An Orientation to Explosive Safety Betty W. Harris Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545 Explosives are chemical or physical systems capable of extremelv raoid exothermic reactions. which are eenerallv accompahiedby high pressures a t the ;eaction zone, products, and a vast amount of heat and light. Such systems can do a tremendous amount of work in very short time. For this reason, explosives are used for domestic, commercial, and military p&poses. Explosives also are used to clear forest areas, to open canals, to build dams, to blast rock in construction projects, to drill for oil, to mine ore, and for submarine activities. Different kinds of explosives are used for different purposes. Initiating explosives are used t o set off more stable explosives. Low explosives are used in some ammunition shells, blasting charges, and fireworks. High explosives (HE'S) are used as main charges, especially in weapon systems. Exdosives are aenerallv svnthesized com~oundsor formulated mixturesof compou&ds. However, some very danaerous exnlosive systems can be accidentally created by both professionals and laymen. Among these &e methane-air mixtures found in landfills or digester gas (sewage) plants located near residential areas, undetected explosive devices left by the military in areas that have since been converted to domestic use, the dust from the storage of grain and fertilizers, and misplaced or lost blasting caps or dynamites from construction projects. Appropriate precautions, safeguards, and expertise must be used to eliminate the hazards arising from these situations.
a
Clasrlficatlon of Explosives Explosives can be classified in many ways ( I ) . We have chosen to limit our classification to the manner and the ease with which they react. Initiating or Primary Explosives Primary explosives are generally sensitive to light, heat, shock. snarks. and static electricitv. Thev are verv unstable and &'be detonated directly when acted upon dy external forces. Examnles are mercurv fulminate. H e ( 0 N c ) ~lead . azide, P ~ ( N & , cyanurictriazide [ c ~ N ~ ( N ~ leadstypb)~J,' nate, [CsH(N02)30~Pb.HzO], and silver acetylide, [AgzCz]. These are usually packaged as disc- or capsule-type detonators such as blasting caps used to stimulate (set off) enereete he use> to ic reactionsinothersystems.'~'hus,lead a ~ i d may initiate a RDX- or I'KrN-(hexahydro-1.3,s.-trinitro-1,3,3triazine and pentaerythritol tetranitrate, respectively) base charge. Special standardized procedures and the proper safeguards must be enforced during the preparation and use of initiating or primary explosives. Low Explosives
Low exnlosives are chemical cornnositions or chemical compounds that deflagrate by a seif-sustaining reaction (burn slowlv. when unconfined) over a eiven neriod of time. when confined, they may react withexplosive violence. Therefore, they are used often in propellants, where controlled burning is important, and in blasting operations. Black powder or gunpowder is an example. Black powder is composed of charcoal, sulfur, and potassium nitrate or sodium nitrate.
Black powder is sensitive to flames, sparks, and friction. When confined, black nowder can be heated to a relativelv high temperature before an explosion will occur. However, it can be ignited easilv by a simple snark. Black-powder fires can cause severe damage to ocher eiplosives and to individuals. Do not attempt to fiaht a black-powder fire. Black powder can be desensitized b; pouring i t into water. Even empty black-powder containers should he washed before discarding. ~ c c i d e n t shave been reported that were caused by contaminated black-powder containers. Hiah (Secondarv - Exolosives . . Exolosives) . Highexplosives (HE'S) arechemicalcompoundsor chemical com~ositionsthat are much more stable than initiatine explosives. These are used almost exclusively for main charges. Some high explosives are insensitive t o mild shock, friction, flames, and heat, and they are generally set off by blasting caps or other initiators. When initiated by a blastingcap, the intense shock wave creates many hot spots within the explosive causing extremely rapid conversion of the solid into gaseous products with the subsequent release of heat and light. The noise one hears from an explosion is the shock wave generated by the hot, rapidly expanding gases moving through the air at the speed of sound. Energy release is almost instantaneous. High explosives are generally more easily handled because of their added stability. Some typical HE'S are nitroglycerin [CH2(N03)CH(N03) CHzN031, trinitrotoluene (TNT), hexahydro-1,3,5-trinitro1,3,5-triazine (RDX), 1,3,5-triamino-2,4-trinitrobenene (TATB), pentaerythritol tetranitrate (PETN), dynamite (principal explosive ingredient ammonium nitrate or nitroglycerin), and Composition B (6013911 wt% RDXITNTI wax). Dvnamite is the high that the lavman is most - exvlosive . likely to encounter. Its composition varies widely with its use. The principal explosive ingredient is ammonium nitrate or nitroglycerin. However, explosive or nonexplosive sensitizers often are added to brine about its detonation. Sensitizers are miatcriak that enhance the detonation wave propagationcharacteristics. thus reducing thecritical (failure)diameter of the explosive. The criticai diameter i s t h e minimum diameter of a cylindrical charge of high explosive required to sustain a high-order, steady-state detonation. The critical diameter is a function of charge confinement, charge density, material particle size, and the initial temperature of the charge. Sensitizers can also he impurities in the explosive. Liquid sensitizers are usually added with carbonaceous absorbent material t o prevent leakage and to obtain a suitable oxveen balance. Ethvlene elvcol dinitrate is an examnle of a successful liquid sensitizer. ~ m o n the g solid sensitizers are nitro-oraanic com~oundssuch as nitrotoluene. nitrostarch. and nitr&aphthaieue. Aluminum and sulfur a;e also added as solid sensitizers. Shock and heat can cause dvnamite mixtures to explode. Some grades of dynamite alsdgive off poisonous aases. - Figure lgives the three basic steps in an explosive train. I t also shows the relationship between primary and secondary explosivrs Ulilsting caps ihemselves, common explosive devices, utilile this relatiunship in their constructton (2).
