Powder Flow Speed Dependence
How flow resistance changes with throughput speed – and whether powder behaviour stays consistent during repeated handling.
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
At the end of the PFSD test, compaction behaviour at four different speeds, cohesion at a reference speed, and flow stability are calculated to characterise how powder resistance changes with process speed and handling.
For each test speed, the positive area under the compaction curve is averaged across two cycles to give the compaction coefficient, which represents the work required to move the blade downward through the powder bed under controlled, compacting conditions.
- An increase in compaction coefficient with speed indicates increasing resistance as throughput rises, which may lead to under-filling or throughput limitations.
- Minimal change with speed indicates speed-independent behaviour.
- A decrease with speed indicates reduced resistance at higher speeds, which can increase the risk of over-filling or loss of control in some processes.
To summarise this behaviour, a Speed Sensitivity Ratio (SSR) is calculated by comparing the compaction work at the highest test speed with that at the lowest test speed. This normalised parameter (also referred to as a High/Low Speed Work Ratio) indicates whether resistance increases or decreases across the tested speed range. Speed dependence therefore reflects the trend of resistance with speed, rather than the absolute resistance at a single condition, making PFSD particularly relevant to high-throughput processing, conveying, and scale-up.
For the Cohesion Coefficient (taken at a reference speed of 50mm/s), the area under the upward negative section of the first and last cycles at 50mm/sec speed curves are averaged. Any increase in cohesiveness or electrostatic forces in the powder would results in a larger negative value and therefore a higher cohesion coefficient.
Flow Stability is calculated by comparing the compaction coefficient measured at the same speed (typically 10 mm/s) at the start and end of the test. A value close to 1.00 indicates that the powder has not changed significantly during handling, while deviation from 1.00 indicates structural change during the test, such as particle breakdown, agglomerate disruption, or densification. Flow Stability therefore highlights whether powder behaviour remains repeatable during processing or evolves with handling history.
Where particle breakdown or friability is suspected, this behaviour can be investigated further using texture analysis with controlled compression or strength testing.
Understanding the measured parameters
Compaction Coefficient – what it means
Tendency of powder to densify under movement at each speed
What this parameter answers:
"How does powder resistance to densification change as process speed increases or decreases?"
| Compaction behaviour across speeds | What it indicates | Likely implications in processing |
| Low and relatively constant | Powder resists densification under motion; packing is stable across speeds. | Consistent bulk density during handling; predictable filling and conveying performance; low risk during scale-up. |
| Increases with speed | Powder densifies more as flow speed increases; resistance builds under dynamic loading. | Density drift at higher throughputs; under-filling risk; increased torque or load at high speeds; throughput limits may emerge. |
| Decreases with speed | Powder becomes easier to move as speed increases; resistance reduces under motion. | Improved flow at higher speeds but potential loss of control; risk of over-filling, flooding, or unstable feed depending on equipment. |
| High at all speeds | Powder packs efficiently even under gentle motion. | Strong sensitivity to handling history; settling during transport; restart behaviour may change after processing or storage. |
Cohesion Coefficient (reference speed) – what it means
Normalised resistance at a defined speed for cross-sample comparison
What this parameter answers:
"How strongly do powder particles resist separation under controlled flow at a representative process speed?"
| Cohesion Coefficient level (at 50 mm/s) | What it indicates | Likely implications in processing |
| Low | Weak particle-particle bonding under dynamic conditions at the reference speed. | Easy flow initiation and sustained movement at this speed. If flow problems occur, they are more likely geometry- or structure-driven rather than cohesion-driven. |
| Moderate | Measurable resistance to separation at the reference speed. | Generally manageable flow, but may show sensitivity to start/stop operation, compaction, or environmental changes. Variability may emerge during scale-up. |
| High | Strong resistance to particle separation at the reference speed. | Difficult flow initiation, sticking to equipment, higher feeder torque, and increased risk of dosing inconsistency or poor restart even during steady operation. |
Speed Sensitivity Ratio (SSR) – what it means
Change in flow resistance between low and high test speeds.
What this parameter answers:
"How does flow resistance change as speed increases or decreases?"
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SSR |
What it indicates |
Likely implications in processing |
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≈ 1.0 |
Speed-insensitive behaviour. |
Powder performance is robust to throughput changes; low scale-up risk related to speed alone. |
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< ~0.85 |
Speed-decrease sensitive behaviour (resistance decreases as speed increases). |
May run well at higher speeds but behave poorly at start-up or low throughput; risk of flooding or loss of control. |
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> ~1.15 |
Speed-increase sensitive behaviour (resistance increases as speed increases). |
Under-filling, throughput loss, or instability at higher speeds; performance may degrade as output demand increases. |
Important usage note for Speed Sensitivity Ratio
The Speed Sensitivity Ratio provides a comparative indicator of speed-dependent behaviour across the tested speed range and should be interpreted alongside other PFSD parameters such as compaction coefficients and flow stability.
