TRANSACTIONS

OF THE

ROYAL SCOTTISH SOCIETY OF ARTS.

On Flour and Malt Dust Fire-Explosions.

By STEVENSON MACADAM, Ph.D., F.R.S.E., &c., Lecturer on Chemistry and Consulting Analytical Chemist, Edinburgh.*

TWENTY years ago, at the corresponding meeting of the year, being the second meeting held in November 1872, I read a paper before the Royal Scottish Society of Arts on flour-mill fire-explosions.

The subject was a novel one. Little or no information on the matter had been previously acquired or published, so that no assistance could be obtained from any prior investigations. The special occasion which had then directed my attention to the subject was stated to be a violent, disastrous, and lamentable fire-explosion which had occurred in a flour-mill in Glasgow. A large building of five floors, besides a sunk flat and attics, had its gable walls blown out, and was fired from top to bottom. The operations which were being carried on were of the most peaceful description, and were simply the cleansing and grinding of grain and the dressing of the flour. All the processes were proceeding in the most regular and harmonious manner, and yet in a space of time not greater than what was required to describe it--in a few seconds-two explosions took place; the mill was in ruins; the débris was simultaneously fired; the site of the mill was a roaring furnace; and, lamentable to relate, more than a dozen human beings were killed-in fact, out of twenty-seven workpeople engaged in or near the mill, fourteen were instantly killed, one was mortally wounded, eleven were more or less severely * Read before the Society, 28th November 1892.

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burned, and only one, who was on an outer gangway, escaped unhurt.

The investigation proved the following general conclusions:

(1) That fine flour-dust, when diffused through ordinary atmospheric air and lighted, formed a violently explosive mixture.

(2) That under certain conditions, such as the feed going off and the stones setting down on each other, or the accidental introduction with the feed of a foreign body, such as a piece of iron, a flinty piece of rock, or a common match, the millstones would strike fire, and lead to the explosion of the mixed flour-dust and air.

(3) That the explosive force of the most suitable mixture of flour-dust and air for forming this fire-explosion was capable of producing a pressure of 8 atmospheres, or 120 lbs. to the square inch; and

(4) That the ordinary daily conditions under which many flour-mills were being worked in 1872 conduced to the recurrence of similar explosions of a more or less disastrous nature from time to time.

The novelty of the subject naturally and properly led to an animated discussion at the meeting of the Society, which was adjourned, and renewed at the succeeding meeting.

The War Office Authorities afterwards communicated with me on the subject, and I gave them the fullest information.

Suggestions were also given by me in the paper for obviating such explosions in flour-mills, or at least for reducing the results to a minimum; and the proposed precautionary measures were adopted by some mills in the country, and the subject thus passed from the theoretical stage in which it was assumed to be when I read my paper before the Society, and to a limited extent became a practical question.

Since 1872, explosions of a more or less disastrous nature have continued to occur in flour-mills, and, where investigated, such have been apparently due to the want of observance of the proper precautions.

Six years after my paper was read-in 1878-attention was

specially directed to the matter by a series of explosions which occurred at Minneapolis in U.S., where the Washburn Cornmills suffered from a fire-explosion due either to the millstones running empty, or to a foreign body getting between, and such inflaming of the flour-dust and air led to explosions in the dust-chambers and thereafter in the general body of the mill itself. This explosion was remarkable as being the primary cause of firing and exploding a second mill-the Diamond Mill-situated 25 feet distant, where apparently the disturbance of the air by the first explosion had raised the flour-dust, and mingled it with the air of the second mill, and then fired it. Still further a third mill-the Humboldt Mill-was almost immediately fired and exploded; and, finally, other three flourmills, situated at a distance of about 50 feet, were fired, if not exploded. Altogether, property of the value of £200,000 was destroyed, and, lamentable to state, eighteen lives were lost.

Again, in 1881, nine years after my paper was read to the Society, a disastrous explosion occurred at a flour-mill in Macclesfield, in England, which laid a large part of the mill in ruins and one man was killed. The Coroner communicated with me on the subject, and I gave him all the information I possessed in regard to such explosions. Mr Thomas J. Richards, Government Inspector of the Board of Trade, also corresponded with me on the matter, and I gave him full reports on all questions relating to such explosions. He then acted as assessor to the Coroner during the inquiry, and subsequently issued a report to Sir William Harcourt, as Secretary of State, and which was presented to both Houses of Parliament.