Volume 64 Number 6 June 1987
541
ALUMiNUM ALLOY CUP
SuLN'
/
/
IGNllION CHARGE (LEAD 5NPHNATE AND ?ARIUM
1
B A ~ CHARGE E
INTBMWIAE CHARGE [LEAD U l D f l
(RDX)
Figure 3. Military Corps of Engineers special no. 8 electric blasting cap
ALUMINUM ALLOY S H Y Figure 1. Basic three-step explosive train.
IGNmON
WRGE,
PRIMING ~ A R G E
BASE CHARGE ,PRN
OPEN END INSERnON SAFEIY N
RUBBER PLUG,
SHY&
LEG
WIRBI
Figure 4. Du Pont no. 6 commerciel nonelectric blasting cap.
IGNnlOu MIX
/
/
SH' WDGE wnE ELE'
SASE
OPEN END WR
'wGE INSERTION OF SAFFil NSE
I
Figure 2. Du Pont no. 6 commerciai electric blasting cap
COPPOl OR ALUMINUMSHN Detonating .Devices
Two general types of devices or methods are used to set off explosives: igniters and detonators. Igniters carry to the explosive mass a flame that lights the explosive mass. Detonators deliver a shockwave that causes the explosive to dissociate, detonate, or burn rapidly. Igniters are squibs (plain or electric), fuses, and delay ieniters. Sauibs are small-diameter tubes of oaner . . or straw filled with a quick-burning powder; they have a relatively slow-hurnine match head attached to one end. Althaueh they are not safe, they are still being used in some coilmining ooerations. Fuses are usuallv fine oarticles of black powder wrapped in a coarse fiber forming a ropelike material. Delay igniters are a combination of igniters and fuses. They are standard and relatively safe for use in metal-mining and tunneling operations. Most detonation devices, even electric blasting caps, are unsafe in coal-mining operations because of the probability~. of igniting the dust or . pas . oresent in the mine. Blasting caps are shells or cups made of copper or aluminum alloy. They are closed a t one end and contain two- or three-layered charges of explosive within. The initiating (verv sensitive) exolosive is near the ooen end of the cun khiie the less senskive base charge is ciosest to the close2 end of the cup. The intermediate or primary charge is in the center. Figures 2-5 show representations of electrical and nonelectrical blasting caos (2). Electrical blasting cans are set off by essentiall; insiantaneous vaporizationuof bridge wires, while nonelectrical blasting caps need a fuse for ignition. Blasting caps are rather small and easily lost. Construction and military personnel sometimes lose live blasting caps in areas where they could be found and handled by children. They look like shiny empty rifle or pistol shells. However, they are very dangerous and could he injurious or even fatal to an individual if accidentally exploded. Therefore, laymen
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542
Journal o f
Chemical Education
IGNlilON CHARGE
PRIMING CHARGE
BASE CHARGE RDX
Figure 5. Military Corps of Engineers Special no. 8 nonelectric blasting cap.
