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EXPLOSION AT HORDEN COLLIERY ON 23rd MARCH, 1953

Report by H. Titman, B.Sc., Ph.D. of the Safety in Mines Research Establishment on the examination of the Plough Face at Horden Colliery and tests on the incendivity of frictional sparking possible with certain pieces of the equipment

1. - EXTENT OF FLAME AREA

The investigation started with the knowledge that there had been two distinct explosions, the first occurring at 9.30 a.m. and the second about 5¼ hours later.

The total area traversed by flame in the two explosions, as determined by direct observations and microscopical examination of specimens, comprised about 342 ft. of the intake outbye the face, the whole 480-ft. length of the face and about 390 ft. of the return. The total spread of flame was thus some 1,212 ft., part of this distance being traversed by both explosions.

The structural damage caused by both explosions was very slight. In the intake a seven-tray stone-dust barrier 348 ft. from the face had collapsed and from 264 ft. to 135 ft. outbye the face a few props had been displaced in an outbye direction. A fall of roof had dislodged props and bars adjacent to the discontinued No. 2 pack on the face. No structural damage was observed in the return but the frames of the two ventilating doors in No. 20 stenton were slightly displaced, in a direction away from the return.

The defining of the extent of each explosion depends mainly on the statements of the rescue teams and of the survivors.

The statements regarding the first explosion indicate that on the return the only damage was to the doors in No. 20 stenton; also, a deputy 450 ft. outbye the face was blown down by blast. A fire extinguisher placed in the return near the face by one of the rescue teams after the first explosion was not disturbed by the second explosion, confirming that the evidence of blast and spread of flame (390 ft.) in the return can only belong to the first explosion. On the intake, only one, the inbye, tray of the barrier was upset. In view of the minor burns suffered by the survivor stated to be at the hoist 210 ft. from the face, it seems unlikely that the first explosion could have spread much further than this.

As regards the second explosion, the undisturbed fire extinguisher indicates negligible blast at the return end of the face, and it is therefore indeterminable how far along the face the explosion spread. The sign of burning near the barrier in the intake and the upsetting of the remaining six trays of the barrier (342 ft. from the face) can be attributed to the second explosion.

The estimated extents of the two explosions are shown in Fig. 1 which indicates also the inert content of specimens of "explosion dust" (i.e. dust raised by the explosion and subsequently deposited) taken during the examinations of the affected area. It was not possible to identify separately the coked dust formed by the two explosions; sectioned coke deposits did not reveal any layering of the coke.

2. - EFFECT OF FLAME AND BLAST

The first explosion expended its main force in the return, and it would seem that this explosion was mainly of firedamp. with coal dust playing little or no part.

The second explosion, also mainly of firedamp. expended its force in the intake, and it is possible that coal dust at the face transfer point and on the conveyor and conveyor structure assisted the spread of this second explosion in some degree. The inert content of the explosion dusts in the intake increased so rapidly outbye the face that the flame of the explosion must have been weakened as it approached the barrier.

3. - DESCRIPTION OF THE STONE-DUST BARRIER

The barrier was of the light (or secondary) Dortmund type, and consisted of seven spaced trays, holding in all about 1,000 lb. of stone dust. The tray furthest inbye was 348 ft. from the coal face. The cross-section of the roadway showing the approximate mounting of the trays is shown in Plate Ia.

The structure of the barrier was said to be as follows. Each tray was 16 in. wide; since the complete barrier extended over a length of 48 ft., the gaps between the trays were on average 6½ ft. A tray was made up of two 6-in, widths and one 4-in. width of 1-in. deal planking. The planks were cut cross-wise into three sections 2 ft., 4 ft. and 6 ft. long. The 2-ft. sections were always located on the conveyor side of the road while the butt joints of the 4-ft. and 6-ft. lengths were staggered. The precise pattern of this staggering is not known. The planks forming each tray rested on wooden cross-members, some four or five in number, which were in turn supported by two light steel boiler tubes about 6 in. apart. These tubes were 12 ft. long and their ends rested on shallow grooves on wooden blocks fitted to steel brackets on the side props. With the exception of the brackets all parts of the trays were collapsible.

