PRODUCTION
SAFETY FOitum
Aluminum dust explosions. T o p , without stone barrier, flame p r o p a g a t i o n is occurring at right. Bottom, with stone b a r r i e r , flame propagation is suppressed
Flame Propagation in Aluminum Dust Explosions Prevented Combination of pressure relief vents a n d barriers of incombustible dust prove effective in suppressing flames T^VURING the recent war, explosions at "•^British aluminum powder plants b e came so common that the Ministry of Supply initiated experiments t o find means to prevent these explosions or to limit their consequences if prevention seemed impossible. These investigations showed that a dense cloud cf stone dust dispersed explosively ahead of the advancing flame front of the explosion would prevent t h e flame from propagating throughout, if t h e duct were fitted with a pressure relief vent between the point of ignition a n d t h e stone dust cloud. Aluminum powder was manufactured in British plants by blowing a jet of air across a jet of molten metal, the product being known as blown or atomized aluminum. T h e bulk of the product was d e posited in containers, while t h e dust w a s drawn from the c h a m b e r and collected in b a g filters. T h e various units of the plant were connected b y ducts which, while t h e plant w a s in operation, contained an airborne cloud of a l u m i n u m powder. T h e inputs of air a n d metal were ad-
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jjusted so that the amount of dust in the «air stream was below t h e m i n i m u m ex]plosive limit. However, it was impossible Ito avoid explosive concentrations in other ]phases of the process, particularly in those w h e r e t h e bulk of the dust was separated jfrom the air. Stringent precautions were Itaken to exclude any source of ignition w h i c h might initiate an explosion, b u t in :spite of this many explosions occurred in w h i c h workers w e r e killed a n d injured ;a n d which had a serious effect on production. It was observed in m a n y cases that explosions which began in one unit of the plant, spread along t h e ducts to other units, a n d the whole p l a n t b e c a m e involved. T h r e e methods of preventing t h e propagation of flame through ducts "were considered: t h e use of inert gases, t h e provision of pressure relief vents, a n d t h e use of barriers of various kinds actuated b y a relay. T h e experiments were carried out in an explosion gallery consisting of lengths of flanged steel piping of 10-inch internal
CHEMICAL
diameter, capable of withstanding pressures of 150 l b . per s q u a r e inch. T u b u l u r e s with screw caps for admission of d u s t and tapped holes for t h e a t t a c h m e n t of instruments w e r e fitted a l o n g the gallery. T h e pressure relief vent for v e n t i n g the pressure developed by t h e explosion was provided for by use of coaxial lengths of gallery separated by gaps. T h e m o v i n g dust cloud of aluminum was introduced into the air stream from a fan. It w a s ignited at the desired point by means of an igniter unit. T h e barrier clouds of stone dust were p r o d u c e d by enclosing t h e dust in paper bags. They were dispersed by a photoelectric relay which d e t e c t e d t h e flame of the aluminum explosion a n d released t h e stone clouds a h e a d of the flame. Flame suppression was judged visually. Tests were c o n d u c t e d in still air a n d in a m o v i n g air stream, t h e latter intended to simulate conditions in an actual blowing p l a n t . Preliminary results in still air indicated that a combination of pressure relief vents a n d explosively projected harriers of incombustible dust would b e necessary to suppress t h e flame satisfactorily in the moving air stream tests. Ten tests w e r e conducted at each of two different concentrations. F l a m e was suppressed in all tests, whereas flame speeds as high as 1400 feet per second were recorded at similar concentrations when t h e pressure relief vents and stone barriers were omitted. T h e complete results of t h e investigation h a v e been published by t h e Ministry of L a b o u r a n d National Service Factory Department in a bulletin entitled "Prevention of t h e Propagation of Flame in Aluminium Dust Explosions." Copies are available at 3 5 cents each from the British Information Services, 3 0 Rockefeller Plaza, New York 20, N. Y.
Accident Rate Down In 1 9 5 1 — S e v e r i t y Up Workers employed b y m e m b e r companies of the National Safety Council had fewer accidents in 1951, but t h e mishaps they d i d have were m o r e serious. I n d u s trial injury rates show a modest reduction in frequency but an equal increase in severity of 1951 accidents as compared with 1950. Twenty-seven of t h e 4 0 basic industry classifications reduced their frequency rates; 20 r e d u c e d severity rates. T h e accident frequency rate for employees in all industries submitting company reports to t h e council, based on t h e number of disabling injuries per million man hours, was 9.06 in 1951. This represents a reduction of 3 r v from t h e year b e fore. T h e communications industry rate of 1.79 ag;iin led all others. This rate represents a 13r/r reduction from 1950. T h e chemical industry with a rate of 5.48, a reduction of 6r/< from 1950, w a s ninth in the field of 4 0 . Communications also had the lowest severity rate with 0.09 d a y s lost per thousand man hours, a reduction of 2 5 % . Chemical industry was nineteenth with a rate of 0.85, an increase of 1 2 % . T h e allindustry figure was 0.97, a 3 % increase.
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ENGINEERING
NEWS
