Bulk density, angle of repose, flowability: the properties that decide the design

Every conveyor, hopper and elevator is sized from a short list of material properties, and most arguments at commissioning trace back to one of them having been assumed rather than measured. This datasheet goes through the properties we ask for, what each one drives in the design, how it is measured and what typical values look like — with the caveat that a typical value is a starting point for a sketch and never the basis for an order. It sits under the system specification guide, whose first step is exactly this list.

Which properties matter, and what does each one drive?

Bulk density

The mass of material per unit volume, air between the particles included. Loose (poured) density is what a belt or screw carries; packed or tapped density is what sits at the bottom of a silo and loads its walls and feeder. Both move with moisture and aeration. It converts tonnes per hour into the cubic metres the equipment must move, and it sets the structural loads.

Particle size distribution and top size

The spread of sizes from a sieve analysis, and the largest lump that will actually arrive — including the occasional one after crusher liners wear. Top size sets belt width, chute openings and elevator bucket choice; the fines fraction sets dust, cohesion and how readily the material segregates. Give both, and say how often the big lumps occur.

Angle of repose and surcharge angle

The angle of repose is the slope a freely poured heap settles at. The surcharge angle is the flatter slope the load takes on a moving troughed belt once the idlers have shaken it down — as a rule of thumb some 5–15° less than the repose angle. Repose sizes stockpiles and chute geometry; surcharge sizes the cross-section a belt carries, and so its capacity.

Moisture

The water content as handled, from drying a sample, expressed on a wet or a dry basis — say which. Moisture changes density, cohesion, adhesion to surfaces and the chute angles that still work; a few per cent can turn a free-flowing fine into one that bridges. Give the seasonal range and the upset condition, not the annual average.

Abrasiveness and hardness

How quickly the material consumes what it touches. Particle hardness, angularity and silica content are the usual indicators, and operating experience on similar plant is the best evidence. It drives liner choice, belt cover grade, screw flight wear allowance and the maintenance interval the plant should expect.

Cohesion and time consolidation

The strength the material develops under its own weight, and how much more it gains after a shift or a weekend at rest. A shear tester measures it as a flow function — unconfined strength against consolidating stress. This is the property that sizes a hopper outlet and decides whether a silo bridges; the silo flow note explains why no heap test reveals it.

Wall friction

The friction between the material and a candidate wall surface — mill-finish steel, polished stainless, a polymer liner — measured in the same shear tester against a coupon of the actual surface. It sets the hopper angle needed for mass flow and the chute angle a wet material will still slide down.

Temperature

The handling temperature and its upset value. It decides belt cover and splice choice, bearing and seal selection, expansion allowances in the structure and whether an elevator needs a chain rather than a belt. Hot material dries and consolidates differently, so its flow properties are tested hot.

Dust and explosivity

How much fine dust the material releases and whether that dust can ignite in air. The safety data sheet is the start; a combustible dust needs its explosion characteristics established by test. It drives enclosure, extraction, the zoning of the plant and the containment of every transfer.

How are they measured?

Most of these tests are quick and inexpensive relative to the equipment they inform. What matters is the sample: representative of the worst condition the plant will see, and tested in that condition rather than dried, sieved and tidied first.

• Bulk density — a known volume filled loosely and weighed, then tapped or compacted and weighed again.
• Particle size — sieve analysis for granular material, laser diffraction for fine powders, a tape and a photograph for the largest lumps.
• Angle of repose — a poured heap or a tilting box, on the as-handled material; the surcharge angle is usually derived from it with the standard offset.
• Moisture — loss on drying, with the basis (wet or dry) stated on the sheet.
• Cohesion, flow function and wall friction — shear cell testing, with a time-consolidation step for anything stored longer than a shift.
• Abrasiveness — mineralogy and hardness, or an abrasion index test, cross-checked against wear on existing plant.
• Dust — the safety data sheet, then explosibility testing where a combustible dust is possible.

What do typical values look like?

The ranges below are the widely quoted textbook figures for the generic material name. They are useful for a first sketch of belt width or silo volume and for checking whether a figure you have been given is plausible. They are not design values: the material in your plant has its own moisture, fines and history, and the spread inside each range is the whole point.

Which four properties change the answer?

Most properties refine a design. Four of them can overturn it, because each one moves the material from one handling family to another.

• Moisture — turns a belt-and-hopper material into one that needs steeper chutes, a mass-flow hopper and a different feeder; it also rules out an airslide entirely.
• Temperature — changes the belt, the bearings, the seals and the structure, and can turn a belt elevator into a chain elevator.
• Abrasiveness — decides between a screw and a belt for the same duty, and sets whether the design has liners, wear allowances and a maintenance interval measured in months or years.
• Cohesion — the difference between a hopper that empties and one that bridges, and the property least likely to appear on the data you are handed.

What should you send us?

The pre-enquiry checklist lists the whole set; for the material alone, five things make the difference between a proposal with margins in the right places and one with margins everywhere. The terms used here are defined in the glossary, and our design and engineering work starts with turning this sheet into a design basis.

• A material data sheet with ranges, not averagesWinter and summer moisture, the fines after a crusher change, the top size when the liners are worn.
• The state the material is handled inFresh from the process or after storage; aerated from a pneumatic line or compacted from a stockpile.
• A sample, or the offer of oneA few kilograms taken at the worst condition are worth more than any table.
• What has already gone wrongBridging, flushing, belt wear, chute build-up — the history is a property in itself.
• The source of every figureMeasured, quoted by a supplier, or estimated — so that we know which numbers to test first.