SAFETY IN MINES RESEARCH BOARD.
SPECIAL REPORT
ON
COAL DUST EXPLOSIONS.
PREFATORY NOTE.
ON 17th February, 1942, when announcing in the House of Commons the
appointment of a Court of Inquiry into the explosion which had occurred at
Sneyd Colliery, Staffordshire, on 1st January, the then Secretary for Mines
(Mr. David Grenfell. C.B.E., MP.) said:
"The present Chief Inspector on Mines is to prepare for me a report reviewing generally the colliery explosions of the past few years, and the Safety in Mines Research Board, at my request, is to review the plans and progress of its researches, which have been going on for many years, into the causes and prevention of coal dust explosions. These reports and the report of the Court of Inquiry will, I hope, provide a sound basis for a reconsideration of existing precautions in the light of the rather disturbing experience of recent years."
The Report of the Court of Inquiry referred to has been published as a Command Paper (Cmd. 6412) and the Special Report from the Safety in Mines Research Board, which had been received shortly before and covers new and different ground, is now also published.
The Report made to the present Minister by the Chief Inspector of Mines indicated that the tendency for explosions to be fewer in number but of greater severity had continued, and, taking its starting point the comprehensive Report on the subject which was made in 1938 by the Royal Commission on Safety in Coal Mines (Cmd. 5890), it drew attention briefly to certain features in which the perspective of that Report had been affected by the explosions of the subsequent five years.
It is not considered necessary to publish this Report of the Chief Inspector separately as the same ground been covered more fully by the Report of the Safety in Mines Research Board and the series of Reports on particular explosions which have been published since 1938.
MINISTRY OF FUEL AND POWER.
March 1943.
SAFETY IN MINES RESEARCH BOARD.
SPECIAL REPORT ON COAL DUST EXPLOSIONS.
Contents
I. COLLIERY DISASTERS IN RECENT YEARS.
For the' last twenty years at least, it has been generally accepted both in this and in other countries that stone-dusting is an effective safeguard against the spread, if not the initiation, of a coal dust explosion in a mine. In presenting evidence to the Royal Commission on Safety in Coal Mines in July, 1936 (the Commission was appointed in December, 1935), the Chairman of the Safety in Mines Research Board stated in the memorandum* which he submitted to the Commission and on which the Board's evidence was based that it had been conclusively shown by the Altofts experiments "that coal dust explosions can be prevented or suppressed by stone dust." This seemed to be confirmed by the experience of the industry up to that time.
Since the general adoption of stone-dusting for the prevention of coal dust. explosions, the spread of four or five major explosions in British mines has been attributed more or less definitely to coal dust, and there have been one or two doubtful cases and a few in which a firedamp explosion may have been somewhat extended by coal dust. These major coal dust explosions were at the Haig Pit (Whitehaven) in 1931, Wharncliffe Woodmoor (Yorkshire) in 1936, Markham (Derbyshire) in 1938, Valleyfleld (Fife) in 1939, and Sneyd (Staffordshire) in 1942.
With the exception of the first, all of these explosions occurred subsequently to the appointment of the Royal Commission. The generally accepted position, therefore, at the time of the sittings of the Royal Commission to hear evidence was that the methods of stone-dusting used in the collieries of this country had apparently almost eliminated coal dust explosions from the mines, and the Commission reported that " The treatment in this way of all roads in coal mines was made the subject of general regulations in 1920, and this form of treatment has stood the test in so far as it has, with only one or two exceptions, prevented the coal dust on the roads from taking any important part in the explosions which have since occurred." +
The work of the Board, they go on to say, had amply confirmed the basic soundness of the principle, but had also shown that the 50 per cent, standard of incombustible matter laid down in the Regulations would not suffice for coals of high volatile content. They did not discuss such matters in detail, for the Mines Department had then in hand modifications of the regulations which, after discussion with the interested parties of the industry, were agreed and published in 1939. The " 50 per cent, standard of incombustible matter was increased for coals of high volatile matter content, and the sampling of the dust on the roadways was to be made more systematic and more frequent (Precautions against coal dust, S.R. and 0. 1939 Nos. 1803 and 1804).
Though the recent experiences have been disturbing, the Board has no ground for disagreeing with the conclusions of the Commission. It has no doubt that stone-dusting, as carried out in the past, has not only prevented many explosions but has limited the spread of others. In the first place, statistics support this conclusion Thus the last column in the following tabulation of major explosions, in decennial periods, indicates in a general way that the average severity of all major explosions (gas and coal dust) was checked at about the time of the general introduction of stone-dusting (1921).
