The groove mortar, a reliable method for testing the flameproofness of certain explosives

Transcription

1 The groove mortar, a reiabe method for testing the fameproofness of certain exposives Jean Bigourd, J.M. Dassonvie To cite this version: Jean Bigourd, J.M. Dassonvie. The groove mortar, a reiabe method for testing the fameproofness of certain exposives. 5. Conférence Internationae des Instituts de Recherches sur a Sécurité dans es Mines, Sep 1993, Pretoria, South Africa. pp , <ineris > HAL Id: ineris Submitted on 3 Apr 014 HAL is a muti-discipinary open access archive for the deposit and dissemination of scientific research documents, whether they are pubished or not. The documents may come from teaching and research institutions in France or abroad, or from pubic or private research centers. L archive ouverte puridiscipinaire HAL, est destinée au dépôt et à a diffusion de documents scientifiques de niveau recherche, pubiés ou non, émanant des étabissements d enseignement et de recherche français ou étrangers, des aboratoires pubics ou privés.

2 93-36 THE GROOVE MORTAR, A REUABLE METHOD FOR TESTING THE FLAMEPROOFNESS OF CERTAIN EXPLOSIVES J.U. Bigourd and J.M. Dassonvie INERIS, B.P. n 0,60550 Vemeui-en-Haatte, France For more than sixty years, cyindrica stee cannons (ong mortar and short mortar) have been used in France to assess the safety of exposives with regard to firedamp. These testing methods are considered expensive mainy because a arge number of shots are needed äs a resut of the ow eve of reproducibiity. Aso, using them with certain exposives, such äs surries and emusions, is somewhat probematica. Taking into aecount the Poish expertise, CERCHAR (now INERIS) deveoped m the sixties a testing method in which the Charge, paced in a stee groove mortar, is isoated from the methane-air mixture by an asbestos-cement pate. This method seems more attractive than those traditionay used, especiay when it comes to distinguishing rock exposives from the more powerfui pennitted exposives. INTRODUCTION The first part of this document examines the factors which encouraged us to ook for a new method for testing the fameproofness of exposives. The foowing parts indicate how the method was deveoped, together with a description of the present tests and a summary of the resuts obtained so far. Exposives in French coa mines Under French reguations, exposives designed for use in Underground coa mines can be divided into three groups : - group (in French "exposife rocher") : rock exposives, which are ony aowed in the east dangerous driftings, - group ("exposifs couche"): coa getting exposives, which are mainy used for basting operations with instantaneous firing, in main Ventiation areas, - group 3 ("exposifs couche ameiores"): improved coa getting exposives which can be used with short deay detonatois, in the most dangerous working paces where basting is pennitted. A these exposives must have a good detonation abiity and a ow production of toxic gases when they detonate. For group 3 exposives, a ow defagrabiity is aso required. With regard to safety in the presence of firedamp or coa dust, there is no requirement for group exposives, unike group and 3 exposives for which tests must be carried out. For group, the approva conditions are not very severe, the main aim being to avoid ignition of a coa dust deposit äs the resut of a bown out shot. The approva conditions are more stringent for group 3. First and foremost, it means preventing the ignition of a coud of coa dust (in an atmosphere which can contain a sma amount of methane) in the case of exposure during deayed basting. Long mortar and short mortar trias Here we are interested in fameproofhess tests for group exposives in the presence of firedamp. Traditionay, two types of cyindrica stee cannons are used to carry out these tests, i.e. ong mortar and short mortar. 5eme Conference Internationae des Instituts de Recherches sur a Securite dans es Mines, Pretoria, septembre 1993, vo., p