are urged to he able to identify blasting caps and notify the local police if one is found in a residential environment. Do not attempt t o destroy t h e blasting cap. Domestlc Explosive Systems
One very common domestic explosive system is the accumulation of large volumes of gas-air mixtures. Sucha system will ignite with explosiveviolence. Sparks from a light switch or telephone, or from static electricity, could cause such mixtures to detonate. Some common sources found in buildings are natural gas (methane), propane, or butane gas leaks, digester gas or landfill gas accumulations in residential sections, and hydrogen-oxygen mixtures near life-support equipment. Fine dust particles from grain can evenly distribute themselves amone the available air in a erain elevator. This hecomes a suitable fuel for a reaction-A spark can ignite this fine erain-dust-air mixture creating" a violent exolosion. Ammonium nitrate fertilizer, when stored in garages or barns. can sometimes become contaminated with oil from lawn mowers or production machinery. The paper bag, ammonium nitrate, and oil constitute an explosive mixture that could be ignited by a cigarette, match; or even a backfire from equipment. Fuels and flammable solvents, such as gasoline, lantern oils, paint thinners, dry cleaning fluids, and adhesives, can be vaporized even at room temperature. The vapors will travel an unimaginable distance in a very short time forming an explosive air-solvent mixture. These vapors can be ignited easily by cigarettes, candles, sparks, or the pilot light of a gas stove or hot water heater or any other source of flames.
Table 1.
Sensitivity of Explosives Determined by Drop-Weight Imnact Test Type 12 H5. (cm)
Explosive
'
300
eDistetancsa 2.5 kg (5.5 Iblweight needs toFall to inltistethematerial 50% of the time.
Table 2. Results of Friction Tests In the Granlte Groove Friction Apparatus Steel Slider Loads kg Explosive
10 20 30 40 50 60 70 80 90 100 150 200
Lead mide E PETN E Blasting Gelatine E 0 LFB-Dynamite Nitro~ell~lo~e RDX Powder 688 "Borenitl' Powder 401 Tetryl Gurit "Nitrolit"
0
E
E
O
O O
K K
K E
0 O
K O
O
K
E
0 0
K O 0
0
been known to penetrate both floors of a two-story building. Both thermostats and safety valves should be checked periodically.
0
Sensitivity to Heat
One of the first sensitivity tests performed for safety purposes is to burn milligram quantities of the explosive inlover an open flame. Many explosives will melt before reaching an exolosion temperature. Others exolode before thev melt. The compound mercury fulminate is an example of the latter. The burnine process is a self-sustainina reaction. The oxygen needed ;lies not come from the air but is found within the compound's structure. Generally, explosive types are arranged according to their thermal stability. Therefore, primary explosives < secondary sensitive H E < secondary insensitive HE (e.g., lead azide < RDX < TATB). More precise data can be obtained using a differential thermal calorimeter. These data are useful in planning for long-term storage of explosives. Sensitivity to Impact
O 0 0
K 0
0
0 = NOaudible or visible elfed: E = Explodon: K = CRickiing noise a minor flash.
An idea of the stability of an explosive t o mechanical impact can he obtained simply by striking a few milligrams, resting on a metal surface, with a hammer. Precise dropweight impact machines have been designed to give more reliable data on impact sensitivity or to establish the critical impact energy necessary to make the compound or mixture explode. Table 1gives results from tested explosives. Sensitivity to Friction
When ignited, the system will explode with violence sometimes destroying houses and killing or maiming individuals in the vicinity. For example, when confined and mixed with the proper volume of air, one pound of gasoline can explode with almost 10 times as much enerm -.liberated as would he liberated form one pound of dynamite, see the example calculation in the Appendix. A gallon of gasoline would liberate about 62 times much energy as pound of dynamite. These chemicals should not be brought into the house nor should they be used for anything other than their stated purpose. Gasoline should not be used t o clean soiled clothing, floors, walls, etc. For flammable solvents intended for use indoors, read labels carefully. All flammahle solvents should be stored in a well-ventilated area. Aerosol cans are pressurized, and they contain a propellant that is easily vaporized. When heated, the pressure within the can becomes so great that the can explodes. Aerosol explosions are not as energetic as those produced from flammable solvents or fuels, but the flying pieces of metal can be lethal. Dry chemicals used by laymen are also potential explosive hazards. Calcium hwochlorite, a solid used to disinfect swimming pools, is % example. The compound liberates chlorine gas when it comes in contact with moisture. Chlorine gas is poisonous and can cause fires and explosions if brought in contact with compounds such as turpentine, ammonia gas, paint, kerosene, rubber, or alcohols. As a general rule, add only a small amount of calcium hypochlorite to a large volume of water, never the reverse. Use a dry, clean scoop t o transfer the chemical and store the original container where it cannot come in contact with moisture. The pressure inside a closed container can exceed the allowable limits, causing the container to explode. A malfunctioning safety valve on a hot water heater can create such an explosive system. Exploding hot water heaters have
as
a
Equipment has been designed to measure the explosive response to the stress and heat generated by friction when an explosive is rubbed between rough surfaces. Soil or sandpaper is sometimes used as an abrasive and a force or load is necessary in this test. Table 2 gives results with some tested explosives (3). TATB would not react under the conditions of the test; however, RDX is sensitive to friction. I t is believed that friction causes heat to concentrate in localized areas creating hot spots within the explosive. This results in a self-propagating reaction that could lead to a detonation. Explosive Reactions
Explosive reactions are exothermic, fast, and give off gaspri~ducts.Theycan be classified as ( 1 ) deflngrntions, (2) explosions, and (3)detonations. Thew can he rmtrolled by varying the composition and the manner in which the explosive is confined. Brief descriptions of these three classes follow. eous
Deflagration
This is the autocombustion of explosive particles. Also, i t is usually a surface phenomenon. In the open air a deflagration is generally slow (rate meterslsecond) with practically no sound. Gases, heat, and light are final products. The deflagration of black powder is an example.