In this report the Government Inspector states very clearly the different views still held then (1881) by millowners regarding these flour-mill explosions. He says:-" Since being directed to investigate the cause of this explosion (Macclesfield), I have made inquiries of a number of millers to ascertain the particulars of explosions which have occurred, and the causes of them. Some informed me that, in their opinion, the causes of explosions in flour-mills have not yet been satisfactorily ascertained. Whilst some millers doubt whether explosions can be produced in stive-rooms by millstones running empty, others disbelieve the possibility of

explosions occurring there altogether; others again, whilst admitting this, are of opinion that the presence of some explosive gas, or extra pressure of air due to the action of the fan, is necessary to cause it, or that a certain electrical condition of the atmosphere must exist. One person of experience informed me that, in his opinion, a stive-room could at all times be safely entered with a light, except when there was observed in it the peculiar odour which is noticed there when one of the millstones has been previously running empty. The fact of such opinions being held is to be regretted, as explosions in corn-mills are liable to be regarded as mysterious, to which they have, in my opinion, no title. The precautions, also, which may obviate or lessen the effects of explosions are liable to be neglected." Then he says:"Professor Macadam some years ago published in a valuable paper the results of some experiments he had made, proving the explosibility of flour-dust when diffused in air, and the views he enunciated are accepted by many millers. Finding, however, amongst some practical millers of long experience the existence of the opinions mentioned above, I considered it desirable to satisfy myself by actual experiment."

The Inspector then details various experiments he made, which were practically a repetition of those which were recorded in my paper, and the results of which entirely corroborated my own experiments and conclusions. He thereafter makes reference to the precautionary measures which were embodied in my paper, and commends their employment for averting and lessening such explosions.

The fire-explosions specially referred to occurred in flourmills, and the explosive mixture was flour in a minute state of division; in fact, flour-dust and air which had been fired by millstones which had gone off the feed, or into which some metallic or flinty particles had passed.

I was quite aware at the time of reading my paper in 1872 that minor explosions had occurred from time to time in breweries and distilleries during the grinding of malt between millstones, and where malt-dust diffused through the air had been fired and exploded, leading to damage to roofs, &c., and occasionally to the scorching of attendants; but these were

minor explosions, probably owing to the less fine state generally of the malt-dust as compared with the flour-dust, and also, no doubt, to the want of special dust in stive-boxes or chambers. Such minor explosions have been also repeatedly noticed since 1872. But it has remained till 1892 for a major or serious explosion to occur in a distillery where malt was being ground, and where a disintegrator, not millstones, was being used.

When the inquiry was placed in my hands, I was informed generally that an explosion had occurred in connection with the disintegrator, shortly after starting such, and before the feed was on, and that a blast of flame had travelled through various floors of the mill, doing much damage to the mill, and scorching more or less severely a number of the workpeople.

The explosion was centred in the grinding-mill proper, the ground floor of which was mainly occupied by machinery, including a powerful disintegrator; the first floor contained five pair of stones, and malt and grain bins; the second floor, including grain and malt bins, with a passage in centre; the third floor also with grain and malt bins and a passage in centre; and the fourth floor, with a stive-box connected with the elevators, box coverings of various bins, and numerous shoots.

Communicating with the mill proper there were :—

On the Ground Floor.-Two doorway openings through the north wall leading to the kiln-furnace vault, neither of which had doors. The kiln-furnace vault immediately adjoining was provided with two iron doors to the outside, but opening in halves inwards, one on the north side and the other on the west side, and opposite the latter there was a doorway on the east side leading into the grain stores.

On the First Floor.-No door communication existed between this floor and any building north or south of the mill proper, but a spiral staircase passed through the floor at the north end.

On the Second Floor.-Oue iron door was constructed on the north wall leading to one of the kiln floors, and the iron door on the south wall leading to the second-floor store.