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Metal powder flammability demands careful handling
PM
TECHNOLOGY
TRENDS
Metal powder flammability demands careful handling he affinity for oxygen of m a n y metal and metal carbide powders m e a n s they pose the risk of fire or even an explosion in powder metallurgy (PM) environments. Various tests have been developed to define the parameters for powder explosions to occur. These assess factors such as the m i n i m u m ignition temp e r a t u r e and energy of a powder, the limiting oxygen concentration to prevent ignition, the m i n i m u m explosible c o n c e n t r a t i o n a n d t h e maxim u m rate of pressure rise. Unfortunately, all the tests have limitations and the results need careful interpretation. They are, for example, depend e n t on the choice of sample a n d its u n i f o r m d i s t r i b u t i o n d u r i n g t h e test. In practice, burning powder creates its own d u s t powder making it a dynamic system. Even if a test indicates t h a t a particular test is n o n - e x p l o s i v e it d o e s n o t mean t h a t u n d e r different conditions it will not burn. An explosion occurs w h e n a metal powder ignites locally and t h e b u r n i n g rate accelerates. The h e a t generated raises the pressure of the surrounding air (and any o t h e r gases), as does the formation of new gaseous products. The rate at which the pressure rises will determine the ferocity of the explosion. For an explosion to occur the following criteria m u s t be
T
fulfilled: • the powder m u s t be combustible; • the powder m u s t be capable of becoming airborne; • the particle size m u s t be capable of propagating flame; • the concentration of powder in the d u s t cloud m u s t fall within the critical explosion range; • the s u s p e n s i o n in contact with ignition source m u s t be capable of initiating and s u s t a i n i n g flame p r o p a g a tion; • the a t m o s p h e r e into which the d u s t is s u s p e n d e d m u s t contain sufficient oxygen to s u p p o r t combustion. The c o m b u s t i b i l i t y of a p o w d e r is a reflection of its oxygen r e q u i r e m e n t , i g n i t i o n t e m p e r a t u r e and the concentration at which an explosion is able to occur. Metals can be categorised as high, m o d e r a t e or low hazard as shown in Table 1. P a r t i c l e size is as i m p o r t a n t as t h e metal's affinity for oxygen in d e t e r m i n i n g w h e t h e r an explosion might occur. The finer the particle size the easier a d u s t cloud is formed and the longer it is s u s t a i n e d . F i n e r particles also need less energy to ignite and give rise to more violent reactions. The effect of particle size on the violence of an explosion is illustrated with d a t a for aluminium in Table 2. P o w d e r c o n c e n t r a t i o n in
the dust cloud affects the viol e n c e o f an e x p l o s i o n . The m a x i m u m effect is at the critical concentration w h e n there is j u s t enough dust to completely r e a c t w i t h all t h e available oxygen. P a r t i c l e s h a p e is also a factor with flaky powder, bec a u s e of its g r e a t e r s u r f a c e area, being more reactive. Turbulence increases the intensity of the initial explosion. It can also give rise to a secondary explosion, which occurs w h e n the force of the initial explosion lifts and ignites dust lying on floors, ledges and elsewhere. A secondary explosion is usually more violent t h a n the primary explosion. For an explosion to occur t h e r e also has to be a s o u r c e of i g n i t i o n . Possible sources include: • h e a t e.g. hot surface; • flame e.g. smoking, welding; • mechanical s p a r k s e.g. grinding; • mechanical impact e.g. striking, friction; • self heat; • e l e c t r i c a l or e l e c t r o s t a t i c discharge. Preventing p o w d e r e x p l o s i o n s starts with good housekeeping This includes ensuring dust is not allowed to accumulate, leaks are d e t e c t e d and repaired and t h a t on staff are properly trained to use e q u i p m e n t and deal with potential hazards. Good p l a n t design also has a place in explosion prevention. Horizontal ledges a n d r o u g h surfaces should be avoided to p r e v e n t d u s t build-up. There should be s h o r t ducting and access for cleaning. E q u i p m e n t should also be e a r t h e d to discharge static electricity. Inert systems also minimize the likelihood of an explosion. Although they are reliable, they do have high initial costs and need careful monitoring. There is also t h e danger t h a t p r o d u c t s
TABLE 1: Classt~ing ~ e hazard level of meal powders High explosion hazard
Moderate explosion hazard
Low explosion hazard
Minimum oxygen requirement
<3%
>3%
-
Minimum ignition temperature
< 600°C
300/800°C
> 700°C
20/50 gm/M 3
100/500 gm/m 3
-
Zr, Mg, AI
Sn, Zn, Fe, Cu, Mn, Si
Mo, Co, Pb
Explosion limits Examples
34 MPR July\August 1993
PM
TECHNOLOGY
TRENDS
TABLE 2: The Particle size (llm)
Minimum ignition
Minimum ignition (m J)
Maximum explosion
(°C)
Minimum explosion ( g m / m a)
(bar)
Maximum rate of (bars)
6
ND
30
13
6.4
1331
17
610
40
28
7
621
100
ND
ND
ND
5.4
135
1000
ND
ND
ND
0
0
will spontaneously combust on emergence. Suppression of an explosion works on the principle t h a t the initial reaction is detected and valves are a c t i v a t e d so t h a t propagation is prevented. Metal powders are too unpredictable for this to be a reliable solution, particularly as it is virtually
Impossible to avoid dust clouds or sources of ignition during powder working. Consequently, s t e p s should be taken to minimize the effects of an explosion should it occur. This includes installing good v e n t i l a t i o n a n d e n s u r i n g fire fighters are trained to handle metal powders and know not to
,!
