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F. A LOVING Eastern Laboratory, E. I. du Pont de Nemours & Co., Inc., Gibbstown, N. J.
Barricading Hazardous Reactions Protection against potential blast and missiles is mostly Q matter of judgment T H E EXPLOSIVES DEPARTMENT Of E. I. du Pont de Nemours & Co. is frequently asked for advice regarding protection of personnel and equipment from explosion hazards connected with experimental work on chemical reactions often carried out a t high pressures and temperatures. Unfortunately, assistance is more often tendered with after-thefact condolence rather than before-thefact recommendations. Any pressurized vessel or exothermic reaction represents a degree of hazard. However, one particularly dangerous reaction area is often overlooked outside the field of explosives research. This area is defined in terms of “oxygen balance.” The oxygen balance of a compound or mixture is the per cent excess or deficiency of oxygen required for complete combustion. Thus,
wt. available 0 2 - wt. 0 reauired for complete combustion Q.B. = total wt. of material
9
x
103
T h e products of complete combustion
are considered as carbon to carbon dioxide, hydrogen to water: combined nitrogen to molecular nitrogen, and metals to their oxides. The oxygen is obviously that available for entering a reduction-oxidation reaction. Such compounds as stable oxides, sulfates, and carbonates are not considered as explosives. However, if the vessel contains such materials as molecular oxygen, reducible oxides, peroxides, nitrogen oxides, nitric acid, nitrates, nitro compounds, or chlorates together with combustible material, the oxygen balance should be considered. The area of particular caution is illustrated in Figure 1 where the energy of various explosives is plotted against oxygen balance. Both compounds and mixtures of maximum potency fall in the oxygen balance range of 0 to -20%, Full scale detonations in reductionoxidation systems can easily occur from oxygen balances of +20 to -1207,. Dangerous reactions however, can occur outside this range of oxygen balance. The area defined here merely calls for particular caution.
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The larger the mass of material, the easier it is to initiate and sustain an explosive reaction. Consequently, where knowledge of stability properties is based on small scale experiments, caution must be exercised in the scale up of either reaction vessels or storage facilities. Barricade Design
A barricade must provide two types of protection. First it must withstand the blast effects of a possible explosion, and second it must arrest or deflect missiles resulting from an explosion. Explosion pressures from combustible gases dispersed within the barricade enclosure as well as pressures from condensed solid or liquid phase explosive mixtures must be considered. Overpressures from gas-phase explosions within a gas-tight barricade may vary from about 100 pounds per square inch for stoichiometric mixtures of methane-air to perhaps 600 pounds for acetylene-oxygen mixtures. These pressures are for starting conditions of room temperature and atmospheric pressure.
5-gallon autoclave exploded
the roof of a nearby building
It is exceedingly costly to construct a barricade chamber of reasonable size to withstand such pressures. Fortunately, large vents are effective in reducing damage from gas-phase explosions, and adequately ventilating the chamber will virtually ensure that the room is never filled with explosive gas mixtures. A practical design pressure may be obtained for condensed (solid or liquid) explosives or explosive mixtures from the equation, P = K -
W V
P = overpressure in pounds per square inch gage; W = Wright of material in pounds; V = chamber volume in cubic feet; and K = 2 X IO4 for T N T (trinitrotoluene), 1.5 X I O 4 for PETN (pentaerythritol tetranitrate), and 7 X 1O3 for 40y0 dynamite. This empirical relationship was derived from measurements of pressure generated by detonating various explosives in the center of chambers of several sizes and shapes. Charges from several grams u p to 10 pounds were used. Observations of both the air pressure near the walls and of the strain produced in the walls were made. The blast or shockwave pressure in itself may not be significant if the walls are massive and the charge relatively small. The shock wave may have such short duration that the wall will not have time to move during the first transient pressure application. Thus, motion of the wall follows an integral of the pressure transients which accompany an explosion. Walls of materials such as l / 2 - to 3/4inch steel plate, and I beam assemblies showed ?trains commensurate with a dynamic load equal to the mean pressure applied for the first 1.5 milliseconds. The pressure derived in Equation 1 is an equivalent hydrostatic pressure which may be used as the design pressure in standard engineering equations. Values
The 100-pound autoclave head, blown through the roof, landed some 100 feet away
of K given have been corrected to allow for dynamic loading but contain no safety factor. The constant used depends on strength and completeness of reaction for the explosive decomposition. The high pressure for TATT was attributed to a secondary reaction of detonation products with air. The rather low pressure from 40% dynamite was caused in part from the incomplete reaction of this material when unconfined, in the quantities tested (up to 9 pounds). The constant to be used should be a sufficiently large one based on the amount and violence of energy release which may occur. The X for PETN (1.5 X IO4) corresponds to an available energy of about 1300 kcal. per kilogram. This pressure estimate is for a room, not a tube or tunnel. The maximum cubical dimension should be no mort than twice the minimum dimension in computing V (Equation 1). In tubes or where detonations occur close to one wall, a local pressure near the explosive considerably in excess of that predicted may easily occur.
