Explosions of Dust Cause Of Big Disasters

Manawatu Times, Volume 61, Issue 6, 8 January 1936, Page 12

Explosions of Dust Cause Of Big Disasters

MUCH CARE NEEDED IN FACTORIES

Wherever dried grains, such, as wheat, corn, oats, or rice are handled in bulk—for example, in elevators, or wherever finely-ground products, such as starch, flour, sugar, sulphur, coal, or aluminium, are made or handled, all these substances give rise to a very flue dust, which, because of its chemical nature, is liable to form an explosive mixture with air. When conditions happen to be favourable, it may suddenly ignite, with disastrous results (says a writer in the Melbourne Age). Experience has shown that in ail such operations, particularly those on a large scale, great care must be exercised iu the design of plant and in the general factory operations. The Corn Products Refining Company, of Illinois, had one of the largest plants of its kind in America—it was exceedingly well designed and managed. The starch-dust hazard was known to bo real, and strict precautions were taken accordingly. Many engineers devoted their full time to safety problems; there was no smoking and no carrying of matches; sparks front external causes and from electrical discharges were rigorously eliminated; all bearings were regularly oiled, and so forth. And yet, on January 4, 1921, the worst dust explosion on record occurred, blowing the starch packing house to pieces, and killing 42 men. And inquiry revealed that a hot bearing was the cause, the oil hole having become clogged with starch.

Not to be Ended This instance serves to emphasise the danger associated with line dusts. At the time, it also served to bring about a more strict enforcement of the safety clauses in similar industries throughout the country. That that risk can never be wholly eliminated is evident from this instance alone, and also from the fact that only a few weeks ago a similar explosion occurred, this time in the Clidcr Soya Beans Products factory in Chicago, wheu about ten people were killed and sixty injured. “The explosion,” so the news item reads, “lifted the six-storey building from the ground, transforming it into a wreck of twisted steel and concrete.”

In Australia dust explosions are not altogether unknown. Some few months ago, one occurred in an empty grain silo, wheu a youth sought to enter in with a naked light. And some years back there was a rather disastrous coaldust explosion in Queensland, when several miners lost their lives. Tne mine, by the way, was quite free from fire-damp at the time. These explosions must not be confused with gaseous explosions, with which everyone is more or less familiar. A comparison, however, is quite apt, since the essential characteristics are much the same iu both cases.

In tho first place, the gas or the dust must consist of a substance which will burn or oxidise readily. It must bo mixed with air—that is, oxygen—a suitable proportions. A flame of some sort must, be introduced to start the action, the heat of which causes the sudden propagation of the flame throuthout the whole mass, with rapid expansion, production of pressure, and, in fact, the general characteristics of an explosion. Ideal Dust.

The more finely divided tho dust, the more dangerous it is likely to be. As the ultimato limit of sub-division is the molecule itself, wo ean actually regard a gas as a sort of ideal dust. Prom this point of view gases are in an ideal state for burning or exploding, and hence are invariably more dangerous than dust, although the latter are dangerous enough. All the factors that apply to infiamable gases are, with small modification, applicable to dusts. This even includes tho secondary detonations, which in the motor car en gine give rise to "knocking." This latter unpleasant effect is due to the explosion llame heating and compressing some of the gas . before it manages to set it alight, with the result that this part of the gas self-iguites or detonates, causing a sudden kick in the otherwise even explosion. All detonations and their harmful effects are entirely eliminated in dust explosions by a simple and perhaps peculiar device. The factories arc built with huge window spaces and glass roofs, so that the initial explosion will blow the glass to pieces, thus relieving the pressure and preventing the added kick due to a detonation.

In recent years considerable attention has been paid not only to be nature of explosive dusts, but to the conditions under which they are likely to become dangerous. ; The inflammability depends on the size and the concentration of the particles, their chemical nature and case of oxidation, the existence of a layer of absorbed oxygen on the surface of the particles, the thermal conductivity, and so forth. . . For example, wheat dust, which is produced by the rubbing together of millions of grains whenever wheat is being handled, particularly when it is being bundled about in a closed space like an elevator, has been subject to careful analysis. Its chief constituents are 34 per cent, of starch and 22 per cent, of crudo fibre or cellulose, both in a very fine state, with tho former tho more dangerous. Fifty Different Dusts.

In England, something like 50 different dust samples have been collected from various factories, including sugar, starch, cocoa, flour, cork, rice, mustard, wheat, elevator dust, sulphur, aluminium and coal, (Esjaerinentj with

these reveal ignition temperatures as low as 500 degrees centigrade, explosion pressures as high as 8001 b per squaro inch, and flame velocities of a mile a second, all serving to emphasise the perilous nature of inflammabie dust.

Speaking more generally, explosive dusts form but a particular example of a number or finely divided substances which can exist in the air. They include fogs, mists, clouds, smokes, and harmless as well as harmful dusts. They can all be produced on a laboratory scale, and as such have been subject to careful investigation. Tobacco smoke, by the way, is quite a favourite study. Of particular importance is the problem of smoke abatement in industrial areas. Apart from the usual settling chambers, dust collectors, and so forth, the most interesting modern treatment is tho electro-static precipitation oi Cottrell, a method which was first suggested just more than 100 years ago. The smoke is passed through a battery of sixty to a hundred,pipes, down each of which runs a high-tension wire, charged with anything tip to 100,000 volts. The steady, silent discharge from the wire electrifies the particles, which are immediately attracted to the insides of the tubes, and they are collected. This electrical cleansing has rapidly, come into favour, and in many cases the material recovered pays for the treatment. One such installation is to bo found in a sulphuric acid plant at Fort Pjrie; another at tho-, briquetting plant at Yallourn,