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Dead-pressing Phenomenon in Emulsion Explosives

Shulin NieSwedish Detonic Research Foundation

Box 32058S-126 11 Stockholm, Sweden

ABSTRACT

The dead-pressing phenomenon in emulsion explosives is wellknown. Research work on this subject has been carried out at the Swedish Detonic Research Foundation for the last few years. Several experiments with emulsion explosives under dynamic pressure condition have been carried out both in iron pipes and in blast holes in rock. The main object was to study the critical amplitude and duration of the pressure causing the explosive to be dead-pressed.

This paper describes two series of experiments, one made in iron pipes and the other in blast holes in an underground mine. The tests in iron pipes show that an emulsion explosive with microballoon Q-CEL 719 can be dead-pressed within 10 ms by a pressure of approximately 10 MPa, while the same emulsion explosive but with microballoon B37/2000 can stand for a pressure higher than 100 MPa. The tests in blast holes show that the detonation in one hole imposes a pressure higher than 50 MPa on the explosives in the neighbouring holes, where the hole spacing is about 20 cm for φ 64 mm holes. With increasing hole spacing, the pressure transferred decreases rapidly.

INTRODUCTION

It is a known phenomenon that industrial explosives such as ANFO and emulsion explosives may be desensitized when they are subjected to external pressure. The desensitization of an explosive means a decrease in its chemical reaction rate and in its reaction extent. In the extreme case, where the reaction rate is decreased to zero, the explosive has lost its detonability totally and become dead-pressed.

Along with the wider use of the emulsion explosives and higher demands on the blasting results, intensive research work on desensitization of the emulsion explosives has been carried out in the past few years. Progress has been made especially in the following aspects:

* Pressure gages have been developed /1, 2/.* Pressure wave profiles have been measured in boreholes in coal /1, 3/, in boreholes

in rock /4/ and in explosive charges underwater /5/.* Detonability of various emulsion explosives has been tested in underwater blast

tests /6/ and with other methods /7/.

Copyright © 2000 International Society of Explosives Engineers1993R - Dead-pressing Phenomenon in Emulsion Explosives - P 1 1 of 7

* A rugged emulsion explosive has been developed /8/.

Studies on the pressure desensitization on emulsion explosives at the Swedish Detonic Research Foundation (SveDeFo) have been carried out in recent years. A pressure gage /2/, based on a piezo ceramic material, and a method to test the detonability of the emulsion explosives in iron pipes have been developed. This paper describes two series of experiments, one of which was carried out in boreholes in rock to measure the cross-hole pressure profile and the other was carried out in iron pipes to test the detonability of emulsion explosives.

2. DYNAMIC PRESSURE ON EXPLOSIVE CHARGES IN BOREHOLES

2.1 Experiment Set-up

A series of experiments has been carried out in a very competent rock of quartz porphyry type in a Swedish underground mine. Efforts have been concentrated on measuring the dynamic pressure, which an explosive charge in a borehole is subjected to when a neighbouring hole is detonated.

Boreholes φ 64 mm were drilled in five drifts in a pattern similar to that used in tunnelling, shown in Fig. 1. The hole lengths varied between 2 and 4 m.

At each blast, one blast hole was detonated and the cross-hole pressure pulses were measured in three neighbouring holes with varied distance to the detonating hole. An example is shown in Fig. 1 where hole No. 2 was detonated and the pressure pulses were measured in hole Nos. 1, 4 and 11. Both the detonating hole and the measuring holes were charged with a bulk emulsion explosive. The detonating hole was initiated by a primer of 25 g plastic PETN at the bottom of the hole.

The fractures or cracks between the detonating hole and the measuring holes were investigated by a so-called water permeability method /4/ prior to the blast.

The pressure gages which were developed at SveDeFo based on a piezo ceramic material were located inside the explosives in the measuring holes.

Totally, nine blasts were fired and pressures were measured in 26 measuring holes. The distance between the detonating hole and the measuring hole varied from 0.22 m to 1.03 m.

