epnm2010 blast and noise mitigation of open air explosions

EPNM2010

Blast and noise mitigation of open air explosions

Tuesday, June 13, 2006

Lay-out

• Blast introduction

• Blast mitigation

• Earlier experiments by others

• New experiments


Cladding industry practice

• Large metal surfaces (5x5 m or 3x8 m)• Large amounts of explosive (hundreds of kg)• Large flat charges

• In open air (enormous noise/bang)• Inside a tunnel (erosion/maintenance/safety)


Blast


Directed blast due to flat shape of explosive

• Simulation of the blast wave form a Ø 2 m plate of detonating TNT (0.1 m thick)


Blast: Bare TNT charge in air

• TNT (internal and kinetic energy)• Air (internal en kinetic energy)

100%

50%

01 ms 2 ms

En

erg

y

Time


Blast mitigation = redistribution of energy

• Kinetic energy in other media than air• Mass; solids (metal, stone) or liquids (water)

• Dissipation of energy (plastic deformation/heat)• Compressible media (sand, saw dust, fibers)

• Heat transfer (heating another medium)• Huge surface needed (small particles or water drops/mist)• Water damp generation takes 2.5 MJ/kg (4.5 MJ/kg TNT)


Blast mitigation

Container after internal explosion

of 0.5 kg RDXSaw dust filled container


Blast mitigation :Water (in bags)

• 55 kg TNT covered using 114 kg water at 0.5 m distance• M/W = 2; Pmax = 62% relative to free expansion

100%

50%

01 ms 2 ms

TNT internal

TNT kinetic

Lucht internal

Lucht kinetic

Water internal

Water kinetic

En

erg

ie

Tijd


Blast mitigation Water canon/sprinkler

• Water drops 50% blast reduction• Mass (kinetic energy)• Cooling capacity (surface area)

• Drawback is continuous water flow

Characteristic

diameter:

5 mm

0.2 mm

< 10 μm


Blast mitigationWater foam is a static water mist• Foam exists of thin water films• In the shockwave films break up into fine drops (micro-mist)


Blast mitigation

• Relation between blast and noise intensity:• Sound pressure level (SPL)

• 20 log ( Y Pa/20 μPa) • If Y = 200 Pa for a blast wave SPL = 140 dB

• Blast mitigation could be used for noise reduction!• Kill the monster while it is “small”

• TNO Defense, Safety and Security core-business!• Protection of ship (internal explosion)• Safety of munition storage • Large blast measuring experience (Australia, Canada,

Sweden)• Mitigation knowledge and techniques developed could be used

for blast/noise-reduction of open air explosions


Blast mitigation Earlier water foam experiments

45 kg at 10 m

Same charge

Right-side using foam blast-mitigation


Blast mitigation Earlier water foam experiments

• Foam expansion ratio 60-200 reduced the distance to a impulse level of 10 psi-ms with a factor 3 (compared to air).

• Peak pressure reduced by an order of magnitude• Noise reduction 12-15 dBA (at 2 km)


Practical application of blast mitigation for the cladding industry

• Effective?• Costs?• Safety?• Cycling time increase?• Site pollution?

• Static or dynamic system?• Mitigating medium?


Out-door experiments at Burbach (D): set-up


Water foam using fire-brigade equipment• Foam in container

• Foam layer thickness about 80 cm

• no optimization of foam or layer thickness

• Foaming agent is biodegradable


Blast wave measurement example 50 kg ANFO at 10 m

P2_10m_ANFO50kg_foam_3 P2_10m_ANFO50kg_4 P2_10m_ANFO50kg_foam_3_impulse P2_10m_ANFO50kg_4_impulse zero

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100kPa

-0.10

-0.05

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10 20 30 40 50 60 70 80 90 100 110

ms

P2_10m_ANFO50kg_foam_3 P2_10m_ANFO50kg_4 P2_10m_ANFO50kg_foam_3_impulse P2_10m_ANFO50kg_4_impulse zero

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0.50kPa.s

12 14 16 18 20 22 24 26 28 30 32

ms


Blast wave measurement example 20 kg RDX powder at 20 m

P3_20m_RDX20kg_foam_5 P3_20m_RDX20kg_6 P3_20m_RDX20kg_foam_5_impulse P3_20m_RDX20kg_6_impulse

-10

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40kPa

-0.10

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0.00

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30 40 50 60 70 80 90 100 110 120 130

ms

P3_20m_RDX20kg_foam_5 P3_20m_RDX20kg_6 P3_20m_RDX20kg_foam_5_impulse P3_20m_RDX20kg_6_impulse

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40kPa

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0.40kPa.s

40 42 44 46 48 50 52 54 56 58 60

ms


Video images of detonation of 20 kg RDX powder


Video of detonation of 20 kg RDX powder with water foam


Video images of detonation of 20 kg RDX powderwithout (left) and with water foam


Video images of detonation of 50 kg cladding explosive without (left) and with water foam


Video images of detonation of 50 kg ANFO explosive without (left) and with water foam


Experimental results: blast measurements


Conclusions

• Water foam seems an useful blast mitigation medium• Cheap• Effective• Environmentally friendly• No large influence on production time-cycle

• Eight shots (4 with water foam) in 3 hours• Blast reduced both in peak pressure and impulse

• ~25% for cladding explosive• ~40% for RDX (no after-burning)• ~20% for ANFO

• Tests are indicative only• No optimization of foam type and thickness

• No noise/acoustic measurements yet …….


Acknowledgements

• Chubb-Ajax is thanked for providing the foaming agent

• DMC-Dynaplat was a great help in this TNO project:

• Allowing TNO to perform these test at their shooting range

• Providing much of the explosives used

• Providing the fire-truck for foam generation

• Enthusiastic help during the tests (on a rainy day)


Blast mitigatieWaterdrops

Massa water 1 kg

Oppervlakte-energie 0.073 J/m2

Dichtheid 1000 kg/m3

snelheid 1000 m/s

temperatuurtoename 100 K

warmtecapaciteit 4190 J/kg*K

verdampingswarmte 2260 kJ/kg

molecuulmassa 18 g/mol

diameter druppels 10 micron

druppel volume 523 ccmicron

druppel oppervlak 3.14E-10 m^2

massa druppel 5.2333E-13 kg

aantal druppels 1.9108E+12

totaal oppervlak 600 m^2

n 55 mol

oppervlakte energie 44 J

kinetische energie 0.5 MJ

thermische energie 0.4 MJ

verdampingswarmte 2.26 MJ

TNT =4.5 MJ/kg


Blast mitigation water bags in detonation chamber (0.13 kg/m3)

• QSP after 1 kg PETN in bunker 130 kPa (100%)• QSP with 0.5 kg water on HE 60 kPa (46%)• QSP with 1 kg water on HE 45 kPa (35%)• QSP with 3 kg water on HE 25 kPa (20%)• QSP with 6 kg water on HE 25 kPa (20%)

• QSP 5 kg water at 50 cm from HE 15 kPa (12%)

epnm2010 blast and noise mitigation of open air explosions

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