Lightning and propane tanks

We’ve had thunderstorms daily for most of the week, this afternoon being no exception. After the latest one I took my mother shopping, and we happened to walk by a wire cage full of those propane tanks meant for grills; google says they’d weigh 20 or 30 pounds each.

“What would happen to the building if that was struck by lightning?” She wondered. I’m not qualified to answer, so I turn to the more weather and um, explosion, savvy dopers.

The building in question a brick 1 story (or maybe 2, however many stories you’d find with a grocery store, drug store and staples etc) strip mall with half a dozen stores. The cage contained roughly 15 propane tanks, and 3/4ths of them looked unused, which means those at the very least were full. This cage was only an inch or two away from the front wall of the store.

If the layout of the plaza matters to anyone trying to visualize the effects it goes: moderately sized grocery store, rent-a-center, curves, shoe store, Riteaid with propane tanks, staples. The tanks are much closer to the shoe store than Staples, since there’s a long blank wall between Riteaid and Staples for no apparent reason and staples’ storefront is a good 10 feet farther forward than the rest of the stores, meaning there’s a corner between the two as well.


Nothing. Unless one was leaking pretty badly.

Do you have any back up for this opinion? I realize metal is a great conductor, but at the kVA levels of even a medium sized lightning bolt, there’s bound to be some resistance or hysteresis heating that could cause relief vent to open or blow off port to rupture. And that sort of accident can be catastrophic as seen in Dallas last year when an acetylene tank storage facility caught fire.

If you are talking about a direct hit, anything could happen. Otherwise, any induced currents are just going to harmlessly flow over the outside of the tank.

The question was what would happen if lightning struck the cage holding the propane tanks.
An average sized lightning bolt carries 40 kA at 3 MV/m (Wiki cite) or 1.2 Gigawatts per meter. And the average lightning bolt is over 1000 meters in length. I don’t have the figures handy to do the math on how much of this would flow through a given tank or how much heat that would generate, but I would want to stand way back from this experiment.

If they’re in a metal cage, surely the cage will act as a lightning rod and direct the lightning safely around the bottles?

Will you stand next to it and find out? Or would you be concerned that in travelling to ground some of that TeraWatt of electrical energy flowing through the cage might not arc from the cage to the bottles? Are you satisfied that the spacing of the wire grid and other openings will constitute a complete Faraday cage and none of the energy will choose one or more of the bottles as part of its path? I’m not, and that’s why I asked if Q.E.D. had any cites for his post.

The question posed was, what would happen if the building were hit by lightning. All of the electrical service to the building is bonded to earth via the grounding conductor. Any structural steel used in the building is bonded to that same point, as is any metallic water piping, which, unless the building is served via a private well, will be at the same electrical potential as earth.

Since lightning takes the best path to earth which it can find, the wire cage sitting on a sidewalk under a canopy is not going to provide a better path than the engineered grounding and bonding system offered in the first paragraph. As Q.E.D. observed, unless one is leaking, nothing much.

Absolutely. No, I don’t have a cite, but what I do have is a complete lack of reports of any incidents of explosion due to an incident such as posited be the OP. Presumably, these sorts of enclosures have been struck before. Also, planes are basically flying fuel tanks; they get hit all the time, but I’ve never heard of one exploding as a result (although other problems do present themselves).

No cite. Just plain old ohm’s law.

The cage will act as a Faraday cage and will shunt most of the current around the tanks. The tanks themselves are conductive metal and will shunt most of any current into them on the skin. Nothing is impossible but I doubt that enough current would be available to cause the metal of the tank to rupture.

If I were standing close to one I would worry much more about electrocution than I would worry about a propane explosion.

Google-fu is failing me as well for finding accounts of cages of barbecue tanks being struck. Around here, at least, those cages are within a building’s drip line - ie: under an overhang or eaves. The building itself will be a far more attractive target for the lightning.

OTOH, if you’re near an uncaged tank, run like hell. A little googling on propane lightning turns up several news stories of large tanks that ignited after being hit by lightning. There’s even a case of a tank that ignited after a fence that was touching the tank was electrified by a downed 27,000 volt powerline.

It’s happened.

Remember also that oxygen is required as well as the gas. So unless something happens to rupture a tank and release the gas, there’s nothing to burn. The article about the fence states that the pressure release valve was activated, I’d guess due to heating the tank causing the gas to expand.

Which is the first possibility I mentioned - heating of the tank causes rupture.

Re-read the OP. They were discussing the cage and asked “What would happen to the building if that was struck by lightning?” (emphasis mine).

After reading this paper from the National Lightning Safety Institute, I am ready to say that the odds of a propane tank explosion from a direct strike to the cage holding them is definitely non-negligible.

Whether you agree or not, the OP’s question was really about what would happen to the building itself. Assuming a tank rupture and subsequent fire causing the remainder of the tanks to explode, what would be the likely damage to the adjacent structure?

The word “that” refers back to the object of the sentence, or the building.