This table takes no account of changes in the numbers of pits or their size or methods of work, or the concentration of workers exposed to the effects of explosions, but such considerations do not affect the broad conclusion.
American experience is also valuable in this connection, for stone-dusting is not compulsory in all States. Harrington (U.S. Bureau of Mines Information Circular No. 6596, 1932) cited seventeen cases in which stone dust prevented or severely limited the spread of serious explosions.
Secondly, in each of the five named coal dust explosions since the Haig Pit explosion in 1931, the area of the explosion was limited to one seam and usually to one district; whereas previously there were many instances of explosions travelling from one seam to another, e.g. at West Stanley (1909) four seams were involved, at Pretoria (Hulton, 1910) three, and at Minnie (1918) two.
Thirdly, it is possible to find many instances of violent firedamp explosions which have not developed into coal dust explosions, and some of the credit for this may be claimed for stone dust. Among recent examples are Upton (1940), Crigglestone (1941), Barnsley Main (1942) and Murton (1942).
Finally, two quotations may be made from the reports on the Inquiries held into the explosions at Markham and Valleyfield. The report on Markham states (Cmd. Rept. 5849 p. 31): "While the feeling of dismay expressed by nearly all parties interested at the failure of the stone-dusting to prevent the spread of this explosion can readily be understood, I think it would be wrong if anything like despair were to replace the confidence which up till recently had accompanied the use of this neutralising element. The explosion in this mine, would, I am convinced, have been much more violent and much more extensive but for the stone dust that had been applied along its roads. The death roll might have been nearly doubled."
In the Valleyfleld report it is stated (Cmd. Rept. 6226 p. 14) that "It is of some significance that……analyses indicate that where there was an adequate percentage of incombustible material the force of the explosion subsided," and in the general conclusions (p. 15): " It is, in my opinion, established ... 2. That the spread of the explosion was caused by coal dust which had accumulated in certain roads. 3. That certain roads through which the explosion spread were insufficiently stone-dusted.''
There must always remain room for discussion as to whether and to what degree the failure of stone dust to prevent the spread of the five explosions named above was due not to failure of the remedy, but to failure in its application due, for example to lack of thoroughness in the application of the stone dust or to faults in its physical nature, but the purpose of the present Report is rather to re-examine the general question and to indicate how great a measure of reliance can be placed on stone dust to prevent disasters in future and what further research the Board is pursuing on the subject.
Contents
II. THE STONE DUST REMEDY FOR COAL DUST EXPLOSIONS.
Choice of the stone dust remedy.-The stone-dust remedy for coal dust explosions has at present no rival. It is impracticable to reduce the coal dust in mines, except locally and usually temporarily, to such a small amount that it would offer no danger, for about one-twentieth of an ounce per cubic foot of air is enough to form an inflammable cloud. It is also impracticable and in respect of other matters undesirable in most British pits, to maintain the roadways throughout so wet that coal dust cannot rise from them. A wetting solution containing calcium chloride may he used successfully to improve the walking conditions of a travelling road but this treatment hardly contributes to the prevention of coal dust explosions except by facilitating the cleaning up of the floors of such roadways.
Experimental results.-It is impossible in a short report to summarise even briefly the results of the enormous number of experiments made on this subject in surface galleries and in underground roadways and workings, in this and other important coal-mining countries. Certain features stand out, however. First, it has been proved that treatment with stone dust of all accessible places throughout the mine roadways is generally preferable to local "barriers,'' which, however ingeniously devised to raise a dense cloud of stone dust in front of the advancing flame of an explosion, may not begin to deal with the explosion at or anywhere near its source and are found by experiment to be not universally effective against explosions of the widely different characters and degrees of violence that may occur in the mines. Barriers are regarded as only supplementary precautions in the United States of America, where they have received prolonged investigation.
Secondly, the least proportion of stone dust to coal dust in a mixed cloud which will prevent propagation of an explosion depends very largely on the strength of the source of ignition, the chemical nature and the fineness of both the coal dust and the stone dust, and on the amount of firedamp (if any) which is present.
Thirdly, the proportion of stone dust to coal dust in a cloud raised in front of an explosion depends on intimacy, or lack of intimacy, of admixture of the dusts on the roadways. Coal dust lying on top of even a large excess of stone dust can be raised preferentially and thus propagate an explosion for at least several hundreds of feet in experimental conditions and doubtless also in similar conditions in a pit. Moreover, coal dust and stone dust laid in short alternating zones (representing irregular scattering of stone dust in a pit) have recently been shown to give rise to a cloud which is more explosive than that raised from the same amounts of well-mixed dusts.
