Troubleshooting

Common Mistakes in Bulk Solids and Aggregate Processing (and How to Catch Them)

The mistakes that quietly wreck bulk solids numbers: loose vs packed density, ignored surcharge angle, dry-basis moisture, and screen efficiency read off the wrong deck.

The single most common error in this category is mixing loose and packed bulk density. Symptom: a Silo Capacity number that overstates tonnage by 15 to 25 percent, or a truck that hits weight limits at 80 percent volume. Root cause: crushed limestone runs about 1.35 t/m3 loose but 1.60 t/m3 tapped, and people grab whichever number is handy. Fix: pin the state before you calculate. Use loose density for conveyor and hopper fill, packed density for stored settled tonnage, and run the Bulk Density Conversion calculator to move between kg/m3, lb/ft3, and t/m3 so a 100 lb/ft3 spec does not get read as 100 kg/m3, an error of 16x.

Ignoring surcharge angle on belt conveyors is the next big miss. Symptom: measured throughput lands 10 to 18 percent below the Conveyor Capacity estimate, and spillage appears at transfer points. Root cause: the theoretical cross-section assumes material heaps to the full surcharge angle, but wet or sticky fines slump to a 5 degree surcharge instead of the 20 degree design value. Fix: cut the load cross-section for real surcharge. A 20 degree assumption on a 35 degree troughed belt might give 0.045 m2; a slumping 5 degree material gives closer to 0.032 m2, so derate capacity by roughly 30 percent before you promise tph.

Belt speed and lump size get set independently, which starves or chokes the line. Symptom: the Belt Load number looks fine on paper but the belt runs half full or plugs the chute. Root cause: a 650 mm belt carrying 150 mm minus feed should run near 1.5 to 2.5 m/s; push it to 4 m/s to chase tph and lumps bounce off, push it to 0.8 m/s and you overload the belt tension. Fix: match speed to lump size and belt width first, then read capacity. Recompute Belt Load at the real speed rather than assuming the drive nameplate speed, since VFDs often run at 70 to 90 percent.

Moisture reported on the wrong basis corrupts every tonnage figure downstream. Symptom: an Aggregate Yield calculation that shorts the customer by 3 to 6 percent per load. Root cause: quarry samples often report dry-basis moisture (mass water over dry solids) while payment is on wet-basis (water over total mass); at 8 percent moisture the two differ by about 0.6 points, and at 20 percent they diverge by nearly 3 points. Fix: state the basis explicitly, convert with wet = dry / (1 + dry), and subtract free moisture before you bill sellable dry tonnes. Test moisture per shift, not per week, when feed comes off an open stockpile.

Screen efficiency read off the wrong deck is a classic troubleshooting trap. Symptom: Screen Efficiency comes out above 100 percent, or product spec fails while the number says 95 percent. Root cause: undersize in the feed is confused with undersize passing the deck, or the analysis uses the top deck cut when the complaint is the bottom deck. Fix: define efficiency as mass of undersize actually passing over mass of undersize in the feed, both from real sieve analysis, not gradation assumptions. World-class near-size handling aside, if your calc exceeds 100 percent you double-counted feed; recheck the feed cut point against the aperture, typically 0.7 to 0.8 of the wire opening.

Screw conveyors get sized at 100 percent fill, which no cohesive material ever reaches. Symptom: measured rate is 55 to 70 percent of the Screw Conveyor Throughput prediction and the trough packs at the outlet. Root cause: loading factor was left at 1.0 when free-flowing grain runs 45 percent and sluggish, cohesive powder runs 15 to 30 percent. Fix: apply the trough loading for the flow class, then derate for incline; a screw at 15 degrees loses about 30 percent capacity and at 25 degrees can lose over half. Also confirm pitch equals diameter for standard flights, since a half-pitch screw carries roughly half the volume per turn.

Crusher throughput gets quoted at the open circuit rate while the plant runs closed circuit. Symptom: the Crusher Throughput figure of, say, 300 tph never shows up because 40 percent recirculates. Root cause: the circulating load from oversize returning to the crusher was ignored; a 40 percent circulating load means fresh feed capacity is only 300 divided by 1.4, about 214 tph. Fix: separate fresh feed from total feed, size the crusher for total, and size the return conveyor and screen for the recirculating tonnes. Confirm closed side setting too, since a CSS that has worn open 10 mm can swing product gradation and change the circulating load by 5 to 15 points.

Powder handling ignores flow behavior until the hopper bridges. Symptom: Powder Flow Rate predictions hold in the lab but the bin rat-holes and Blending Time balloons because material will not discharge evenly. Root cause: fine powders below about 100 microns are cohesive, and mass flow was assumed when the hopper geometry only supports funnel flow. Fix: check the wall friction angle and half-angle against the flow function before trusting steady discharge; a 30 percent bump in cohesion can turn reliable flow into intermittent flooding. For blends, verify the coefficient of variation drops below 5 percent at the sampled Blending Time rather than assuming the mixer curve from a different powder.

The quiet killer across all of these is unit drift between metric and imperial. Symptom: a number that is off by 1.1, 2.2, or 16, the classic conversion multiples. Root cause: short tons (2000 lb) mixed with metric tonnes (2205 lb), or lb/ft3 confused with kg/m3. Fix: carry units through every step and let the Bulk Density Conversion calculator anchor the material property. A quick sanity check catches most of it: aggregate near 1.6 t/m3, water at 1.0, so any bulk solid density under 0.3 or over 3.0 t/m3 deserves a second look before it flows into Conveyor Capacity or Silo Capacity.

Published 2026-07-01.