The stone dust used was a Durham dolomitic limestone containing about 80 per cent. of dolomite. The acid-insoluble material in one sample was as high as Il per cent. and the moisture content up to 2 per cent. Ash, after removal of carbon dioxide, was about 46 per cent. The fineness of the dust was quite satisfactory in that only a small proportion (less than 6 per cent.) was retained on a 60-mesh B.S. sieve. The dust was readily dispersable: that used on the trays, said to be the same as that used for general stone dusting at the pit, had been placed there five months prior to the explosion. The stone dust on the other barriers that had been erected in the district at the same time appeared to be in an easily dispersable condition and free of crust.

4. - EFFECTS OF THE EXPLOSIONS ON THE BARRIERS

A general view of the collapsed barrier looking outbye is shown in Plate lb. The appearance of the collapsed barrier confirmed that both explosions had been very mild. The steel tubes on which the trays had been supported were displaced only a few feet outbye. The trays themselves had been displaced outbye for varying distances. The furthest distance from the last tray at which any piece from the barrier was found was said to be 87 ft.. but no planks were found beyond 56 ft. A prop in the middle of the road which had been supporting a roof bar 8½ ft. outbye the first tray had been thrown down; there were, however, no marks on it to indicate that it had been dislodged by a violent blow.

5. - DISPERSAL OF THE BARRIER DUST

It was evident that it would be impracticable to determine how the stone dust which had fallen from the barrier was distributed along the rough and obstructed floor, but that a useful index of the distribution might be obtained from an examination of the dust lying on the conveyor belt. The conveyor had not been moved after the collapse of the barrier and, since the ventilation was low, little erosion could have taken place. The dust on the belt was therefore in the same condition as it had been immediately after the explosion. There was not much coal on the belt.

Moving outbye from the face the amount of dust on the belt up to the first tray was only slight. It was no more than 1/20 in. thick at a point 100 ft. inbye the first tray; there it was a mixture of stone and coal dusts, and was therefore not likely to contain barrier dust. It had the typical appearance of explosion dust. The first heavy deposit of stone dust on the floor was immediately under the first tray: there was much stone dust on the conveyor structure 3½ ft. further outbye, while the first deposit (1 in. thick) on the conveyor belt was at l0½ ft. Thereafter and up to 18 ft. outbye the last tray the dust on the belt lay in heaps up to 2 in. or more in thickness. It consisted of stone dust with a thin uniform overlay of dark explosion dust. In this region there were heavy deposits of barrier dust both on the floor and on the conveyor structure.

Moving further outbye the last tray the dust on the belt was once again fairly regular. Down the centre line of the belt it was at first about ½ in. thick, gradually thinning to 1/20 in. at about 280 ft. outbye the last tray. This distance marks the end of the zone over which the barrier dust was measurably deposited. Throughout the part of the roadway in the region of the barrier there was little fresh stone dust on the floor towards the sides, indicating perhaps that the planks fell inwards towards the middle of the road and deposited most of their dust there.

For some distance outbye the zone over which the barrier dust was deposited in measurable amount, the dust on the belt appeared to be an intimate mixture of stone and coal dusts, and most of it was probably explosion dust.

6. - DISTRIBUTION OF DUST ON THE BELT

The dust along the belt was weighed after brushing it off across the full width and in various lengths from 3 to 60 in. according to the concentration and regularity of the deposit. The weight distribution determined in this manner is shown graphically in Fig. 2.

The total weight of dust on the belt under the barrier was 73 pounds. The total. weight between the first tray and a point 286 ft. outbye the last tray was 125 pounds, this representing roughly the limits of spread of the barrier dust. Subsequent analyses showed that the amount of coal present in this dust was small. Approximately 58 per cent, of the belt dust in the affected zone lay under the barrier (48 ft.), 28 per cent, lay beyond the last tray over the next 34 ft. and the remaining 14 per cent. was blown further outbye. It appeared that no substantial quantity of dust had been deposited or sucked inbye the first tray.

Since the roadway was six times as wide as the belt the amount of stone dust on the belt should be about one-sixth that of the total weight of barrier dust. The fraction found was one-eighth, a result in reasonably good agreement with the estimate in view of the uneven fall of dust across the roadway and the position of the conveyor to one side of the roadway.

