Cause and prevention of dust explosions in grain elevators

Powder Technology, 8 (1973) 1 3 7 - - 1 4 7 
© Elsevier Sequoia S.A., Lausanne -- Printed in T h e N e t h e r l a n d s 
Cause and prevent ion o f dust exp los ions in gr~in elevators* 
and f lour m i l l g * * 
O.F. THEIMER 
Becker-Gundhal-Str. 16, 8 Munich 711 (Germany) 
(Received Oc tober I 0 , 1972; accepted Apr i l I 0 , 1973) 
S u m m a r y 
Grain and f l o u r dus t s can f o r m e x p l o s i v e 
c louds and, because e x p l o s i o n s f r o m such 
c louds have caused s o m e o f t he w o r s t indus- 
trial acc idents , it is necessary to d r a w p u b l i c 
a t t e n t i o n to this p r o b l e m . S ta t i s t i c s o f d u s t 
e xp lo s ions in t he U n i t e d S t a t e s s h o w tha t 
m o r e than 5 0 % o f all e x p l o s i o n s o f c o m b u s - 
t ible dus t s have o c c u r r e d in grain e leva tors 
and f l our mills. D u s t s f r o m w h e a t and o t h e r 
grains, as we l l as f r o m f lour , ign i t e a n d p r o p - 
agate f l a m e s readi ly because t he source o f 
hea t r equ i red is r e l a t w e l y small. T h e a u t h o r 
ana lyzes c o n d i t i o n s a n d causes o f d u s t e x p l o - 
s ions to d e t e r m i n e crit ical c o n c e n t r a t i o n s o f 
dus t and air a n d t he t e m p e r a t u r e s necessary 
to ignite an exp los ion . H e descr ibes the e f f e c t 
and d e s t r u c t i o n o f d u s t e x p l o s i o n s in e x a m - 
p l e s o f r e c e n t disasters in t he U n i t e d S t a t e s 
and E u r o p e a n d r e c o m m e n d s p r e v e n t i v e mea- 
sures, as we l l as s t ruc tura l f e a t u r e s o f bu i ld ing 
and p lan t , to avo id t h e m a n y injuries a n d 
deaths, as we l l as mater ia l losses w o r t h mil- 
l ions o f dollars, resu l t ing f r o m such e x p l o - 
sions. 
* In the Uni ted S ta tes and Canada, the bins for bu lk 
s torage o f grain are called "gra in e levators" , whereas 
in E n g l a n d a n d c o n t i n e n t a l Europe t hey are cal led 
"'grain si los". In t h e United States , the word " 'si lo" 
per tains pr imar i ly to fa rm storage tanks used largely 
for si lage. 
** Cont r ibu ted b y the Materials Handl ing Engineering 
Divis ion o f The Amer ican S o c i e t y o f Mechanical 
Engineers for representa t ion at t h e 2 n d S y m p o s i u m 
o n S t o r a g e and F l o w o f So l ids , Chicago , I l l . , S e p t e m - 
ber 1 7 - - 2 0 , 1 9 7 2 . 
INTRODUCTION 
E x p e r i e n c e a n d l a b o r a t o r y t e s t s h a v e 
showx~ t h a t f i n e l y d i v i d e d p a r t i c l e s o f v i r t u a l l y 
a l l c o m b u s t i b l e m a t e r i n l ~ c a n p r o d u c e v i o l e n t 
e x p l o s i o n s i f t h e p a r t i c l e s a r e s u s p e n d e d i n 
t h e a i r a n d i g n i t e d . A t t h e t u r n o f t h e c e n t u r y 
t h e m a j o r d u s t e x p l o s i o n h a z a r d s w e r e f a i r l y 
c o n f i n e d t o c o a l m i n e s , milling p r o p e r t i e s , 
a n d a f e w c h e m i c a l o p e r a t i o n s . S i n c e t h a t 
t i m e m a n y o t h e r d u s t s h a v e b e e n i n t r o d u c e d 
a n d h a v e b e c o m e m o r e p r e v a l e n t . T h e s e i n - 
c l u d e c h e m i c a l s , m e t a l s , p l a s t i c s , s o a p s a n d 
a d d i t i o n a l p r o d u c t s i n a g r i c u l t u r a l m i l l i n g . 
