Powder Flow Speed Dependence
What is Powder Flow Speed Dependence (PFSD)?
Powder flow behaviour can change substantially as process speed increases or decreases. Some powders become more resistant to movement at higher speeds, leading to under-filling, throughput limits, and feeder overload. Others become dramatically easier to move as speed increases – which sounds beneficial but can cause over-filling, flooding, or loss of control at high production rates. Many powders also change behaviour during extended handling, drifting gradually from their initial state as particle structure evolves under repeated stress.
These effects are not visible in static or single-speed tests. The Powder Flow Analyser (PFA) PFSD test quantifies them directly by measuring the work required to move the blade across a defined speed range, and tracking whether that behaviour remains stable across the test sequence.
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Powder Flow Speed Dependence testing answers the question: "How does powder flow behaviour change across a defined range of process speeds, and is that behaviour stable?" |
How the PFSD test works
The test begins with two conditioning cycles to remove any user loading variation and to normalise the powder column after filling.
The powder flow speed dependency test provides 5 sets of 2 cycles at increasing speeds. The downward parts of the cycles compact the powder and the upward stroke of the cycle uses a lifting action.
The test measures resistance of a powder sample as controlled flow is imposed at different speeds. Powders that flow freely will transfer very little resistance through the powder column in either a downward or an upward direction. Conversely, poorly flowing powders exhibit substantial amounts of force in either direction.
Measured parameters
- Compaction Coefficient (at multiple speeds) (g.mm) – how readily the powder compacts as speed increases, indicating sensitivity to dynamic loading and aeration effects
- Cohesion Coefficient (at 50 mm/s) (g.mm) – a normalised measure of resistance to flow that allows comparison between powders and batches
- Speed Dependence (Speed Sensitivity Ratio) – the change in flow resistance or compaction behaviour across the tested speed range, highlighting powders that become more cohesive or unstable at higher speeds
- Flow Stability – comparison of the work required to move the blade at the same speed at the start and end of the test, indicating whether powder behaviour changes with conditioning or handling history
- Conditioned Bulk Density (g/ml) – bulk density after controlled preparation (split vessel)
Interpretation of the graph profile
Understanding the measured parameters
Compaction Coefficient – what it means
Cohesion Coefficient (reference speed) – what it means
Speed Sensitivity Ratio (SSR) – what it means
Flow Stability – what it means
Bulk Density – what it means
Note: No single parameter describes powder behaviour. Caking, PFSD, and Cohesion parameters should be interpreted together to understand how a powder behaves during movement, over time, and after rest.
When is a PFSD test most useful?
A PFSD test is most useful when powders run acceptably at one speed but show under-filling, over-filling, drift, or instability as throughput changes. It quantifies how flow resistance varies with speed and whether powder behaviour remains stable during repeated handling. PFSD is particularly relevant for conveying, high-throughput processing, and scale-up, where speed-related effects often emerge that are not visible in static or single-speed tests.
What to test next based on your PFSD results
PFSD identifies whether a powder’s flow behaviour is speed-sensitive. The most appropriate follow-up tests depend on whether resistance increases, decreases, or remains stable with speed.
Low or negligible PFSD
Resistance increases with speed
Resistance decreases with speed
When PFSD testing is essential
Why PFSD should not be used alone
Sample data and its interpretation
Tabulated data and its meaning
Charts
PFSD behaviour classification – Speed Sensitivity × Flow Stability
How the PFSD test compares with other powder flow measurements
PFSD vs Cohesion
- PFSD measures speed sensitivity.
- Cohesion measures baseline resistance to movement.
- Together, they distinguish whether poor flow is inherent or process-dependent.
PFSD vs Caking
- PFSD focuses on dynamic behaviour during movement.
- Caking focuses on strength development during rest.
- A powder may show minimal PFSD effects but still fail after storage.
PFSD vs Compressibility
- Compressibility assesses packing under load.
- PFSD assesses resistance during motion.
- Highly compressible powders often show strong PFSD effects, but this is not universal.
FAQs
What does PFSD actually measure?
Is PFSD the same as cohesion testing at different speeds?
Why does speed dependence matter in processing?
Does a low PFSD value always mean good flow?
Is PFSD suitable for quality control?