Translation of the experimental results into practice. -A consideration of the methods of stone-dusting in pits will show that the ideal state of precaution, i.e., complete admixture of the stone dust and coal dust, is not and can not be attained in practice and accordingly that stone-dusting while it does provide a large, cannot always provide a complete, measure of security against some spread of coal dust explosions. The whole process of stone-dusting and the means of checking it are no more than a rather rough approximation over a far-flung area of roadways, in contrast, for example, with safeguards against mechanical and electrical dangers, which are precise, localised, and fully controllable and yet are never completely safe.
The intimate mixing of the two dusts which is so highly desirable cannot always be fully provided beforehand in pit conditions but must depend in part on the effects of the rush of air ahead of the flame in raising sufficient stone dust and mixing it with the raised coal dust before the flame of the explosion arrives.
Furthermore, it is not practicable as with mechanical dangers and even with ventilation, to provide any substantial factor of safety to cover a combination of unfavourable conditions, for even under experimental conditions where intimate mixture of the coal dust and stone dust is pre-assured and there is no firedamp in the air, the proportion of stone dust required to prevent an explosion is commonly as high as 65 per cent., and sometimes as high as 75 per cent.
Therefore, in translating the experimental results into practice it is difficult to provide any margin of safety and impossible to provide more than a small one.
It was no doubt from such considerations that, when the current Regulations (SR. and 0. 1939, Nos. 1803 and 1804) laying down certain minimum requirements for the amount of incombustible matter in mine dusts, were issued 1939, the covering letter (M.D. Circular No. 127) stated that the Secretary for Mines feels sure that colliery owners and managements, in accordance with the appeal which was made to them last summer by him and by their respective National Associations, will continue their efforts to achieve higher standards of stone dusting wherever these are desirable and practicable."
In one respect the experimental conditions are usually more severe than those underground, in that a much finer coal dust than that ordinarily found in the roads is used. When, however, there exists underground a rare coincidence of unfavourable circumstances (fine coal dust, poor admixture with stone dust, more or less firedamp, an intense source of ignition, physical obstructions which increase turbulence) the conditions may be more severe than those of any experiments yet made.
In these circumstances, the safe- guard of stone-dusting must be less sure in itself and more difficult of effective application than, for example, ventilation to dilute gas; and though improvements in the practice of stone-dusting at the collieries will bring full security nearer and nearer, they may not always ensure it.
The Royal Commission believed (Report 1938 p. 354) "that reliance on the efficacy of stone-dusting as a means of neutralizing the explosive quality of coal dust has had the effect of encouraging some mine-managements to take too complacent an attitude towards the accumulation of dust."
In their opinion that was to be regretted, for they felt sure that quite apart from the explosion risk, travelling or working in an atmosphere vitiated by clouds of dust, whether coal dust or stone dust, which are raised by the passage of men, must in the long run have a prejudicial effect on the workman's health, as well as being a nuisance and a cause of general discomfort."
Reference is made in paragraph III (g) below to the researches carried out recently by various organisations on the reduction of the dust nuisance, and the translation of the results of such researches to the pits should help substantially towards the attainment of safer conditions as regards coal dust explosions.
Definition by Regulations of requirements for stone-dusting. The development of preventive measures against coal dust explosions has been expressed in statutory requirements, from time to time.
In the period 1911-20, under the Coal Mines Act, 1911, section 62, the floor, roof and sides of roadways were to be cleared as far as possible to prevent coal dust accumulating; the roadways were to be watered or otherwise treated as laid down in Regulations; and they were to be examined daily and a report made on their condition as to coal dust and on the steps taken to mitigate danger arising therefrom. Visual observation alone was relied on. In the period 1920-24, under General Regulations of 30th. July, 1920 (S.R. & 0. 1920 No. 1423), the cleaning up of coal dust (except for a different purpose on main travelling roads) and watering as such were no longer obligatory, and stone-dusting took their place.
The new requirement, based on the experiments and recommendations of the Explosions in Mines Committee (Reports Nos. 1 to 7, 1912 to 1915) was that the dust on the floor, roof and sides should always throughout consist of a mixture containing not more than 50 per cent, of combustible matter. The check on the observance of this requirement was, however, no more than representative tests made once a month, and the tests each month did not, in fact, cover more than certain lengths of certain roadways. The requirement as to the daily examination of roadways on their condition as to coal dust and the report thereon fell into disuse; for although by such examination a competent observer could judge the efficacy of cleaning up and watering, it was an imperfect and sometimes very misleading means of judging quantitatively the adequacy of stone- dusting. That was to be done by sampling and analysis.
