US2887054A - Blasting initiator - Google Patents

Description

May 19,1959- Pf JQB'RYAN 2,887,054

BLASTING INITIATOR v Filed Jan. 15, 1956 INVENTOR PAUL J. BRYAN ATTORNEY United States Patent BLASTIN G INITIATOR Paul I. Bryan, Philadelphia, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware This invention relates to delay electric blasting initiators having increased resistance to failure as the result of arcing during initiation.

Delay electric blasting initiators are used in quarrying, mining and construction operations to provide precision rotation tiring of a plurality of explosive charges. Precision rotation tiring aiords improved fragmentation, reduced concussion and vibration, and greater eiiciency per unit quantity of explosive. The delays between the various charges must be accurate in order to achieve the maximum elfectiveness, and failures must be avoided.

The delay electric blasting initiators consist essentially of a detonating main charge, an ignition charge ignitable by a hot wire, and a delay element interposed between the main charge and the ignition charge. and the delay element are retained in proper position within an elongated metal shell integrally closed at one endl and sealed with la plug which also retains the electrical ignition means at the other end.' The delay element usually consists of a thick-Walled metal tube containing a column of a composition or mixture capable of self-sustained decomposition at a known and reproducible rate.

The early delay elements consisted of columns of deilagrating explosive compositions, notably black powder, and such delay elements are in use today. The deagrating explosives are characterized by the production of considerable volumes of gas upon decomposition, so that provision must be made for venting these gases in order to avoid premature rupture of the shell or greatly accelerated burning rates of the composition due to the increasing pressures. Such venting is normally provided by perforating the shell in the area of the ignition charge and sealing the perforation with some readily rupturable material. The vented delay electric initiators 1are subject to desensitization of the ignition composition if they are immersed in water and subjected to a shock from an adjacent or nearby detonation before the delay element has been ignited. Further, a danger of igniting the vexplosive charge by the escaping gases before the desired delay has been obtained does exist.

For the foregoing reasons, kconsiderable eifort has been made to produce ventless delay electric initiators in which a delay composition producing little or no gas is used. While no compositions having the desired burning characteristics and completely free of gas production are known, a number of mixtures of an oxidizing agent and a metal fuel rare known which burn at reproducible rates and which produce only small amounts of gaseous products. At the present time, most of the delay electric initiators used, particularly in the presence of water, are of the ventless type.

In many operations Where machinery is powered by electricity, the operators prefer to use directly the same source of electrical energy to carry out their blasting operations,` thus avoiding4 the need of additional equipment. The electricalenergy available at these sites may The chargesV lCC be either alternating current, up to 550 volts, or direct current, up to 250 volts. In other operations, the blasters wish to use portable sources of electrical energy such as batteries and blasting machines. Experience has shown that personnel using blasting initiators are unwilling to select initiators on the basis of the electrical characteristics of the power source they will use, and, therefore, the initiators must be designed to function at both low and high voltages.

At high voltages, i.e., 220 volts or more A.C. or 200 volts D.C., the bridge wire will be volatilized and an arc will form between the leg wires projecting below the plug, i.e., the bridge posts. This arc will continue until the leg wires have been burned back into the plug, and, in some cases, will continue for a distance into the plug. In the case of instantaneous initiators, the arc is of no consequence, because the detonation of the main charge occurs and the initiator is disintegrated. ln the case of delay initiators, however, the consequences of such arciug can be very serious. The `arc will frequently rupture the shell wall adjacent to the bridge posts, thus permitting the entrance of fluids into the shell. In the ventless type of delay initiator, the delay cornposition is usually loosely packed, and may be blown out by the sudden venting of the shell. Sudden release of pressure also will extinguish carriers that are packed to a high density. In the case of the vented or unvented shell, ignition of the delay charge may not occur before rupture. In either type, the delay period is adversely affected by the undesired venting of the' shell.

Accordingly, an object of the present invention is to provide a delay initiator not subject to failure due to arcing. A further object is to provide such initiator wherein the means for eliminating arcing failure is loW in cost and simple to install. Further objects will become apparent as this invention is more fully described.

