Internal friction and shear testing measures how a powder resists movement under defined stress and wall-contact conditions. The results help assess whether storage can strengthen the powder enough to disrupt discharge from a production hopper.
For pneumatic conveying, reliable hopper discharge is an upstream requirement: the feeder must receive and meter material consistently before conveying air can transport it. Shear data support the design of this storage-to-feed interface.
Measure Strength at Relevant Stress
A shear test consolidates a bulk sample under a controlled normal load and measures the resistance to failure. Repeating the test across a relevant stress range creates a yield relationship and supports calculation of unconfined yield strength.
The stress range should represent the planned vessel. A laboratory result obtained only at high load may not describe a shallow hopper, while a low-stress result may miss the strength developed in a tall silo. Sample preparation, consolidation time and test direction should be documented.
The result is not simply an “angle.” It is a stress-dependent description of when the powder yields. This distinction is essential when evaluating arching, ratholing and outlet size.
Test Wall Friction on the Actual Surface
Material sliding against a hopper wall behaves differently from material shearing internally. Wall-friction tests should use the proposed liner, material grade, surface finish and orientation. A polished laboratory coupon may not represent welded seams, fabrication tolerance or a surface roughened by service.
Temperature and moisture can change adhesion. Polymer liners may also deform or age. Where these effects are credible, the test programme should include the relevant condition rather than relying on a generic coefficient.
Include Time Consolidation
Powders can gain strength while stored. Fine particles rearrange, moisture migrates and chemical or mechanical bonds develop. A hopper that empties immediately after filling may fail after a weekend shutdown.
Time-consolidation tests should reflect credible residence periods. The design can then define whether the outlet, wall angle, flow aid or operating procedure must change. A fixed restart timer does not replace evidence that the stored material can yield.
Separate Hopper Capacity from Discharge Reliability
Vessel volume determines how much material can be stored; it does not prove that the entire inventory will move. Mass-flow and funnel-flow patterns create different residence times, wall contact and stagnant regions. Eccentric outlets or inserts can further change the stress field.
Empirical discharge-rate equations may be useful within their validated material and geometry ranges, but they should not be copied into a general project as interchangeable design rules. A specialist should select the appropriate method using measured material data.
Stabilize the Feeder Boundary
The feeder must accept the hopper discharge without creating a local restriction. Its inlet length, extraction pattern and speed influence whether the outlet is activated uniformly. A conveyor used as a feeder must control rate rather than merely carry whatever material arrives.
When hopper discharge surges, the pneumatic line experiences changing solids loading and pressure. Increasing conveying airflow may raise particle velocity while leaving the actual storage problem unresolved.
Transfer Verified Data into the Conveying Duty
The system specification should state the guaranteed feeder-rate range and bulk-density states supported by hopper testing. The pipeline model then converts that controlled solids input into airflow and differential pressure.
Verify the air-supply duty against its flow, speed, power and discharge-temperature limits after the feeder-rate range is established. Keep shear results with the hopper and feeder design record so transport-air equipment is not used to compensate for unverified storage behaviour.
Glossary
Consolidation stress: Compressive stress that changes packing and strength in a stored bulk solid.
Flow function: Relationship between unconfined yield strength and major consolidation stress for a tested bulk material.
Internal friction: Resistance developed when one region of a bulk solid shears against another.
Time consolidation: Increase or change in bulk-solid strength during storage under load.
Unconfined yield strength: Stress required to fail a consolidated bulk-solid specimen when no lateral confinement is applied.
Wall friction: Resistance to sliding between a bulk solid and a specified equipment surface.
Yield locus: Measured relationship between normal and shear stress at material failure for a defined condition.