Sunday, June 28, 2015

DANGER ZONE: Liquid and Gas Explosions with John Gilstrap

Flammable Liquid
Flammable Liquid (Photo credit: Wikipedia)
We are chatting about things that go BOOM! in the night with New York Times bestselling author John Gilstrap.

John, will you tell my fellow Thrillwriters and readers why you're my go-to guy for all things explosive?

In my "other" life--my non-writing life--I was a safety engineer. I got my masters degree in that field a while ago, and have been a safety professional for 35 years. My expertise, starting way back at the beginning of me career, deals with explosives, hazardous materials and hazardous waste. During my 15 years in the fire and rescue service, I was a member of the HazMat team.


I am making making my way through all of your books and one of the things that I enjoy most is when you blow things to smithereens. We have discussed explosions before in THIS ARTICLE, but we decided to devote another article to gasses and liquids. Can you start with a primer on liquid and gas explosions? 

John Gilstrap
John - 
Let's start with some basic science. In reality, there really is no such thing as a flammable or combustible liquid. No liquids burn. Only gases and vapors burn. When gasoline burns, it's actually the vapors surrounding the gasoline that are burning. As the fire creates more heat, the rate of evaporation (vapor creation) increases, and the fire gets bigger. At the same time, because the liquid is evaporating, the volume of liquid decreases. When there’s no more liquid to produce vapor, the fire goes out. The difference between a flammable and combustible liquid, is the temperature at which the liquid creates

enough vapor to burn. That temperature is called the "flash point" (it has nothing to do with a "flash" like a lightbulb; “flash” is the chemistry term for the act of transforming from liquid to vapor).

A flammable liquid is defined by a liquid whose flash point is less than 100 degrees F. A combustible liquid has a flash point between 100 and 200 degrees F. Gasoline has a flash point of around -43 degrees F, so it is considered a flammable liquid. Diesel fuel’s flash point is around 123 degrees F, so it is considered a combustible liquid. 

Once they start to burn, the difference is purely academic. So, building on what we talked about in our last chat, it is difficult to get an explosions from flammable liquids. Depending on how much vapor as accumulated at the time of ignition, you can get a pretty good whump when they first ignite, but I can’t think of a way to get a really big bag. 

Gases, on the other hand (like propane) are gaseous at atmospheric temperature and pressure. To use them (say, in our gas grills), we compress the gases into tanks and convert their physical state to a liquid via condensation. If the pressure vessel is ruptured, that gas reconverts at a ratio of several hundred to one back to its gaseous form. If the gas is flammable (as opposed to, say, nitrogen, which is not), that big gas cloud will ignite all at once.

That rapid expansion and ignition can and often has caused low-order explosions. It's very difficult to get gases to detonate, however. Remember, a detonation is a flame front that travels at supersonic speed.

Fiona -
Could you define low order explosions? What would they look/feel like if present as one goes off? And as a follow up, is there medium and high order? If yes, how are they differentiated and experienced?

John -
Low order = a subsonic transmission of energy. 

High order = supersonic transmission of energy (i.e., a detonation) 

Up close, like standing on the surface, the difference is academic. As you move farther away, however, a low order explosion loses its destructive energy much sooner. With a detonation, the blast effects are much more widespread.

Fiona - 

So subsonic you don't hear a BOOM! ? 

John - 
Oh, there’ll be a boom. When an airliner crashes, the resulting fireball is a low-order explosion. There's still a boom, because sound and pressure are the same thing. What you won't get is a destructive shock wave.

Bursting a balloon is a very low order explosion.

Fiona -
The myth about exploding a gas tank by shooting it with a bullet. Why is this impossible?

John - 
An auto gas tank is not a pressure vessel. It just holds liquid. If a hole is poked, the liquid will leak out. If it's a flammable liquid, it will leak out burning. The fire will not propagate back into the tank through the hole for several reasons, probably the most important of which is because there won’t be enough oxygen among the vapors inside the tank to support combustion. You can get a *whump* and a fireball, but you won't get a *bang*. It's just physically not possible.

Fiona - 
What are some mistakes that you've either read in books or seen in movies that you would like us to avert? John - Remember that scene in Bourne Identity where Jason Bourne uses a shotgun to create an explosion when he shoots a diesel tank in the back yard? That. In movies in particular, where firefighters are working a fire, or where our heroes are trying to survive. All of that pretty fire along the floor is wrong. Heat rises. In fact, here's a video of what the inside of a fire really looks like.

Fiona - 
Thank you. I've been doing Citizens Fire Academy, and we got to see a flashover in their fire building. Very scary stuff. 

John - 
Yeah, I've been way too close to a couple of those over the years. You find out very quickly where your exposed skin is.

Fiona - 
Have you ever used gas or liquids in your novels? I know Big Guy is awfully fond of his C4 - have your characters ever needed to fabricate a bomb on the spot with found ingredients? 

John - 
Not in the Grave novels, no. But I think maybe in AT ALL COSTS, my second novel. That was my hazmat novel. Flammable liquids (FL) are very inefficient weapons. They are damaging only to the degree that people get splashed. Or, I guess, from the radiant heat. If you can create a BLEVE, though (boiling liquid expanding vapor explosion), you can do a lot of harm.

More science

Remember, I said before that liquified gas reconverts to its gaseous form instantaneously. Well, imagine a tank of propane that is exposed to fire. As the tank heats, the liquefied gas inside also heats, which causes vapor pressure to increase. The hotter the contents, the higher the pressure. 

Meanwhile, continuing flame impingement causes the steel of the tank to weaken. As the pressure inside increases. Sooner or later, the tank will weaken to the point where it can no longer contain the internal pressure, and it will unzip, at which point you get this instantaneous liquid-to-gas conversion that ignites all at once. The resulting (low order) explosion has been known to throw railroad tank cars over 600 feet. That can happen at the BBQ-bottle scale as well. 