The monthly tests by this means could clearly not ensure that the roadways were properly stone-dusted, at all times and in all places, and the stipulation that the coal dust and the stone dust should always he mixed was not a practical possibility, but the notion grew that most of the dust on a roadway would be raised into the air as the flame of an explosion approached it, and that so long as the average composition of the dust was safe its pre-mixing was of little moment.
Some experiments of the Explosions in Mines Committee had been made with layered dusts (the coal dust being on top of the stone dust) and had shown that whilst a mild explosion could be staged in such a manner that the top dust would rise and propagate flame, the bottom dust was also raised as soon as violence developed. This was, in effect, to rely upon a coal dust explosion developing violence as the means of bringing the preventive agent (stone dust) into operation although, possibly, it was thought that the experiments represented extreme conditions that could not occur underground, and that coal dust would never, in fact, lie so deeply as to be raised in dangerous amounts without sufficient stone dust rising at the same time.
In the period 1924-39, the General Regulations of November, 1924, (S.R. & 0. 1924, No. 1364) required that the dusts on the floor, roof and sides were to be sampled separately and the average content of each of the samples was to be the criterion of the efficiency of the stone-dusting.
Since 1st January, 1940, General Regulations have abandoned the requirement of a mixture of coal dust and stone dust in favour of a requirement that "the dust which can be raised into the air shall be of a certain (average) composition." Yet the weakness of relying on "average composition" and the uncertainty as to what dust would be raised to feed the flame of an explosion were recognised by a stipulation that the monthly sampling should include only the dust collected to a depth of ¼ in. as near as may be on the roof and sides and a depth not exceeding 1 in. on the floor.
As the indefiniteness of the second of these figures indicates, there was no experimental basis for them and it was moreover, an assumption that the dust which can be raised is the same as the dust which would be raised to feed a flame or as the dust in the top inch or quarter inch.
The difficulty of defining the boundary between safe and unsafe dust conditions lies primarily in the fact that all tests in the pit must be indirect, i.e. that the condition cannot be tested by direct trial but must be deduced by comparison with experimental results and with the deductions made from necessarily incomplete observations after an underground disaster. In the present state of knowledge it is clear that the average content of incombustible matter in road dusts should be kept high, and that the higher it is the greater can be the confidence reposed in stone-dusting. Any lack of intimate mixture between the coal dust and the stone dust introduces however a corresponding uncertainty and in addition to a high average percentage of stone dust it is necessary for security that the surface layers of dust should be satisfactory. If, for example, as well as a correct average percentage of stone dust the top fiftieth of an inch were satisfactory day after day, no explosion of coal dust could occur (in the absence of a material contribution from firedamp). The nature of the top layer throughout a roadway cannot be ascertained by monthly analysis and can only be kept right by visual inspection and attention, perhaps daily or even from one part of a shift to another.
This analysis of the problem shows that monthly sampling of road dusts though still absolutely essential is not enough, and suggests that visual inspection is necessary daily and in some conditions even more frequently, and should be directed to the condition of the top layer of the dust and be followed wherever necessary by appropriate and immediate action.
It is necessary to emphasise that in the present state of our knowledge and experience this visual inspection can serve little more than the limited purpose of forestalling or detecting dangerous surface conditions of the dust, but there seems no reason why, with growing experience and with the aid of simple instruments, there should not be developed a technique of "examining for coal dust" comparable with the simple and effective technique of examining for the other explosion danger - firedamp. The need is for some simple method of sampling truly the dust on the roadways, and of making an approximate test of samples on the spot as one tests for firedamp with the flame safety lamp.
There is no proven way, but work is in hand by the Board's staff and the Board commend the matter to the attention also of others, and more particularly of colliery chemists, whose experience and ingenuity may find, or help towards, an effective solution.
As already indicated, the necessity for monthly sampling and analysis as a check on the "average composition" of the road dust remains, and in that respect also there is need for better methods. The simpler the procedure can be made and the more numerous the samples that can be taken, the more reliable will be the information as to the state of the stone dusting.
