Reducing the risk of blasting
“Hot Holes” July 2008
Why undertake this work?
• Many surface operations on coal mines are moving
into areas where there are underground fires
• There have been incidents reported where primed
holes (thankfully not charged!) have detonated
prematurely
• Most mines are developing operating procedures to
reduce the risk associated with blasting in these hot
areas
– The threat is real
– Precautions must be taken
– Adherence to correct standards and procedures
Procedures…
• The procedure defines three cut off areas:
– 40oC.
• Below 40oC is considered a normal
hole
• Above treated according to procedure
– Between 60oC and 80oC will be
considered a hot hole and treated by
cooling agent, till below 60oC
– Above 80oC will not be considered for
charging at all
Procedures…
• AEL has sat with the mines and conducted
risk assessments for drafting of procedures:
– Measure temperature of holes, identify
number of holes to be blasted
– Cool
– Measure again
– Charge from cold to hot holes but
maintain a pattern so that blast can occur
as required according to the risk
– Minimum number people, equipment at
distance
Procedures
• AEL’s recommendations have been to use
emulsion based explosives, shock tube (On
surface) and detonating cord & booster (Toe
primed)
• AEL needed to ascertain the safety issues
related to explosives and initiating systems
when subjected to prolonged periods of
elevated temperatures
Tube to keep transducer at
a distance from pipe
Pressure
transducer
Lid
Testing
Gasket
Bolts to attach lid
Pipe
(Pressure +
6 bar)
Data
capture
system
Explosive
Thermocouple 1
Booster
Detonator
Thermocouple 2
Gas
burner
Solenoid
connected
to data
capture
system
Gas
Cylinder
Test Setup
Test No.
Description
1
Detonator only. Oil placed inside pipe for heat
transfer
2
Powercord only – oil in pipe
3
Detonator only (repeat #1) – oil in pipe
4
400g Pentolite booster – no oil
5
Booster & detonator
6
Gassed P100
7
Gassed P700
8
Detonator, booster & P700
Detonator only
Test #1: Detonator only (in oil)
800
Erratic behaviour of
flame temperature
probably because of
wind
700
Temperature (degC)
600
Emuls
Oil Temp
500
Flame Temp
400
Flame cools rapidly
after gas turned off
300
200
Steady rise in
temperature of the oil
until detonator initiates
at 160degC after
about
15mins
about 15
mins
100
0
0.00
200.00
400.00
600.00
Time (secs)
800.00
1000.00
1200.00
Emulsion only
Pipe pressurised
atvalue
an initial
value ofThis
5.5bar. This
Pipe pressurised
at an initial
of 5.5bar.
rises to about 7bar as the sample is heated.
rises to about 7bar as the sample is heated.
Test #8: P700,
Booster
& Detonator
Test
#6: Gassed
Test #6:
Gassed
P100 P100
700 600
600
8
10
8
P100 Temp
P100 Temp
Emuls
Temp
Flame Temp
Flame Temp
Temp
Pressure
Pressure
9
600
500
8
300
Sample temperature rises
Sample temperature rises
rapidly from 40 to 200degC.
300
rapidly from 40 to 200degC.
300
200
200
100100
0
4
200
0
0
0
0 0.00
These temperature fluctuations are probably as a result
These temperature fluctuations are probably as a result
of cold product slumping into the zone w here the
of cold product slumping into the zone w here the
thermocouple is located and then being heated.
thermocouple is located and then being heated.
500
500
200.00
1000
5
4
3
100
Pressure (bar)
56
400
Temp (degC)
Temp (degC)
400
Temp (degC)
7
400
1000 400.00 1500
2000
2500
800.00
1500
2000 600.00 2500
3000
Time
(secs)
Time
(secs)
Tim e (secs)
3000
3500
1000.00
3500
4000
4000
1200.00
4500
had significant
6 hadWind
Wind
significant
effect on the lame
effect on the lame
temperature
temperature
5
4
Pressure (bar)
6
500
Pressure (bar)
500
7
7
3
Product did not detonate.
did not
detonate.
