WO2001029499A1

WO2001029499A1 - A blast initiation system

Info

Publication number: WO2001029499A1
Application number: PCT/IB2000/001511
Authority: WO
Inventors: Mark Lee Thomas
Original assignee: Chemical Holdings Int. Ltd.
Priority date: 1999-10-20
Filing date: 2000-10-20
Publication date: 2001-04-26
Also published as: AU7937300A

Abstract

This invention relates to a blast initiation assembly (10) including a length of shock tube (20), and lengths of safety cord (18) (22) which provide a time delay for propagating an explosives reaction. The assembly (10) includes one length of shock tube (20) with a first length of safety cord (18) connected to one end thereof, and a second length of safety cord (22) connected to the other end thereof. A detonator (12) for detonating an explosives composition is attached to the end of the first length of safety cord (18), and a detonator (28) with a clip (32) attached thereto is attached to the second length of safety cord (22). The first and second lengths of safety cord are selected to provide a required time delay. The clip (32) is arranged is for clipping to a length of shock tube on a similar blast initiation assembly, to join assemblies for sequential rock blasting.

Description

A BLAST INITIATION SYSTEM

BACKGROUND OF THE INVENTION

THIS invention relates to a blast initiation system and to a time delay unit therefor.

When blasting a rock face in a mine, a pattern of holes is drilled into the rock face and explosive charges are loaded into the holes. The charges within each hole are then detonated sequentially to ensure proper blasting of the rock face.

Currently, a number of detonation systems are used to detonate the charges in the holes. These include so-called "safety fuse systems" and so-called "shock tube systems".

In a typical safety fuse system, a detonator to which is connected a length of safety fuse, which has a free end, is placed in each hole in the rock face. The free end of the safety fuse from each detonator is connected to a common length of igniter cord. The igniter cord in use ignites the safety fuse connected to each detonator sequentially. Each detonator is provided with a time delay by the rate of burning of the safety fuse to ensure that, ideally, the holes are detonated sequentially. Although the system is relatively inexpensive, it has a relatively poor time delay accuracy when compared to more sophisticated systems. In addition, in certain circumstances, the so-called burn front is not sufficiently spaced from the sequential detonations to prevent disruption of subsequent ignitions being initiated.

In a typical shock tube system, a first "in hole" detonator to which is connected a length of shock tube is loaded into each hole in the rock face. The ends of shock tube protruding from each hole are connected to one another by an "out of hole" detonator and lengths of shock tube. The respective in hole and out of hole detonators typically have very short time delays of the order of 4000ms and 200ms, respectively. On ignition, the shock tube ignites the detonators in rapid succession to blast the holes sequentially. Although this system has a better time delay accuracy, and is generally more reliable, it has the drawback of being substantially more expensive than a typical safety fuse system.

It is an object of this invention to provide a blast initiation system that addresses these problems. SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a delay unit for a detonator, the unit including:

a length of safety cord; and a connector for connecting a length of shock tube to the safety cord;

wherein the safety cord is selected to provide a required time delay for propagating an explosives reaction.

Preferably, the connector includes a transition element for transmitting a shock wave from a shock tube connected to the connector to the safety cord.

According to a second aspect of the invention there is provided a blast initiation assembly including:

a length of shock tube having a first end and a second end; a first length of safety cord having a first end connected to the first end of the shock tube by way of a first connector, and a second end; and a first detonator connected to the second end of the first length of safety cord;

wherein the first length of safety cord is selected to provide a required time delay for the propagation of an explosives reaction from the shock tube to the first detonator.

Preferably, a second length of safety cord is connected to the second end of the shock tube by way of a second connector, wherein the second length of safety cord is selected to provide a required time delay for the propagation of an explosives reaction from the shock tube to a second detonator. Advantageously, the first and second connectors include a transition element for transmitting a shock wave from the shock tube connected to the connector, to the safety cord.

The time delay of the first length of safety cord should be greater than that of the second length of safety cord. Generally, the time delay of the first length of safety cord will be 10 to 30 times, preferably 20 times, greater than that of the second length of safety cord. Typically, the first length of safety cord has a length of about 340mm to provide a time delay of about 100 seconds and the second length of safety cord has a length of about 50mm and a time delay of about 5 seconds.