3 Thc ong mortar method has bcen practiscd in France since The current prowdure dates back to the 1930s (Audibert, 1931). The mortar bore is metcrs ong and has a diameter of 38mm when new. The ength of the Charge varies from one test to another. It consists of one or more 30mm diameter cartridges. Inversey primed, it is pushed to the back of the mortar. The shot is fired without stemming. The short mortar method was deveoped in the 1930s and the current procedure dates back to the 1950s (Loison, 1954). The mortar bore is 0.m ong with a diameter of 38mm when new. The Charge is 0.18m ong with a diameter of 30mm. Inversey primed, it is paced in the centre of the mortar. A stee disk, whose diameter is sighty smaer than that of the bore, is paced at the mouth of the mortar, sünuating stemming. Its thickness can vary from one test to another, from to 10mm or more. Over the ast forty years, our institute has carried out a very arge number of ong mortar and short mortar exposive tests, for the approva of new formuae and, in particuar, for the inspection of exposives used in coa mines. With time, it has become increasingy obvious that both methods pose a number of probems in the case of reativey powerfü exposives and do not aways enabe groups and to be distinguished. When using the ong mortar test, for exampe, figure represents the probabiity of firedamp ignition äs a fünction of the Charge ength. The soid ine shows the behaviour of a typica group exposive according to the ignition frequencies obtained in about 500 tests in which the Charge had 10 different engths ranging from 0.1 to 1.8m. The dotted ine shows what it was thought happened in the past. Consequenty, it was beieved that by carrying out a reasonabe number of shots, the characteristics of a given exposive coud be determined by its Charge imit (maximum Charge which does not cause ignition in a fixed number of tests, usuay five). When compared with what reay happens, there is obviousy no basis for the notion of Charge imit. What is more, the state of wear of the mortar has a certain infuence on the resuts, eading to a ack of reproducibiity since it is not financiay possibe to change the mortar for each test. Thus, the ignitions observed with the smaest charges were mosty obtained when the mortar was new (first ten shots). - Figure - Long mortar; probabiity (P) of firedamp ignition äs a fünction of the ength (L) of the Charge; (I) od theory; (R) reaity. Tests versus reaity Originay, according to Coward (196), the stee mortar was chosen, in preference to bocks ofrock or coa, for simpidty in preparation and with the reasonabe hope of greater reproducibiity of resuts. The tests carried out in the different experimenta mines (Nagy, 1959 ; Meerbach, 1961, Cybuski, 1963) enabes the officia tests to be compared with the practica conditions. From these tests, it emerges that, generay speaking, the use of stee makes the tests more severe but, in certain cases, woud seem to introduce artifacts. For exampe, it has been observed that Variation in a parameter can have the opposite effect in a test and under the corresponding practica conditions. However, the Poish sot mortar test, where a ayer of cay separates the exposive Charge from the methane and air mixture is an exception. It gives resuts in the vicinity of those obtained --

4 under süniar conditions, for rock and coa, in the Barbara experimenta mine (Cybuski, 1963). In 1965, we started deveoping a simiar test which woud enabe us to do away with ong and short mortar for the approva of group exposives. This is the test which is presented here. DEVELOPMENT OF THE GROOVE MORTAR TEST The Poish sot mortar is a cyindrica stee mortar, a sector of which is removed over a certain ength. It is therefore reativey fragie when powerfü exposives are fired. To obtain a more resistant apparatus (figure ), we mied a shaow, trough-shaped groove on one side of a bock of stee. This groove, designed for 30mm diameter cartridges, has a depth of 40mm and a width of 50mm (figure ). Provided the Charge is correety paced in the groove section using a suitabe device, a free space of at east 10mm can be eft between the Charge and the stee, in order to reduce the wear of the mortar. By covering the groove with a pate of materia comparabe to rock, we can simuate the case of a mine hoe whose atera confinement is reduced during firing of the round. To encourage atera expansion of the gas rather than axia expansion (figure 3), the groove is fied with cay tamping (humidity 15 %), over a ength of 100mm, on either side of the Charge (m a ong mortar shot, this amount of cay, used äs stemming, argey reduces the risk of firedamp ignition). We conducted severa tests m which the groove was covered with a cay pate. We obtained comparabe resuts to those of the Barbara experimenta mine. With a group exposive, a thickness of 10 to 0mm of cay was sufficient to prevent ignition of the firedamp when the ength of the Charge was between 00 and 500mm. However, making the cay pates was difficut and expensive. We decided to repace the cay by asbestos-cement, a materia which is commerciay avaiabe in pates of various thicknesses and which simuates rock fairy we. Both materias have süniar densities but behave differenty when an exposive Charge detonates into contact with them. X-ray fashes show that basting of asbestos-cement is faster than that of cay, for the same thickness of materia. Since the shot fümes are not contained äs ong, it can be expected that the asbestos-cement wi be a itte ess effective than the cay in preventing firedamp ignition. In our first tests, we used 80mm wide asbestos-cement pates.' Firedamp ignition occurred for the group exposives, regardess of the thickness of the asbestos-cement pate (6 to 60mm), even for sma charges (100mm ong). /5th second photographs, taken with an argon fash, showed that the basting fumes get between the asbestos-cement and the stee, causing a arge amount of uminosity on either side of the pate. This probaby expains the ignitions observed. By increasing the width of the pates, we then obtained resuts which had a coser resembance to those of the Barbara experimenta mine. The width of the asbestos-cement pate is now fixed at 00mm. We aso make sure that the pate is in dose contact with the stee. When the edges of the groove are too uneven for this to be possibe, we grind the surfäce of the stee bock or we change the groove. We varied the thickness of the asbestoscement pate so äs to detemüne the minimum thickness which does not resut in ignition after Oshots, for a fixed Charge ength. These tests were carried out with different batches of a group exposives. For these batches, the baistic mortar work ranged from 78 to 87% of the picric acid strength. For charges with a ength of 300mm or more, the minimum thickness varied consideraby from one batch to another, increasing with the baistic mortar power measured. For charges of 100 or 00mm, the resuts were more reguär -a minimum thickness of 6 or 10mm. We chose to impose an asbestos-cement pate thickness of 6mm and vary the ength of the Charge from now on m order to determine the Charge imit. -3-