The problem is, it doesn’t happen. From a fire standpoint, the cage has two kinds of tanks: full and empty ones. The empty ones contain very little fuel, and any overpressure will vent via the safety relief valve. The volume of fire produced will be minimal, after which you have nothing which will “rupture”.

The full ones can be made to rupture, but it’s properly referred to as a BLEVE. To cause that, you must apply sufficient heat to the vessel that the outer shell is weakened, and overpressure from boiling contents causes a failure of the vessel, together with release of the contents, with accompanying fire, should the contents be combustible or flammable.

To accomplish this in the shortest space of time, apply heat to the vessel above the liquid line, which is quite high in a full tank. Otherwise, the heat applied will be absorbed by the liquid commodity within. In either case, the relief valve will operate. If the venting product does not immediately ignite, an explosion hazard will be established, as a source of ignition can then flash back to the point of release. Whether or not that explosion could damage other containers is up to an engineer to predict. I know of no tests conducted by NIST or UL on that exact scenario. If the venting product does ignite, then you have a flamethrower. What the flame impinges upon determines where things go from there. That being said, the original heat applied to the vessel has to increase internal pressure faster than the rate of venting reduces it to approach a BLEVE condition.
The likelihood is extremely low, in part because the premise is based upon no one doing anything to interrupt the chain of events.

I agree with the rest of the posters saying that it’s going to be very difficult to get anything to happen; propane tanks are pretty robust for a reason. But, since I work in safety basis, let’s assume we’re going to have an explosion. It’s most likely going to start with a BLEVE, as danceswithcats points out. For the purposes of this calculation, let’s assume that all BLEVEs happen simultaneously, that the BLEVEs do not impart any additional energy into the explosion, and the lightning lasts long enough to detonate the mixture.

The heat of combustion is about 50* MJ/kg for propane, compared to 4.652 MJ/kg for TNT. This means that, with perfect efficiency, propane would have an equivalence of 10.75. However, there is also an efficiency correction, which might be as low as .04 or as high as .22 (I cheated on this a bit. My explosive textbooks are at home, and the highest and lowest efficiency numbers I found with a cursory search and worked backwards. Just trying to develop a familiarity with the process).

This gives us an equivalent between .46 and 10.75. As you can see, the efficiency is a huge factor. Assuming 12 full propane tanks weighing 25 pounds, this gives us about 300 pounds of propane, or between 150 and 3,000 pounds (68kg and 1360kg, respectively) equivalent.

Using the formula:
scaled distance = actual distance / ((mass TNT)^1/3), we find the scaled distances for the structure 1 meter away from the BLEVEs to be .244 for the smaller yield, and .090 for the larger yield (remember, being closer will give a larger pressure. These numbers are expressed in units of m*kg^1/3).

Referring to the chart found on this page, we can see that these cases would give us values of between 100 and 300 bar. Keep in mind that those numbers are ridiculously high because we’re assuming all tanks fail simultaneously and pressure drops of with respect to the cubed distance.

For a more realistic scenario, let’s assume only one tank reaches BLEVE state, and the BLEVE does not impart any additional energy into the resulting explosion. For a 30 pound (13.6 kg) propane explosion with a realistic yield of four percent, this gives us a TNT equivalence of 7 kilograms, for an equivalent standoff distance of 1/2. The still high 10 bar pressure resulting from this would easily blow through the brick wall, and possibly damage the other tanks (but that’s beyond the scope of this problem ;)). At this size explosion, we’d have a 50/50 possibility of living (from overpressure alone, assuming no shrapnel) at 10 meters, and a nearly 100% chance of living at 20 meters.

Those equations are all fine and dandy, but I still think QED’s initial response is the correct one, as others have pointed out. I was just going through the thought exercise of “What If?”.

This sentence is a bit ambiguous, but I just wanted to clarify that a BLEVE (boiling liquid expanding vapor explosion) can happen when the internal fluid is not combustable; I believe danceswithcats was referring to the accompanying fire.

Stupid board timeouts. A couple edits:

Also, the efficiency yield assumes that all the vapor cloud is available instantaneously at concentrations in the air between 2.1 and 9.5%. In practice, there’s much more likely to be a deflagration which would produce substantially lower pressures, but even causing that is going to be a stretch.

Since I’m double posting anyway, here are the calcs for a BLEVE of the tanks:

E = (Pi-Pf)V / (1 - gamma), where gamma is the ratio of specific heats, or 1.13, and assuming 100 degrees F for a pressure of 172, gives us:

E = (1.19e6 - .1e6) * .026 / (1 - 1.13) = 218kJ per tank.

I don’t disagree with your analysis, but you need to re-re-read the OP. The word “that” refers to the wire cage the OP+Mom were walking by at the time the question was asked. At least that’s how I read it. I assume OP’s Mom would have said “it” if she meant the building.

I think we have a great candidate for a Mythbusters segment.

No, the “that” in Mom’s question was in reference to the cage we were walking by. Otherwise she would have asked what would happen to the building if it were struck by lightning.

Interesting answers, thanks!