A n e x a m i n a t i o n o f t h e R e p o r t o f I m p o r - 
t an t D u s t E x p l o s i o n s I p u b l i s h e d b y t h e N a - 
t i o n a l F i r e P r o t e c t i o n A s s o c i a t i o n ( N F P A ) 
s h o w s t h a t d u s t i g n i t i o n s h a v e o c c u r r e d p r i - 
m a r i l y i n p l a n t s h a n d l i n g g r a i n , f e e d , s t a r c h , 
s u g a r , p o w d e r e d m i l k , f e r t i l i z e r , w o o d a n d 
p a p e r d u s t s . T h e n a t u r e o f t h e c o m b u s t i b l e 
m a t e r i a l i n f l u e n c e s t h e e x p l o s i o n s . I n t h e r e - 
p o r t 1 1 2 0 e x p l o s i o n s a r e m e n t i o n e d i n t h e 
U n i t e d S t a t e s d u r i n g t h e p e r i o d f r o m 1 9 0 0 t o 
1 9 5 6 , o f w h i c h 5 3 6 ( 4 8 % o f t h e t o t a l n u m b e r 
o f e x p l o s i o n s ) h a v e o c c u r r e d i n i n d u s t r i e s 
h a n d l i n g g r a i n , f e e d a n d f l o u r . I n t h e s e 5 3 6 
e x p l o s i o n s , 3 9 2 p e r s o n s w e r e k i l l e d , 1 0 1 5 
w e r e i n j u r e d a n d t h e m a t e r i a l l o s s e s a m o u n t e d 
t o $ 7 5 , 8 8 8 , 5 0 0 . T h i s i n d i c a t e s t h a t d u s t f r o m 
g r a i n p r o d u c t s i s m o s t d a n g e r o u s b e c a u s e i t 
i g n i t e s a n d p r o p a g a t e s f l a m e s e a s i l y , t h e 
s o u r c e o f h e a t r e q u i r e d b e i n g s m a l l . I n c l u d e d 
i n t h i s c a t e g o r y a r e f l o u r , d u s t f r o m w h e a t 
a n d o t h e r g r a i n s a n d s e e d s , w h i c h a r e t h e m a - 
teriAl~ o f f l o u r m i l l i n g a n d i n g r e d i e n t s o f t h e 
f o o d i n d u s t r i e s . 
138 
W h e n a m a s s o f s o l i d f l a m m a b l e m a t e r i a l is 
h e a t e d , i t b u r n s a w a y s l o w l y , o w i n g t o t h e 
l i m i t e d s u r f a c e e x p o s e d t o t h e o x y g e n o f t h e 
air . T h e e n e r g y p r o d u c e d is r e l e a s e d s l o w l y 
a n d h a r m l e s s l y t o t h e s u r r o u n d i n g a t m o - 
s p h e r e . B u t w h e n t h e s a m e m a s s o f m a t e r i a l is 
g r o u n d t o a f i n e p o w d e r a n d m i x e d w i t h a i r i n 
t h e f o r m o f a d u s t c l o u d , a n d i f i g n i t i o n n o w 
o c c u r s , t h e w h o l e m a t e r i a l w i l l b u r n a t o n c e 
v i o l e n t l y . T h e e n e r g y is n o w r e l e a s e d s u d d e n - 
ly w i t h t h e e v o l u t i o n o f l a rge q u a n t i t i e s o f 
c h e m i c a l r e a c t i o n p r o d u c t s . T h i s s u d d e n re- 
l ease o f p r e s s u r e a n d h e a t is t e r m e d as d u s t 
e x p l o s i o n . 
D u s t e x p l o s i o n s a r e s i m i l a r t o gas e x p l o - 
s i ons , a n d t h e s i m i l a r i t y is m o r e p r o n o u n c e d 
as t h e p a r t i c l e s i ze d e c r e a s e s . T h e f i n e r o r 
s m a l l e r t h e s i ze o f t h e p a r t i c l e s , t h e m o r e 
v i o l e n t is t h e e x p l o s i o n , t h e less e n e r g y ( h e a t ) 
is r e q u i r e d t o i g n i t e i t , a n d t h e m o r e l i k e l y i t 
is t h a t t h e d u s t w i l l r e m a i n i n s u s p e n s i o n i n 
t h e a i r f o r a l o n g e r p e r i o d o f t i m e . T h e d u s t 
c a n t r a v e l f r o m r o o m t o r o o m , f l o o r t o f l o o r , 
a n d b u i l d i n g t o b u i l d i n g , i f s t i r r e d u p b y a 
s m a l l p r i m a r y e x p l o s i o n . T h e d a m a g e s f r o m 
t h e e n s u i n g s e c o n d a r y e x p l o s i o n c a n b e d i sas - 
t r o u s , as w i l l b e s h o w n i n p i c t u r e s i n a f o l l o w - 
i n g p a r a g r a p h . 
CAUSES OF DUST EXPLOSIONS 
T h e r e a r e t h r e e p r i n c i p a l f a c t o r s r e s p o n - 
s ib l e f o r r e l e a s i n g d u s t e x p l o s i o n s : 
(1) a m i x t u r e o f d u s t a n d a i r i n t h e f r o m o f 
a d u s t c l o u d , 
(2) a s o u r c e o f i g n i t i o n , s u c h as h o t su r - 
faces , s p a r k s a n d f l a m e s , a n d 
(3)