Powder Flowability Index – Turning “Does it Flow?” into Numbers You Can Design Around

This topic is part of the SG Systems Global powder handling, flow and dry-ingredient operations glossary.

Updated December 2025 • Bulk Density Testing, Powder Cohesiveness Classification, Fines & Coarse Particle Distribution, Powder Conditioning, Silo Rat-Holing & Bridging, Loss-in-Weight Feeder Calibration, Vibratory Conveying Dynamics • Ingredients & dry mixes, bakery premix, nutraceuticals, pharma, agrochemicals, detergents, plastics, minerals

Powder Flowability Index is a catch-all term for numerical measures that describe how easily a powder flows under gravity or mechanical force. Instead of arguing whether something is “a pig to move” or “flows fine most of the time,” flowability indices put numbers on that experience: how much it compacts, how quickly it discharges, how much stress it needs to start moving. