Embed Size (px)
Citation preview
RWkW
Safety considerations in conveying of bulk solids and powders
Stanley S. Grossel HofSmann-La Roche Inc., Corporate Engineering, 340 Kingsland Street, Building 105, Nutley, NJ 07110, USA
This review covers a wide range of safety factors which need to be considered when handling bulk solids and powders. The physical dangers of the mechanical equipment are well covered by published standards and codes, as are noise levels and the use of electrical equipment. Dust explosions caused both by static electricity and other ignition sources, are more complex. The plant requires careful investigation to ensure that explosion hazards are kept to a minimum and suitable protective measures installed. Different types of conveyers, e.g. belt, pneumatic and bucket elevators, each pose their own hazard and the system chosen should be considered carefully in the light of the material to be handled.
(Keywords: bulk solids handling: safety factors; dust explosions)
There are a wide range of safety factors to be considered in the handling of bulk solids, from the safety of the machinery used to the toxicity and explosibility of fine powders. This review looks at these different safety aspects.
Conveyor personnel protection considerations
Machinery guarding All bulk solids/powders conveyors have a large number of moving parts, including power transmission machin- ery and equipment (shafts, pulleys, couplings, speed reducers, etc). In accordance with OSHA and ANSI requirements guards must be provided to protect per- sonnel and state and local codes and regulations must also be satisfied.
The OSHA and ANSI standards listed below should be consulted for details.
0 OSHA Safety and Health Standards (29CFR 1910): General Industry, Paragraph 1910.219 (1981) - Mechanical Power - Transmission Apparatus.
0 ANSI B15.1 (1972) - Safety Standard for Mechan- ical Power Transmission Apparatus.
0 ANSI/ASME B20.1 (1984) - Safety Standard for Conveyors and Related Equipment.
Noise exposure Equipment and machinery noise levels must be kept below the values given in OSHA Safety and Health Standards (29CFR 1910), Paragraph 1910.95 - Occupa- tional Noise Exposure (see Table I). If equipment/
Received 19 January 1988
OSSO-4230/%%/020062-13S3.00 0 1988 Butterworth & Co. (Publishers) Ltd
62 J. Loss Prev. Process Jnd., 1988, Vol 1, April
Table 1 Permissible noise exposures
Sound level dBA slow
Duration Per day fhl ~l3SpO”Sl3
8 90 6 92 4 95 3 97 2 100
1: 102 1 105
: 110 : or less 115
When the daily noise exposure is two or more periods of noise exposure of different levels their combined effect should be considered, rather than the individual effect of each. If the sum of the following fractions: Cl/T, + G/T2 G/T, exceeds unity, then, the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and Tn indicates the total time of exposure permitted at that level. Exposure to impulsive or impact noise should not exceed 140 d6 peak sound pressure level.
machinery cannot be obtained with noise levels comply- ing with OSHA standards, then engineering controls, such as sound mufllers, must be installed. If this does not bring the sound levels within the limits listed in Table I, then suitable protective equipment must be provided for the operators.
Electrical equipment All electrical equipment (e.g. motors, switchgear, wiring,) must be designed and installed to comply with the latest edition of the National Electrical Code (NEC), and paragraphs 1910.301 to .399 of OSHA Safety and Health Standards (29CFR 1910).
Safety considerations in conveying bulk solids and powders: S. Grossel
When electrical equipment needs to be repaired or maintained, it must be locked out and tagged to prevent injury to personnel. There is currently no OSHA stand- ard, but legislation has been proposed and submitted for review, entitled ‘Control of Hazardous Energy Sources’ (Lockout/Tagout). There is also an ANSI consensus standard 2244.1 (1982).
Static electricity and dust explosion hazards
Static electricity hazards
When conveying bulk solids and powders, especially organic ones, static electrical charges can develop. These charges arise from contacts made between sur- faces during the movement of the particles. The charge on a powder particle is governed by three factors: the charge production rate; the charge leakage rate when the particle is in contact with a ground; and the electrical breakdown of air initiated by the high field around the charged particle.
There are five fundamental quantities in an under- standing of electrostatics. The most basic is the electric charge that is transferred to a material, usually by friction. When an object is charged it exerts a force on any other charged object, and is then said to have an electric potential or voltage, k’. The rate of change of voltage with distance or potential gradient is the electric held, E. The potential reached by an object having a charge q depends on its electrical capacity, C. The higher the capacity the more charge is needed to achieve a given potential.
The rate at which charge dissipates depends primarily on the electrical resistance R between the stored charge and ground. An electrostatic spark occurs when an isolated charged object is suddenly grounded. The accumulation of static electricity on an object produces an electric field around it and a spark will occur if the field strength exceeds the breakdown value of the sur- rounding atmosphere. For air, this is approximately 3000 kV/m.
Electrostatic sparks can cause dust explosions if they achieve a minimum ignition energy and the dust cloud in the air is within the explosive concentration range. The minimum ignition energy is the energy which just ignites the most easily ignited mixture and is usually measured with a capacitor discharge by varying the charge quan- tity, the capacitance and the electrode separation at standard temperature and pressure conditions (1 bar, 2O”C), or when applicable, in saturated vapour.
The minimum ignition energy of a dust-air mixture (with the exception of explosives and other reactive materials is lo-100 mJ and is therefore 50 to 1000 times greater than those of gas-air or vapour-air mixtures. Determining the minimum ignition energy is, with other data, an important aid in quantifying the potential hazard of electrostatic charges.
Cross and Farrer ’ present an excellent discussion of static electricity phenomena and measurement tech- niques for minimum ignition energy. Also, the book by Haase’ is a good source of information on electrostatic
hazards. Palmer3 lists minimum ignition energies for many bulk solids and powders. These values are listed in Table 2.
Dust explosion hazards The subject of dust explosions is too large and com- plicated to cover in depth in this review, but certain aspects of it will be discussed below to present some fundamentals and background materials. For further reading on the subject, the following books are recom- mended: Cross and Farrer’, Palmer’, Bartknecht4, Field’, and Nagy and Verakis6.
A dust explosion results when finely divided combus- tible matter is dispersed in an atmosphere containing sufficient oxygen to permit combustion and a source of ignition of appropriate energy is present. Dust explosions have certain similarities to gas explosions, especially with regard to the chemical processes involved and in cases where the particle size of the dust is less than 5 pm. However, there are significant differences which make the study of dust explosions extremely difficult.
For a dust explosion to occur there must be a degree of turbulence, if only to disperse the dust into a suspension. Gas explosions can occur when the gas is in a quiescent state, the mixture being homogeneous and consisting of molecular-size particles. The suspensions of dusts encountered in dust explosions are, however, unlikely to be homogeneous, and would normally con- tain a range of concentrations of particles which are many orders of magnitude larger and heavier than gas molecules and which settle out of suspension due to gravity.