use w a t e r and w h e n to stop. The E u r o p e a n P o w d e r Metallurgy A s s o c i a t i o n (EPMA) h a s recently u p d a t e d its 'Guide to EC I~gislation and Health & Safety in the E u r o p e a n PM Industry' to include a section on the flammability and explosivity of metal powders, a
Source: EPMA News
ceramitec'94 6th International Trade Fair of Machinery, Equipment, Plant, Processes and Raw Materials for Ceramics and Powder Metallurgy
J
0 "U "I=
Munich, 11-15 October 1994
i
Information: Pattern Ltd. London House, 243-253 Lower Mortlake Road, Richmond, Surrey TW9 2LL, Tel. 081 940 4625, Tx 927051 Lonfin G, Fax 081 948 1442. Contact: Roger D. Sherman
Organizer: M0nchener Messe- und Ausstellungsgesellschaft mbH, Postfach !2 10 09, D - 8000 M0nchen 12, < 17.1993:D-80325 MOnchen > Tel. (8ct) 51 07-0, Fax (89) 51 07-172, Tx 5212086 ameg d.
Please send further information O Exhibitor
O Visitor
Name/Company Street, P.O. Box
.Jill MESSEMONCHEN~
C o u p o n c e r a m i t e c '94
INTERNATIONAL
Town, PostalCode
MPR July\August 1993 35
TECHNOLOGY
TRENDS
Metal powder flammability demands careful handling he affinity for oxygen of m a n y metal and metal carbide powders m e a n s they pose the risk of fire or even an explosion in powder metallurgy (PM) environments. Various tests have been developed to define the parameters for powder explosions to occur. These assess factors such as the m i n i m u m ignition temp e r a t u r e and energy of a powder, the limiting oxygen concentration to prevent ignition, the m i n i m u m explosible c o n c e n t r a t i o n a n d t h e maxim u m rate of pressure rise. Unfortunately, all the tests have limitations and the results need careful interpretation. They are, for example, depend e n t on the choice of sample a n d its u n i f o r m d i s t r i b u t i o n d u r i n g t h e test. In practice, burning powder creates its own d u s t powder making it a dynamic system. Even if a test indicates t h a t a particular test is n o n - e x p l o s i v e it d o e s n o t mean t h a t u n d e r different conditions it will not burn. An explosion occurs w h e n a metal powder ignites locally and t h e b u r n i n g rate accelerates. The h e a t generated raises the pressure of the surrounding air (and any o t h e r gases), as does the formation of new gaseous products. The rate at which the pressure rises will determine the ferocity of the explosion. For an explosion to occur the following criteria m u s t be
T
fulfilled: • the powder m u s t be combustible; • the powder m u s t be capable of becoming airborne; • the particle size m u s t be capable of propagating flame; • the concentration of powder in the d u s t cloud m u s t fall within the critical explosion range; • the s u s p e n s i o n in contact with ignition source m u s t be capable of initiating and s u s t a i n i n g flame p r o p a g a tion; • the a t m o s p h e r e into which the d u s t is s u s p e n d e d m u s t contain sufficient oxygen to s u p p o r t combustion. The c o m b u s t i b i l i t y of a p o w d e r is a reflection of its oxygen r e q u i r e m e n t , i g n i t i o n t e m p e r a t u r e and the concentration at which an explosion is able to occur. Metals can be categorised as high, m o d e r a t e or low hazard as shown in Table 1. P a r t i c l e size is as i m p o r t a n t as t h e metal's affinity for oxygen in d e t e r m i n i n g w h e t h e r an explosion might occur. The finer the particle size the easier a d u s t cloud is formed and the longer it is s u s t a i n e d . F i n e r particles also need less energy to ignite and give rise to more violent reactions. The effect of particle size on the violence of an explosion is illustrated with d a t a for aluminium in Table 2. P o w d e r c o n c e n t r a t i o n in
the dust cloud affects the viol e n c e o f an e x p l o s i o n . The m a x i m u m effect is at the critical concentration w h e n there is j u s t enough dust to completely r e a c t w i t h all t h e available oxygen. P a r t i c l e s h a p e is also a factor with flaky powder, bec a u s e of its g r e a t e r s u r f a c e area, being more reactive. Turbulence increases the intensity of the initial explosion. It can also give rise to a secondary explosion, which occurs w h e n the force of the initial explosion lifts and ignites dust lying on floors, ledges and elsewhere. A secondary explosion is usually more violent t h a n the primary explosion. For an explosion to occur t h e r e also has to be a s o u r c e of i g n i t i o n . Possible sources include: • h e a t e.g. hot surface; • flame e.g. smoking, welding; • mechanical s p a r k s e.g. grinding; • mechanical impact e.g. striking, friction; • self heat; • e l e c t r i c a l or e l e c t r o s t a t i c discharge. Preventing p o w d e r e x p l o s i o n s starts with good housekeeping This includes ensuring dust is not allowed to accumulate, leaks are d e t e c t e d and repaired and t h a t on staff are properly trained to use e q u i p m e n t and deal with potential hazards. Good p l a n t design also has a place in explosion prevention. Horizontal ledges a n d r o u g h surfaces should be avoided to p r e v e n t d u s t build-up. There should be s h o r t ducting and access for cleaning. E q u i p m e n t should also be e a r t h e d to discharge static electricity. Inert systems also minimize the likelihood of an explosion. Although they are reliable, they do have high initial costs and need careful monitoring. There is also t h e danger t h a t p r o d u c t s
TABLE 1: Classt~ing ~ e hazard level of meal powders High explosion hazard
Moderate explosion hazard
Low explosion hazard
Minimum oxygen requirement
<3%
>3%
-
Minimum ignition temperature
< 600°C
300/800°C
> 700°C
20/50 gm/M 3
100/500 gm/m 3
-
Zr, Mg, AI
Sn, Zn, Fe, Cu, Mn, Si
Mo, Co, Pb
Explosion limits Examples
34 MPR July\August 1993
PM
TECHNOLOGY
TRENDS
TABLE 2: The Particle size (llm)
Minimum ignition
Minimum ignition (m J)
Maximum explosion
(°C)
Minimum explosion ( g m / m a)
(bar)
Maximum rate of (bars)
6
ND
30
13
6.4
1331
17
610
40
28
7
621
100
ND
ND
ND
5.4
135
1000
ND
ND
ND
0
0
will spontaneously combust on emergence. Suppression of an explosion works on the principle t h a t the initial reaction is detected and valves are a c t i v a t e d so t h a t propagation is prevented. Metal powders are too unpredictable for this to be a reliable solution, particularly as it is virtually
Impossible to avoid dust clouds or sources of ignition during powder working. Consequently, s t e p s should be taken to minimize the effects of an explosion should it occur. This includes installing good v e n t i l a t i o n a n d e n s u r i n g fire fighters are trained to handle metal powders and know not to
,!
use w a t e r and w h e n to stop. The E u r o p e a n P o w d e r Metallurgy A s s o c i a t i o n (EPMA) h a s recently u p d a t e d its 'Guide to EC I~gislation and Health & Safety in the E u r o p e a n PM Industry' to include a section on the flammability and explosivity of metal powders, a
Source: EPMA News
ceramitec'94 6th International Trade Fair of Machinery, Equipment, Plant, Processes and Raw Materials for Ceramics and Powder Metallurgy
J
0 "U "I=
Munich, 11-15 October 1994
i
Information: Pattern Ltd. London House, 243-253 Lower Mortlake Road, Richmond, Surrey TW9 2LL, Tel. 081 940 4625, Tx 927051 Lonfin G, Fax 081 948 1442. Contact: Roger D. Sherman
Organizer: M0nchener Messe- und Ausstellungsgesellschaft mbH, Postfach !2 10 09, D - 8000 M0nchen 12, < 17.1993:D-80325 MOnchen > Tel. (8ct) 51 07-0, Fax (89) 51 07-172, Tx 5212086 ameg d.
Please send further information O Exhibitor
O Visitor
Name/Company Street, P.O. Box
.Jill MESSEMONCHEN~
C o u p o n c e r a m i t e c '94
INTERNATIONAL
Town, PostalCode
MPR July\August 1993 35