I have found that the foregoing objects are attained when l provide a shunt across the leg wires so located that no damage to the delay portion of the initiator will result from the heat generated at the shunt, the shunt being so designed as to provide considerable resistance at low voltages and only negligible resistance at high voltages. I have found that the preferred location for the shunt is Within the sealing plug. By placing the shunt within the sealing plug, the possibility of accidental dislocation or damage to the shunt is minimized. Further, break-through of the shell wall because of arcing atthe shunt is less likely because of the insulating eiect of the sealing plug. Finally, even if a rupture of the shell wall is produced, the working portions of the delay initiator continue to remain enclosed. As the shunt having high resistance at low voltages and low resistance at high voltages, I prefer to use a two layer strip, one layer consisting of metal and the adjacent layer consisting of a fusible resistive lm, the thickness and width of iilm being such as to provide a resistance of from 25-250 ohms. By positioning the thus designed shunt with the resistive hlm layer in Contact with the leg wires of the initiator, the desired shunting action is achieved. When a low Voltage is applied, as, for example, in firing the initiator by means of a blasting machine or battery, the resistance of at least 25 ohms at the shunt insures that the current diverted from the bridge wire will not affect ignition-a shunt having a resistance of 25 ohms requires an increase of only 0.02 ampere in the tiring current, a negligible value. When a high voltage is applied, for example 220 or 440 volts A.C., the fusible resistive lm Will be burned or melted away almost instantly, so that the natural pressure of the sealing plug material will force the metal layer in contact with the leg wires, providing a short-circuit having negligible resistance. During the period required for the resistive lm to be fused, the bridge Wire will be energized to initiate the ignition composition. After the fusing of the resistive film, essentially all of the electrical energy will'be shunted across the leg wires within the sealing plug, thus preventing arcing at the bridge posts.

In order to more fully illustrate'my invention, reference is now made to the accompanying drawings,l in' Which- Figure l represents an assembledl delay electric initiator in accordance with this invention;

Figures 2 and 3 represent alternative shunt applications; and l Figure 4 represents a greatly enlarged view of the shunt of this invention.

Referring now to the figures in detail, lis a tubular metal shell, for example of aluminum or gilding metal, 2 is a base charge, for example of PETN or RDX, 3 is a primer charge, for example lead azide. or a fulminate, 4 is a delay element tube, for example, of lead, 5 is an essentially gasless delay composition such as a barium peroxide-selenium mixture or a boron-red lead mixture, 6 is an ignition composition such as boron-red lead or a loose mixture of bismuth, selenium and potassium chlorate, 7 is a resistance bridge wire, 8 are the leg wires, 9 is a sealing plug, for example of natural or a synthetic rubber, 10 are peripheral crimps in the shell wall, 11 is the shunt of the present invention having a metal layer 11A, and a fusible resistive lm layer 11B, and 12 is insulation about the leg wires 8.

As shown in Figures 2, 3, and 4, the shunt may be applied in a number of ways. yfolded about the wires to form a double span in which the resistive lm layer is on the inside. In Figure 3, a

double span is provided by using two shunts parallel tol each other. In Figure 4, a single span is illustrated. I have found that the arrangement illustrated in Figure 2 is preferable when the sealing plug is formed about the leg wires While that illustrated in Figure 3 is preferable when performed plug halves are assembled to form the sealing plug. The arrangement illustrated in Figure 4, while operative, is disadvantageous because dislocation of the shunt during assembly is more likely and the shunt must be greater in width in order to provide the currentcarrying capacity required.

With the construction described, a delay electric initiator is provided which is not altered in performance by application of high voltage energy because of arcing. To illustrate the improvement obtained with the present invention and the critical features thereof, reference is now made to the following tables.

In Table I, delay initiators identical in every respect except for the presence of the shunt of the present invention are compared. The tests were made by connecting 10 initiators in parallel and applying 440 volts A.C. to the circuit.

Table l Number Number No. having Shunt Present of Initla- Deto- Number shells ruptors nated Failed tured by Tested arcing Yes l 30 0 o No 2O 10 10 17 1 Double-span shunt 34 in. wide consisting of 0.004 in. resistive nlm ofa conductive rubber vulcanized on 0.002 1n. shim brass-average resistance, 30-100 ohms, as measured on sample ignition assemblies.

In Figure 2, the shunt wasl l metal layer of the shunt. The tests were conducted as described for Table I.

, current.

Table Il Number of Number Inltiators Number having Metal Layer, Tested Detonated shell rupture by arcmg Tests also were ymade using copper andaluminum with satisfactory results. Shim brass was selected for most experiments because of its availability and ease of handling.

Table III illustrates theA criticality of providing sutlicient cross-sectional area in the metal layerrto carry the The tests were made as described for Table I, using a double-span shunt of a resistivelm of conductive vinyl resin cemented on'shim brass.