A dust explosion involves such a high rate of combus- tion that individual particles and agglomerates are either consumed or oxidized. The combustion of carbon in organic material produces gaseous products which in themselves take up more space than the solids of the parent material. An expanding flame front will also result from the ignition of flammable gases produced by the decomposition of the dust. A dust explosion there- fore requires more space because of the expansion of the hot gaseous products.
In industrial plant, the heat released during a dust explosion is likely to exceed the natural rate of cooling and consequently an explosion would be accompanied by significant, and, in some cases, uncontrolled expan- sion effects. In an unconfined situation, there would be mainly localized flames and pressure effects. However, in the confined situations commonly found in plant handling particulate matter, the expansion effects are likely to be sufficient to burst through the confines of the plant equipment and/or piping.
The following conditions must exist for a dust explosion to occur:
l The dust must be combustible. l The dust must be in suspension in the atmosphere
which must contain sufficient oxygen to support combustion.
J. Loss Prev. Process lnd., 1988, Vol I, April 63
Safety considerations in come ying bulk solids and powders: S. Grossel
Table 2 Dust explosion parameters
Dust
Maximum
oxygen Minimum co”ce”-
ignition Minimum Maximum tration temperature explosible Minimum Maximum rate of
f°C) to prevent
CO”CB”- ignition explosion pressure ignition tration energy pressure rise 1% by
cloud layer fgl ‘I (mJ) lib I” ? (lb in >sl volume) References Notes
Acetamide
Aceto acetanilide
Acetoacet-p-phenetedtde
Acetoecet-o-toluidine
2 Acetylamino-5-nitro
thiazole
Acetyl-p-nitro-o-
toluidine
Adipic acid
Alfalfa
Almond shell
Aluminium, atomized
Aluminium, flake
Aluminium-cobalt alloy
Aluminium-copper alloy
Aluminium-iron alloy
Aluminium-lithium alloy
Aluminium-magnesium
alloy
Aluminium-nickel alloy
Aluminium-silicon alloy
Aluminium acetate
560 - 560 ~ 560 -
710 -
450 450
450 -
550 -
460 200
440 200
650 760
610 320
950 570
~ 830
550 450
470 400
430 480
950 540
670 -
560 640
Aluminium octoate
Aluminium stearate
2 Amino-5-nitrothiazole
Anthracene
Anthranilic acid
Anthraquinone
Antimony
Antipyrin
Asphalt
Aspirin
Azelaic acid
awl’ Azo isobutyronitrile
Barley
Benzethonium chloride
Benzoic acid
Benzotriazole
Benzoyl peroxide
Beryllium
460 -
400 380
460 460
505 Melts
580 -
670 ~
420 330
405 Melts
510 500
550 Melts
610 ~
430 350
370 -
380 410
600 Melts
440 -
Beryllium acetate, basic
910 540
620 -
Bis (2hydroxy-5-
chlorophenyl)-methane
Bis (2-hydroxy-3. 5,6,-
trichlorophenyl).methane
Bone meal
570 -
Did not 450
ignite
490 230
Boron 730 390
Bread
Brunswick green
P-t-butyl benzoic acid
Cadmium
Cadmium yellow
Calcium carbide
Calcium citrate
Calcium gluconate
Calcium DL pantothenate
Calcium propionate
Calcium silicide
Calcium stearate
450 -
360 -
560 -
570 250
390
555 325
470 -
550 -
520 -
530
540 540
400 -
_ 0.030
0.030
0.160
- 20
10 _
40
- -
0.035 60
0.100 320
0.065 80
0.045 50
0.045 10
0.180 100 0.100 100
- _
<O.l 140
0.020 80
0.190 80
0.040 60 - _
0.015
0.075 _
0.030 _
0.420
0.025
0.015
0.025
0.015
0.020
0.011
0.030 _
_
10
30
35 _
1 920
25
16
25
25
60
12
30
21 -
0.080 100
0.040 60
_ _
Did not
ignite
_
_
0.020 _
_
_
_
_
0.050 _
0.060
0.025
25
4oocl
_
80 _
150
15
_ 90 87 _
137
4 800
>10000 -
9 000
_
95 4 000
88 1 100
101 1 400
84 > 20 000
127 > 20 000
92 11 000
95 4 000
36 300
96 6000 86 10000
96 10000
85 7 500
59 950
-
86
110
68
84
28
53
94
87
76
134
-
>lOOOO
5 600
700
8 500 -
300 -
4 800 7 700
4 700
8 000 _
91
95
103
6 700
10 300
9 200 _
Did not Ignite
87 2 200
70 2 000
_ -
11 100
41 200
_ -
-
88 6 500
7 100 _ -
13 - _ -
-
105 4 600
90 1 900
86 20 000
97 >lOOOO
_
_ - _
_
_ _ - _ -
- _
_ _
- _ - _
- _ _
_ _ _ _ _
15
13
_
_
_
-
_
_
_
_
_
_
_
8
4.8 5
5
3
3
3
3
3
3
3
3
3
1
8
1
1
2.8 1
8
3
2
7
8
4
4
8
1
8
1
8
3
1
1
1
6
3
8
8
4
3
8 2
8
8 1
8
3
1
Group (bl dust
Guncotton ignitton
source in pressure
test
Contained 8 per
cent oxide
Inert gas carbon
dioxide
Inert gas carbon
dioxide
Guncotton ignition
source in pressure
test
Guncotton ignition
source in pressure
test
Group (bl dust
Group (bl dust
64 J. Loss Prev. Process Ind., 1988, Vol 7, April
Safety considerations in conveying bulk solids and powders: S. Grossel
Table 2 (continued)
Dust
Maximum
oxygen
Minimum co”ce”-
ignition Minimum Maximum nation
temperature explosible Minimum Maximum rate of to prevent
(OC) concen- ignition explosion pressure ignition
tration energy pressure rise (% by
cloud layer (gl ‘1 ImJl (lb in ‘) (lb in * s.1 volume) References Notes
Caprolactam 43b - 0.07
Carbon, activated 660 270 0.100
Carbon, black
Carboxy methyl cellulose
Carboxy methyl hydroxy
ethyl cellulose
Carboxy polymethylene
Casein
Cellulose
Cellulose acetate
Cellulose acetate butyrate
Cellulose proprionate
Cellulose triacetate
Cellulose tripropionate
Charcoal
Chloramine-T
510 -
460 310
380 -
_ _ _ _
0.060 140 130 5 000
0.200 960 83 800
520 ~ 0.115
460 - -
410 300 0.045
340 - 0.035
370 - 0.025
460 - 0.025
390 ~ 0.035
460 ~ 0.025
530 180 0.140
540 150 _
o-Chlorobenzmalono nitrile
o-Chloroaceto acetanilide
p-Chloroaceto acetanilide
Chloro amino toluene
sulphonic acid
4-Chloro-2 nitro aniline
