Xcivembpr, 1927
IN11 (ISTRIAL A N D I3NGINEI3Xl;VG ( ~--H ~ M I ~Y~ T i Z
1293
Apparatus for Studying the Ignition Process of Inflammable Gas-Air Mistures by Explosives’” By G . St. J. Perrott and U. 13. Gawthrop I’iirsuonali l ! x , ~ s i ~ * u i i ssi*Tio*(, i. 1:.
5.
il”lil*ii
o v -\IINES, PiTTSeilniili, PA.
An upparutus is described f o r photogruphilzg on a nrodiiig filiiz the plzenmnena taking place when a charge of explosive i s fired izzto a combustible gus-air &xlure. Typical plzotographs of ignitions are shown. XI’LOSIONS caused by the ignition of gas and dust in coal mines are often the cause of appalling loss of life and great damage to property. These disasters may sometimes be traced to the explosives used in blasting the coni. particularly when black blast,ing powder is employed. “Permissible” explosives have been developed to meet the need fur an explosive less likely to ignite the explosive mirturcs of pas and air or coal dust with air often present in coal mines. An explosive becomes permissible when it has passed a series of prescribed tests made by the Bureau of Xines a t the Explosives Experiment Station. Bruceton, Pa., and it rcrnnins permissible in blasting coal only when used in the prescribed manner. One series of tests, known a s “gallery tests,” requires that a certain weight of explosive Inast not cause ignition of ail explosive rriixtnre of gas and air, or gas, coal dust, and air when the charge is fired from a steel cannon into the gallery. The gallery test in slight,ly modified f o r m is the main criterion of safety of an explosive as an initiator of mine explosions in England, Belgium, and Germany, as well as in tlie Uuited States. It has been criticized as “empirical ” bot it bas not yet been found possible to express t.he relative safety of explosives as initiators of coal-mine cxplosioris in terms of properties expressible i n fiindamental units as, for example, Aaine temperature, rate of detonat,ion, or pressure. A s part of a program of research which is hoped to lead to quantitative evaluation of the several factors affecting such safety, the apparatus described in this paper was devised as a means of mapping out the ignition process irom the time the flame of the explosive issues from the borehole to the moment of ignition of the idammabk? mixture and the propagation of flame through the mixture. Description of Apparatus
The apparatus (Figure 1, A and B ) consists of a large steel tube or gallery made of %-inch (19-mm.) boiler plate. The gallery is F.1 metem (20 feet) long and 1.93 meters (6 feet 4 inclies) in diameter. At one end a steel cannon is mounted on n truck in such a position that a charge of explosive may be fired from the cannon through a hole in the center of tlrc gallery. The borehole of the cannaii is 5.7 em. io dinmetcr and 55 em. in leiigth. A horizontal slot: 3.89 meters long. cut in the shell of the gallery permits photojyaphs to be tnkcn nf the travel of flame along the center line of the gallcry. The slot is closed by twenty plate glass windows, 15 CUI. long, 10 em. wide, and 1.9 em. thick, so spaced that there i. a distance of about 3.5 cm. between each vinriow. I3craosc of the :mgk between caiinon end of the gallery a i d t,lie cam~ria Iciit: Llio first wiiirlow d o ~ snot show on the photograplis. ‘The cannon renders the gallery gas-tight at one end and a paper diaphragm (Figure 1, R) closes tlie climber a t a point just, beyond the twentieth window. At a distance of G.1 1 Presented before the Division of Cas nnd F u e l Chemistry at t h e i 3 i d Meeting of the Arnericsn Chemical Society. Richmond. Vn., April 11 to 16, 1927. Published with approval of t h e Director, U. S . Bureau of Minen.
