Bagchi, A. and Dutta, P.K. (1975) An assessment of electrostatic hazard during ANFO Loading by a pneumatic device. Journal of Mines, Metals & Fuels, 23 (11). pp. 504-519. ISSN 0022-2755

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Normally pneumatic loaders are used in forcing the ANFO mixture in the blast holes at a pressure of about 4-5 kg./cm2 . it is possible that under such conditions, a static electric charge is built up, resulting in development of high potential, which, if discharged through the blasting cap, can cause premature firing of explosives. There are two distinct situation where the danger of premature ignition of blasting cap can occur normally. 1. Ignition by passage of spark between the detonator shell and the fuse head. 2. Ignition by passage of discharging current through the lead wire and the fuse head from a charged body to the ground. When the detonators are embedded into the explosive cartridges, the charges of the ANFO mix may penetrate through the explosive material to the detonator shell, and the conditions of discharge may occur. The margin between the minimum ignition pulse of detonation and the static energy stored into the system determines the degree of safety of the operation. The maximum static energy that can be stored in any system is given by ½ CV2 in joules where C is the capacitance and V is the potential difference. Under the worst condition the capacitances comprising of the various components of the loading system may represent shunt capacitances and provide the storage for the static electric energy. In the experimental programme it was considered that the capacitances of all the components of the system should be determined under actual operational conditions. The potential differences developed due to accumulation of charges at various points in the system were measured. The worst possible condition was simulated with the ANFO mix loaded into a galvanized iron pipe having a copper rod along its length, and the static energy developed at the time of loading at the rate of 35 kg per minute was measure, For the purpose of safety at the measurement stage in actual blast hole loading some dummy detonators were made having almost the same capacitance characteristics as those of the detonators. These were placed into the holes and the static voltage developed were measured between various components of the loading system and the earth during the ANFO loading operation. The worst possible situation of production of high voltage and high capacitance was taken into consideration in the analytical treatment, computations have been made of the various capacitances in the loading system and of the energy levels that occur in various points of potentially dangerous discharge. Under the most ideal condition of static electricity generation in the experimental set up, i.e. at 50 volts, the maximum ignition energy has been found to be as only 0.00375 milli joules across the loading hose and a copper wire in the ANFO column. This value is well below he minimum ignition energy reported as 0.5 milli joules. It has been found that to obtain this minimum energy level of 0.5 milli joules, the voltage should be about 579 across the loading hose and the leg wire. The analysis of the circuitry has indicated that the most dangerous conditions can occur when the fuse wire in the detonator is broken. It is because the long leg wires and the shotfiring cables altogether cause an enormous increase of the capacitance of the system. One of the desirable features of the detonators to be used under static electrical conditions is that it should have much greater static and arcing protection than is normally provided.

Item Type: Article
Subjects: Flame Proof Laboratory
Depositing User: Dr. Satyendra Kumar Singh
Date Deposited: 27 May 2012 05:18
Last Modified: 27 May 2012 05:18

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