explosives

[Chris Conlon]

Apologies in advance for the rather non-EFI content.

> >Propane is not an explosive???  Go back to elementary school young man.  Or,
> >at least back to basic Jr.  High chemistry!  Propane IS an explosive.  My
> 
> Many fuels can burn in an explosive manner under certain
> conditions--methane, gasoline, propane, even diesel fuel. However, they are
> definitely NOT explosives. By definition, an explosive is a chemical or a
> mixture of chemicals which contains its own oxidizer. As in nitroglycerine,
> or diesel fuel after it is mixed with ammonium nitrate prell. Ask Timmy

First, the DOT, BATF, chemists and explosives chemists all have slightly
different definitions of "explosive". I'm using the terms from the POV
of an explosives chemist. Second, the original point, that propane in air
*can* be hazardous, is the real point, IMHO.

Now. An explosive, or explosive mixture is basically something that can
"burn" *by itself*. Pure propane cannot burn or explode, you need an
oxidizer like O2, air or fluorine. I put "burn" in quotes to cover all
forms of rapid oxidation. For instance pure acetylene *can* explode,
without air or any outside oxidizer. (Before someone screams, I didn't
say it *will* explode - generally it has to be under high pressure.)
The c-c triple bond is very high energy and can decompose exothermically,
rapidly, under some conditions. Other weird explosives, e.g. nitrogen
tri-iodide, also explode without oxygen being involved at all.

Explosives are classed as low or high, according to their velocity.
Low explosives *deflagrate*, basically, they burn in the ordinary way
we're used to, perhaps quickly. Deflagration velocities are on the
order of 1-10 meters/sec. Gas in an engine is usually like this.
High explosives *detonate*, a qualitatively different process.
Detonation velocities are on the order of 1000-10,000 m/sec. Lower
velocity produces a "pushing" effect, good for moving earth and the
like. Higher velocities produce more of a shattering effect.
ANFO (ammonium nitrate + fuel oil) is a low velocity example, C4
and PETN are high velocity examples. (BTW I believe McVeigh used
AN and *nitromethane*, an interesting twist on plain old ANFO.)

Detonation is not "just" fast deflagration. In deflagration, the
immediate reaction products (gases, etc) move in a direction *opposite*
to the flame front's travel. In detonation, the immediate reaction
products move in the *same* direction as the shock wave's travel.
The reaction speed is much faster in detonation, and more dependent
on shock than flame. (High speed photos of explosives often show very
little recognizable flame until well after the shock front has passed.)

Some, perhaps many, explosives may deflagrate or detonate depending
on how they are initiated (set off). Some low explosives can detonate,
but only under extreme conditions, i.e. smokeless powder in a gun
barrel if the barrel is jammed. This is very much like detonation in
an engine, and perhaps even more quickly destructive. Gas-air is one
of these explosives. It may burn, or detonate, depending on conditions.
Nitroglycerin, in the open, will burn peacefully if lit. But if confined
and lit, it may detonate, and if struck, it will surely detonate.
Just some examples that an explosive may do different things, depending.
Even the detonation velocity will depend on how strongly the explosive
is initiated, in some cases it will vary over an 5:1 or 10:1 range,
while still detonating in each case.

Okay I'll shut up now. But please check and define your nomenclature
before flaming each other over semantics, ok? I think we all agree
that propane in air *can* be a hazard... again IMHO that's the only
real point of this.

   Chris C.