US3175492A - Electrical safety detonator - Google Patents

Description

M. REYNE ETAL Filed June 4 1962 INVENTORS 13W 226w ATTORNEY ELECTR I CAL SAFETY DETONATOR i A wwwwww mwwmn w QM M \.x\ 6 y w March 30, 1965 United States Patent 9 Claims. (Cl. 102-28) The invention relates to electrical safety detonators for use in well bores to detonate explosives or shaped charges which provide a penetration in earth formations. In particular, this invention relates to detonators which are useful where well bore pressures and temperatures are very high.

Perforating apparatus for use in well bores typically includes a shaped charged device which is detonated by a blasting cord. The blasting cord is in turn detonated by a blasting cap or detonator. Detonators or blasting caps in this type of use are often exposed to the fluids in the well bore and consequently, the pressures and temperature of the fluid in the well bore. Conventional detonators are not practical for use where the pressures exceed about 14,000 p.s.i. and the temperatures exceed about 200 C. Moreover, in well bores where gas is present under considerable pressures, conventional detonators, which generally have rubber or neoprene seals, are not gas-tight so that their operation is frequently faulty.

Also, detonators of known types are normally fitted with low-resistance igniter wires to ensure their very quick ignition. In order to protect these detonators from being prematurely detonated by stray currents and voltages, resistors of appreciable value are coupled to the outside of the detonators in an electrical series connection with the igniting wire. These additional exterior resistors are sometimes inadvertently shorted or opened to cause a detonator failure. Igniting compounds of a very low sensitivity are sometimes used to avoid premature detonation of a detonator from such things as atmospheric electrical discharges. Such igniting compounds like aluminothermic mixtures (25% Al+75% Fe O or 20% Al+80% CuO for instance) react only at temperatures from 1000 C. and require ignition currents of several amperes. Cables conventionally used in well bores are not designed to carry such currents so such igniting compounds of low sensitivity are not usually used in well bores. Moreover, in conventional detonators it is possible to develop relatively high static charges on the detonator as a result of friction with the metal walls. Hence, it is possible for a spark to leap between the detonator jacket or wall and one of the terminals of the heating resistor to prematurely ignite the detonator. This, of course, can be eliminated by a high loss insulator, such as rubber of high carbon content covering the detonator but nonetheless, the hazard is still present.

It is an object of this invention to provide new and improved detonators which operate satisfactorily at temperatures in excees of 200 C. and at pressures in excess of about 14,000 psi.

Another object of this invention is to provide new and improved detonators which are hermetically sealed.

It is a further object of this invention to provide new and improved detonators whose safety in the presence of stray currents or of static charges is excellent.

It is another object of this invention to provide a new and improved safety detonator needing a low igniting current.

Still another object of this invention is to provide new and improved detonators which are easy and safe to use.

3,175,492 Patented Mar. 30, 1965 "ice An electrical detonator according to the invention, comprises a hollow metal receptacle with a suitable wall thickness. Sealed in the receptacle by a metal cover are a detonating or explosive charge, an inflaming or igniting compound wherein the igniter is connected to at least one conductor traversing the cover closing the receptacle. The conductor is sealed in the cover by an insulating glass bead. The igniting compound is more particularly a sensitive mixture of the aluminothermic type.

According to another feature of the invention, the elec trical igniter is made of a helically wound wire whose resistance is of some few dozen ohms which prevents untimely operation of the detonator in the presence of interference currents or charges.

The novel features of the present invention are set forth with particularly in the appended claims. The present invention, both as to its organization and manner of operation together with further objects and advantages thereof, may best be understood by way of illustration and example of certain embodiments when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a view in cross-section of the detonator casing of the present invention;

FIG. 2 is a view in cross section of the igniter subassembly for the detonator; and

FIG. 3 is a view in cross section of the assembled detonator.

According to FIG. 1, a hollow, cylindrically shaped receptacle 10 is shown constructed, for example, of steel of sufficient thickness to withstand high pressures. The receptacle can, for example, be 1.97 inches long with a wall 12 of 0.12 inch thickness and a central bore 13 which is 0.24 inch in diameter. The opening of the receptacle has a counterbore 13a which forms a shoulder 14.

A metal cylindrical closure member 18 (shown in FIGS. 2 and 3) is sized for reception in the counterbore 13a and has two passages 19, 19a extending therethrough. The lower end of the member 18 is counterbored to receive a thin, tubular tube 15 constructed of brass which is brazed to the member 18 as shown at numeral 16.

Each of the passages 19, 19a receives a bare wire conductor 11, 11a made of an iron-cobalt (15-20%) and nickel (25-30%) alloy which is hermetically sealed therein by a glass bead 10a. The beads 10:: are obtained by a well-known technique in electronics, by inserting a little glass tube into each passage with a conductor and melting the glass which by dissolving alloy oxides forms hard welding. While cooling, the passage 19 hoops the glass bead. Within the tube 15, the ends of conductors 11, 11a are attached to the ends of a helically wound wire resistor 24 which has an electrical resistance of 50 ohms. Resistor 24 lies approximately along the center line of tube 15. The tube 15 is filled with an alumino-thermic igniting mixture 25, for example, a weakly compressed homogeneous mixture of aluminum and barium peroxide powder in roughly stoichiometric proportions (Al:20% and BaO A lid 26 is provided for the lower end of tube 15.

