CA2139799A1 - Detonator protector - Google Patents

Abstract

A protector for use in the transportation and storage of detonators comprising, a detonator holder which is open at one end for insertion of a detonator and closed at the other end, and which radially encloses the base charge of said detonator, at least one detonator retaining means integral with the detonator holder, and a first wall which is radially spaced around the holder, and wherein the holder and wall define a space, wherein in use, the detonator retaining means holds the detonator within the holder such that a free volume is provided around the base charge of the detonator.

Description

` 2139793 DETONATOR PROTECTOR

The present invention relates to a protector for detonators and a method of packaging detonators for transport and storage.

Detonators may be broadly divided into two types, either electric or non-5 electric, depending on the manner in which they are initiated. When detonatorsare attached to an initiation means such as electric leg-wires or signal tubing, the assembled unit is commonly termed a detonator assembly.

Electric detonators generally have a fusehead within the detonator shell, the fusehead being initiated by electric current transmitted along the legwires.10 Non-electric detonators are commonly attached to a length of signal tubing, which comprises plastic tubing, lightly coated on the inside with a reactive powder which is capable of transmitting a shock wave to the detonator and thus initiating it.
While many different types of detonators exist, those used for civilian purposes generally comprise only a delay element (which is optional), a priming 15 charge and a base charge which are enclosed within a metal shell. When an initi~ting shock wave or electric current is transmitted to the detonator, the delay element burns, then the priming charge is initiated and finally the base charge in the closed end of the detonator shell explodes.

The storage and transport of dangerous goods such as detonators and 20 detonator assemblies generally requires licenses and is regulated by law in almost all countries. Often manufacturers and large mine sites need to store hundreds of detonators in their maga_ines. Statutory requirements usually dictate many parameters such as the material of construction of the m~g~7ine, type of lightning protection, security standards and the lllinilllulll distances of the m~g~7ine from 25 other constructions.

Detonators are often transported from one site to another in specially marked and fitted trucks which are exclusively dedicated to the transportation of explosives. Depending on local law, these trucks may be limited in the quantities `- 2139793 of detonators carried, the times of travel on public roads and the number of escort vehicles required.

Legislation in most countries includes such specific requirements for the transportation of explosives to try and ",i~ e the likelihood of initiation of a5 detonator in transit. Such events can have many causes including mishandling, lightning strike or road accident. Most countries base their legislative requirements for storage and transportation of explosives on the "Recommendations on the Transport of Dangerous Goods" issued by the United - Nations plus the United Nations' prescribed testing codes for establishing the acceptability of various pack~ging and transportation methods for dangerous goods.

With respect to the transportation and storage of detonators, the United Nations' 1.4B Code of testing is generally accepted as the relevant criteria. Ingeneral it is preferable that when detonators are packed together for storage and transport, inadvertent initiation of one detonator will not lead to mass initiation of other detonators. In other words, compliance at least in part with the followingcriteria is preferable;

1. initiation of a single detonator in a box of assemblies will not cause the instantaneous initiation of a ~ignifir~nt number of other units (sometimes referred to as the "Case Test") and 2. if a box of assemblies is burnt, initiation of an individual unit will not cause instantaneous initiation of a significant number of other units (sometimes lefel,cd to as the "Bonfire Test").

To achieve the first objective the p~cl~ging must necessarily pre~ellt a 25 detonator from initiating either the base charge or the initiation tubing of a significant number of other detonators in the bonfire test. With respect to non-electric detonator assemblies, initiation tube (which is generally made of plastic) will melt and become non-functional before the temperature of the fire is high ` ~13979~
, enough within the detonator to cause thermal initiation. Hence direct initiation of detonators by shrapnel and shock is the major incident to consider.

When an unprotected deton~tor is initiated, the majority of shrapnel occurs in two distinct areas. The first is in a ring that flows out from near the top of the 5 detonator base charge and the second is in the form of a slug emitted axially from the bottom of the detonator. In air, radial shrapnel can cause initiation of an adjacent detonator at 120 mm separation and the axial shrapnel can cause initiation at least up to 300 mm separation. In other, low shrapnel areas, initiation of a receptor detonator appears to be of the order of S0 mm.

Various attempts have been made in the past to minimi~e the likelihood of cross propagation of detonators whilst maintaining a practical packing density.
For example, polystyrene packages have been approved by the UK and Australian authorities for the transport of detonators colllplisillg No.8* delay elements (with no initiation tubing). These packages essentially consisted of a poly~ly.elle lid and 15 top which included cavities into which the detonators were laid 'top to tail'. The separation between the base charges of the detonators was about S0 mm and the package had overall dimensions of a~r~ tely 300 x lS0 x 30 mm. Huwc;~el this type of packaging was not commercially practical for non-electric detonatorassemblies as it would lead to uneconomic p~ckine density.