2.2 The Dynamic Pressure Measured in a Borehole

The duration of the pressure pulse measured in the explosive in a borehole varied from 5 to 39 ms with an average value of approximately 20 ms.

The amplitude of the pressure pulse has been plotted versus the distance between the detonating and the measuring hole in Fig. 2. A regression line in log-log plot may be expressed by:


-1.95P = 0.91 R

where P = pressure amplitude in explosive in a borehole (MPa) d = distance from the borehole to the detonating hole (m)

The pressure amplitude decreases very rapidly with the increasing distance to the detonating hole. For example, when the distance is 0.22 m, the detonation in the detonating hole can impose a pressure higher than 50 MPa on the explosive in the measuring hole. When the distance is increased to 1 m, the corresponding pressure is reduced to approximately 1 MPa.

3. DETONABILITY OF EMULSION EXPLOSIVES UNDER DYNAMIC PRESSURE

3.1 Testing Method

A method to test the detonability of emulsion explosives under dynamic pressure has been developed. The configuration is illustrated in Fig. 3.

Explosives are tested in a iron pipe measuring 52 mm in inner diameter, 4 mm in wall thickness and approximately 1.5 m in length. A clay mass divides the pipe into two compartments. In the smaller compartment, ca 50 mm long, a black powder charge is loaded. In the larger compartment, about 4 kg emulsion explosive is loaded. A 50 g pressed PETN primer containing a No. 8 electrical detonator with a certain delay time is placed at the end of the explosive column opposite the black powder. Thereafter, the iron pipe is closed with hoods.

The black powder and the detonator are initiated simultaneously. Thus, the burning of the black powder generates a dynamic pressure which pressurizes the explosive before it is initiated by the primer with the delayed detonator.

Two SveDeFo pressure gages are mounted in the explosive to register the pressure. The signals through the amplifiers are recorded by a LeCroy 7200 digital oscilloscope.

The pulse width of the pressure signal has been measured earlier to a value between 35 and 140 ms.

Up to now, two explosives with the same matrix but different glass microballoons (GMB) have been tested. The first explosive contains GMB Q-CEL 719 from The PQ Corporation and the second one contains B37/2000 from 3M.

The detonability of the tested explosives was determined by the visual examination and the sound level from the blast. If the explosive was dead-pressed, a length, usually about 1 m, of the iron pipe with the explosive inside remained after the blast. Otherwise, neither


the pipe nor the explosive could be recovered on the test site.

In the case that the explosive was dead-pressed, the remainder of the pipe with the explosive was recovered and a second initiation with a primer of the same type was carried out after a time interval.

3.2 Results

The detonability of the explosive with GMB B37/2000 and Q-CEL 719 are plotted in Fig. 4 and 5 respectively. Detailed explanations are given in the figure captions.

Since it is very difficult to achieve a pressure higher than 100 MPa in the iron pipes due to the leakage from the cable holes, the maximum pressure tested was approximately 100 MPa.

The results depicted in the figures show that the breakage of the GMBs is the reason for the dead-pressing of the explosives. For example, the explosive with GMB Q-CEL 719 is dead-pressed at a pressure of approximately 10 MPa while the one with GMB B37/2000 can stand a pressure of 100 MPa. In other words, the matrix in the tested explosive is strong enough to withstand a pressure of at least 100 MPa.

When the explosive with GMB Q-CEL 719 was dead-pressed, it took place very rapidly, within a time less than 10 ms.

The area to the right of the broken line A in Fig. 5 shows an interesting and important result. That is, the remainder of the dead-pressed explosive trade somehow regained its detonability when the second initiation was carried out. However, this recovery time is very long, about 30 minutes. Therefore, this feature of the emulsion explosives may not be utilized in a normal blast round. The mechanism of this recovery is still unknown.