The speedier the procedure of taking and testing a batch. of samples the less will be delay in bringing to the notice of the management dangerous or precarious conditions which call for immediate remedy. Two simpler and speedier methods of examination of dust samples which have proved their worth in suitable circumstances are described below in paragraph III (f), but these are not the end of the matter and the Board looks forward to, and for its own part is seeking, something better.
Contents
III. CURRENT AND FUTURE RESEARCH ON COAL DUST EXPLOSIONS.
The evidence presented by the late Professor R. V. Wheeler, on behalf of the Board, to the Royal Commission reviewed very briefly the results of the Board's researches on coal dust explosions and indicated lines upon which it was hoped that the work would develop. These included a closer study of the constitution of coals or of their reactivities, to lead to a more certain measure of their inflammabilities than is provided by their volatile matter contents; certain studies on the dispersability and caking of incombustible dusts, and on the specific action of certain salts as inhibitors of the combustion of coal dust; and a further study of the wetting of coal dust.
The actual course taken by the Board's research has been different from this in certain important respects, which were indicated partly by questions raised acutely by the Markham explosion.
(a) Relative efficacies of incombustible dusts-The need for a more certain measure of the inflammabilities of the dusts of various coals has appeared to be of less immediate importance than the measurement of the efficacies of various incombustible dusts. Several series of careful trials have shown that, at least in various conditions of test in the 4 ft. diameter explosion gallery at the Buxton Experimental Station, limestone dust is somewhat more efficacious than shale, and that gypsum is more than twice as efficacious as shale. These results are to be checked by trials with a strong source of ignition in the large underground roadway at Buxton.
Common salt, in similar experiments, has proved to be from ten to twenty times as efficacious as shale, according to its degree of fineness. The value of salt is, however, much limited by its tendency to cake and even to " liquefy" except in very dry conditions, and the possibility of making use of its remarkable powers has not yet been fully explored, for attempts to make preparations of it which will resist caking have not so far given sufficiently promising results to warrant trials under pit conditions.
The commonly used stone dusts also have a tendency to cake though in far smaller degree than common salt. This tendency is somewhat greater with gypsum and limestone dusts than with most shales, and in consequence they may in some circumstances be less useful than a good shale dust. In the case of limestone dust, the tendency to cake has been overcome by a waterproofing process in which the particles of the dust are prevented from becoming damp and cohering by an exceedingly thin coating of a fatty acid. The waterproofed dust can be used to best advantage in damp places, for it remains dispersible for many weeks where any ordinary stone dust rapidly becomes caked and useless. It then neutralises freshly-deposited coal dust, which cakes slowly in damp places. The small extra cost of this waterproofed dust is amply off-set by its resistance to moisture. The waterproofing of other incombustible dusts for use in mines has not yet been developed as a commercial process.
(b) The layering of coal dust on stone dust-A second line of enquiry has been into the possibility that a coal dust explosion might be propagated when a thin layer of coal dust has been deposited on stone dust which would have been ample in amount, if uniformly mixed with the coal dust, to suppress an explosion. The results of the tests, made in the underground roadway at Buxton, are given in the Annual Report of the Board for 1938. In these tests, coal dust was laid one-eighth of an inch thick on top of an adequacy of stone dust on shelving, the roadway being usually too wet for pit conditions to be better imitated, and flame was propagated throughout the 700 ft. length of the roadway. Though, as explained in the report quoted, the relation between these experiments and mining conditions is not very close, the experiments are sufficient to show, in a general way, the danger of a layer of coal dust on an otherwise adequate amount of stone dust.
(c) The weathering of stone dust in use.-A third line of enquiry is into the question of the maintenance of efficiency of incombustible dusts during exposure to the atmosphere underground. The problem is whether with lapse of time (with conditions short of actual wetness) the dusts "compact" or cake to a degree which markedly impairs their efficiency and if so how this undesirable property may be eliminated or mitigated. This question is being approached by laying stone dust and coal dust in short alternating sections or zones in the 4 ft. diameter gallery, thus exaggerating the defects of imperfect pre-mixing of the dusts but allowing the stone dust to be separately exposed to the air in the only way to ensure reproducible and comparable conditions. The experiments have not been carried far enough for general conclusions to be drawn on the effect of weathering, but the preliminary experiments have shown incidentally that for two incombustible dusts freshly laid the quantity required (per lb. of coal dust) for preventing an explosion was nearly twice as much when the dusts were laid in zones as when they were uniformly mixed under the conditions chosen. This result emphasises the need for the closest possible mixing of the stone dust and the coal dust.