According to a third aspect of the invention there is provided a method of propagating a time-delayed explosives reaction wherein a detonator is connected to shock tube via a length of safety cord with the length of the safety cord being selected to provide a required time delay of the propagation of an explosives reaction between the shock tube and the detonator.

The safety cord preferably comprises a core of incendiary material wrapped in a non-explosives wrapping material, such as a textile yarn material, plastics material or paper, for example.

In particular, the safety cord is a safety fuse comprising a core of black powder.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a plan view of a blast initiation assembly 10 according to the invention; and Figure 2 shows a partly cut away view of a section of the blast initiation assembly shown in Figure 1.

DETAILED DESCRIPTION

Referring to Figure 1 , a blast initiation assembly 10 according to the invention includes a first detonator 12 which is arranged to be placed into a hole drilled in a rock face, for detonating an explosives charge in the hole. The first detonator 12 in this case is crimped at 14 onto a first length of safety cord 16, preferably so-called safety fuse. Although crimping is preferred, any appropriate connecting means may be used to connect the detonator 12 to the safety cord 16.

Typically, the safety cord 16 is safety fuse which consists of a core of incendiary material, in particular black powder in the case of conventional safety fuse, surrounded by non-explosive wrapping materials, such as textile yarns, synthetic plastics, paper and the like. An additional plastic layer may be present externally of the wrapping material to protect it from the environment. In the case of this in-hole safety cord 16, the rate at which the core burns is preferably in the range of about 150 to 320 s/m, in particular about 300 s/m.

The first length of safety cord 16 is connected via a first shock tube connector 18 to a length of shock tube 20 by way of crimping. Shock tube 20 of this kind typically consists of a length of hollow plastic tubing which has an inner surface coating, typically of a reactive agent such as HMX, for example. Upon initiation, a shock front is propagated along the length of the tube 20 at a rate typically in the order of 2000 m/s. In a similar manner, the shock tube 20 is connected via a second shock tube connector 22 to a second length of safety cord 24 which in turn is connected to a second detonator 28. The second length of safety cord 24 is also preferably conventional safety fuse which is preferably arranged such that its core burns at a rate in the range of about 100 to 150 s/m, in particular about 100 s/m. It will be appreciated that the rate of burning of the safety cord 24 is selected to be greater than that of the safety cord 16 to ensure a burn front which is sufficiently spaced from the sequential detonations to prevent disruptions of subsequent ignitions. The detonator 28 is connected to a clip 32 which is arranged to be connected to a shock tube of an adjacent blast initiation assembly 10 to provide for sequential blasting of the rock face.

Referring to Figure 2, the shock tube connector 18/22 comprises a housing 34 and houses free ends of the safety fuse 16/24 and shock tube 20. In this case, a transition element 36 joins the safety fuse 16/24 to the shock tube 20. Although there may be direct initiation of the safety cord with shock tube, it has been found that the energetic pulse from the shock tube (which comprises hot gases and hot solid particles) may damage the safety cord and adversely affect the safety cord burn rate. The transition element 36 contains an energetic material that serves either to provide a shock tube output barrier or to modify the shock output signal, and thus to present a reliable and consistent ignition stimulus to the safety fuse. In the case of a shock tube output barrier, the barrier may be a mechanical barrier as in the case of a percussion primer or may be a densely packed pyrotechnic powder column. The shock tube output signal modifier involves the weakening of the impulse from the shock tube so that it does not harm the safety cord structural integrity. For example, the shock tube output could fire into an expansion volume to effectively lower the shock tube impulse. This could also be accomplished by a gas flow restriction i.e. an orifice between the shock tube and the safety cord. Similarly, an object placed in- between the shock tube output central vector and the safety cord core would also serve to diffuse the impulse. The housing 34 is crimped onto the shock tube 20 and the safety fuse 16/24. As mentioned previously, any appropriate connector or connection means may be used. In order to provide a better seal, a sealing element 38 is provided between the housing 34 and the shock tube 20 at the point of crimping.