p-Chloro o-toluidine
hydrochloride
Chocolate crumb
Chromium
Cinnamon
Cttrus peel
Coal, brown
Coal, 8 per cent volatiles
Coal, 12 per cent volatiles
Coal, 25 per cent volatiles
Coal, 37 per cent volatiles
64; ~
650 ~
650 -
0.025
0.035
0.035 _
co.750 _
0.23;
0.060
0.060 _
_
_
0.120
0.055
Coal, 43 per cent volatiles
Cobalt
Cocoa coconut
Coconut shell
Coffee
0.050 -
0.065 _
0.035
0.085
Coffee, extract
Coffee, instant
Coke
Coke, petroleum, 13 per
cent volatiles
590 120
650 -
340 -
580 400
440 230
500 330
485 230
730 -
670 240
605 210
610 170
575 180
760 370
500 200
450 280
470 220
360 270
600 -
410 350
>750 430
670 ~
_
0.280
Colophony
Copal
Copper
325 Melts
330 Melts
700 -
Copper-zinc, gold bronze
Cork
Corn cob
Corn dextrine Cornflour
Cornstarch
Cotton flock
370 190
460 210
450 240
410 390
390 -
390
470 -
_
1 .oo
_
_
_
1 .oo
0.035
0.045
0.040
0.040
0.050
60 79 1 700 8 8
92 1 700 _ 7
640 76 1 200 _ 89 1 200
40 117 8 000
20 114 6 500
30 81 2 700
60 105 4 700
30 107 4 300
45 88 4 000
20 100 1 800 _ 7 150
30
20 _
140
90 >10000
94 3 900
85 5500 _ _
123 3500 - _
1
1’
1
_
_ _
40 56
30 121
00 51 _ _
_ _
_ _
20 62
60 90
50 _
92 2 000 _ _
69 1 200 _ _
115 4 200
38 150
120
60
160
_ Did not
ignite _
Did not
30 145 9 500
25 94 6 000
_
5000
3900
1 200 -
_
_
400
2300
47
68
_
500
36
_
200
-
68 _
Did not Did not ignite ignite
44 1 300
96 7 500
127 3 700
124 7 000
_
_
_
_
_
_
5
7 _
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
10
_
_
_
_
-
_
_
_
_
_
_
_
7
4
4
4
8
4
4.8 4
4
4,8 4
7
1
1
1
1
8
1
8
;
5
2
5
5.8
2
5
2
7
2
2
3
Guncotton ignition
source in min. expl.
cont. and max.
expl. pressure tests
Inert gas nitrogen
Guncotton lgnitlon
source in pressure
test
See also Lignite
Standard Pittsburgh
coal
Inert gas carbon
dioxide
Guncotton ignition
source in min. expl.
cow. and max.
expl. pressure tests
See also Gum
manila
icontinoedl
J. Loss Prev. Process Ind., 1988, Vol 1, April 65
Safety considerations in conveying bulk solids and powders: S. Grosset
Table 2 (continued)
Dust
Maximum oxygen
Minimum CO”CB”- ignition Minimum Maximum tration
temperature explosible Minimum Maximum rate of f°C)
to prevent concen- ignition explosion pressure ignition tration energy pressure rise 1% by
cloud layer fgl II fmJ) (lb in ‘1 (lb in-*s) volume) References Notes
Cotton linters Cottonseed meal Coumarone-indene resin Crystal violet Cyclohexanone peroxide Dehydroacetic acid Dextrin Dextrose monohydrate Diallyl phthalate Diamino stilbene
drsulphonic acid Diazo aminobenzene Di-t-butyl-p-cresol Dibutyl tin maleate Dibutyl tin oxide Dichlorophene 2.4.Dichlorophenoxy ethyl
benzoate Dicyclopentadiene dioxide Dihydrostreptomycrn
sulphate 3-3’ Dimethoxy 4-4’
diamino diphenyl Dimethylacridan Dimethyl drphenyl urea Dimethyl isophthalate Dimethyl terephthalate S-S’-Dimethyl xanthogene- thylene bis dithiocarba- mate Dinitro aniline 3, 5-Dinitrobenzamide 3. 5-Dinitrobenzoic acid Dinitrobenzoyl chloride Drnitrocresol 4. 4’.Drnitro-sym-diphenyl
urea Drnitro stilbene disulphonic
acid Dinitrotoluamide Diphenyl 4,4’-Diphenyl di
sulphonylazide Drphenylol propane
(BisphenoCA) Egg white Epoxy resin Esparto grass Ethyl cellulose Ethylene diamine tetra
acetic acid Ethyl hydroxyethyl
cellulose Ferric ammonium
ferrocyanide Ferrrc dimethyl dithio
carbamate Ferric ferrocyanide Ferrochromium Ferromanganese Ferrosilicon
(45 per cent Sif Ferrosilicon
(90 per cent Si) Ferrotitanium Ferrous ferrocyanide Ferrovanadrum
520 - 530 200 550 - 475 Melts
- _
430 - 410 440 350 - 480 - 550 -
0.50 0.055 0.015
_ _
0.030 0.050
-
0.030 _
550 - 0.015 420 - 0.015 600 - _
530 - - 770 - _
540 ~ 0.045
420 - 0.015 600 230 0.520
0.030
540 490 ~ 580 - 570 - 400 -
_ _
0.025 0.030 0.300
470 ~ 500 Melts 460 - 380 - 340 Melts 550 -
0.040 0.050
0.030 0.095
450 ~
500 - 630 - 590 140
570 ~
610 - 490 -
340 330 450 -
390 -
390 210
280 150
370 - 790 670 450 290 640 -
0.050 15 153 >lOOOO 0.015 20 82 3 700 0.065 30 143 5 500
0.012
0.14 0.015
0.025 0.075
0.020
1.500
0.055 25
_
2.00 0.130
_
Did not 980 ignite
370 400 380 190 440 400
0.240 1280
0.140 80 0.400 _ 1.300 400
1920 73 80 89 10 93 _ _
21 84 15 87 40 99
20 90 _ _
20 15 _ _ _
60
30 _
114 79
_
72 84
89 9 500 42 200
_ 82
_
15 20
3200
_ _
84 105
84
_
45 45 - -
60
_ _
163 6 500 139 4300
_
102
-
11
640 9
_
15 50
30
81 11 800
58 500 94 8 500 94 7 300
112 7 000 106 3 000
_
_
80 _
94 2 200
17 100
86 6 300
82 1 000
62 5oDO _ _
113
55 _ _
400 2 200
11000 _
5 600 8 000 9 000
-
8 500 _
>10000 13000
_ -
3 000 2 200
>10000
_ _
8 000 12 000
1 500
_
2 500
_
3 500
9 500 _ _
5 - 11 _ _ _
_ _ _
-
9 _
- _
_
7
_
- -
s
- - _
_ _
_
_ _ _
5
_
_
_
_
- _
_
_
_ _
5 5 4 2 8 1 6 8 1 8 Group fbf dust
1 4 8 8 1 1
4 1
1
8 1 1 8
1.2 1
8
6 1 1
4.8 Inert gas nitrogen
5 4.8
8 4 1
6
1
1
1 3 3 2
3
3 1 3
66 J. Loss Prev. Process Ind., 1988, Vol I, April
Safety considerations in conveying bulk solids and powders: S. Grossel
Dust
Maximum
oxygen Minimum concan-
ignition Minimum Maximum tration
temperature explosible Mintmum Maximum rate of
(OC)
to prevent concen- ignition explosion pressure Ignition tration energy pressure rise (% by
cloud layer (gl ‘I (mJ) (Ibin 2, (lb in 2 s) volume) References Notes
Fish meal
Fumaric acid
Garlic Gelatin, dried
Gilsonite
Graphite
Grass
Gum arabic
Gum Karaya
Gum manila (copal)
Gum tragacanth
Hexa methylene tetramlne
Horseradish
Hydrazine acid tartrate