In the bottom of receptacle 10 is a thin-walled container 17 enclosing a booster explosive 28 such as lead nitride. Container 17 is closed at its upper end by a thinwalled crimped disc 29.

The assembly comprising the tube 15 of aluminothermic powder -and the cover 18 practically occupies all of the free space within the receptacle 10 above the lead n-i- .tride booster charge 28 and is jammed into tight contact with the case 17. Cover 18 has an outer recessed surface 21 to form 'an annular recess with the receptacle 10 which permits a metal seal 30 to be made between the cover 18 and the receptacle 10. Seal 30 is thus a hermetic se-al. Cover 18 is brazed onto receptacle 10 by high frequency or induction heating which does not affect explosive mixture 25 since the explosive mixture requires higher temperatures for detonation.

With such an embodiment the metal seal 30 and the wire-welded glass beads 110a seal the detonator very tightly so that the assembly is gas proof and fluid tight, even for hydrocarbon gases under very high pressures, since these external high pres-sures are transmitted by the metal seal 30 to the cover body thereby increasing the hooping of the glass beads against the wall of the passages 19 and 19a. In electronics, glass beads and iron cobalt-nickel conductors ensure insulation of the said conductors but a high degree of fluid-tightness is neither required nor obtained. But, by a good choice of the cover shape and the seal position one obtains an increasing of the bead hooping by the high pressures applied on the detonator, thereby allowing the said detonator to withstand the said pressures without any drawback.

Furthermore, the steel wall of receptacle has a thickness of ample dimensions to withstand any high pressures.

Operation of the detonator acocrding to the invention at temperatures in excess of 200 C. is ensured. Indeed, the alumino-thermic mixture is perfectly stable at a temperature up to 677-700 C. and lead nitride 28 withstands without detonating, a temperature of 270 C. The safety margin thus is great compared with ambient temperatures encountered in a well bore.

During the lowering of a perforating apparatus into the well bore, heavy vibrations occur. Because the alumino-thermic mixture 25 and the lead nitride 28 are separately enclosed, abrasive contact of the explosive with metal parts is prevented. Hence, the detonator is insensitive to any such vibrations.

With regard to stray electrical currents and static charges, the protection of the detonator according to the invention, is directly ensured by the high resistance of the igniter resistor 24. While in the instance described, the resistor 24 has a resistance of 50 ohms, its value may vary according to cases, from to 80 ohms. Static voltages typically found in well bores generate in resistor 24 only currents which are too Weak to develop sufiicient temperature to ignite the explosive mixture 25. The same holds true with respect to stray currents which may be induced by adjacent electrical conductors passing the detonator.

Another advantage of a detonator according to the invention, in the presence of static charges which have been stored by friction against the outer sides of the detonator, results from the alumino-thermic mixture which itself provides a low resistance path to discharge static charges as they occur.

If a control voltage of 25 volts is applied from a power source (not shown), a 0.5 amp. current flows through resistor 24. The resistor 24 is heated slowly and after approximately 1 second, its temperature rises to some 700 C. At the same time, the alumino-thermic mixture 25 and the lead nitride 28 still remain at the pressure conditions of the assembly because the thickness of wall 12 of the receptacle prevents the external pressure from significantly alerting the internal pressure, and, of course, no gas or liquid under high pressure can enter the receptacle. As soon as the critical temperature of 600700 C. for igniting the mixture 25 of aluminium and barium peroxide is reached, a reaction occurs at the surface of the resistor 24. This reaction is quickly transmitted to the remainder of the mixture 25 releasing a great quantity of heat energy. This great quantity of heat is confined in receptacle 10 and detonates the lead nitride mass 28., It is Well-known that the barium peroxide BaO is an unstable product which easily becomes bariurn oxide (BaO) and oxygen. This feature explains Why an aluminumbarium peroxide mixture reacts from 600700 C. instead of 1000 C. like usual alum-ino thermic mixtures, needing thus a low igniting current.

Thus, a detonator according to the invention, is capable of operation under very severe temperature and pressure conditions encountered in well bores.

It is well understood that the invention is not limited to the examples described and illustrated above which have solely been used as non-restrictive examples, but on the contrary the invention may be made in other alternative manners.

Thus, one can use a single head traversed either by both conductors or only one, as in the case where one end of the igniter is connected to the jacket or wall of the detonator.

Likewise, the attachment of cover 18 to receptacle 10 may be made in another way than by brazing, for example, an autogenous soldering or a metal-to-metal seal which can be accomplished by pressure setting or shrinkfitting methods.