Fiona - 
At what temperature does steel weaken and will this happen in a normal fire? I'm wondering about the construction of fire-proof safes/file cabinets. 

John - 
There are quite a few variables, but 1500 F is in the ballpark. At 2000 F you're talking imminent collapse in most cases.

But remember, that's the temp of the steel itself. These days, there are all kinds of ways to insulate the steel structure. Fiona - Going back to your last response, BBQ scale propane - could someone effect this by starting a small fire and setting the tank on top? Or is that not enough heat? John - Okay. More science . . . All liquid--even liquefied flammable gases--are heat sinks (a heat sink is a passive heat exchanger that cools a device by dissipating heat into the surrounding medium). That means flame impingement on the liquid space will never get the steel hot enough to weaken.

However, if you can direct a flame to the vapor space at the top of the container, you'll get an efficient transfer of heat and the tank will melt. Or, you could shoot a propane bottle for more or less the same effect. (Stand back a ways, though.) 

Fiona - 
How far back? 

John -
Hey, a quick plug: I'll be teaching a course on all of this at CraftFest in New York on July 8, 2015.

How far back is dependent on how full the tank is, but I'd be twitchy at much closer than 100 yards

Fiona - 
Directing a flame to the vapor space - how might that be done and conversely if someone was trying to protect a tank what could they do to protect it from exploding? 

John - 
I'll take that in two parts. 

One: How to make it happen. Imagine a welding cart, where two bottles of flammable gas are right next to each other. If you crack the valve of one and direct the burning gas stream to the vapor space of the bottle next to it, you've got a pretty good shot. Or, in your scenario of putting a five-gallon propane tank on a fire, as the pressure increases, the pressure relief valve will release and cause direct flame impingement on the vapor space of the bottle. 

Then the question is whether there’s enough time of impingement to cause the explosion. There’s no way to predict that.

Two: To protect it. Well, that's tougher. My SOP in the fire service was to pull back and drown the bottle with an indirect water stream. But for something bigger, we wouldn't even do that. We'd evacuate the area and put in earplugs. 

BLEVEs are extremely unpredictable. The only way to stop the inevitable once you have flame impingement is to keep the pressure vessel cooled to a temperature below its critical temp.

Fiona - 
What household/garage-held liquids/gases might serve a danger or conversely a MacGyver-type last ditch effort explosive? As an example I've seen hairspray used as a blow torch before (long story about a rat - don't ask). 

John - 
ANY compressed gas cylinder will work--including hair spray, but the magnitude of a household aerosol can is just not enough to do real harm. I've had them popping off all around me in fires. They scare the bejeebers out of you, but there's not enough potential energy to do much harm. 

Fiona - 
We've talked about solids as explosives in our last article and this one is about liquid/gas. I'm curious about the combination of the two. Are there particular products (not to teach people how to cook bombs for sure) but products that people should know not to store in the same place unless, for example, a bad guy shoots a hole in a container and all of a sudden product A leaks into product B - and if you would be so kind a science lesson? 

John - 
Okay . . . Fuels and oxidizers do not get along. Pretty much anything in your garage that ends with "nitrate" in its chemical name should be kept away from fuel sources because nitrates are very strong oxidizers. 

Petroleum products react spontaneously with strong oxidizers to create a fire. Medicinal O2 and fuels = bad stuff. 

Oddly, some of the most stable items to store are the things you'd think you'd have to worry about. Stuff like ammunition is very stable, and difficult to cook off. To shoot a box of ammo is to put a hole through the box and spill powder. Nothing dramatic. If you store gasoline in your house, you're not thinking things through. 

In Oklahoma City, the Murrah Building was brought down with a mixture of ammonium nitrate (fertilizer) and fuel oil. It's called ANFO, and it's a very useful and powerful explosive. It's the addition of the oxidizer that makes such a big boom. Very, very high order. Rocket propellant is essentially polymer and oxidizer, with a little magic and voodoo thrown in. 

Fiona -
Voodoo being the essential ingredient.

I have been to a house where my friend was on O2 as you come to her door there is a big sign "no smoking allowed" If a non-English speaking person was smoking and walked into the house what if anything would happen?

John - 
Oh, there's a great/horrifying story about this. Supposedly, there was an old lady in an oxygen tent back in the day who, when preparing for the arrival of her family, combed her hair. A tiny static spark immolated her. The astronauts of Apollo 1 were incinerated on the pad because of a tiny spark in a high-O2 atmosphere. Very, very dangerous.

Fiona - 
Carbon monoxide is lethal because it displaces oxygen in the bloodstream, is it also flammable?

John - 
It is. 

Fiona - 
What did I miss that you feel we should know? 

John - 
I encourage everyone to research this kind of stuff before you put it down on paper. Whether it's explosives or guns or knee surgery, there are SO MANY resources available through the Interwebs. And people should feel free to reach out to me if they want to bounce an idea or two. 

Fiona - Thank you kindly for sharing your expertise - as always it was fabulous learning from you. Now, you have a new book. Can you tell us what it's all about?

Read it Now

John - 
Jonathan Grave finds it hard to believe that a fellow combat vet has gone rogue, killing American agents and leaking sensitive intel to hostile foreign interests. With black ops assassins on the trail of his old friend, Grave sets out to get to him first. He finds far more than he bargained for. Not only the wily operative, but evidence of a conspiracy so dangerous, so far-reaching, that an unthinkable tragedy is in-motion. Grave and his elite team of specialists must expose a deadly high-level secret —and do it in time to avert a catastrophe of historic proportions…

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