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Comment by jjk166 | Hacker News Reader

A random person being scared of something near their home does not make it actually dangerous. LNG doesn't explode like in the movies, it needs to vaporize and mix with oxygen to burn. A leak in an enclosed space can pose an explosion risk, but that's not an issue for a rail car sitting outside. Boiloff can potentially cause a tank to rupture, but this is easily prevented with a pressure release valve. If there is a leak, there is of course a fire hazard, but such a hazard is no worse than the leak of an oil tank. In fact, where an oil leak could potentially allow a large amount of fuel to pool up creating an explosive hazard prior to ignition, natural gas will mostly dissipate harmlessly prior to ignition. LNG transport is generally safer than crude oil transport, and a crude oil spill has much worse environmental effects. If you are comfortable with oil being transported through densely populated areas, you should be fine with this.

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The problem isn’t a train with a single car of LNG, it’s a derailment of a train carrying mostly LNG. Trains of Oil, LNG, or other flammable liquid can become a major hazard in ways that a train full of say lumber isn’t.

Forty-two people were confirmed dead, with five more missing and presumed dead.[3] More than 30 buildings in the town's centre, roughly half of the downtown area, were destroyed,[2][4] and all but three of the thirty-nine remaining downtown buildings had to be demolished due to petroleum contamination of the townsite. https://en.wikipedia.org/wiki/Lac-Mégantic_rail_disaster

Exactly! And none of that would happen with LNG because natural gas is lighter than air, it would all rapidly rise into the sky and make a very pretty, and harmless, fire.

And speaking of lumber: https://bangordailynews.com/2020/07/29/news/a-scene-from-hel... (I need a better link, but basically the cars with hazardous material did not leak, and the fire was from lumber.)

Surprise surprise, Lumber is actually more dangerous than LNG! Because none of that would have happened with LNG. With LNG it all dissipates very fast.

https://en.m.wikipedia.org/wiki/Viareggio_train_derailment

32 dead because of derailment of LPG transporting train. Is the hazard posed by LPG much different to LNG?

Yes. Propane is heavier than air, so it sits on the ground and just burns and burns.

LNG rises up into the air and is gone.

Petroleum gas is heavier than air so it stays at ground level for an extended period of time, natural gas is lighter than air so it is dispersed quickly. The gasses in petroleum gas also have significantly lower ignition temperatures.

Yeah, but we already have trains of flammable liquid, and most are far more dangerous than LNG. Had a LNG train been involved in Lac-Mégantic there would have been no flaming tsunami of oil nor any petroleum contamination.

That sounds more like an argument against allowing rail transport of petroleum rather than an argument for allowing rail transport of LNG.

It very well could be if you don't feel the benefits outweigh the risks. But it's dumb to be fine with rail transport of petroleum and not LNG.

Latent heat of vaporization means huge quantities of LNG stay liquid for plenty long enough to get a flaming tsunami. Your not going to end up with petroleum contamination, but in terms of loss of life their about equally dangerous.

Anyway rail transport of oil through populated areas is also a major risk. Which is why it’s also banned in some places.

If you had a tsunami of LNG it would be more likely to freeze you than burn you. However the latent heat of vaporization for LNG is very low, it will vaporize quickly as it spreads out, unlike crude oil. You certainly won't get a flaming tsunami - you can actually extinguish a flame in LNG.

Transport of oil through populated areas is quite common. Local governments that have restricted its transport are welcome to do the same for LNG.

Latent heat of vaporization of LNG is 512 kJ/kg vs 2257 kJ/kg for water. Which is basically irrelevant when your talking into the million’s of kg of the stuff.

Do some math and even just a single 35,000 gallon tanker load is going to flow around like a liquid for quite some time.

3 kg of dry dirt at 20 deg C has enough energy to vaporize a kg of LNG (it takes slightly less concrete). Assuming on the timescales involved it can come into thermal contact with the top inch of soil, all the LNG from that tanker should dissipate once it spreads over a surface of 3000 m^2, or about 60% of the area a football field. It vaporizes faster when you take into account other sources of heat like the air, and way faster if you light it on fire. At most it'll be on the ground for a few minutes, but for a real spill it could be just seconds.

Sure, if it ended up in a nice flat cornfield somewhere then no problem, but the urban landscape is designed to channel ran. Thus it’s more a question of dispersal not duration.