7. - SIZE AND COMPOSITION OF DUST ON THE BELT

During the fall of the barrier dust, its subjection to the blast would be expected to cause some degree of elutriation. The finer granules would remain suspended in the air for longer than the coarser ones; they would also acquire a higher horizontal velocity, and so would reach and in part be deposited further outbye. This is a matter of importance because it is only the dust which is in suspension that can quench the flame of an explosion, and the finer particles are the more effective.

Accordingly samples of dust from various points shown on Fig. 2 on the belt were analysed to determine the percentage of stone dust present and also its size distribution. The results obtained are shown in Table l.

TABLE 1

Analysis of samples of dust from conveyor belt for fineness and proportion of stone dust

	Location		Stone dust in	Fineness of stone
			"thro' 60-mesh	dust thro'
Sample	From face	From barrier	fraction"	240-mesh
Number	ft.	ft.	Per cent.	Per cent.
1	317	Inbye 31	42	84
2	353	Under barrier	54	90
3	361	Under barrier	100	61
4	369	Under barrier	100	62
5	378	Under barrier	100	63
6	394	Under barrier	98	63
7	406	Outbye 10	97	52
8	430	Outbye 34	90	69
9	482	Outbye 86	84	80
10	582	Outbye 186	82	76
11	682	Outbye 286	54	79
Bagged stone dust				65

The thick deposits of dust found under the barrier (samples 3, 4, 5 and 6) were substantially pure and unelutriated stone dust. The heavy deposit found up to 1 8 ft. outbye the last tray was slightly coarser and had had a proportion of its fine fractions stripped from it (sample 7). Samples 8, 9, 10 and 11 are seen to be rich in fine particles which probably came from the dust which was elutriated by the blast in its fall from the barrier. Samples I and 2 were of characteristic explosion dust.

An estimate of the amount of dust on the barrier that was dispersed as a cloud can be obtained by weighing the amount of dust on the belt outbye the point at which the last masses fell. When allowance is made for the road dust that was picked up in the explosion and afterwards deposited on the belt, a reasonable estimate is that about 10 per cent, of the barrier dust was dispersed: that is, about 100 pounds in weight.

8. - DISCUSSION

It was established from evidence that the barrier was dismembered and collapsed at some stage during the second explosion. The effect of a barrier upon the explosion that causes it to be dislodged depends on the time-interval between the upset of the barrier and the arrival of the flame at the barrier zone. We may divide such explosions roughly into groups depending upon the magnitude of that time-interval. If it is not too short, the action of the barrier would affect the flame, and indeed suppress it if a sufficiently large proportion of the dust was in the air to meet the flame. If it is so long that most or all of the dust has fallen before the flame arrives, the barrier dust deposited on the belt, floor and structure is an effective blanket for any coal dust trapped underneath it, so preventing its participation in spreading the explosion. There may even be redispersal of this new protecting dust to help in suppressing the explosion, whether of firedamp or coal dust. Finally, the barrier provides little or no protection if the time-interval is so short that the flame arrives at the barrier zone before the dislodged stone dust has had time to cover the whole cross-section of the roadway, and form a protective plug.

In the explosion at Horden, the bulk of the dust fell in masses and these would each take about second to reach the floor. Since all the trays were probably not tipped at the same instant and since, also, the dust from a tray would be released over a period of time, dust was probably falling through the air in the barrier zone for several seconds, heavily in places during the first two seconds. During most of this period the concentration of dust in the air was probably high over the whole cross-section of the road; towards the end of the period there would be a thick carpet of stone dust on the floor. The critical period was the time (about half-a-second) before the dust curtain completely covered the cross-section.

Unfortunately, it is not possible to say where the flame was when the barrier was tipped, or how long it then took to reach the vicinity of the barrier. It is known, however, that flame reached the barrier, but did not pass far, if at all, beyond it. From analyses of dust deposits, it must be doubted whether the road outbye the barrier could by its own dust have supported a coal-dust explosion. Although, therefore, it cannot be said that the barrier halted a developing coal-dust explosion, it is not unreasonable to conclude that it did at least reduce the spread of flame in an explosion dying through lack of fuel. Indeed, to the extent that it was called upon to do so, the barrier supplied adequate protection.