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Explosions
Explosions are extremely rapid reactions characterized by a sharp increase in pressure a t the reaction site. Gaseous products expand into a volume much greater than the original unreacted material and can severely deform or totally destroy objects in the vicinity. Explosions are accompanied by a loud sound. The confinement of black powder in a blasting cap creates this kind of a system. Volume 64
Number 6
June 1987
543
Detonations Detonations are ex~losionspropagating a t a constant rate that exceeds the speed of so&d'(rate of a few kilometers1 second) in a given substance. Detonations are almost always associated w&h a shock wave traveling a t a given v e ~ o c % ~ (detonation velocity) (4). Although usually associated with high explosives, some less sensitive explosives such as propellants and ammonium nitrate can be made to detonate if properly confined. The difference between an explosion and a detonation can be shown best by describing their effects upon a physical object. If an explosion is made to occur inside a 5-in. X 3-in. X l-in. wooden block, using a fuse and a metal sleeve filled with black powder, the hlock is broken up into large chunks. Using a blasting cap and following the same procedure the system can he made to detonate. The wooden hlock is then blown into sawdust-size particles. Although the energy in hoth systems is nearly the same, the effects are quite different. In the detonation, the energy is delivered in a much shorter period of time. This results in a much higher pressure, which is very destructive.
good example. To the military, explosives are a means of maintaining defense. Basic knowledge about explosive safety is of a definite benefit to laymen who might accidentally create an explosive atmosphere, or handle explosives occasionally. However, no one should attempt to use explosives who has not been trained to do so. T o recoenize a situation involving explosives and to know how to d e z with it cansave many lives. This brief introduction to explosives, their recognition, classification, reactions, and safety aspeets, can he used in conjunction with other resource books such as the Dangerous Properties of Industrial Materials (5),Explosiues (6), and Terminal Ballistics (7).
Emergency Response The time and manner in which one responds to a potentially explosive situation or to an explosion can save hoth lives and property. Most major facilities, such as factories, government laboratories, and military establishments, have standard operating procedures (SOP'S), which outline in detail a sequence of emergency responses. For the layman, two things are basic, (1) if possible, leave the hazard area, and (2) notify the proper authorities, the police, the fire department, or local military ordnance group. In many areas, the emergency telephone number is 911.
W LC*? ..... 6. Meye,. R. Explosives: Verlsg Chemie: Deerfield, FL, 1981.
Conclusion For many, explosives are sources of otherwise unavailable energy to do work. They are used by farmers instead of manpower to remove trees, shatter boulders, and drill wells. T o the construction worker, they are a means of welding metal. of hlastine for road construction. or of dieeine canals. To industry, explosives are big business in various aspects of uroduction.. use.. and trans~ortation.The mace industrv. .. especially in the use of solid:propellants rocket boosters, is a "W
544
Journal of Chemical Education
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~
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Literature Cited 1. One example ia found in Military Erpiorima: Departments of the Army and Air Force Teehniesl Manual. U.S. Government Printing Offim: Washington, DC. Nov. 1967:
1 9 7 0 : ~137. 4. Fair, H. 0.;Walker, R. F. Ener#etic Moteriais; Plenum: New Yark, 1977: p 431. 6. Sax, N. I. Dongerour Propertieao(Indudrio1 Moleriois; Reinhold: New York, 1979:pp 7. Bsckman, M. E. Terminal Roilistics; Naval Weapons Center: Chins Lake, CA, 1976.
Appendix: Example Caiculatlon Dynamite Detonation Energy = 1Kcallg 453 gilb X 1 Kcallg = 453 Kealflh Gasoline (n-Octane) Heat of combustion = 112,460 BTUIgal
Comparison 4,325.4 Kcalflb gasoline = 9.6 453 Kcalllb dynamite
