I'd expand this to look at how much of a factor elevated temperature
might be, as well as possibly also elevated pressure in a non-vented
container. The idea being to pin down both whether heating in a fire
can cause no-fuel AP explosions at all, and what the actual factors are
- altered structure, heat, and/or pressure - the sublimed 30% has to go
somewhere - if confined, might that be what detonates first (analogous
to peroxide vapor) even at relatively modest pressures?
Assuming sufficient funding, of course. The test matrix would get
somewhat larger. Heat samples in both vented and unvented containers,
monitor pressures and whether either goes bang, then when cooled check
AP structure and test for shock sensitivity. Then heat samples in both
vented and unvented containers and check for shock sensitivity at
temperature. For a range of temperatures, if affordable - depending on
the shock generation method, multiple shocks as temperature (and
pressure in the unvented sample) rises could yield much more data per
test run. Although that could leave questions as to whether previous
shocks were altering structure and thus results - final confirmation
runs with single shocks below and above any apparent threshold would be
useful validation.
The results of whatever fraction of this might be affordable could be
very practical - how better to store AP safely, when to run if a fire
does start, and whether any AP left post-fire presents an increased
hazard even after cooled to ambient.
On 7/28/2015 3:24 AM, James Padfield wrote:
Draft MSc project proposal below. Comments (constructive ones!) welcome...
*Investigation of Shock Sensitivity and Critical Diameter*
*in Fire-Damaged Ammonium Perchlorate*
**
*Background*
Ammonium perchlorate has been involved in a number of accidental
explosions, some of them fatal, most notably the PEPCON disaster in
Nevada in 1988.AP is normally considered an oxidiser at larger particle
sizes (e.g. 200 um), and is not considered an explosive until it is
reduced to a very fine particle size (< 15 um).The cause of AP
explosions is often attributed to mixing of AP with a fuel (e.g.
asphalt, plastic storage drums) during a fire, thus creating an intimate
fuel-oxidiser mixture which is much more sensitive and with a smaller
critical diameter than AP alone.
However, when heated up it is well known that AP partially decomposes
and sublimes, losing approximately 30% of its mass, leaving a very
porous crystal structure.We speculate that this porous AP could be
significantly more sensitive and have a reduced critical diameter when
compared to the original material, and that this could in fact be the
cause of explosions of AP of a nominally large particle size that is
involved in accidental fires.
This project will look to investigate whether this is a plausible
mechanism for explosions of AP in a fire.
**
*Project Plan*
1. Heat treat a quantity of AP to obtain samples of “porous” AP samples.
2. Characterise this “porous” AP in terms of particle size, surface
area, bulk density, …
3. Subject the “porous” AP to critical diameter tests and subsequently,
assuming that we can get the “porous” AP to detonate at all,
shock-sensitivity (gap) tests.
4. Possibly Fragment Attack tests (maybe the EMTAP small-scale version,
dependent on critical diameter)…
*Risks & Potential Problems*
1. Any tests that do not fully detonate the AP samples will scatter
unreacted material all over the range, upsetting the
tree-huggers.Potentially quite a bit of AP…
2. We can’t get the “porous” AP to detonate at all.It won’t be much of a
project for the student…
On 23 July 2015 at 16:50, James Padfield <james.padfield@xxxxxxxxx
<mailto:james.padfield@xxxxxxxxx>> wrote:
Peter, if you don't mind I'll email you off list - I feel, as
interesting as this might be to some people, that we may be drifting
a bit OT. Unless anyone thinks it is relevant and asks us to keep
it on-list...
On 23 July 2015 at 16:26, Peter Fairbrother <zenadsl6186@xxxxxxxxx
<mailto:zenadsl6186@xxxxxxxxx>> wrote:
On 23/07/15 13:58, James Padfield wrote:
Peter, that sounds like it could be an interesting student
project...
Investigate the shock sensitivity and critical diameter of
"pristine" AP
crystals, and then heat treat them to produce the strange porous
crystals we are talking about. What do you think? I still
have a good
relationship with my previous employer, perhaps we could
come up with a
plan, I can contact them and we could do some research into
the matter.
Sounds good to me, please let me know if I can help. Can do math :).
I can't do this myself, for legal (and practical) reasons, here
in the UK - if I could, I would.
btw, if there's a published paper, I wouldn't mind 4th author -
I need to up my publication status by mid-2017 if possible. Not
something I usually care a jot about, but it has become a little
more relevant recently.
-- Peter Fairbrother
On 23 July 2015 at 13:55, Peter Fairbrother
<zenadsl6186@xxxxxxxxx <mailto:zenadsl6186@xxxxxxxxx>
<mailto:zenadsl6186@xxxxxxxxx
<mailto:zenadsl6186@xxxxxxxxx>>> wrote:
On 23/07/15 11:39, James Padfield wrote:
Peter: I helped with a student project a couple of
years ago at the
university I was working at. She was examining
baking AP (I don't
recall the initial particle size) at fairly high
temperatures (200,
maybe 220 degC) to alter the particle porosity. As
you say,
some mass
is lost, 30% sounds about right, and the resulting
particles are no
longer regular crystals, but porous. She took some
nice Scanning
Electron Microscope photos of the particles. She then
formulated them
(IIRC) with PVC/DOA binder system (easier than
mucking about with a
curable HTPB/IPDI system). Don't recall how far
she progressed
with the
project, I don't believe it got as far as measuring
burnrates...
James: What I'd really like to know is how the
detonatability
changes. The product has submicron particles, which
would be easy to
detonate if they were normal crystals - but they
aren't, and I don't
know what happens.
IMO this could be very significant, for AP in fires ..
John: Thinking again about AP flowing. while it does
not melt in
bulk, it may flow as a bulk powder in some
circumstances. If it is
for instance on top of and reacting with tarmac or
polythene, the
reaction will produce lots of gas, at a prodigious rate.
This effusion of gas may be fast enough to fluidise any
overlying AP
powder, which would then flow almost like a liquid.
-- Peter Fairbrother