Let’s say 10 tanker cars so 350,000 gallons or 560,000kg are spread down a water channel at the side of a street at 15mph. At 4 minutes their already covering 1 mile which easily takes them out of most towns making longer durations largely meaningless. If the channel is V shaped with a depth of 2 feet and a width of 10 feet. That only adds up to ~4,5000 square meters, and by your calculations most of it should still be there.

It doesn’t last as long if it’s on fire, but there’s going to be an air gap of unmixed natural gas over a likely several feel thick channel of LNG. So even then it’s going to take a while to burn off meanwhile the wave of fire is destroying everything around it.

10 tanker cars all instantaneously emptying directly into a 2 feet deep and 10 feet wide v shaped channel at the same point is not at all realistic. For starters, your V shaped channel is perfectly full when the LNG is spread out over a mile - over the preceding 4 minutes the LNG was spilling over the side of the channel onto the surrounding flat ground, dramatically increasing the amount of ambient heat it would have absorbed.

In a real scenario, the cars are going to be spread out over an area, their ruptures will be relatively small orifices that the fluid must flow through, it's going to spill out rapidly in all directions from that point, any area where a train is passing through is going to be decently wide and flat, worst case scenario probably a 50 foot wide street.

You can look up pictures of derailed tanker cars, for example [0], and see the area where the liquid spills over. By the time the contents of a tanker car have spread over an area of 1 acre, the fluid is only 1in deep, even if none of it vaporizes it's limited in how far it can meaningfully spread. The spill of a cryogenic fluid will by necessity cover less area. As you can see, the spill of 322,000 gallons of flammable liquid affects an area comparable to that threatened by the derailed train cars themselves. That fire lasted 36 hours, wouldn't it be great if all the fuel evaporated in 36 minutes?

[0] https://www.progressiverailroading.com/federal_legislation_r...

Would a crash be sufficient to cause a sudden aerosolization of the lng?

It seems to me that it could, but I am not an expert. Natural gas has a smell added because it has historically exploded...

> If you are comfortable with oil being transported through densely populated areas, you should be fine with this.

What if I'm not comfortable moving crude but train?

>Would a crash be sufficient to cause a sudden aerosolization of the lng?

Well yes, but actually no.

You can't get a freight train up to the kind of speed you need to create an "instantly dump the contents of the pressure vessel" type crash without creating a "slowly vent the contents out" type crash first.

You would need to use a bomb to rupture a pressure vessel to get the kind of fuel/air mix you need for an explosion or fireball. Fuel-air explosives operate in this manner. But they are devices with countless engineering man-hours spent making them actually work and fairly precise manufacturing. Parking ye ol' Winnebago (or Penske, if you're a history buff) beside the tracks and waiting for the train to go by is not going to be a very productive approach. Look at people blowing up propane tanks on YouTube. At best they get a fireball.

I'd much rather have nat-gas wafting up and away rather than crude oil, propane, or some other volatile that sits at ground level.

You get real differences of scale which makes smaller models inconsistent with what happens in a train derailment. Lac-Mégantic rail disaster for example had some serious explosions from oil.

According to the Wikipedia article there was much destruction, but... that happened when the oil flowed where it shouldn't and then burned in places where one really doesn't want it to burn.

Lighter-than-air gas is crucially different. If you let that leak out it won't flow downhill through the city, it'll go straight up. Of course you can argue that gas is dangerous, but not by example of an accident where most of the flammable material flowed away (and caused harm elsewhere) instead of either burning or exploding at the site.

LNG isn’t a gas, it’s a cryogenic liquid like liquid nitrogen except flammable. If we are talking say 100+ DOT-111 tank car‘s that’s 3,450,000+ gallons which isn’t going to simply fly into the air instantly. The phase change from liquid to has takes significant energy at the scale which takes time.

Again it’s a question of scale, you don’t think of say molasses as fast moving liquid but increase the scale to 2.3 million gallons and you can be talking about a 25 foot tall 35 mph wave. https://en.wikipedia.org/wiki/Great_Molasses_Flood

AIUI it's not flammable. It is flammable when mixed with air in the right proportion, but if it's mixed with air it is a gas and rises straight up, rather than flow as a wave across the landscape.

You can't have your cake and eat it.