(d) Dispersability of stone dust. - Stone dusts are generally much less dipersable than coal dust and the Board have been aiming for some time to find means for improving the dispersabilities of stone dusts. The first stage was the development of laboratory tests which would give a numerical value to the dispersability of a dust when subjected to air-blasts such as precede the flame of an explosion. The second stage was the grading of stone dusts by means of these tests. The third stage, now in hand, is an attempt to discover the cause for the differences, to ascertain for example how far improvements in dispersability depend on the nature of the material and how far they depend on the method of producing the dust. The fourth stage will then be the production of highly dispersab1e stone dusts.
(e) Distribution of the stone dust underground. An alternative or supplement to the general broadcasting of stone dust underground has been the subject of many of experiments on the Continent and in America. So-called ''barriers" of dust heaped on supports from which it is discharged automatically in front of the advancing flame were first designed by Taffanel in France and were later elaborated in many forms in the United States, Germany and Poland. In no country is reliance placed on harriers alone however; at the best they are recommended as supplementary safeguards. The problem is to devise a barrier or a system of barriers that shall be operated at the right moment to be effective in the event of either a fast or a slow explosion.
The Boards officers experimented before the war with a new type of barrier in the form of a mortar from which an explosive propelled a quantity of stone dust just ahead of an advancing explosion. It was successful in stopping weak and strong explosions of firedamp in the 4ft. diameter gallery at Buxton. But failed to stop strong explosions of coal dust in the underground roadway. It is possible that other means to use the same principle would be more successful , but since the war it has not been possible with the depleted staff to pursue the subject, which, according to the American experience with barriers, would require very thorough and elaborate research.
(f) Examination of the condition of dust in mine roadways. Regulations have hitherto required that representative samples of the dust in roadways shall be taken according to certain prescribed procedures and shall be analysed to ascertain the percentage of incombustible matter present. The Regulations of 1939 called for a large increase in the number of samples to be taken and analysed as well as defining more closely than hitherto the manner and place of taking samples. To facilitate and expedite the work of analysis, the Board have examined and recommended to the Mines Department two processes (1) a physical analysis by an instrument known as the ''volumeter," (2) a "visual colour test" which will enable samples that are well above the prescribed minimum of stone dust content, to he passed on simple comparison with standard samples. Both these processes are on trial at a few collieries, and where they are adopted should not merely save labour but should have the important result of more quickly calling attention to an imperfectly stone-dusted part of a road. As a corollary, rapid method of taking a representative sample of road dust required. No generally satisfactory answer has been found to the question as to how deeply the dust on a roadway (especially the floor) should be sampled. What is wanted is a knowledge of the composition of the dust that will be raised by the rush of air preceding the flame of an explosion in its earliest stages. The Problem is complicated by the fact that a violent ignition will disturb the dust deposits more deeply than will a weaker ignition. Experiments on layered dusts quoted earlier suggests that it is the top ¼ in. or even one-eighth inch which is the most significant.
It is clear that in some circumstances, and perhaps more usually than is now realised, the nature of the dust raised in advance of an explosion or an ignition of firedamp is not the same as that shown by analyses of the road dusts as now required by the Regulations, and in addition to examining the changing state of dust in typical roadways underground it is proposed to make tests in the mine of the composition (physical as well as chemical) of clouds of dust raised by suitable blasts of air and to compare the results with analyses of samples of the road dusts taken in situ.
(g) Reduction in the amount of coal dust.-Whilst it is as a rule impossible to reduce the formation of coal dust in a mine to an amount that would present no danger, it is easier to safeguard small amounts than large amounts and all efforts to reduce the coal dust in a mine are therefore indirectly contributory to the reduction of the explosion hazard.
The Royal Commission on Safety in Coal Mines (Report, p. 498) recommended in their summary, emphasising the recommendation by italics, that "In dealing with coal dust below ground much more attention should be paid to preventing its formation so far as possible, collecting it at prolific points of origin, and removing or consolidating any which still remains. The practical problems involved in applying these principles should be actively pursued.'' Two pages later they call for investigation of the problem of limiting the production of dust on mechanised working-faces, for a joint committee to work out appropriate methods for dealing with dust-production at conveyor junctions and loading points, and for a study of the avoidance of dust- production from loaded tubs.
The joint committee suggested was in fact formed, but the war caused its meetings to be suspended. However, the problems mentioned have received much experimental study at the hands of investigators specially appointed by Committees formed in the various coal mining districts to investigate these problems. Their published reports are named in the list of references appended, but give an incomplete picture of the amount of attention given to the subject, for other districts have done work which is yet unpublished.