Referring to Figure 3, during a blasting operation, a series of holes 42 to 48 are drilled into a rock face 50. A blast initiation assembly 10 as illustrated in Figure 1 is then loaded into each hole with the first detonator 12 extending into an explosive charge (not shown) in the hole and the clips 32 protruding from the holes. Each clip 32 is connected to the shock tube 20 of a blast initiation assembly 10 in an adjacent hole. Blasting is initiated at I using a suitable conventional blast initiation product such as a capped fuse unit or an electric detonator. The shock tube 20 of the assembly 10 in hole 42 ignites the length of safety fuse 16 which provides a delay before initiating the detonator 12 in the hole 42. The shock tube 20 of the assembly 10 in the hole 42 also ignites the length of safety cord 24 connected thereto which provides a time delay before igniting the detonator 28 which is connected to the shock tube 20 of the assembly 10 in the hole 44 by way of a clip 32. The detonator 28 from the assembly 10 in the hole 42 then initiates the shock tube 20 of the assembly 10 in the hole 44 and the connected assembly 10 are initiated sequentially.

Thus, the sequential propagation of an explosives reaction from hole to hole is determined by the manner in which the initiation assemblies are connected together and is controlled by time delays caused by the selection of particular first and second lengths of safety fuse 16 and 24.

In a preferred example the first length of safety fuse 16 has a length of 340mm and burns at a rate of 300 s/m, providing an approximate 100 second delay. The second length of safety fuse 24 has a length of 50mm and burns at a rate of 100 s/m, providing an approximate 5 second time delay. This type of system has a burn front of about twenty sequential delays or blast holes or combinations of blast holes. In order to increase the burn front to the order of fifty sequential delays, for instance, the second length of safety fuse 24 can be adapted to provide a 2 second time delay.

In tests carried out by the applicant, it has been found that the blast initiation assembly of the invention combines effective blasting operations comparable to more elaborate systems whilst being more cost effective.

Claims

1. A delay unit for a detonator, the unit including:

2. The delay unit according to claim 1 wherein the connector includes a transition element for transmitting a shock wave from a shock tube connected to the connector, to the safety cord.

3. The delay unit according to claim 2 wherein the safety cord is safety fuse.

4. A blast initiation assembly including:

5. A blast initiation assembly according to claim 4 wherein the first connector includes a transition element for transmitting a shock wave from the shock tube connected to the first connector, to the first length of safety cord.

6. A blast initiation assembly according to claim 4 wherein a second length of safety cord is connected to the second end of the shock tube by way of a second connector, and wherein the second length of safety cord is selected to provide a required time delay for the propagation of an explosives reaction from the shock tube to a second detonator.

7. A blast initiation assembly according to claim 6 wherein the second connector includes a transition element for transmitting a shock wave from a shock tube connected to the second connector, to the second length of safety cord.

8. A blast initiation assembly according to claim 7 wherein the time delay of the first length of safety cord is greater than the time delay of the second length of safety cord.

9. A blast initiation assembly according to claim 8 wherein the time delay of the first length of safety cord is from 10 to 30 times greater than the time delay of the second length of safety cord.

10. A blast initiation assembly according to claim 9 wherein the time delay of the first length of safety cord is 20 times greater than the time delay of the second length of safety cord.

11. A blast initiation assembly according to claim 10 wherein the first length of safety cord has a time delay of 100 seconds and the second length of safety cord a time delay of 5 seconds.

12. A blast initiation assembly according to claim 6 including a clip, connected to the second detonator, for connecting the second detonator to a length of shock tube on another blast initiation system.

13. A blast initiation assembly according to claim 4 wherein the safety cord is safety fuse.

14. A method of propagating a time-delayed explosives reaction wherein a detonator is connected to shock tube via a length of safety cord with the length of the safety cord being selected to provide a required time delay of the propagation of an explosives reaction between the shock tube and the detonator.

15. A method of propagating a reaction according to claim 14 wherein the safety cord is safety fuse