By comparing Fig. 4 and 5 with Fig. 2, one can conclude that the explosive with GMB Q-CEL 719 will not function properly in a blast pattern where the hole diameter is 64 mm and the hole spacing is less than or about 0.3 m, while the explosive with GMB B37/2000 will.

A quantitative visual observation has also been made on the explosion gases during the experiments. It has been seen that a pressure desensitized explosive, including the one with recovered detonability, produced much darker gas clouds. This reveals a lower chemical reaction extent of the explosive.

4. CONCLUSIONS

- Matrix and GMBs are the two components in an emulsion explosive. The strength of the weakest one determines the pressure durability of the explosive. In the tests, the matrix tolerates more than 100 MPa. Therefore, the breakage of the GMBs is the reason for the dead-pressing.


- The explosive containing GMB Q-CEL 719 can be dead-pressed at a pressure level of 10 MPa, while the one containing GMB B37/2000 can withstand more than 100 MPa.

- It takes a very short time, less than 10 ms, to dead-press an emulsion explosive. However; it requires a very long time, about 30 minutes, for the dead-pressed emulsion explosive to recover its detonability.

- The reaction extent in a pressure desensitized explosive is lower than in a normal explosive.

- An explosive charge in a borehole in rock can be exposed to a very high pressure, e.g. higher than 50 Mpa, depending on the distance to the detonating hole.

- The knowledge of the pressure durability of an explosive and the pressure this explosive can be subjected to in a borehole provides a guideline for both the blasters and the explosive manufacturers to match the explosive with the actual blast conditions. This can result in a better blasting quality and lower explosive costs.

REFERENCES

1, Wieland M S: Cross Borehole Stress Wave Measurements in Underground Coal. S-E-E*, Anaheim, California. 1988.

2, Nie S, et al: Measuring Dynamic Pressure in ANFO and Emulsion Explosives - Experiences in Developing a Pressure Gauge Based on a Piezo Ceramic Material. SveDeFo Report DS 1990:1. Sweden. 1990.

3, Wieland M S: The Laboratory Determination of Dynamic Pressure Resistance of Cap-sensitive Explosives. S-E-E*, Orlando, Florida. 1990.

4, Nie S, et al: Pressure Effects on Explosives in Boreholes. SveDeFo Report DS 1991:5G. Sweden. 1991. (In Swedish).

5, Mohanty B and Deshaies R: Pressure Effects on Density of Small Diameter Explosives. S-E-E*, New Orleans, Louisiana. 1989.

6, Matsuzawa T, et al: Detonability of Emulsion Explosives under Various Pressures. Journal of the Industrial Explosives Society, Japan. Vol 43, No. 5. 1982.

7, Huidobro J and Austin M: Shock Sensitivity of Various Permissible Explosives. S-E-E*, Orlando, Florida. 1992.

8, Ruhe T C and Wieland M S: Rugged Emulsion Explosive Formulation #37 Candidate Permissible. Proceedings of the Annual Meeting of Society of


Explosives Engineers, Orlando, Florida. 1992.

Fig. 4. Detonability of the tested emulsion explosive containing GMB B37/2000 under dynamic pressure.


Waiting time = the delay time between the arrival of the pressure at and the initiation of the explosive.

Fig. 5. Detonability of the tested emulsion explosive containing GMB Q-CEL 719 under dynamic pressure.

Waiting time = the delay time between the arrival of the pressure at and the initiation of the explosive.

The area to the left of the broken line A contains the results from the first initiation. The value of the pressure is the pressure generated by the black powder which pressurizes the explosive before it is initiated by the primer, see Fig. 3.

The area to the right of the broken line A summarizes the results from the second initiation, i.e. the dead-pressed explosives left from the first initiation have been reinitiated. In those cases, the explosives trade actually been pressurized by two pressure pulses, one generated by the black powder and the other by the first primer. However, the pressure from the primer is too high to be measured by the gages. Therefore, in this area, the pressure value is still the pressure from the black powder which the explosive had been subjected to at the first initiation.


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