Bulk density is a basic parameter for the design of solids handling equipment, but rarely has a specific value. It can vary with particle size, compaction and moisture content, amongst other things, so the measurement chosen should reflect its composition, the relevant circumstances of the application and the purpose for which it is to be used. In general, a low value should be used for storage calculations and a high value for power and structural design. One of the most important features of the bulk density values is their correlation with flow condition. A shown by Jenike, (1), the way that the ‘flowability’ changes with bulk density is a key feature of hopper design and a Horslev surface, (2),
Indicates how this changes with stress conditions. Damp and wet materials are particularly sensitive to wet conditions, especially close to the region of saturation. Circumstances that can have a major influence are moisture migration and ambient conditions, such as rain and temperature changes,
The actual value may well vary at origin and change in its route through origin to completion of an application, so the range of appropriate values should be considered. The size of the container used to measure bulk density should reflects the particle size of the media as wall effect influences the particle packing structure near the contact surface. Descriptions such as ‘loose poured’, ‘settled’ and ‘compacted’ are widely used, but the stress conditions that apply in these circumstances can vary widely according to the rate of feed, velocity of impact, scale and design of storage container, vibration, moisture, ambient conditions and the like. Data base values can give a rough guide, but should be treated with great caution as the detailed stress circumstances are rarely given.
The bulk solids industry has a rather dismal history of performance efficiency compared with plants that handle liquids and gasses, (3), (4). A relatively recent expensive failure, (5), suggests that if problems can still arise on large projects, then it is likely that there will be many more that do not see the public light of day. With and application or project involving bulk handling it would be a wise precautions to conduct a flow audit by an experienced person.
Standards:
ASTM D6683 – 08 Standard Test for Measuring Bulk Density Values of Powders
and Other Bulk Solids as Function of Compressive Stress
ASTM D7481 – 09 Standard Test Methods for Determining Loose and Tapped Bulk
Densities of Powders using a Graduated Cylinder
BS EN 1097-3:1998 Tests for mechanical and physical properties of aggregates.
Determination of loose bulk density and voids
Refs.
1 Jenike.A.W. ‘Storage and flow of solids’ Bul.123 Vol. 53. No. 26. Nov.1964
2 Horslev.M.J ‘Time lag and soil permeability in ground water observations’
Waterways Exp, Stn. U<S.Corps. of Engs. Vicksburg.April 1951.
3 Merrow.E.W. ‘Understanding cost growth and performance shortfalls in
Pioneering process plants’. Rand report. 1981
4 Merrow.E.W. ‘Linking R & D problems experienced in solids processing’
Rand report 1984
5 Wellwood.G.A ’Bulk solids handling – A chemical engineers perspective’.
Jenike & Johanson Pty.Belmont. Aus.