UK and Australian authorities have also approved a method of p~ n~
electric detonators having long lead wires which method consisted of forming thelead wires into a "figure 8" coil and locating the detonator as far as possible inside the wire bundle. While this concept can be applied to electric detonators it is not suitable for non-electric detonators fitted with signal tube as the signal tube is an active component of the assembly.

In another attempt to provide a method of safe transportation of detonators, No.8 detonators with no initiation tubing attached have been stored in packages or blocks constructed of compressed paper. These packages are appru~ ately 290 x S0 x 20 mm and holes 11 mm in diameter are drilled through / 21397~9 the block in a pattern on 30 mm centres. The bottom of the block has a cardboard sheet stapled on and the top is covered with a cardboard sheet into which crosses are cut to line up with each hole. This allows the detonators to be pushed into the holes and be retained in place.

In yet another attempt to provide a safe transportation holder and package for detonators US Patent No. 5,133,258 discloses the use of a holder having an internal cavity and surrounded by concentric, cylindrical elastomeric walls which are spaced radially from one another by concentric cylindrical spaces or air gaps.
In use, the detonator shell is flush with the walls of the cavity and this snug fit is sufflcient to prevent the holder falling off the detonator.

It has now been found that detonators may be packaged in such a manner as to reduce the likelihood of detonator to detonator propagation of an explosion.
This is primarily achieved by providing a free volume around the base charge of the detonator to allow for initial eYpan~ion of the products of a detonation .

The cullenl invention therefore provides a protector for use in the transportation and storage of detonators conl~lisillg, a detonator holder which is open at one end for insertion of a detonator and closed at the other end, and which radially encloses the base charge of said detonator, at least one detonator retaining means integral with the detonator holder, and a first wall which is radially spaced around the holder, and wherein the holder and wall define a space, wherein in use, the detonator retaining means holds the detonator within the holder such that a free volume is provided around the base charge of the detonator.

213979~

~, There is further provided a method of pacl~ging detonator assemblies by inserting detonators in the protector of the current invention and placing the protected detonators on either side of an insert in a packing case such that thedetonators point away from the detonator initiation means. Using this p~ ein~
5 method the shrapnel and shock waves from an exploding base charge is unlikely to impinge upon the initiation means of the detonator assemblies.

Where the initiation means is signal tubing it is important that the detonators are positioned such that if one detonator explodes it will not cause a shock front to pass along the signal tubing of neighbouring assemblies and initiate 10 their attached detonators. While this packing arrangement can also be used for electric detonators, the packing arrangement is clearly not critical because no matter how close a detonator explodes to leg-wires, the detonation will not cause any flow of electric current in the leg-wires.

The protector of the cullenl invention provides for a free volume around 15 the base charge of the detonator and this free volume may optionally extend along the entire length of the detonator shell.

The protector may optionally colllplise at least one outer wall which is radially spaced about the first wall, each pair of adjacent walls defining a space.
In a particularly preferred embodiment the protector of the current invention 20 comprises one outer wall and the protector is constructed of high density polyethylene (HDPE). The provision of a free volume around the detonator, an outer wall and the use of HDPE as con~lluction material has been found to be particularly advantageous. Using this embodiment of the protector, number 8*
detonators (which col~lise 780 milligrams of p~ le~ylhlilol tetranitrate, PETN) 25 may be packed with an inter-detonator spacing of as little as 2 centimetres before a shock front from one detonator will initiate neighbouring detonators. Where noouter wall is provided the minimum inter-detonator spacing is 3.5 centimetres.

` 2139799 .~

The detonator relail~ing means may COlllpliSC any collveniently configured projections on the interior of the holder. Fins extending the length of the interior of the holder are particularly pre~lled.

The protector of the current invention may be used as an independent unit 5 or two or more may be joined together or manufactured together to form a set.
The protectors may be joined together by any col,~enient means such as stirking,heat welding or solvent welding protectors. Altelnalively a single moulding or similar process may be used to form a set. Where the material of construction isan elastomeric substance the individual protectors may be joined by a web of the10 elastomer. Using sets of several protectors joined together may be advantageous where illlploved packing density is required.

