Federation of European Explosives Manufacturers
BLASTING PRACTICE
TECHNICAL BULLETIN No. 6
ELECTROSTATIC HAZARDS IN
PNEUMATIC LOADING OF ANFO
CONTENTS
1. INTRODUCTION
2. ALLOWABLE LEVEL OF RESISTANCE
3. IMPORTANT INFORMATION ON SEMICONDUCTIVE HOSES
4. MAKING THE SYSTEM CONDUCTIVE
5. SUMMARY OF RISK FACTORS
The TECHNICAL BULLETIN N°6 has been published by the FEEM Blasting Working Group
in October 2005
FEEM is affiliated to CEFIC (European Chemical Industry Council)
Route1, Avenue F Van Nieuwenhuyse 4 – B1160 Bruxelles
TECHNICAL BULLETIN No. 6
ELECTROSTATIC HAZARDS IN
PNEUMATIC LOADING OF ANFO
INTRODUCTION
In the pneumatic loading of small-particle bulk blasting agents such as ammonium nitrate-fuel oil (ANFO), it is possible to generate sufficient static electricity to cause damage or premature initiation of electric, electronic and non-electric detonators.
Small electrical charges can build up during the contact-separation process of the prills as they are pneumatically conveyed through the loading device. The electric currents generated are extremely small and do not constitute a hazard to blasting operations if not allowed to accumulate. However, if these charges become stored and discharge suddenly, sufficient energy may be available to initiate a detonator. The absence of earthing of the equipment is a very dangerous situation, as very high voltages will appear on the loading equipment and the operator. In some instances voltages as high as 60,000 volts can be generated. Generally, metal loading equipment resting directly on a wet, conductive mine floor provides sufficient ground. If auxiliary grounding is required, metal straps or cables should be used.
It is important to realize that a hazardous situation can arise when loading ANFO. There are four points in a pneumatic loading operation at which electrical energy can be stored: (1) on the operator; (2) on the blasting agent loader and accessory equipment; (3) in the borehole and (4) on the detonator leg wires.
If a semi-conductive path is maintained between these points and ground, the energy is dissipated before it can build up to dangerous levels. In a pneumatic loading system this semi-conductive path for the bleed off of potentially dangerous static electricity can only be assured if a semi-conductive loading hose in good condition is used. The semi-conductive hose must do two things; (1) it must be conductive enough to harmlessly dissipate static electricity generated during the pneumatic loading of ANFO through properly grounded equipment; and (2) it also must have enough resistance so not to provide a low-resistance path by which hazardous levels of stray current can reach the detonators. The hose should have equal resistivity in radial and linear directions.
It is recommended that all pneumatic loading systems have semi- conductive loading hoses and grounded equipment that satisfy these requirements.
Electric detonator leg wires should be shunted during the loading operation to minimize any risks.
ALLOWABLE LEVEL OF RESISTANCE
Minimum Allowable Resistance of the Hose
The minimum allowable resistance of apneumatic loading hose should be 10 000 ohms. A hose with a resistance of less than 10 000 ohms is considered too conductive because it provides low-resistance electrical paths to the borehole for stray and galvanic currents.
Practically this implies that the resistivity of the hose should be more than 1 kohm/m.
Maximum Allowable Resistance of the Hose
The maximum allowable resistance of a hose is governed by the amount of current developed during the loading operation and the system’s highest capacitance (ability to store electrical charge). The resistance of the hose itself should not exceed 100 Mohms during the loading operation. It should be noted that the resistance of the total system may be influenced by external factors, like for example: humidity; loading rate and duration; and capacitance of the system.
- Humidity
The amount of electrical charge that can be developed by the blasting agent in any type of pneumatic loading system is related to the humidity of the conveying air. As the moisture in the conveying air is increased, the electrical charge on the blasting agent is decreased and vice versa.
2 Loading Rate and Duration
The total current developed by the ANFO is also proportional to the loading rate. Higher loading rates produce higher currents. The amount of static electricity generated increases with the loading duration. Additives such as aluminium or polystyrene can influence the generation of static electricity. The equipment should be operated according to the manufacturer´s specification
3.Capacitance
The use of borehole liners may significantly affect the capacitance (ability to store electrical charge) of the loading system.
Measuring the System’s Resistance (Conductivity).
The conductivity of the entire system can be measured by the simple test described in Figure 1. It should be apparent that this test measures the total resistance through the blasting agent, the rock, the blasting agent container, and the loading hose. The system resistance should be re-measured if any substantial environmental or work changes occur.
It is recommended to follow any national regulations. It is important to establish the resistivity at each site and periodically check for any variations.
IMPORTANT INFORMATION ON SEMICONDUCTIVE HOSES
Operators should be familiar with the following important cautions regarding semi- conductive hoses and their use.
(1)All semi-conductive (rubber or plastic) hoses can lose their conductivity as indicated by an increase in linear resistance under conditions of use. Factors affecting retention or loss of conductivity are:
a. Wear and Tear. There is evidence that repeated flexing and abrasion during use raises resistance over a period of time (lowering vital conductivity).
b. Oil exposure from ANFO mixtures raises resistance (lowers vital conductivity).
c. Impurities may affect the resistance.
(2)If it is suspected that the resistance of the hose has been altered it is advisable that its resistance should be re-checked or the hose replaced. Periodic testing of hoses in service will show operators if conductivity is being lost and how fast. Any tests must be made after the inside of the hose is washed with a detergent and thoroughly air dried for a true reading.
(3)Many non-conductive hoses look approximately like the semi-conductive types used for ANFO loading. These must be prevented from mistakenly being substituted for the proper semi-conductive type by positive identification.
(4)A non-conductive hose cannotbe rendered conductive or semi-conductive with the application of paint, coatings or the addition of wires.
MAKING THE SYSTEM CONDUCTIVE
It is important that the entire loading system is conductive between the tip of the hose and the ground via the loading unit to dissipate any charges. This conductivity can be determined by measuring the resistance of the entire system as used in the field.
In summary, a suitable conductive loading and borehole system can be obtained if: (1) semi conductive hoses are used; (2) the blasting agent container is metal and well grounded; (3) either the rock has sufficient conductivity or the humidity is high enough to dissipate the energy from the borehole; and (4) semi conductive liners are used in the borehole. Although both cap and fuse and non-electric delay caps have higher resistance to static initiation than electric or electronic detonators, tests show that these systems too can be initiated by static electricity. Therefore, it is recommended that semi conductive loading systems are used even with non-electric initiating methods.
In low conductive rock, such as salt, gypsum etc. sufficient conductivity may not be achieved and therefore special precautions should be taken. Special guidance should be sought from the supplier of the charging equipment.
SUMMARY OF RISK FACTORS
The risk is dependent of the electrostatic sensitivity of the detonator system.
The minimum resistance of the hose
The maximum resistance of the hose
Humidity
Loading rate and duration
Borehole liner
1