p-Hydroxy benzoic acid
Hydroxyethyl cellulose
Hydroxyethyl methyl
cellulose
Hydroxy propyl cellulose
Iron
Iron, carbonyl
Iron pyrites
lsatoic anhydride
lsinglass
lsophthalic acid
Kelp
Lactalbumin
Lampblack
Lauryl peroxide
Lead
Leather
Lignin
Lignite
Lycopodium
Magnesium Maize husk
Maize starch
Maleic anhydride
Malt barley
Manganese
Manganese ethylene bis
dithio carbamate
Maniac
Mannitol
Melamine formaldehyde
resin
DL Methionine
1 -Methylamino
anthraquinone
Methyl cellulose
2. 2-Methylene bis-4-ethyl-
6-tmbutyl phenol
Milk
Milk, skimmed
Milk sugar
Molybdenum
Molybdenum disulphide
Monochlorecetic acid
Monosodium salt of
trichloro ethyl phosphate
Moss, Irish
Naphthalene
P-Naphthalene-azo-
dimethyl aniline
P-Naphthol
Naphthol yellow
485 ~
520 -
360 ~
620 480
580 500
730 580
500 260
520 240
360 390
490 260
410 -
_ _ 0.085 35
0.10 240
570 -
620 -
410 -
410 -
co.5
0.020 25 _ - _ _
0.060 100 0.100 180
0.030 30 0.040 45
0.015 10
<0.100 _
0.175 460 0.040 _
0.025 40 _ _
400 ~
430 240
420 230
380 280
700 -
520 -
700 ~
570 220
570 240
730 -
0.020 30 _ _
0.105 100
1 .oo 8 200 0.035 25
_ -
0.035 25 Did not ionite
790 290
0.040 _
_
_
390 ~
450 -
450 200
480 310
560 430
430 - 410 -
600 Melts
400 260
460 240
270 -
0.040
0.030
0.025
0.030 _
_
0.055
0.125
0.07
430 - _
460 - 0.065
410 ~ 0.02
370 360 0.025
830 Melts 0.055
360 340 0.030 310 - _
440 ~
490 200
450 Melts
720 360
570 290
620 ~
540 ~
0.050 _
_
_
530 230
575 Melts
510 Melts
670 ~
415 395
Did not ignite _ _
0.020 50
50 _
12
Did not
ignite
20
30
40
40 _
-
35
305
35
_
40
50
35
50
20 _
50 _
_
_
_
_
_ 103
57
78
78 _
56
117
116
89
123
98
96
30
37
106 _
_ 3 000
1 300
1 200
4 500 -
400
3 000
2 500
6 000
5 000
11000
1 600
200 -
2 600
96 _
47
5
80 Nil
78
19
97 _
90
3
2 900
8 000
100
4 900 Nil
3 100
200
3 500
6 400
100
_
102
94
75
116
75 _
95
53 -
_
5 000
8 000
3 100
15000
700 _
_
4 400
4 900 _
- _
97 2 800
93 1 800
119
71
133
76
5 700
3 300
6 000
7 300
_
95
31 _
_
_
_
21
87
70
_
2 300 _
_
-
_
300 -
2 300
_
-
_ _ _
_ _ _ _ _
11 _
_
_
_
_
_
_
_
_
_
_
7
9 _
_
_
_
_
_
_
_
7 _
_
_
_
_
_
_
-
-
_
_
2
4
5 1
7
7
8 4
4
4
4
4
6
1
1
6
8
6
2
3
3
8
4
5
4
7
8
3 Flame ignition
source in pressure
test
8
4
7
5
3
8
2
2
5
3
8
8 1
4.8
4
8
8
5
2
3
6
8
8 Group lb) dust
5
2
J. Loss Prev. Process Ind., 7988, Vol 1, April 67
Safety considerations in conveying bulk solids and powders: S. Grossel
Table 2 (continued1
Maximum
Dust
Minimum oxygen
concen- ignition Minimum Maximum nation
temperature
(OC)
explosible Minimum Maximum rate of to prevent concen- ignition explosion pressure ignition tration energy pressure. rise
cloud layer (gl II (o/o by
(mJ) (lb in *) (lb I” ‘sl volume) References Notes
Nigrosine hydrochloride
p-Nitro-o-anisidene
pmN$y-benzene arsenic
Nitrocellulose
Nitro diphenylamine
Nitro furfural semi
carbazone
Nitropyridone
p-Nitro-o-toluidine
m-Nitro-p-toluidine
Nylon
Oilcake meal
Onion, dehydrated
630 -
400 --
360 280
480 ~
240 -
430 Melts 470 -
470 -
600 430
470 285
410 -
Paper
Para formaldehyde
Peanut hull
Peat
Peat, sphagnum Pectin
Penicillin, N-ethyl
piperidine salt of
Penta erythritol
Phenol formaldehyde
Phenol furfural resin
Phenothiazine
p-Phenylene diamine
Phosphorus, red
Phosphorus pentasulphide
Phthalic acid
Phthalic anhydride
Phthalimide
Phthalodinitrile
Phytosterol
Piperazine
Pitch
Polyacetal
Polyacrylamide
Polyacrylonitrile
Polycarbonate
Polyethylene
Polyethylene oxide
Polyethylene terephthalate
Poly isobutyl methacrylate
Poly methacrylic acid
Polymethyl methacrylate
Polymonochlorotrifluoro
ethylene
Polypropylene
Polystyrene
Polytetrafluoro ethylene
Polyurethane foam
Polyurethane foam, fire
retardant
Polyvinylacetate
440 270
410 -
460 210
420 295
460 240
410 200
310 -
450 -
460 ~
530 -
540 -
620 -
360 305 280 270
650 Melts
605 Melts
630 -
2700 Melts
330 Melts
480 - 710 -
440 ~
410 240
500 460 710 -
390 -
350 -
500 -
500 280
450 290
440 -
600 720
420 -
600 500
670 570
510 440
550 390
450 -
Polywnyl alcohol 450 Melts
Polyvinyl butyral 390 - Polyvinyl chloride 670 -
Polyvinylidene chloride
Polyvinyl pyrrolidone
Potassium hydrogen
tartrate
Potassium sorbate
Potato, dried
670 ~
465 Melts
520 -
380 180
450 ~
_ _ _ _
0.195 480
_
._ 30
_
0.045 _
_
0.030
35 _
0.130
0.055
0.040
0.045
0.045
0.075 _
_
20 _
Did not
ignite
60
20
50 _
50 35 -
0.030
0.015
0.025
0.030
0.025
10
10
10
_
0.050 -
0.015
0.030
0.025 _
0.035
0.035
0.040
0.025
0.025
0.020
0.030
0.040
0.020
0.045
0.020
Did
30 _
15 _
15
50 _
10 -
20
20
30
20
25
10
30
35
40
100
15
not
0.020 30 0.020 15
Did not 0.030 20
0.025 15
0.040
0.020
160
10
Did not ignite
_ _ _ _
0.120 60 _ _
_ 77 900
> 256 _
>143
> 20 900
8 600
111 _
>10000 _
_
95
_
4 000 _
35
-
500
96 3 600
133 13000
116 8 000 _ _
104 2 200
132 8 000 _
90 9 500
107 6 500
88 8 500
56 3 000
94 11 000 _
64
62
72
89
43
76
72
88
113
85
89
96
80
106
98
74
97
101
>10000 _
4 200
4 800 _
> 10000
1 400
6 000
4 100
2500
11 000
4 700
7 500
2 100
5500
2800
1800
1 800
ignite
76 5 500
100 7 000
ignite
87 3 700
96 3 700
69 1 000
78 _
84 2 000
38 500
_ _
15 _
_ _
79 9 500
97 1 000
_ _
8
8
1
_ 8 - 8 _ 8
_ 1 - 8 _ 8
6 4 _ 2 _ 5
_
_
_
_
_
7 _
-
-
._
_
_
11 _
_
_
_
_
_
_
_
5 _
_
_
7 _
_
_
_
_
11
_
5 _
_
-
-
_
4
8.4 4
1
4
2
1
2
2,4 1
2
1
1
7
4
4
4
4
4.8 4
4
4
4
4
4
4.8
2
4
4
8
2
1
1
8
Inert gas carbon
dioxide
Flame ignition
source
Group lb) dust
66 J. Loss Prev. Process Ind., 1988, Vol 7, April
