Tankers
May 20, 2015 3:44 AM Subscribe
Why is it that trucks,railway cars even your cig lighter are cylindrical.
Is it something to do with the flammable contents?
A cylindrical shape can withstand a lot of pressure, including from within. That is why fuel bottles, spray cans and many other items that hold flammable liquids or liquids/gases under pressure are cylindrical. Spherical would be even better but that's hard to make and hard to store.
(I've never seen a cylindrical truck though.)
posted by Too-Ticky at 3:56 AM on May 20, 2015 [2 favorites]
(I've never seen a cylindrical truck though.)
posted by Too-Ticky at 3:56 AM on May 20, 2015 [2 favorites]
A cylinder is an easily manufactured form that reduces the number of edges, which makes it stronger. For trucks and trains, it also makes for a convenient way to drain the container, since the product will naturally migrate to the lowest area. If you look at a truck or train closely, you will notice that it usually is slanted very slightly towards pipes at the bottom. This is especially easy to see on fuel trucks, which often carry several different types of fuel in subcompartments.
posted by jgreco at 4:00 AM on May 20, 2015 [3 favorites]
posted by jgreco at 4:00 AM on May 20, 2015 [3 favorites]
Flammability is somewhat of a red herring here: _most_ tankers are cylindrical, even the ones intended for non-flammable substances. As others have noted, it's a matter of structural strength.
posted by Dr Dracator at 4:02 AM on May 20, 2015 [2 favorites]
posted by Dr Dracator at 4:02 AM on May 20, 2015 [2 favorites]
My friend Bill Hammack, The Engineer Guy, just did a video on why soda cans are cylindrical, which you can extrapolate to other cylindrical containers for things under pressure.
posted by MsMolly at 4:11 AM on May 20, 2015 [3 favorites]
posted by MsMolly at 4:11 AM on May 20, 2015 [3 favorites]
In addition to the above points the failure modes of cylindrical tanks are potentially "friendlier" to nearby humans than other designs.
For a classic implosion video demonstration see: https://www.youtube.com/watch?v=Zz95_VvTxZM&hd=1
For cylinders used for storing gases at high pressure they will fail at the regulator (if well designed) which is the little gizmo at the top which lets gas in and out and shows remaining pressure etc. This leads to the classic rocket situation which you will hear stories of in many chemistry and physics labs where a cylinder went through several walls etc. Remarkably I've never really heard of people getting hurt in these instances but I think that is sheer luck.
You might think that the rocket situation is not exactly fail safe but when you have a lot of stored energy that needs to be got rid of quickly it is vastly preferably to a fragmentation of the cylinder casing (effectively a bomb) which I fear could quite easily happen with a, for example, cuboid container. Or a least you get a huge sideways blast of extremely high pressure gas cutting like a knife from the edge that failed. You also see the rocket situation when a shop which sells propane tanks/canisters catches fires and after the fire department set up the exclusion zone you will see propane tanks rocketing into the air.
Of course, with oil carriages with a heat source underneath, you get a particularly impressive Boiling Liquid Expanding Vapour Explosion (BLEVE skip to ~46s for the lazy) which I wouldn't want be within a couple of miles of but in that sort of situation I don't think there is much to be done unless you get there early and put out the fire plus cool the tank with water but risky as all get out.
posted by Beware of the leopard at 5:00 AM on May 20, 2015 [2 favorites]
For a classic implosion video demonstration see: https://www.youtube.com/watch?v=Zz95_VvTxZM&hd=1
For cylinders used for storing gases at high pressure they will fail at the regulator (if well designed) which is the little gizmo at the top which lets gas in and out and shows remaining pressure etc. This leads to the classic rocket situation which you will hear stories of in many chemistry and physics labs where a cylinder went through several walls etc. Remarkably I've never really heard of people getting hurt in these instances but I think that is sheer luck.
You might think that the rocket situation is not exactly fail safe but when you have a lot of stored energy that needs to be got rid of quickly it is vastly preferably to a fragmentation of the cylinder casing (effectively a bomb) which I fear could quite easily happen with a, for example, cuboid container. Or a least you get a huge sideways blast of extremely high pressure gas cutting like a knife from the edge that failed. You also see the rocket situation when a shop which sells propane tanks/canisters catches fires and after the fire department set up the exclusion zone you will see propane tanks rocketing into the air.
Of course, with oil carriages with a heat source underneath, you get a particularly impressive Boiling Liquid Expanding Vapour Explosion (BLEVE skip to ~46s for the lazy) which I wouldn't want be within a couple of miles of but in that sort of situation I don't think there is much to be done unless you get there early and put out the fire plus cool the tank with water but risky as all get out.
posted by Beware of the leopard at 5:00 AM on May 20, 2015 [2 favorites]
The reason they are cylindrical is twofold:
(fun fact: I trained as a nuclear pressure vessel designer, but I got better.)
posted by scruss at 5:54 AM on May 20, 2015 [7 favorites]
- The stresses in a cylindrical [pressure] vessel with hemispherical ends are well understood, and relatively easy to design for. The mathematical proof of design really only came into being in the mid 20th century, so early pressure vessels (like this BOC lorry) were spherical, as spheres are the simplest pressure vessel to design.
- Spheres on a truck bed — being as wide as they are tall — are difficult to fit under bridges. My grandfather was an engineer at BOC Cricklewood during WW2, and he said the old spherical vessels had to take special routes to avoid low bridges. The load distribution and the intense cold from the liquid oxygen used to crack the truck chassis. Cylindrical pressure vessels avoid the height and point load problem.
(fun fact: I trained as a nuclear pressure vessel designer, but I got better.)
posted by scruss at 5:54 AM on May 20, 2015 [7 favorites]
Scruss' point about spheres being the best shape is correct and he's also correct that cylinders are used because they fit under bridges.
That being said, spheres are still the preferred method and are used where height restrictions are not a concern. For example: LNG carrier and storage facilities.
posted by Confess, Fletch at 6:06 AM on May 20, 2015 [1 favorite]
That being said, spheres are still the preferred method and are used where height restrictions are not a concern. For example: LNG carrier and storage facilities.
posted by Confess, Fletch at 6:06 AM on May 20, 2015 [1 favorite]
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For a cigarette lighter it's so that you can fit it in without positioning it.
posted by gorcha at 3:55 AM on May 20, 2015 [5 favorites]