A History of Research and a Review of Recent Developments

62
grain is in the form of powder, which forms a suspended dust with an explosive
strength much higher than that of coal dust. Between 1962 and 1975 there
were 23 agricultural dust explosions, killing 8 people and injuring 49. The
main cause was sparks from welding processes in bucket elevators, chutes,
bins and pipes. Because of this a number of university experimental research
programmes were undertaken, using apparatus to generate uniform dust clouds
over a wide range of concentrations. In one of these studies, reported by
Enomoto [3.9], the apparatus consisted of a cylindrical explosion chamber,
diameter 270mm, 10 litres volume, that could be rotated to make a uniform
dust cloud within it. Ignition at the centre of the chamber was by 0.1 gram of
guncotton, detonated electrically. It was found that a maximum explosion
pressure of about 6.5 kg/cm 2 occurred at a dust concentration of approximately
1000 g/m 3 for cornstarch, potato starch and wheat flour. Maize and wheat
dust produced explosion pressures between 4.5 and 5.0 kg/cm 2 over a range
of concentrations between 1000 and 4000 g/m 3 . The rate of pressure rise for
wheat dust reached a maximum of 60 kg/cm 2 /sec at a concentration of about
1500 g/m 3 , but the average rise was about half this. The relationship between
the maximum rate of pressure rise (dp/dt) m and the ratio of the peak pressure
(p m) and initial pressure (p i) is given for many types of dust by the equation
and the average rate by
Propellant, dust, gas and vapour
Figure 3.1 Comparison of theoretical and experimental ignition points for coal dust
(from Kurosawa, ref. 3.8).
(dp/dt) m =0.7(p m /p i ) 2.5 ,
(dp/dt)av=0.3(p m /p i ) 2.5 .
(3.3)
(3.4)
An investigation of maximum explosion pressures of dispersions of sugars in
air was carried out by Meek and Dallavalle [3.10] in 1954, and has been