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 ~l3SpOSl3
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, 2OC), 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.
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