Again small scale vs large scale. If your talking a square foot of the stuff then it’s going to rise like your suggesting. Cover a few acres in LNG you get complex boundary layer interactions and giant swirls of natural gas mixed with air.

Read up on how you can get temperature inversion layers in the atmosphere. In your model that kind of effect shouldn’t happen, but actual fluid dynamics gets complicated.

Explain how you get spontaneous combustion in the spots where the mixture is right.

Or were you thinking about transporting LNG together with something that that would help start a fire?

Train crashes have plentiful ignition sources as metal scrapes across metal. Oddly enough it’s surprisingly common for train crashes to result in fires ex: https://youtu.be/vww5V-xmtzs or https://baltimore.cbslocal.com/2016/07/06/recent-derailments... or the original disaster I mentioned.

At which point those secondary fires can easily ignite the literal pools on LNG being described.

I've thought about this a few times now.

Would you describe this as a noticeable risk compared to driving around in a car, in urban traffic, with gasoline? Or are you comparing it rather to an alternative that more or less avoids transport of LNG?

Natural gas is lighter than air.

Any such aerosolization would rapidly go up into the sky.

It would make a fire, but not a disaster.

It could. But so could a crash of a truck or a train carrying any type of liquid/gaseous fuel (e.g. truck carrying the gas to your local gas station). The article talk about LNG requiring thicker tanks than ethylene and ethane, both of which could also 100% explode if in a wreck.

I think the worry is that the volume of shipments for LNG would be much higher, and hence the probability of something happening is higher. A singular shipment of LNG would be no worse than a single shipment of car gas.

It's not enough to just aerosolize the LNG, it needs to mix with air in a rather narrow fuel-air ratio of 5-15% to burn. Further the flame front speed of methane in open air is only 4 mph - it won't explode, though as the flame front spreads it might kind of look like an explosion (think about what it looks like when you light a gas stove or grill). Such a fireball can potentially ignite things in close proximity, but there won't be a pressure wave that damages structures, so you can't have a situation where one tank 'exploding' causes other tanks to explode, though in a crash it's easily possible multiple tanks are damaged.

Natural gas is an explosion hazard in enclosed spaces. If you walk into your house and smell gas, that's very dangerous. Note also that even if there is no explosion hazard, fire is still dangerous.

If you are uncomfortable with rail transport of crude, that's a perfectly legitimate stance to take. Honestly, transporting any flammable substance (and many non-flammable ones as well) does pose risks. If you know the risks and benefits and think the former outweigh the latter, let people and especially your representatives in government know. However, society generally seems to accept rail transport of oil and similar chemicals, so outrage over the transport of a less dangerous substance seems like it would be more likely to come from a place of ignorance than rational analysis.

Your statements do not seem to be consistent with those in the article.

> Ray Mentzer, a chemical engineer at Purdue University, spent his career working on LNG projects for Exxon Mobil. He says the specially designed containers that transport hydrocarbons have a good safety record. But he says transporting the gas through densely populated areas increases the risk if there's a leak.

> "It's not flammable until it's vaporized, but it's going to be vaporized pretty darn quickly and then it's going to seek an ignition source," he says. "Believe me, it will find an ignition source pretty darn readily."

I will also note that those train tracks in the town in the photos had a derailment only a few years back. I am familiar with the area and know people who live there.

https://response.restoration.noaa.gov/about/media/train-dera...

There is no inconsistency. Natural gas is most certainly flammable once it's vaporized (it would be a very poor fuel if it wasn't), but there's a big difference between something being flammable and something being explosive. If you have a big leaking tank, it is very likely that it will catch fire, and no one really wants a big raging fire in a residential area, but that fire will only burn at the leak where fuel and air are mixing. It's very difficult out in the open to form cloud of natural gas and air with a uniform mixture such that it can all burn at the same time, and even then the flame front is too slow to cause a damaging explosion.

I would also note that the engineer's use of informal language here is clearly alarmist.

A Boiling liquid expanding vapor explosion (BLEVE) "blev-ee" is what I'd be worried about. Not for an explosion causing damage via shrapnel but from the radiant heat which can scorch things a kilometre away.

Here is a PDF with seems to have the longest url I've ever seen: https://pdf.sciencedirectassets.com/278653/1-s2.0-S187770581...

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