3 Product
Detonator
at material and
Pinkfired
granular
Pink
granularafter
material
and
205degC
2
a liquid w20
hich fudged on
2
aminutes
liquid w hich
fudged onThe
of
heating.
cooling, remained after
2 cooling, remained
after
bulk explosive
was
the test as
w ell as black,
the
test
as
w ell as black,
partiallyburnt
initiated
but
material.
1
1 1 burnt
largematerial.
amounts of unfired
product remained in the
pipe.
0
00
4500 1400.00 5000
5000
Detonator, Booster & Emulsion
Test #8: P700, Booster & Detonator
10
700
Emuls Temp
Flame Temp
9
Pressure
600
8
7
6
400
5
300
4
3
200
2
100
1
0
0.00
200.00
400.00
800.00
600.00
Time (secs)
1000.00
1200.00
0
1400.00
Pressure (bar)
Temp (degC)
500
Detonator fired at
205degC after 20
minutes of heating. The
bulk explosive was
partially initiated but
large amounts of unfired
product remained in the
pipe.
Summary of Testing – Day 1
Test # Description
Comments
1
Detonator only
Detonation at 160ºC
2
Powercord
No detonation. The Powercord immersed in the oil
was burnt away and the tip of the remaining sample
was severely melted
3
Detonator only
Detonation at 180ºC
4
400g Pentolite
booster
No event. Could not see what had happened to the
sample but suspected that the booster had melted
and that the Pentolite had run into the bottom of the
pipe. Pipe destroyed by detonating a new booster
inside the pipe
5
Booster +
detonator
Detonation at 203ºC. Damage to the pipe was
significantly less than that caused by an intact
booster. The welds at the bottom of the pipe were
blow open
Summary of Testing – Day 2
Test # Description
Comments
6
Gassed P100
No detonation
7
Gassed P700
No detonation
8
Detonator,
booster + P700
Detonation occurred at 205ºC. Lid of the pipe
was detached and the bolts sheared. Pipe
remained intact and significant quantities of
emulsion remained.
Summary
• In all three tests where a detonator was
subjected to elevated temperatures, a
detonation occurred
– Temperatures varying between 160ºC and
205ºC
• Neither the straight emulsion or the blend
reacted in a vigorous manner
– Significant temperatures and pressures
– Explosive did not undergo a violent
exothermic reaction which would have
caused it to “run away” and detonate
Water displaced during charging
Field Tests
Gassing/stemming? Equipment removal
Equipment set-up
Field Test
60
4
2
50
0
-2
30
-4
-6
20
-8
10
0
11:09:36
-10
11:16:48
11:24:00
11:31:12
11:38:24
11:45:36
11:52:48
12:00:00
Time
Temp 1
Temp 2
Temp 3
Temp 4
Pressure 1
Pressure 2
12:07:12
-12
12:14:24
Pressure
Temperature
40
Other tests
• Minimum burning pressure tests
• Test chamber to 70 bar
• Apply instantaneous heat (+1000oC)
– AEL & Competitor bulk products tested
(at their request) do not detonate
• Developed for pumping but applicable to the
current situation
– How confined will the explosives be in the
hole? (Fissures, venting between holes,
old workings etc)
Recommendations…
• A good assumption would be that if there is
an initiating system with a detonator present
that detonates, the bulk explosive will also
be initiated
• These tests show that the detonator is the
component that has the lowest ignition
temperature
• Detonating cord, boosters and bulk
explosives have a lower risk to detonate
when heated under conditions resembling
those found in “hot” holes in coal mines
Recommendations
• It would therefore seem logical that areas in
the Witbank area which are deemed “hot”
use detonating cord as the initiating system
down the hole
• AEL’s recommendations have been to use
emulsion based explosives, shock tube (On
surface) and detonating cord & booster (Toe
primed)
• A move to top priming should be considered
to further reduce the risk particularly from
the booster (Heat exposure)