This reinforces that SSR is useful but not standalone and that it complements, rather than replaces, PFSD analysis
Flow Stability – what it means
Whether powder behaviour changes with conditioning and repeated handling.
What this parameter answers:
"Does powder behaviour remain consistent as it is repeatedly handled?"
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Flow Stability value and behaviour |
What it indicates |
Likely implications in processing |
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≤ 0.9: Flow-stable |
Flow resistance remains consistent during the test; little change with handling. |
Predictable behaviour during long runs; low drift risk; handling history has limited impact. |
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0.9 –1.2: Moderately stable |
Some change in resistance as the powder is handled. |
Gradual drift during production runs; sensitivity to recirculation or extended operation. |
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1.2 –1.6: Flow-unstable |
Meaningful change in resistance during the test. |
High risk of performance drift, inconsistent dosing, and throughput variability. |
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> 1.6: Strongly flow-unstable |
Large changes in resistance with handling; powder structure evolves significantly. |
Very high drift risk; "runs fine then changes" complaints; strong link to attrition or rearrangement. |
Bulk Density – what it means
What this parameter answers
"What density does the powder adopt after controlled, repeatable preparation?"
Bulk density reflects how particles arrange and pack under their own weight and gentle conditioning, not how they flow or fail.
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Bulk Density behaviour |
What it indicates |
Likely implications in processing |
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Low |
Inefficient packing; high voidage. |
Larger pack volumes; higher fill variability if packing changes. |
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Moderate, consistent |
Repeatable packing under controlled preparation. |
Reliable filling and QC trending. |
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High |
Efficient packing with little void space. |
Smaller packs possible; increased consolidation sensitivity may occur. |
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Variable between samples |
Handling-sensitive packing behaviour. |
Density drift, feeder instability, batch-to-batch variation. |
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
Typical behaviour:
Flow resistance remains consistent across the tested speed range.
Likely risks:
- Flow issues are not driven by operating speed.
- Problems may originate from cohesion, caking, or consolidation instead.
Recommended next tests:
- Cohesion testing – to assess baseline resistance to movement
- Caking – if restart or storage issues are observed
Resistance increases with speed
Typical behaviour:
Powder becomes harder to move as speed increases.
Likely risks:
- Poor performance at high throughput
- Overloading of feeders or drives
- Sensitivity during ramp-up
Recommended next tests:
- Cohesion – to assess inherent resistance to movement
- Compressibility – to evaluate packing and consolidation effects
This behaviour is often associated with frictional or contact-dominated mechanisms.
Resistance decreases with speed
Typical behaviour:
Powder flows more easily as speed increases.
Likely risks:
- Inconsistent behaviour during start/stop
- Sensitivity at low speeds or during gentle handling
Recommended next tests:
- Cohesion at low speeds – to assess start-up resistance
- Caking – if problems arise after rest or storage
This pattern often reflects structural breakdown or partial fluidisation effects.
When PFSD testing is essential
PFSD testing is particularly valuable when:
- Flow problems appear only at certain operating speeds
- Start-up and steady-state behaviour differ
- Throughput changes trigger failures
- Scale-up introduces unexpected flow issues
Why PFSD should not be used alone
PFSD explains how behaviour changes with speed, not why resistance exists. Combining PFSD with cohesion and caking results allows speed effects to be interpreted in the correct physical context.