Safety considerations in conveying bulk solids and powders: S. Grossel
Table 2 (conrinued)
Dust
Maximum
oxygen
Minimum concen-
ignition Minimum Maximum fration
temperature explosible Minimum Maximum rate of to prevent
CC) concen- ignition explosion pESSU,e ignition
tration energy pressure rise (% by
cloud layer fgl ‘I (mJI (lb in ‘) (lb in ‘s) volume1 References Notes
Potato starch
Provender
Pyrethrum
Quillaia bark
Rape seed meal
Tayon. viscose
Rayon, flock
Rice
Rosin
Rubber
Rubber, crude, hard
Rubber, crumb
Rubber, vulcanized
Rye flour
Saccharin
Salicylanilide
Salicylic acid
Sawdust
Sebacic acid
Se”“a
Shellac
Silicon
Soap
Sodium acetate
Sodium amatol
Sodium benzoate
Sodium carboxymethyl
cellulose Sodium 2-chloro-5 nitro-
benzene sulphonate
Sodium 2,2-dichloro
propionate
430 -
370 -
460 210
450 ~
465 ~
420 -
440 240
390 -
380 -
350 ~
440 ~
360 -
415 325
690 -
610 Melts
590 -
430 ~
440 -
400 -
Did not 760 ignite
430 600
590 ~
580 Melts
560 680
320 -
550 440
500 ~
Sodium dihydroxy 510 -
naphthalene dlsulphonate
Sodium glucaspaldrate
Sodium glucoheptonate
Sodium monochloracetate
Sodium m-nitrobenzene
sulphonate
Sodium m-nitrobenzoate
Sodium pentachlorophenate
Sodium propionate
Sodium secobarbital
Sodium sorbate
Sodium thiosulphate
Sodium toluene sulphonate 530 -
Sodium xylene sulphonate 490 -
soot >690 535
Sorbic acid 440 460
L-Sorbose 370 ~
Soya flour 550 340
Soya protein 540 -
Starch 470 -
Starch, cold water 490 -
Stearic acid 290 -
Steel 450 ~ Streptomycin sulphate 700 -
Sucrose 420 Melts
Sugar 370 400
Sulphur 190 220
Tantalum 630 300
Tartaric acid 350 ~
Tea 500 -
Tea, instant 580 340
600 -
600 -
550 -
_
Did not 360 ignite
479 -
520 -
400 140
510 330
- _
0.100
_
_
0.03
0.050
0.015 _
0.025
_ _ _ 93
80 95 _ _ _ _ - _ _ _
50 105
10 87
_ 1 400
1 500 _
_
_
_
2 700
12000
_
_
0.040
0.025
50 _
_
_
_
20
0.010
0.020
<O.lO
_
105
10
80
80 3 800
84 3 300 40 _
35 _
_ _
73 4 800
84 8 800
97 2 000
74 400
49 300 73 3 600
94 13000
0.085 100 77 2 800 0.030 35 90 4 600
0.140 _ 65 800
0.050 80 91 3 700 1.10 440 49 400
_ _ _
0.260 220 68
_ _ _
_
_
_
_
_
_
_
_
_
_
_
92
87
_
500
_
_
_
_
400
2 900 _ _ Drd not ignite
_ _
0.100 960 0.050 30
70 700 76 800 87 6 500
11 <lOO
_
_
_
0.020
0.065
0.060
0.050 _
-
_
_
_
_
15
80
100
60 _
25
_ _ _ _
Did not ignite
106 >lOOOO
76 4 700
94 800 98 6 500 - _
80 8 500
_
0.045
0.045
0.035
< 0.20 _
_
_
40
30
15
120 _
_
- -
86 5 500 109 5 000
78 4 700 55 4 400 _ _
93 1 700
48 400 Did not ignite
_
_ _ _ _ _
13
_
_
_
-
9 _
_
5
_
_
_
_
_
_
_
_
_
_
_
_
_
_
5
9
9 -
_
_
-
_
-
_
_
_
_
2
a 1 a 2
8
8
5
4
8
4
8
2
2
1
1
4.6 8
8
8 4
3
1
1
a Group lb1 dust
8
a 8
1
1
1
8
1
1
1 Guncotton ignition
source in pressure
test 8
8
2
i,a Inert gas nitrogen
1
5
5
8
8
1
8
8 1
5
1
3
8
8
5 lcontinuedl
J. Loss Prev. Process lnd., 1988, Vol 7, April 69
Safety considerations in conveving bulk solids and powders: S. Grossel
Dust
Maximum
oxygen
Minimum concen-
ignition Minimum Maximum tration
temperature explosible Minimum Maximum rate of
(“C)
to prevent
concen- ignition explosion pressure ignition
tration energy pressure rise f% by cloud layer fgl ‘1 fmJ1 (lb in *) (lb in *s) volume) References Notes
Tellurium 550 340 Terephthalic acid 680 - Tetranitro carbazole 395 Melts Thiourea 420 Melts Thorium 270 280 Thorium hydride 260 20 Tin 630 430 Titanium 375 290
- _ 0.050 20
_ -
0.075
_
5 0.080 3 0.190 80 0.045 15
_ 84 8 000 _ -
29 100 79 5 500 81 12000 48 1 700 85 11 000
Titanium hydride
Tobacco
Tobacco, dried
Tobacco, stem
Tribromosalicyl anilide
Trinitro toluene
s-Trioxane
cv, cx’-Trithiobis (N, N-
dimethyl-thioformamide)
Tung
Tungsten
Uranium
Uranium hydride
Urea
Urea formaldehyde
moulding powder
Urea formaldehyde resin Vanadium
Vitamin Bl mononitrate
Vitamin C
Walnut shell
Wax. accra
Wax. carnauba
Wax, paraffin
Wheat, flour
Wheat, grain dust
Wheat starch
Wood
Wood, bark
Wood, flour
Wood, hard
Wood, soft
Yeast
Zinc
Zinc ethylene dithio- carbamate
Zinc stearate Zirconium
480 540 485 290 320 - 420 230 880 Melts
480 ~ 280 230
540 240 730 470
20 100 20 20
900 -, 460 -
0.070 60
_ _
Did not ignite _ -
0.070 75 0.143 _
0.060 35
0.070 240 _ -
0.060 45 0.060 5
Did not 0.085 80
121 12000 _
85 1 000
53 400 63 2 100 _
85 600
96 6 000
74 1 900
Did not ignite
69 5 000
74 9 000
ignite
89 3 600
430 - 500 490 380 190 460 280 420 210 260 - 340 - 340 - 380 360 420 290 430 ~ 360 - 450 250 430 - 420 315 440 325 520 260 680 460 480 180
315 Melts 20 220
0.02 0.220 0.035 0.070 0.035
_ _ _
0.050 _
0.045
34 60 35 60 60
110 1 600 57 1 000
120 9 000 88 4 800
121 5 500 _
_
50 _
25
0.020 60 0.050 20
_ _ _ _
0.050 50 0.500 960
- _
109 3 700 43 _
100 6 500 90 5 700
103 7 500 94 8 500 66 63
123 3 500 70 1 800 45 300
0.020 0.045
10 5
80 >10000 75 11 000
Zirconium hydride 350 270 0.085 60 90 9 500
_ _ _ _ _
Ignites in
carbon droxide
3 _ -
_ _ _ _
_ _ -
9
10 - _ _ _ _
_ _
_
Ignites in
carbon dioxide
3
3 4 2
3 3 3
2.3
5 3 3 3
4.8 Group (b) dust 4
8 3 1 1 5 8 8 8 5 2 5 8 4 4 2 2 5 3 1
1.2
3
3
Reprinted by permission of Chapman and Hall Lrd
The dust must have a particle size distribution that will propagate flame. The dust concentration in the suspension must be within the explosible range. The dust suspension must be in contact with an ignition source of sufficient energy.
Under these conditions the hazard from a dust explosion depends upon the explosibility of the dust, the