The Board itself is supporting by grants the work of the British Colliery Owners' Research Association on the suppression of coal dusts in pits. This wonk is being carried out under the direction of Mr. J. lvon Graham, and has so far been concerned chiefly with the following matters -the reduction of dust at loading points and during cutting in coal, the prevention of dust accumulations in the vicinity' of conveyor belt rollers, the reduction of dust passing into return airways, particularly from a mechanised coal face, and to a less extent the reduction of dust on haulage roads by means of water sprays fed from high pressure water mains and by mists.
A promising means for the suppression of dust produced in coal cutting and at conveyor junctions and loading points appears to be the use of a spray or mist of water, and the design and performance of various devices for spraying have been examined and improvements which have been made in them are under test. The most effective lay-out of such devices, largely a matter of trial and error, is being ascertained at first in the laboratory and later in pits.
Jets of water, in quantity much larger than that used in sprays, have been made to play on the jib, chain and picks during cutting, and have been successful in reducing the production of dust. A different method has recently been the subject of successful trials in anthracite mines: it consists of the injection of water under pressure through bore-holes into the coal face, with the object of wetting the dust in the "slips" and in zones of crushed coal. Trials of this method in bituminous coal seams are planned.
Deposits of dust are often found below the return belt of a conveyor, mainly near the rollers. Several devices for removing the dust from the belt as near as possible to the loading point, and gathering it in collectors, have been made and tested, but agreement as to the most effective means has not yet been reached. Means for the reduction of dust passing into returns, more particularly by the use of low-resistance filters which are kept clean by sprays of water re-circulated by a pump, are under trial in a pit.
Contents
IV. SUMMARY AND CONCLUSIONS.
Stone dusting unquestionably provides a large measure of security against the occurrence of coal dust explosions. The security can be progressively increased by control, and reduction in the amount, of the coal dust itself, and by improvements in the stone dust, in the degree and methods of the application of stone dust, and in the technique of examination and testing.
But the measure of security, large as it may be, will not be complete. Research into the coal dust problem must, therefore, go on and as far as war conditions permit, will go on.
Stone dusting is the only remedy in sight. It may be that some other remedy more sure or more easily applied will one day come to light as the result of further research, but for the present the chief road to a still higher standard of safety is the more effective application of stone dusting.
This depends on the individual manager of each mine; and the nearer he can bring the condition of his roads to the ideal, that is to a dominant superiority of stone dust over coal dust, the nearer he will have brought the mine to complete safety against coal dust explosions.
MALCOLM DELEVINGNE,
Chairman.
November, 1942.
* Minutes of Evidence taken before the Royal Commission on Safety in Coal Mines, p. 616. para . 2 (1936).
+ Royal Commission on Safety in Coal Mines Report, in (Cmd. 5890), p. 354.
Contents
LIST OF REFERENCES.
From the great quantity of publications on coal dust explosions, the following
have been selected as most pertinent to the present note: -
GENERAL..
Royal Commission on Safety in Coal Mines. Report, 1938, Cmd. 5890. 7s. 6d. (8s.), pp 20 - 22, 353-359.
Minutes of Evidence taken before the Royal Commission on Safety in Coal Mines, 1936. Certain daily parts are still available from H.M. Stationery Office. Particulars on application.
Coal Mines General Regulations (Precautions against Coal Dust), as temporarily amended S.R.O. 1939, No. 1803, 2d. (3d.) and No. 1804, 1d. (2d.).
Safety in Mines Research Board, Annual Reports, 1921-194I . Reports for 1939, 1940 and 1941, each 1s. (1s. 2d.).
Reports on the explosions mentioned on page 4 of this Report:
Haig Pit (Whitehaven, Cumberland), 1931, - Commissioner's Rept. Cmd. 3858. Is. (1s. 2d.).
Wharncliffe Woodmoor (Yorkshire), 1936 - Commissioner's Rept. Cmd. 5503. 2s. (2s. 3d.).
Markham (Derbyshire), 1938. - Commissioner's Rept. Cmd. 5849. 2s. 6d. (2s. 9d.).
Valleyfield (Fife), 1939, - Commissioner' s Rept. Cmd. 6226. ls. 6d. (1s. 9d.).
Sneyd (Staffordshire), 1942. - Commissioner's Rept. Cmd. 6412. 2s. 6d. (2s. 9d.).
PREVENTION AND SUPPRESSION of DUST.