It is particularly l~refelled that the individual protectors or sets be located in alternate packing rows. Where used herein the term "alternate packing rows"
refers to the packing in which no protector is located directly in alignment with 15 other protectors in the rows above or below. In a plefelled embodiment the protectors are packed in storage containers such as cardboard boxes or the like,by arranging a base layer of units in a close packed or square packed array, then placing a second layer of inverted units such that they fill or point towards the interstitial spaces of the base layer. A third layer of units repeats the 20 arrangement of the base layer and a forth layer repeats the arrangement of the second layer.

With this p~çking arrangement there is a low probability that one detonator may be located in direct alignment with a second unit and that the second unit could be initiated. However, whilst two units could fire, propagation 25 to further units is unlikely.

This method of packaging is particularly useful for detonators without attached initiation means. Various packing densities can be obtained depending on various parameters including the size of the detonator, the configuration of the ~139799 _`

protector, the dimensions of the packing case and the method of packaging of the~;ullellt invention.

The ~;ullellt invention will now be described with refelellce to the drawings in which Figure 1 shows sectional views of one embodiment of the protector;
5 Figure 2 depicts a plan view of an array of protectors; Figure 3 shows a sectional view along BB' of Figure 2; Figure 4 is a perspective view of detonator assemblies in a p~cl~gine case.

Figure l(a) clearly shows a cylindrical detonator holder (1) having an open end (la) and a closed end (lb) within which is located a detonator retaining 10 means compli~ g several fins (2) which are integral with the interior of the detonator holder. A wall (3) is radially spaced around the holder and the holderand wall define a space (4). The top of the wall and the holder form a junction (5)-Figure l(b) is a cross sectional view along AA' of Figure l(a) and shows 15 the wall (3), the holder (1) and the fins (2). In use a detonator would be pushed into the holder and held in place by the fins, the space between the detonator shell and the holder forming a free volume.

Figure 2 depicts a set of individual protectors (6) held together by a web of material (7) to form an array. Where the protectors are constructed of plastic, 20 the entire array could be formed by a single injection moulding process with a web of the plastic joining individual protectors. In use detonator shells would be placed in the holders (1), the tops of the detonator shells projecting above thetops of the protectors. When another set of protectors is placed on top of the first, the tops of the detonator shells would be located in the interstitial space (8).

Figure 3 is a cross sectional view along BB' of Figure 2 with detonators inserted in the protectors. The detonators are depicted in cross section to showeach base charge (9a), priming charge (9b) and delay element (9c) located withineach detonator shell (9d).

. . 213979g -Figure 4 depicts detonator assemblies packed in a box (10) using a prefel,~d p~ ging method. The box includes an insert (11) about which the detonator assemblies (12) are packed. The initiation means (12b) of each assembly is formed into a loop and each detonator (12a) is located in a protector 5 (13)-Bundles of detonator assemblies are held together by a rubber band(12c) located around their looped initiation means. As shown in the diagram, bundles of detonator assemblies are packed with their detonators on alternate sides of the insert, the detonators pointing away from the loops of initiation 1 0 means.

While the invention has been explained in relation to its ~lefelled embodiments it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such 15 modifications as fall within the scope of the appended claims.

Claims (11)

1. A protector for use in the transportation and storage of detonators comprising, a detonator holder which is open at one end for insertion of a detonator and closed at the other end, and which radially encloses the base charge of said detonator, at least one detonator retaining means integral with the detonator holder, and a first wall which is radially spaced around the holder, and wherein the holder and wall define a space, wherein in use, the detonator retaining means holds the detonator within the holder such that a free volume is provided around the base charge of the detonator.

2. A protector according to claim 1 wherein the free volume extends along the entire length of the detonator shell.

3. A protector according to claims 1 or 2 wherein the detonator retaining means comprises projections on the interior of the detonator holder.

4. A protector for use in the transportation and storage of detonators according to any of claims 1 to 3 wherein at least two protectors are joined together to form a set of protectors.

5. A protector according to any of the preceding claims wherein the material of construction is high density polyethylene.

6. A protector for use in the transportation and storage of detonators according to any of the preceding claims which further comprises at least one outer wall which is radially spaced about the first wall, each pair of adjacent walls defining a space.

7. A method of packaging detonator assemblies in a packing case, each detonator assembly comprising an initiation means and detonator wherein the detonator is located in the protector of any of the preceding claims and wherein the protector enclosed detonators are placed on either side of an insert in a packing case.

8. A method of packaging using the protector of any of claims 1 to 6 wherein protectors are placed in a storage container in alternate packing rows such that no protector is located directly in alignment with other protectors in the rows above or below.

9. A method of packaging according to claim 8 wherein at least two protectors are joined together to form a set of protectors.

10. A detonator protector substantially as herein described with reference to the Figures.

11. A method of packaging detonators substantially as herein described with reference to the Figures.