Table III Number Number Shells Cross-sectionalarea of metal of Deton- Number Rnptured in shunt (Total) Imtiators ated Failed By Tested Arelng 320v circular mils -.f--f.-.. 95 15 640 circular mllsm.- 60 60 0 0 960 circular mlls 160 o Table IV illustrates the use .ofl dilferent resistive films in the shunt. .commercial product consisting of carbon particles in a rubber matrix and having a resistance of 3 ohms-sq. cm., was vulcanized to shim brass. The conductive vinyl resin lm,

, also a standard commercial product consisting of carbon particles in .a vinyl Iresin matrixand having a resistance of 1 ohm-sq. cm., was cemented to shim brass with a Table V illustrates the effect of the resistance of the resistive film on the performance of the shunt of this invention. The shunts were prepared by vulcanzing conductive rubber ilm of different thickness onto shim brass, and a double-span shunt was used in each case. The resistance was determined by measurement of several sample ignition assemblies in which the bridge wire was omitted. The tests were made as described for Table I.

Table V Number of Number of Resistance of Bhunt Inltlators Number Shells Rup- Tested Detonated tured by Areing ao-so ohms 6o so o 20 20 0 40 40 o 1,000 ohms 30 28 7 summarizing the ndings illustrated in the tables, the shunt of the present invention consists of (l) a metal layer in which the cross-sectional area is adequate to carry the current long enough to prevent arcing at the bridge posts, `(2) afusible resistive. lm layer of such The conductive rubber lm, a standardr dimension that the resistance of theshunt is less than 1000 ohms. The choice of metal or of fusible resistive film is not critical when the above requirements are met. As previously indicated, the resistance of the shunt must be at least 25 ohms to insure an adequate bridge wire current from low energy sources. Preferably, the resistance of the shunt will be between about 25 ohms and about 250 ohms.

Inasmuch as brass is representative of metals generally, the total cross-sectional area of the metal should be at least 500 circular mils (250 mils for each span when a double-span is used). The maximum cross-sectional area is limited only by the space available in the initiator.

The fusible resistive ilm is preferably laminated or cemented to the metal layer for ease of assembly. As previously indicated, it is the resistance of the shunt assembly and not of the lm itself which is critical. Thus the selection of the ilm thickness will depend upon the width of the shunt and the amount of cement, if any, between the lm and the metal layer.

'Ihe invention has been fully described in the foregoing, and many variations and modifications will be apparent to those skilled in the art. Accordingly, I intend to be limited only by the following claims.

I claim:

l. In an electric delay blasting initiator of the type wherein the leg wires of the electric ignition system pass through a sealing plug of con-conductive material closing the open end of the metal shell, the improvement comprising a two-layer shunt entirely within said sealing plug and spanning said leg wires, said shunt having a metal layer and a layer of a resistive fusible lm and being so positioned that only the layer of resistive lm is in contact with a bared portion of each leg wire.

2. A delay electric initiator as claimed in claim 1 wherein said shunt encircles said leg wires.

3. A delay electric initiator as claimed in claim 1 wherein the resistance of the intact shunt is between 25 and 250 ohms.

4. A delay electric initiator as claimed in claim 1 wherein the resistive film layer consists of conductive rubber.

5. A delay electric initiator as claimed in claim 1 wherein the resistive tlm layer consists of a conductive vinyl resin.

6. A delay electric initiator as claimed in claim 1 wherein the metal layer has a cross-sectional area of at least 500 circular mils.

References Cited in the le of this patent UNITED STATES PATENTS 1,606,419 Grant Nov. 9, 1926 1,777,916 Schatller-Glossl Oct. 7, 1930 2,247,384 Huyett July 1, 1941 2,408,125 Rolfes Sept. 24, 1946 UNITED l STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noa 2,887,054 May 19, 1959 Paul J, Bryan Patent should read as corrected below.

Column 4, line 30, Table III, under Jche heading "Number Failed", opposite "960 circular mils" insert en; column 5, 1ine'28, for "Cone eonduotive" read non=conduotive Signed and sealed this 8th day of December 1959.,

(SEAL) Attest:

KARL AXLINE Attesting Oicer ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION .'Pafent No 2,88%054 May 19, 1959 Paul J., Bryan Column. 4, line 30, Table III, under Jalle heading "Number Failed", Opposite "960 circular mils" insert O n; column 5, line 28, for "Gonf- Oonduetive" read mnonuconduetve me.

Signed and sealed this 8th day of December 3.959.,

Attest: KARL HfXLNE ROBERT C. WATSON Attesting Officer Commissioner Of Patents

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