Sample data and its interpretation
Tabulated data and its meaning
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Sample |
PFSD test data (SSR + Flow Stability) |
Key message and likely processing issues |
Cross-reference testing |
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Wallpaper adhesive |
SSR 0.46 Flow Stability 0.85 |
Speed-decrease sensitive; unstable with handling. May appear to flow more easily at higher speeds, but behaviour changes with handling; unreliable over time; strong drift during runs and recirculation. |
Explains why this material performs poorly despite apparent improvement at speed. Cross-check Caking / Consolidation for severe set-up after storage and Cohesion (Bridging Factor) for discharge risk. |
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Sesame seed |
SSR 1.01 Flow Stability 0.94 |
Speed-insensitive; stable with handling. Predictable dynamic behaviour across throughput changes; low scale-up risk; issues more likely geometry-related than material-driven. |
Typically shows low cohesion and weak caking. Useful reference material for stable PFSD behaviour. |
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Granulated sugar |
SSR 1.56 Flow Stability 1.94 |
Speed-increase sensitive; unstable with handling. Resistance increases at higher speeds and behaviour drifts during extended running; performance degrades as powder is worked. |
Cross-check Compressibility (packing sensitivity) and Caking / Consolidation (set-up after rest). Cohesion alone may underestimate risk. |
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Bird sand |
SSR 1.40 Flow Stability 1.68 |
Speed-increase sensitive; unstable with handling. Strong sensitivity to speed and handling; geometry-driven problems dominate; resistance increases under dynamic loading. |
Matches very high Bridging Factor in cohesion testing. Cross-check Cohesion and Caking if storage-related issues are also reported. |
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Ethylcellulose |
SSR 0.24 Flow Stability 1.24 |
Strongly speed-decrease sensitive; unstable with handling. Works at speed but poorly at start-up; loss of control or flooding risk at higher speeds; behaviour changes with handling. |
Resolves contradictions seen in Cohesion (1 speed). PFSD and Cohesion at 4 speeds are essential to understand rate effects. |
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Sugar soap |
SSR 0.72 Flow Stability 0.65 |
Speed-decrease sensitive; stable with handling. Speed changes influence resistance, but behaviour remains repeatable; generally robust dynamic performance. |
Often aligns with moderate cohesion and weak caking. Cross-check Bulk Density if fill-weight variation is the primary concern. |
SSR > 1: resistance increases with speed (speed-increase sensitive)
SSR < 1: resistance decreases with speed (speed-decrease sensitive)
Higher Flow Stability: behaviour changes more with handling during the test
Charts
PFSD (Powder Flow Speed Dependence) comparisons
PFSD quantifies dynamic resistance (Compaction Coefficient) across multiple speeds (10/20/50/100 mm/s) and reports Flow Stability (change in resistance during cycling).
Two graph types work best: (1) Compaction Coefficient vs speed line plots, and (2) bar plots for Speed dependence (Comp100/Comp10) and Flow Stability.
Baseline resistance across speeds (extremes) – Wallpaper adhesive (very high resistance), Bird sand (high resistance that rises with speed), and Sesame seed (low, near speed-independent).
Speed sensitivity and drift (filling-relevant behaviour)
These four show distinct behaviours that customers recognise in filling: Granulated sugar and Bird sand become more resistant at speed (ratio > 1), Ethylcellulose becomes much easier at speed (ratio < 1), and Sugar soap becomes easier and also breaks down during cycling.
This graph shows whether resistance increases or decreases with speed – a direct indicator of filling sensitivity to line speed.
Flow stability comparison for a range of samples
PFSD behaviour classification – Speed Sensitivity × Flow Stability
Speed Sensitivity – Does behaviour change with throughput?
Flow Stability – Does behaviour change with repeated handling?
This table shows how Speed Sensitivity Ratio and Flow Stability should be interpreted together to assess dynamic flow risk during processing. Both parameters are required to distinguish speed-driven effects from handling-driven drift.
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Flow-stable with handling (low drift during test) |
Flow-unstable with handling (behaviour changes during test) |
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Speed-insensitive (SSR ≈ 1) |
Robust dynamic behaviour
Typical outcome: predictable feeding and conveying |
Handling-driven drift
Typical outcome: drift during long production runs |
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Speed-sensitive (SSR ≠ 1) |
Speed-dependent but controllable
Typical outcome: works well when speed is controlled |
Highest dynamic risk
Typical outcome: unstable feeding, dosing drift, start-up issues |
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?
PFSD measures how a powder’s resistance to flow changes as test speed increases. It identifies whether a material behaves consistently across operating speeds or whether flow behaviour is strongly speed-dependent.
Is PFSD the same as cohesion testing at different speeds?
Not exactly. While PFSD involves testing at multiple speeds, its purpose is to quantify speed sensitivity, not absolute cohesion. Two powders with similar Cohesion Index values may show very different PFSD behaviour.
Why does speed dependence matter in processing?
Many powders behave differently at low versus high shear rates. A material that flows well slowly may resist movement at higher speeds – or vice versa. PFSD helps explain problems that appear only during ramp-up, high throughput, or transient conditions.
Does a low PFSD value always mean good flow?
Not necessarily. Low PFSD indicates consistent behaviour across speeds, but a powder may still be highly cohesive or prone to caking. PFSD should be interpreted alongside cohesion and caking results.
Is PFSD suitable for quality control?
PFSD is most valuable for process understanding and troubleshooting, rather than routine QC. It is particularly useful when flow problems occur only under certain operating conditions.
