too. While the bend eases unloading, as NMech covers amply, it also prohibits entirely filling; there is an airspace above the load that simply cannot be filled, short of drawing a vacuum on the tank (which the Mythbusters showed is a bad idea).

Loads have different volumes at different temperatures. Notice the steam-heating apparatus in the cutaway on the drawing. That means the car is intended for loads that may be too thick to flow without heating. That material's volume will increase while you are heating it, or simply the inevitable effect of 16 hours in the Arizona sun on a car painted flat-black. You do not want the car to "hydraulic lock"; that would send pressure up very rapidly and crack the tank.

That "unfillable" volume assures there is head space for the liquid to expand into; of course this compresses the gas there somewhat, increasing pressure on the whole vessel; but nothing it can't handle.

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So any sediment will collect there as part of cleaning.

Then other designs have a more pronounced bend to allow for the bogies and also keep the height down - some tunnels are lower.

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