Missile Hazards A more difficult problem than blast pressures must be considered-namely, hazards from missiles. Serious injuries other than burns from accidental explosions are almost always the result of missiles. The mass, velocity, and shape of missiles from autoclaves or other reaction vessels are so varied that no rule of thumb can be given for providing adequate protection. However, some observations may be helpful : 1. Steel Walls. Double walls are preferred both for stopping power and for protection against spalling. Tests with explosive-propelled missiles have shown that two l/g-inch steel plates spaced several inches apart were superior to one 1-inch steel plate. A wall comprising two '/z-inch steel plates will effectively arrest light weight, irregularly shaped missiles such as nuts, bolt heads, pipe fragments, and light-gage metal container fragments. Steel walls should be supported by welding. Rivets or bolts which fail in tension may become missiles themselves. 2. Ballistically Shaped Missiles. Shafts, bolts, pipe sections, valve-handle VOL. 49, NO. 10
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extension rods, and such may be accelerated ballistically so as to strike a barricade with considerable penetrating power. One effective material for arresting such missiles is dry sand. A 50caliber pointed projectile travelling a t 2800 feet per second is arrested by about 10 inches of travel through fine dry sand. Wet sand is less effective. 3. Massive Missiles. Massive missiles such as autoclave heads, large pieces of pressure vessels, heavy pipe flanges, which may be accelerated to substantial velocities, are the most difficult to arrest and may be exceedingly destructive. Their direction of travel can usually be predicted from the equipment layout, and critical segments may often be aimed toward unoccupied areas. T h e cheapest protection against such missiles is probably several feet of earth or sand cover. Vents Effectiveness of vents in a barricaded chamber or rupture disks in a reaction vessel depends on the vent area, of course, but more importantly it depends on the reaction rate of any decomposition which may occur. For autoclaves, rocker bombs, and similar equipment, such venting is effective only for slow pressure
rises in the explosive sense. Fast deflagrations and detonations, even of the lowest order, may not be safely vented even by a rupture disk equal to the vessel diameter in size. A similar situation is found in barricade design. Large vents aid ventilation and are effective in reducing damage from gas-phase explosions or from a slow-acting explosive such as black powder. For a high rate process, however, venting cannot be depended upon to reduce the effective pressure indicated in Equation 1. Severe missile damage, shown in the accompanying illustrations, is the result of an equipment failure because of high pressure in a 5-gallon autoclavr. A reaction got out of control and the 100-pound autoclave head was blown off and out through the roof. In addition to wrecking the roof, this missile sheared and deflected an 8-inch I beam trolley rail in its path. The head, without causing further damage, landed some 150 feet away. A portion of the rupture disk assembly ejected in this explosion penetrated the roof of a nearby building. The small size of the rupture disk vent was no doubt a prime factor in the failure of this device to protect the vessel. .4 ’jd-inch thick steel barricade prevented lateral spread of missiles but the need for protection both on the sides and overhead is clearly illustrated here.
Summary and Conclusions
Barricade design is largely a matter of judgment. The empirical expression for an equibalent hydrostatic pressure from explosions has been verified sufficiently by experiment to merit some confidence. However, potentialities of missiles are exceedingly hard to predict, and each installation must be carefully examined. The design of an adequate and economical barricade should include the following considerations: 1. If a potentially explosive solid or liquid mixture is involved, a charge limit should be established by using Equation 1 and an appropriate safety factor for the structural design. 2. hIissile protection should be provided on the sides and top, by double steel walls and/or substantial earth- or sand-filled “sandwich” walls. Equipment should be oriented so that potential missiles will be propelled in harmless directions. 3. Large vent areas, frangible walls, and copious ventilation should be used to minimize pressures from gas-phase ignitions. RECEIVED for review April 7, 1957 ACCEPTED July 20, 195; Division of Industrial and Engineering Chemistry, Symposium on Safety in the Chemical Industry, 131st Meeting, ACS, hliami, Fla., April 1957.
In these autoclave bays built at Du Pont’s Eastern Laboratory in accordance with criteria given here, each unit i s ventilated independently and equipment under pressure i s operated with safety disks
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