meters from t,lie gallery is mounted 3 rotating drum camera which makes it possible to take photographs of the ignition process on a moving film. The d m n is 0.5 meter in circumCerence. A peripheral speed of 15 meters per second has been employed in most of the work hero, and this is inensured by means of a tachometer of the vibrating reed type, connected electrically to a commutator segment on the drum shaft.. Procedure
In making an experiment the paper diaphragm is first put in place. The cannon is then loaded with a charge of tho desired explosive, rolled up against the gallery, and the
h
INDUSTIUAL AND ENGINEERING C H E M I S T R Y
1294
Vol. 19, No. 11
charge (indirect initiation) and the explosive was loaded "density-of-one"-that is, it was squeezed hack into the hole so that no air space existed between the borehole walls and the charge of explosive. Figure 2,A is a photograph taken with the camera facing the side of the gallery and shows the passage of flame along the length of the gallery. The film is moving downward, and time is measured from the instant the flame appears a t the month of the borehole. The flame from t.he explosive is projected a distance of about 0.8 meter and has a duration of about 1.1 milliseconds. There is then an interval of 1.7 milliseconds during which there is little visible reaction, after which flame appears in the third winduw and is propagated along the gallery to the seventh window a t a speed of ahout 500 meters a second. The intensity of the reaction ie somewhat lessened until at 6.4 milliseconds from zero time a wave of reaction begins a t the third winduw and is propagated back to tlie cannon and forward to about the eighth window a t a speed around 500 meters per second, There is some evidence of a third phenomenon of this kind and then a t the tenth window the flame is propagated slowly, but at. an accelerat.ing speed which averages GO meters a second between the tenth and thirteenth windows. The explanation of this phenomenon may he deduced from Figure 2,B as. follows: Explanation of Reaction Phenomenon When a charge of explosive is detonated in the borehole of a cannon, a spherical pressure wave first issues from the cannon and close behind it are the expanding gaseous products of the explusiorr. Tlme products are rapidly cooled by expansion, as iu evidenced by the diminution of flame intensity in Figures 2,A and 2,B. During this period some mixing with the atmosphere of the gallery takes place, Blgure 2-50-Gram
Charee of 40 Per Cent ShsUht Nitroglycerin Dynamite without Stemmin~
electrical connections are made. The required amount of natural gas is admitted into the gallery and mixed with the air by a circulating fan. A safety switch in the firing line is then closed, the drum is brought up to speed by means of a cont.ro1 rheost.at.,arid the charge fired by opening a switch which releases a metal plate or shutter placed above the lens and arranged to drop vertically in front of tlie Icns. As the shutter falls it closes the firing circuit through a firing contact on the drum shaft and then corers the lens after the drum has made one revolution. 1)uring this period (30 millisbcondsi the flame has usuallv trareled the entire length of the gallery. When it is desired to ubotomauh the iatcral tritvcl of tlie flame, the camera, suitjbly proiecterl, is mountcd facing the end of the gallery. A paper diiiphragm liaviiiq it horizontal cellophane slot across the center is employed aud tbe photograph is taken through t,liis slot. \A sertical slot and camera with a drum mountcd on a vertical axis may also bc employed. Photographic Results of 50-Gram Charge
Typical photographs are shown in Figures 2 to 4. Those in Figure 2 are of a gas-ignition caused by firing a charge of 50 grains of 40 per cent stmight nitroglycerin dynamite, (composition: H,O, 0.7; nitroglycerin, 39.3; NnKOl, 43.8; anti-acid, 0.6; carbonaceous material, 15.6 per cent; 5.5 g r a m wrapper per 100 grams of explosive) without stemming, into the gallery which contained a mixture of 8 per cent natural gas (approximate composition: CH,, 88; C. Ha, 7; CaHa, 2.5; C,H,,, 1; N2,1.5 per cent; "8 per cent" means 8 per cent CH,+C,H, as calculated from an Orsat analysis) and air. The electric detonator was a t the back of the
INDUSI'RIAI, A N D ENGINEEE'ING Cf3EMISTEY
November, 1927
and when the pressure wave reflected from the gallery walls passes through the gases they are again raised to the eomhustion temperature. The path of this wave and its speed are shown in Figure 2,B, beginning at a time near 3 milliseconds. It is nioving at a s p e d of around 750 meters a second. There is evidently an actual compression of the reacting gases as evidenced by the narrower width of the flame at a point around 4 milliseconds. There is then another exDansion which reaches its fullest &tent at about 5.8 milliseconds. During this expansion the reaction of the gases is again diminished, as shown by the dark area in the photograph. The compression which again follows acrrl~ratesthe reaction. 0 05 f Figure 2,C shows an interestinE Distance. meters nhot&ranh of a similar shot fired w u r e 4- 1 5 0 - G r s m Into tce 'gallery when it contained Charge of Petml8aible Ex"tneius ~ i . ~ r. a. nl. m.. ...~ ~ t -.. ~ onlv t ~air. . The ~ ~gases from this exGas plosive contain considerable quanti2!: 2: ties of carbon monoxide and hydrogen, and they are ignited in an "after-flame" similar in characteristics to the ignition taking place when the gallery contains gas. When the cannon is fired into the open air, where unlimited facilities for expansion exist, au after-flame is also produced, hut the compressions and rarefactions noticed in the gallery photographs do not take place. The photographs Figures 2,B and 3,B have h e n trimmed by making use of the data for position of flame from Figures 2,.4 arid 3,A so that the edge of the print represents approximately the position of the gallery walls with respect to the flame. It is evident that, when the camera faces the end of the gallery, a given lateral distance at the far end of the gallery or cannon end will produce a smaller image on the film than the same distance at the near end or diaphragm end. Hence, as the flame movm toward the camera the position of the gallery walls a t the flame front moves outward.