70 J. Loss Prev. Process Ind., 1988, Vol 7, April
volume and characteristics of the vessel or chamber containing the dust suspension, the dispersion and concentration of the dust suspension and the degree of turbulence in the vessel.
The explosibility of a dust can be determined by tests which are described by Field’. The tests specific to dust explosion venting are described by Field’ and Schofield ‘.
Generally speaking, the explosibility of a combustible
Safety considerations in conveying bulk solids and powders: S. Grossel
dust is greater the smaller the particle size. The minimum ignition energy is reduced and the maximum explosion pressure and rate of pressure rise are increased with a decrease in particle size. In addition, fine particles more readily stay in suspension increasing the probability of producing an explosible concen- tration. Particles greater than 2500 pm diameter are unlikely to cause dust explosions, although the possi- bility of coarser materials producing fine dust by attrition during handling must be anticipated.
Minimum explosible concentrations in air are typ- ically in the range lo-500 gme3 and some values are given in Table 2 together with other explosion param- eters. Explosible concentrations are much higher than those associated with toxic hazards or nuisance prob- lems (which might range from = 1 to 10 mgme3) and such explosible concentrations would most likely occur very close to a dust source or within an enclosed space where the dust cloud cannot spread. Indeed, enclosure and confinement of dust sources to solve occupational hygiene or nuisance problems may well concentrate the dust cloud increasing the risk of explosion. This danger is often overlooked by environmental protection engi- neers.
Ignition sources of sufficient energy to cause a dust explosion are many but the main ones are listed below:
Flames Hot surfaces Incandescent material Spontaneous heating Welding or cutting operations Friction heating or sparks Impact sparks Electrical sparks Electrostatic discharge sparks
Precautions against dust explosions may be for preven- tion or protection and are summarized in Table 3.
The following general approach to dust explosion precautions is recommended:
Where possible select less dusty alternatives for materials and minimize attrition. Minimize handling of dusty materials and design the handling system to minimize dust generation and the size of dust clouds. Avoid the accumulation of dust (which can be dis- turbed to form a dust cloud) by the detailed design of equipment, building and working practices. Anticipate possible ignition sources and eliminate them, as far as is reasonably practicable, by appro- priate equipment design, grounding, maintenance and working practices. Take appropriate additional precautions, where practicable, such as inerting, containment, venting or suppression. Isolate vulnerable plant where appropriate.
Prevention and protective measures wi!l be described in detail below.
Pneumatic conveying systems
Pneumatic conveying systems cause the highest risk of dust explosions and fires for the following reasons:
Static electricity is generated by contact between particles themselves and between particles and the pipewall. Dust concentrations within the explosible range can arise at the delivery point where the dust is separated from the air (silos, cyclones, baghouses). Heated particles which are created during grinding or drying may be carried in a pneumatic transport system and fanned to a glow by the high air velocity. These particles can then cause an ignition in the storage or collection system at the end of the pneumatic transport. Tramp metal in pneumatic systems may also cause frictional heating or sparks as it passes through the system.
Table 3 Summary of dust explosion precautions
Method Comments
Prevention (of an explosion occurring) Exclusion of dust cloud Material can be rendered less dusty and handling system designed to minimize dust. Impossible to
guarantee total dust free environment short of changing to wet process. Exclusion of ignition sources All practical measures must be taken to exclude ignition sources but because sources are often
unknown It is difficult to guarantee so other precautions usually taken. Exclusion of oxygen (inerting) Reduces oxygen content below minimum necessary to support combustion (typically c6-15%). using Nz, CO2 or other suitable Requires continuous monitoring of oxygen content. Usually requires closed system to conserve gas. inert gas. Expensive. Diluent dust addition - to Non-combustible diluent, well mixed with dust, acts as heat sink thus reducing explosibility of reduce explosibility of dust dust. Limited application because of contamination and expense. (Typically the amount of inert dust
exceeds 50%). Containment Vessel and associated pipework etc built sufficiently strong to withstand the maximum explosion
pressure. Expensive in all but the smallest systems. Venting Vents provided in walls of vessel to allow escape of dust and combustion products to limit pressure
nse to an acceptable level. Widely used. Suppression Start of explosion detected by instruments which trigger release of fire suppressants. Useful where
venting is unacceptable or impracticable e.g. when the dust is toxic.
