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Old 02-09-2005, 04:48 PM
tracer tracer is offline
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Why are gasoline truck tanks oval in cross-section?

The big trucks that carry most kinds of liquid -- water, orange juice, liquefied propane, etc. -- have storage tanks that are circular in cross-section. The tank looks like a big circle from behind.

So, then, why do gasoline trucks -- and only fuel trucks, it seems -- have storage tanks that are elliptical in cross-section? Why aren't these tanks circular? And if there's a good reason for them to be oval shaped, then why aren't other truck liquid storage tanks oval shaped too?
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Old 02-09-2005, 04:53 PM
sleeepy2 sleeepy2 is offline
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I'd guess lower center of gravity, less tippy.
I believe they refill from overhead from a boom thingy they drive under, so maybe the height had to be standardized?

Not bad for pulling it out of my ass...
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Old 02-09-2005, 04:56 PM
GorillaMan GorillaMan is offline
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Two WAGs - it could be to do with the position of internal separations to stop the liquid sloshing around, or to deal with the build-up of vapour (the latter would be one with particular importance to gasoline)
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Old 02-09-2005, 04:58 PM
robby robby is online now
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I don't have a definitive answer, but this website indicates that tanks with an oval-cross section are non-pressurized, while circular tanks often are pressurized, especially if they have reinforcing ribs.

This makes sense, as a circular cross-section is stronger.

An oval tank may be the result of attempting to maximize the volume of an unpressurized tank while keeping the tank height under a certain value.
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Old 02-09-2005, 06:33 PM
danceswithcats danceswithcats is offline
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Type MC-305,306 elliptical tankers transport flammable/combustible commodities with low vapor pressures.
Type MC-307 round tankers handle commodities with medium vapor pressure.
Type MC-312 round tankers with external ribs are typically used for corrosives, poisons.
Type MC-331 round tankers with spherical ends are pressure vessels used for transportation of commodities such as LP.

The tanker type seems to be associated with vapor pressure (round vs. elliptical).
For further information, consult 49 CFR 178, et. al.
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Old 02-10-2005, 01:16 AM
tracer tracer is offline
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So ... does this mean that flammable substances with low vapor pressures (e.g. gasoline) are somehow safer to transport in an elliptical tank rather than a circular tank?
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Old 02-10-2005, 03:02 AM
Xema Xema is offline
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It's probably that gasoline (and similar) doesn't need the strength per volume that a circular cross-section gives and can get by with a section that occupies the full legal width of a semi-trailer while staying within the normal maximum height.
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Old 02-10-2005, 07:38 AM
drachillix drachillix is offline
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Quote:
Originally Posted by danceswithcats
Type MC-312 round tankers with external ribs are typically used for corrosives, poisons.

The tanker type seems to be associated with vapor pressure (round vs. elliptical).
For further information, consult 49 CFR 178, et. al.
Ribbed tankers for corrosives are ribbed for pressure resistance. They are unloaded by pumping air into them and forcing the fluid out via a siphon arrangement because it is very difficult to make a pump that various corrosives will not eat, a hose is a little easier.

If you look at one the next time you see one you will notice a conspicuous lack of valve assemblies except the one cap on top.
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Old 02-10-2005, 07:58 AM
Fear Itself Fear Itself is online now
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I have another related question that could probably be answered here. How come gasoline tankers don't explode more often? When they are emptied, doesn't the displacement of gas by air as the tank drains create an explosive mixture? I should think a partially or mostly empty gasoline tanker is a rolling bomb. How do they minimize the danger from explosion triggered by static electricity or other random ignition source?
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  #10  
Old 02-10-2005, 08:17 AM
Quercus Quercus is offline
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The main thing to remember is that gasoline vapor is only explosive within a fairly narrow range of concentrations (despite what Hollywood would lead you to believe). You can often drop a burning match into a bucket of gasoline and it will just go out [Don't try this at home, unless you're prepared to deal with a burning bucket of gasoline, of course; results are not guaranteed].

But gas tankers are also designed to eliminate anything that might spark inside the tank -- everything is well grounded to everything else, there aren't any sparking metals exposed, and so on. So no sparks, no explosions.
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  #11  
Old 02-10-2005, 08:24 AM
galen galen is offline
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I think that the shape of gasoline tanks is the natural shape that a non-pressurized tank full of liquid "wants" to assume, therefore making it stronger. Imagine taking a cylindrical balloon and filling it with water and laying it on its side. It will assume the approximate shape of a gasoline tanker. I'm sure someone here can do the math.
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