5 At present, the groove mortar consists of a stee bock attached to a cart. The bock is 1.m ong and has a square section with a side of 0.35m. It has a groove on each of the four main faces, so that its Position can be changed by rotating the bock one quarter turn. The groove used is that facing the cosed end of the gaery when the mortar is positioned in the firing chamber (figure 4). During the test, the distance between the groove and the cosed end ofthe gaery is,5m. - Figures and 3 - Groove mortar; cross and ongitudina sections ; (a) stee bock; (b) cay tamping; (c) Charge; (d) detonator; (e) ug; (f) iron wire device; (p) asbestos-cement pate; (r) groove. THE CURRENT GROOVE MORTAR TEST Our firedamp test gaery consists of a 1m ong stee outer wa with a diameter of m, cosed at one end and open at the other. A sheet of paper, inserted in a sot ocated 5m from the cosed end divides the gaery into two sections - the Bring chamber between the paper and the cosed end, and the expansion chamber. The firing chamber, where the an- and methane mixtures are produced, has a voume of im 3. The sides are protected with stee armour at the cosed end and with an inside concrete coating on the cyindrica part. The asbestos-cement pate'used is 6mm thick, 00mm wide and at east 00mm onger than the Charge being tested. Every 100mm aong its main axis, the pate has two sma hoes (mm diameter), 30mm apart and symmetrica around the axis. A thin wire device (1mm diameter) through the two hoes is used to attach the Charge (cartridge diameter 30mm) to the pate. The wire device has two 10mm ong ugs to ensurc that the Charge wi be centred in the groove. The groove is fied with cay (15% humidity) aong 100mm on either side of the füture ocation of the Charge. The charge-pate assemby is then paced and hed paced with adhesive tape. After the methane-air mixture has been produced, firing takes pace. The ength of the Charge can take a series of vaues in an approximatey geometric Progression (..., 80, 100, 15, 160, 00mm,...). It is varied from one test to another, in order to determine the Charge imit. Tabe gives the Charge imit obtained for various group and exposives, äs we äs the density of the exposive and its baistic mortar strength (in % of the picric acid work). Exposives a, b, c, d and A are TNT sensitised exposives. Exposives e, f, g and B are NG exposives. Exposive h is an emusion. In group, exposives A and B contain approximatey 0 % sodium Choride. 4-

6 - Figure 4 - Test gaery with groove mortar in test position; (a) stee bock; (b) stee pating; (c) cart; (p) sheet ofpaper; (r) concrete coating; (v) stee she. Tabe Group ^ 1 JL Exposive a b c d e f S h A (batch ) A (batch ) A (batch 3) Density Baistic mortar work Charge ünit (mm) : r i6o 1160 f B (batch ) B (batch )

7 The Charge imit has a ength equa to or greater than 15mm for group exposives, and ess than or equa to 50mm for typica group exposives. The Charge imit (100mm) is between the two previous groups in the case of the emusion h which is not especiay designed to be a safety exposive. These resuts seem fairy we reated to the power of the exposives determined by baistic mortar. CONCLUSION Determining the Charge imit using groove mortar with a 6mm thick asbestos-cement pate is a method which coud repace our traditiona ong and short mortar methods for evauating the fameproofness of group exposives in the presence of firedamp. It has the foowing advantages over traditiona methods: - it can be considered more reaistic in the ight of information obtained äs the resut of tests carried out in various experimenta mines; - it is sufficienty reproducibe and enabes group exposives to be distinguished from cassica group exposives with ony a sma number of tests; - its use with powerfü exposives of the emusion type pose itte probem, since it uses a reativey resistant apparatus; - äs a resut of the ast two points, it is ess expensive. REFERENCES Audibert E., 1931, "The testing of exposives", paper presented at the First Conference, Buxton. Coward H.F., 196, "Research on the mcendivity of Coa-Mine Exposives : A Review", S.M.R.E. research report no. 10. Cybuski W., 1963, "Investigations on safety of exposives in presence of methane and coa dust, in coa and sandstone äs we äs in stee mortars", paper no. 4 at the 1th Conference, Aix es Bains. Loison R. and Thouseau G., 1954, "Risque d'infammation du grisou par e tir au mortier avec piaquette de bourrage", Annaes des Mines, (Engish version: S.M.R.E., paper no. 6,1955). Meerbach H. et a., 1961, "Versuche zur Zündung von Schagwettern unter Grubenbedingungen", paper no. 17 at the th Conference, Warsaw. Nagy J. et a., 1959, "Hazards of Cut-off Exposive Charges in Mutipe Basting of Coa", paper no. 19 at the Oth Conference, Pittsburgh. -6