As for the composition of the igniting compound, black powder which is stable up to 300 C. can be also used if the detonator is carefully made. With special cables which can withstand large currents, usual alumino-thermic mixtures can be used.

Likewise, the lead nitride 28 may be replaced by another primary blasting agent, or even by a secondary one, if sufficient quantities, provided the the explosives be stable at temperatures in excess of 200 C.

As for the receptacle 10, it can be made of material other than steel, provided its resistance to the pressure is great enough and its dimensions are not excessive.

Furthermore, instead of being made of an alloy of iron-cobalt and nickel, Wires 11 and 11a can be made of tungsten or molybdenum oxides of which are also dissolved by melting of a suitable glass, and thus form a hard welded bond.

An alternative concerning the receptacle 10 is employed when a detonator according to the invention is used as a detonator for a quick fuse or a shaped charge. In these cases, the bottom of receptacle 10 is machined for direct attachment to the tip of said quick fuse or shaped charge.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made Without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. A detonator comprising: a hollow metal receptacle having an open and a closed end; a booster explosive received in said closed end of said receptacle, said explosive being stable up to a temperature substantially not less than 200 C.; and means for detonating said booster explosive including an igniting composition separately disposed in said receptacle proximate to said booster explosive, said igniting composition being stable up to a temperature substantially not less than 600 C., an electrical resistor disposed Within said igniting composition, said resistor having an electrical conductor leading from one end thereof, a metal cover having a glass tubular electrical insulator sealingly engaged therein, said conductor being passed through said tubular insulator and sealingly engaged therein, said cover being sealingly engaged with said receptacle to hermetically close said open end thereof; said receptacle, cover and insulator being adapted and engaged one to the other to withstand high pressures external thereto, the resistance of said resistor being in the range of 15-80 ohms and capable of being heated to temperatures in excess of 600 C. in approximately one second in response to a sustained application of electrical current, yet being insensitive to stray electrical currents.

2. A detonator comprising: a hollow metal receptacle having an open and a closed end; a first hollow case received in said closed end of said receptacle; a booster explosive disposed in said first hollow case, said explosive being stable up to a temperature substantially not less than 200 C.; means for detonating said booster explosive including a second hollow case received in said receptacle and disposed with one end thereof in detonating proximity to said booster explosive, an igniting composition disposed in said hollow case, said igniting composition being stable up to a temperature substantially not less than 600 C., an electrical resistor disposed within said igniting composition, a metal cover received in said open end, means for conducting current through said resistor including a metal electrical conductor passed through said metal cover and attached to said resistor, electrical insulation means sealing said conductor relative to said metal cover, and means sealing said cover relative to said receptacle; the resistance of said resistor being in the range of 15-80 ohms and capable of being heated to temperatures in excess of 600 C. in approximately one second in response to a sustained application of electrical current, yet being insensitive to stray electrical currents.

3. The detonator of claim 2 wherein said resistor has a resistance of 50 ohms.

4. An igniter for a blasting cap including an enclosure member for receipt within a blasting cap container, an igniting composition disposed in said enclosure member, said igniting composition being stable up to a temperature References Cited by the Examiner UNITED STATES PATENTS 991,373 5/11 Rennie et al. 102-28 1,617,675 2/27 Djidics 102-28 2,240,438 4/41 Durant 10228 2,882,820 4/59 Young 10228 2,926,566 3/60 Atkins et a1 10228 X 2,953,443 9/60 Lloyd 102-28 X 2,987,997 6/61 Ireland 10228 3,040,660 6/62 Johnston 10228 3,054,351 9/62 Apodaca et al. l0270 3,059,576 10/62 Haefner 10228 SAMUEL FEINBERG, Primary Examiner.

Claims (1)

1. 4. AN INITER FOR A BLASTING CAP INCLUDING AN ENCLOSURE MEMBER FOR RECEIPT WITHIN A BLASTING CAP CONTAINER, AN IGNITING COMPOSITION DISPOSED IN SAID ENCLOSURE MEMBER, SAID IGNITING OCMPOSITION BEING STABLE UP TO A TEMPERATURE SUBSTANTIALLY NOT LESS THAN 600*C., AN ELECTRICAL RESISTOR DISPOSED WITHIN SID IGNITING COMPOSITION, AN ELECTRICAL CONDUCTOR MEANS ATTACHED TO SAID RESISTOR FOR CONDUCTING CURRENT THROUGH SAID RESISTOR, THE RESISTANCE OF SAID RESISTOR BEING IN THE RANGE OF 15-80 OHMS AND CAPABLE OF BEING HEATED TO TEMPERATURES IN EXCESS OF 600*C. IN APPROXIMATELY ONE SECOND IN RESPONSE TO A SUBSTAINED APPLICATION OF ELECTRICAL CURRENT, EYT BEING INSENSITIVE TO STRAY ELECTRICAL CURRENTS.

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