The Reduction of dust in the vicinity of Underground Loading Points. J. I. Graham, D.G. Skinner and W. H. Walton. Trans. Inst. Min. Eng. 1937-8, 94, 260.
Reduction of Dust in Coal Mines. J. I. Graham. Proc. Nat. Assoc. Coll. Man. 1940, 37, 250.
The Reduction of Dust at the Coal Face. Dan Jones, Colliery Guardian, 1943, 166, 213.
Report of the No. 2 Sub Committee (Coal Dust) of the Yorkshire Advisory Committee on Mining Research. First Report, 1938. Second Report, 1939.
Reports on Coal Dust Research of the Midland Counties Colliery Owners' Association. First Interim Report, 1941. First Report: The Problem of Coal Dust deposited beneath Underground Gate Belt Conveyers, 1941. Second Report: The use of Sprays to suppress Dust on Underground Conveyors, May, 1942.
Reports of the Coal Dust Research Committee of the Monmouthshire and South Wales Coal Owners' Association. First Report: The Design and Testing of a Dust Collector for a Conveyor Loading Point in an Intake Roadway, n.d.
Second Report: Sprays, n.d. Third Report: The Suppression of Dust at the Main Gate Transfer
Point of a Conveyor District, n.d.
Fourth Report: The Removal of Dust Clouds from Main Gate Loading Stations by Utilisation of Waste Workings, 1942.
Fifth Report: Dust Reduction and Suppression on Machine Cut Faces, 1942.
EXPERIMENTAL EXPLOSIONS OF COAL DUST.
Stone Dust as Preventive of Coal Dust Explosions. Comparative Tests. S.M.R.B. Paper No. 13 (1925).
Coal Dust Explosions. The Effect of the Release of Pressure on their Development. S.M.R.B. Paper No. 14 (1925).
The Inflammation of Coal Dusts: The Effect of the Chemical Composition of the Dust. S.M.R.B. Paper No. 33 (1927).
The Relative Inflammability and Explosibility of Coal Dusts. S.M.R.B. Paper No. 48 (1928).
The Inflammation of Coal Dusts: The Effect of the Presence of Firedamp. S.M.R.B. Paper No. 64 (1931).
The Inflammation of Coal Dusts: The Effect of the Nature of Added Incombustible Dust. S.M.R.B. Paper No. 79 (1933).
The Inflammation of Coal Dusts: The Effect of the Fineness of tile Dust. S.M.R.B. Paper No. 95 (1936).
The inflammation of Coal Dusts: The Value of the Presence of Carbon Dioxide and Combined Water in the Dusts. S.M.R.B. Paper No. 96 (1936).
APPLICATION OF STONE DUST IN MINES.
The Application of Stone Dust in Coal Mines. S.M.R.B. Paper No. 2 (1923).
LABORATORY TESTS OF DUSTS.
Laboratory Methods of Determining the Inflammability of Coal Dusts. S.M.R.B. Paper No. 31 (1927).
The Relative Inflammability of Coal Dusts: A Laboratory Study. S.M.R.B. Paper No. 56 (1929).
A Routine Test of the Inflammability of Mine Dusts. S.M.R.B. Paper No. 68 (1931).
The Combustion of Coal Dust. S.M.R.B. Paper No. 73 (1932).
The Routine Method for Determining the Inflammability of Mine Dusts: A modified Form of the Test. S.M.R.B. Paper No. 87 (1934).
The Analysis of Mine Dusts. S.M.R.B. Paper No. 101 (1938).
LAYING of DUSTS BY WATER.
Dust Laying on Mine Roadways. F. V. Tideswell and R. V. Wheeler. Trans. Inst. Min. Eng. 1933-4, 87, I.
The Treatment of Dust on Travelling Roads. J. L. Hay and F. V. Tideswell. Trans. Inst. Min. Eng. 1935-6, 90, 213.
DISPERSIBILITY OF DUSTS.
Dispersibility of Mine Dusts. F. V. Tideswell and R. V. Wheeler. Trans. Inst. Min. Eng. 1937-8, 94, 140.
Further Notes on the Dispersability of Mine Dusts. F. V. Tideswell and R. V. Wheeler. Trans. Inst Min. Eng. 1938-9, 97. 176.
Stone Dusts of high Dispersability. R. Hattersley and F. V. Tideswell. Trans. Inst Min. Eng. 1941-2, 101, 367.
Publications for which prices are shewn may be obtained from H. M. Stationery Office.
Prices in brackets include postage.
|
