Safe to store small propane bottles on roof rack?

[netcoyote]

Getting ready for a road trip and almost packed. I have a small propane stove I'm going to use and don't have room in my Jeep for a large propane tank, so I opted to carry several small tanks, the small Coleman tanks. Reading the fine print on the container, it warns against exposing them to temperatures above 120 deg. I'm thinking that having them wrapped in a canvas tarp in a rack on top of a Jeep with a black roof will get them at least to that temperature.

Anyone have experience with this, good or bad?
Storing them inside where it will be cooler just doesn't seem like a good option.

[MADMAX]

Personally I'd put them in a box in the back like a plastic milk crate and keep them from rolling around

[Kittman]

Store the bottles in a well ventilated space where there is no source of ignition, and make sure the pressure relief valve is positioned above the liquid level in the cylinder/bottles.

[netcoyote]

Store the bottles in a well ventilated space where there is no source of ignition, and make sure the pressure relief valve is positioned above the liquid level in the cylinder/bottles.

These are the type of tanks I'm talking about. Do they still need to be stored vertically since there seems to be a relief/fill fitting near the bottle fitting?
There will be no source of ignition near them on the roof rack but they will be getting pretty warm sitting in the sun under a tarp.

[92xj]

I just throw those bottles wherever they fit in any orientation. I've never had an issue and never heard of anyone having an issue. I might not be as safe as some people will tell me I should be. The only thing that would make me think twice with those bottles is storing them outside of the vehicle in direct sunlight, no thanks.

[Kittman]

Delving deeper into this is what I've dug up on the subject:

How much pressure does propane generate? It depends on the temperature:
•At a temperature of 70° F, propane's vapor pressure is 124 PSI.
•At a temperature of 162° F, propane's vapor pressure is 390 PSI.

Portable tank valves release at about 250-275 psi and the disposable bottles at 375 psi.

Probably why you see the small ones on the store shelves, but the larger ones must kept stored outside.  It would seem unlikely you would have any problem if you store them upright, out of the direct sunlight and in a spark free and ventilated place.  Personally, I store them inside my slide in camper cabinet in a small cardboard box in the upright position.  Never a problem, with all the alarms in there to boot. 

[Firedogg]

If they are new ones you are good to go keeping them almosnt anywhere.

  If they are ones you have refilled yourself off of a big tank, then I would make sure to store them outside as the valve has a tendancy to not completely seat leaving a small leak. I have gotten brass screw on tops with "0" rings for mine for this reason.

 Edited for better price...

http://www.campingworld.com/shopping/item/propane-tank-brass-cap-2-pack/55148

[Knocker of rocks]

Delving deeper into this is what I've dug up on the subject:

How much pressure does propane generate? It depends on the temperature:
•At a temperature of 70° F, propane's vapor pressure is 124 PSI.
•At a temperature of 162° F, propane's vapor pressure is 390 PSI.

Portable tank valves release at about 250-275 psi and the disposable bottles at 375 psi.

Probably why you see the small ones on the store shelves, but the larger ones must kept stored outside.  It would seem unlikely you would have any problem if you store them upright, out of the direct sunlight and in a spark free and ventilated place.  Personally, I store them inside my slide in camper cabinet in a small cardboard box in the upright position.  Never a problem, with all the alarms in there to boot.

That is different than the pressure in a bottle with two phases present. 

I'll come up with the answer later

[Kittman]

with a little more digging- Liquefied Petroleum Gas Code, 2011 edition of NFPA 58, page 198 states vapor pressures at 120 degrees F is 1,669.3 kN/m2, or 242.1 psia.

[CAMPMEAT]

Put them any place you want. How many hippies have died lately from their VW buses blowing up ? None that I've heard about. 

[netcoyote]

Guess you could say I've been "field testing" all day. Drove with them all day to Pendleton where air temp is 80 deg. The Jeep roof might be hotter but whenever I felt it, it just didn't seem that hot. Got the bottles in white reflective bag and have a canvas tarp over it with some plastic between the bag and the canvas so it's insulated a bit from any direct heat.
Sounds like even if they got hot, the worst case is they would vent out excess pressure and I would loose some propane. There is no ignition source so that's not a problem. I'll keep an eye on it as I go further south with higher temps.