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The dark lines in Figures 2,B aiid 3,B are opaque pieces of paper pasted across the cellophane slot in the diaphragm. Disappearance of these dark lines indicates the breaking of the diaphragm. Photographic Results of l5O-Gram Charge Figure 3 shows similar photographs with a 150-gram charge of the same explosive. The flame from the explosive is projected a distance of 1.5 meters and has a total duration of about 1.5 milliseconds. There is then an interval or lag of about 1 millisecond when little flame is evident, after which flame appears in the fifth window and is propagated along the gallery a t a speed around 700 meters a second to the ninth window, after which it progresses no farther because of the heginning of a lateral expansion of the gases, as shown in Figure 3,B. There is evidence of a second and third similar phenomenon, after which the flame is propagated along the gallery at a slowly accelerating speed averaging ahout 130 meters a second. The bright areas which appear periodically at the firat windows may he the result of pressure wave8 reflected from the diaphragm, hut are more probably caused by the alternate compressions and expansion8 indicated in Figures 2,B and 3,B which tend to force the gases back toward the cannon as well as ahead toward the diaphragm end of the gallery. These are the subject of further study. Figure 4 shows a charge of I50 grams of a permissible explosive which did not cause a gas ignition. A future paper will give detailed results of shots of permissible explosives of various degrees of safety, fired both with and without stemming, into mixtures of gas and air and coal dust and air, and will endeavor to show the meaning of these ignition pictures in terms of the relative safety of the explosives. Acknowledgment
Thanks are due J. E. Tiffany, explosives engineer of this Station, who designed the gallery and moving film camera.
Improved Micro-Kjeldahl Ammonia Distillation Apparatus' By George Kemrnerer and L. T. Hallett Tim i:mv~$rsiru 01 I~ISCOXSIN, hlnnisou, WIS.
N STUDYIKG the fertilizers present iii lake water the Kjeldahl nitrogen was determined in t.he field, using samples that had been stored for only a few hours. The difficulty of transporting arid using the regular Kjeldahl apparatiis iii t,he field suggested the determination of the Kjolduhl nitrogen in the residues obtained hy the evaporation of the lake water. Since most of the lakes of northern 1%consin contain soft Knter, the small residues obtained ilcmandad a micro method.
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Preparation of Samples
Samples uf the water residues obtained by evaporating several liters of the lake water a t 80" (:. were dried at the same temperature. The dried material was then removed from the porcelain eraporating dish, usiiig great care to remove the last traces. These samples were then weighed as a whole and stored in small homeopathic vials. 1
Kcceircd
Jliiic
13, 1027
Usiug a micro balance, l0-mg. samples uf this residue were weiglied on small cover glasses riiadc by cutting the ends from test lubes. Each cover glass and sample was then placed in a small Pyrex digestion tube, and 2 cc. of sulfuric acid, 0.,5 gram of potassium sulfate. and a small crystal of copper sulfate were added to each digestion tube and tlre samples digest,eil over micro buriierfi mtil practically colorless. which required from 1 to 2 hours. The amnioiiia was then distilled horn tlie snmplcs. The cover gl helixd to premmt humpiiig during digestion. Divtillatiuri
The preliminary work WRS carried out with a distillation apparatus designed by Parnas and Wagner and recommended by l'regl.s The results were accurate and agreed well with the Kjoldahl determination carried out on the original water. The apparatus, however, has several rubber connections which soften very quickly when in contact with steam and 8
"Q,,antil&tive Oreanic Micrornslysis." 13. 97,irandhted by Pyiemnn.