J. Loss Prev. Process Ind., 1988, Vol 1, April 71
Safety considerations in conveying bulk solids and powders: S. Grosset
Design considerations to minimize hazards Pneumatic conveying systems should possess the following general characteristics:
They should be air tight, to prevent the escape of dust from the system where it might present a fire, explosion or health hazard. If operating under a negative pressure the system should be air tight to prevent pulling in air or other contaminants. They should be strong enough to remain intact and air tight under normal operating conditions, includ- ing vibration; and, in some cases, to withstand or contain explosive pressures. Static electrical charges should be minimized by grounding, including bonding across joints where necessary. Eletrically isolated metallic objects within the system may accumulate dangerous static charges. Wire braid within rubber-covered transfer hose may act as a static accumulator.
Electrically conductive bags may be used in bag filters handling extremely static-sensitive materials. Care must be taken to ensure that the conductive bags do not become ungrounded.
0
0
0
a
0
a
An electrical installation must meet the electrical classification imposed by the conveyed materials as well as the surrounding environment. Suitable construction materials compatible with the materials handled and the surrounding environment should be used. Screens, magnets and metal detectors should be installed for the detection or removal of any foreign material which might create hazards in the system. Where appropriate special materials should be used, such as non-ferrous metals, to minimize mechanical sparking in the event of misalignment or failure of moving parts with the process stream. An adequate programme of maintenance and inspec- tion must be instigated to ensure proper alignment of drives, proper clearances, dust tightness, electrical bonding and grounding and control of ignition sour- ces. Joints and openings should be suitably located to facilitate cleaning or unplugging. High velocities (3000 to 4000 fpm) will minimize settling and there- fore reduce frequency of cleaning.
Fire and explosion protective measures All potentially dangerous pneumatic conveying systems which transport bulk solids should be designed to contain an explosion or provided with explosion vents. Two alternative protective measures are: to use an inert gas, usually nitrogen, to transport the solids; and to provide explosion suppression systems.
The sizing and design of explosion vents is covered in detail in NFPA 68’, and this document should be consulted when explosion venting is to be provided for the conveying piping and/or downstream collection or storage facility (cyclone, baghouse, silo). With respect to explosion containment, design of the conveying
piping and even cyclones for this is feasible. Usually, large-volume baghouses are not designed to contain explosions, for economic reasons. In recent years, how- ever, several baghouse vendors have been offering small-to-medium volume units designed for up to 150 psig, which can contain dust explosions.
Explosion suppression is a technique which has been in use for many years and is highly regarded. The shock wave is detected by pressure transducers, located strate- gically in and on the plant item where an explosion is anticipated. The pressure wave is subsequently attacked by a very fast injection of a high concentration of Halon or other suppressant. The vaporization of this Halon breaks down the flame front and leaves behind an inert atmosphere, which prevents a secondary fire being started by any residual glowing particles within the vessel. Plant vibrations and pressure .fluctuations may disturb this sort of system. It is also quite expensive, especially when spurious trips occur and the container has to be refilled. A good discussion of explosion suppression systems is presented in NFPA6gY.
Another protection system has recently been developed which can detect glowing particles at temp- eratures down to 400°C (radiation detector operating in the wavelength 1.5-3.0 cm). The detector activates a fire water injection system which extinguishes the glow- ing particles lo.
Several other good discussions of fire and explosion protection of pneumatic conveying systems and their components have been published3V”-‘3.
Mechanical conveyors
Screw conveyors Screw conveyors have minimal free volumes so that dust suspensions cannot form and thus dust explosions are not usually a problem. However, screw conveyors may occasionally act as a source of ignition by generating friction, which can lead to subsequent overheating and ignition of the conveyed material. This hazard can be minimized by installing an overload trip on the motor driving the screw.
Belt conveyors A belt conveyor system presents two principal fire hazards: first, those of the belt itself, and second, those of the material being conveyed. Belts made of rubber or synthetic products are combustible. Their combus- tibility and the extent to which the heat is released from the belt can cause additional damage to housing struc- tures covering the conveyors.
Some belts are less easily ignited than others, but they are still essentially combustible. Those made of poly- vinyl chloride are one example. Those meeting the fire retardancy standards of the US Bureau of Mines or the British and Canadian equivalents are specific examples. Tests in the Factory Mutual material testing calorimeter indicate extremely high heat release rates once ignited. Several of the more promising types of ‘fire retardant’
72 J. Loss Prev. Process Ind., 7988, Vol I, April
Safety considerations in conveying bulk solids and powders: S. Grosset
belting, meeting the specifications of the US Bureau of Mines, etc., were ignited in tests conducted at FM Research Corporation and, once ignited, burned to completion with high heat release rate. However, use of these will reduce fire frequency because they are more difficult to ignite.
Belt conveyors can also overheat either because of a jammed idler roller or, if the belt jams, as a result of drive rollers continuing to run. The belt can also generate static electricity, and should therefore be of anti-static material.
Belt conveyors used for dusty systems should be enclosed. The free volume within the enclosure is likely to be much greater than with a drag link conveyor and the explosion hazard is thereby increased. The drag link type should be used in preference. If a belt conveyor is chosen, then full protection against explosion should be provided in accordance with NFPA 68.
Factory Mutual recommends a number of protective measures for belt conveyors’4: Automatic sprinklers or water spray protection should be installed for all com- bustible conveyor belts. The belt drive should be inter- locked to shut down on sprinkler water flow.
0 The system should be hydraulically designed for the operation of ten automatic sprinklers and two small hose streams (sprinkler spacing 100 ft’ per head). The system should be designed for a flow pressure of 10 psi on the end sprinkler. (This will result in about 200 gpm for sprinklers and the same for small hose.) In a conveyor enclosure less than 15 ft wide, this can usually be accomplished by installing a single con- tinuous supply line with an unlimited number of sprinklers.
0 Water supplies should be adequate for one hour of use.
l High initial water pressures should be used to over- come friction and elevation loss. A constant dis- charge pressure system would be one method of providing such initial high pressure.
A small hose or equivalent should be provided at suitable intervals with sufficient FM-approved li in hose at all locations to reach any part of the conveyor system. Small hose connections can be made to the sprinkler piping. A suitable alternative to this is the provision of hydrants and easy access to the conveyor by fire fighting equipment. All weeds, brush and trees should be cleared from underneath and at least 25 ft from both sides of conveyor supports. Unprotected combustible buildings and similar exposures should be removed. Each con- veyor belt system should be provided with tamperproof devices arranged to automatically shut off driving power in the event of greater than 20% belt slowdown or misalignment of belts. In addition, interlocking devices should be arranged to shut off power on all contributing conveyors. Where conveyor belts are critical to plant operations, spare belts in quantity consistent with expected fire damage (with recom- mended protection in service) should be kept on hand.
Some damage will occur even with recommended pro- tection in service.
Bucket elevators Bucket elevators are often subject to explosions and fires, and numerous examples have occurred in the past, particularly with vegetable dusts, for which these elevators have customarily been used3. The design of these elevators lead to dust clouds being continuously present during working, particularly at the head and the boot of the elevator. The buckets are also regularly subject to impact and the belt supporting the buckets can slip on the pulleys and generate frictional heat. As a result, a source of ignition and a dust suspension can be present simultaneously, causing explosion or fire. Modern high capacity elevators, with separate delivery and return legs, have a reduced risk because of the reduced volume per unit weight of dust conveyed. In the general case, with explosible dusts, other types of elevator are preferable, and are particularly necessary for dusts of more severe explosibility, e.g., those giving maximum rates of pressure rise in excess of 5000 lb in-’ s (35000 kN m-‘s) in small scale test appa- ratus. Use of elevators should also be avoided for dusts known to be readily ignited by friction, e.g., sulphur.
Where the use of bucket elevators is unavoidable, their positioning should be carefully considered and regular maintenance is essential. The elevators should be mounted outside the building, e.g., supported by an outside wall, and the intake and delivery points should preferably be isolated from the rest of the dust handling plant by means of chokes. The elevator casing should be a fire resistant construction, sufficient to retain a fire, dust-tight and of sufficient strength not to rupture in the event of an explosion. To meet the strength require- ments, the casing should be provided either with auto- matic suppression or with explosion relief at the head and the boot, with vent areas calculated per NFPA 68. Long elevators, say more than 6 m (20 ft), may require additional relief at intervals along the casing to ensure that no point is too remote from a vent.
Particular care should be taken to ensure that flame burning dust, etc., discharged from the vents during an explosion cannot injure operators or damage nearby plant. Provision of ducting or deflectors over the vents may be required. Where it is unavoidable to site a bucket elevator inside a building it is desirable for the internal pressure to be slightly below atmospheric to minimize leakage of dust. Discharge of combustion products from vents to the outside of the building is essential, and the design requirements of ducting from vents should be in accordance with NFPA 68. The need for dust tightness and adequate casing strength and fire resistance should be rigorously met.
Steps should also be taken when designing bucket elevators to minimize the generation of ignition sources. These steps may include the provision of strong fixing for the buckets and strong bearings for all shafts, external to the casing, provided with detectors for
J. Loss Prev. Process lnd., 1988, Vol I, April 73
Safety considerations in conveying bulk solids and powders: S. Grossel
overheating. The main drive to the elevator should be external to the casing. Belt slip within the casing can be detected by belt speed meters, and anit-runback devices can also be installed. Development of friction within the casing can thereby be reduced.
Exhaust ventilation can be applied to the casing of bucket elevators which assists the removal of suspension of dust in air. These suspensions contain the dust fractions of smaller particle size, and ventilation would also give a slight negative pressure relative to atmos- phere, within the casing. The ventilation system should be conducted to a dust collection unit in a safe area, and should be provided with explosion protection on a similar basis to that in the elevator casing itself. Care should be taken to ensure that the air flow is sufficient to prevent deposition of dust in the ducting of the collec- tion unit, and also sufficient to permit the collector to function efficiently. Dust collection systems are con- sidered in more detail in the literature3*“.
Automatic explosion suppression can be used to protect bucket elevators when explosion relief is not practicable, often because of the situation of the el- evator being located within a building. The design of the suppression system should be done by a qualified vendor.
Gillis and Fishlock conducted experimental investiga- tions on bucket elevator explosions to determine ways to limit their effects, or to extinguish the explosion’5. The purpose of these tests was twofold: first, to obtain data and information on how bucket elevators may be vented to eliminate physical damage and to minimize explosion propagation; and second, to demonstrate the effect- iveness of an explosion suppression system in detecting and extinguishing a dust explosion within the bucket elevator.
Halon suppression systems were used successfully to isolate flames from propagating in the head and boot section during explosion vent tests. A specially designed flame retarding distributor, supplied by Union Iron Works, was tested and found effective in limiting flame propagation to a single path when all but one outlet was sealed. Various types of optical detectors were tested as means of detecting the initiation of an explosion and compared with a pressure detector. Results from this
research should shed new knowledge on how to vent duct-like volumes containing obstructions and shows that supression systems can be effectively used to stop explosions.
En Masse Conveyors
The explosion hazard situation in en masse conveyors is similar to that in bucket elevators but more complex especially when operated vertically. In the elevating leg the channel will be full of material, probably above the upper explosive limit, but during start-up, or when the inlet is starved, an explosible concentration could occur. It is impracticable to vent the elevating leg so this should be strengthened and vented at the top.
Exposion vents should be provided on the empty return leg in the same way as for bucket elevators.
References
I Cross, J. and Farrer, D. ‘Dust Explosions’, Plenum Press, New York. 1982
2 Haase, H., ‘Electrostatic Hazards: Their Evaluation and Control’, Verlag Chemie, Weinheim, West Germany and New York, 1977, English translation by M. Wald
3 Palmer, K. N., ‘Dust Explosions and Fires’, Chapman and Hall, London. UK. 1973
4 Bartknecht, W., ‘Explosions-Course. Prevention, Protection’. Springer Verlag, Berlin, Germany, and New York, 1981, English translation
5 Field, P., ‘Dust Explosion’, Handbook of Powder Technology, Volume 4, Elsevier, Amsterdam, The Netherlands, 1982
6 Nagy, J. and Verakis, H. C., ‘Development and Control of Dust Explosions’, Marcel Dekker, Inc., New York, 1983
7 Schofield, C., ‘Guide to Dust Explosion Prevention and Protec- tion, Part 1 - Venting’, IChemE. Rugby, UK, 1984
8 NFPA 68 ‘Guide for Venting of Deflagrations’, National Fire Protection Association, Quincy, MA, USA, 1988
9 NFPA 69, Standard on Explosion Prevention Systems’, National Fire Protection Association. Ouincv. MA. USA. 1986
IO Forsyth, V. G., ‘Dust Explosi& Prc%ection in Pneumatic Convey- ing Processes’, Fire Prevention, No. 135, March 1980, pp. 25-30 Bennett, N., ‘Explosion Protection for Fabric Dust Collectors’, Specifying Engineer, September 1982, pp. 81-83 soo. s. L.. J. PiDPlimY 1981. I. 57
II
12 13
14
15
NFPA 650. Pneimatic Conveying Systems for Handling Com- bustible Materials’, National Fire Protection Association, Quincy, MA, USA, 1984 Factory Mutual Loss Prevention Data Sheet 7-1 I, ‘Fire Protection for Belt Conveyors’, August 1972 Gillis, J. P. and Fishlock, F. H., J. Powder ond Bulk Solids Technology 1982, 6(2), 5
74 J. Loss Prev. Process Ind., 7988, Vol 1, April
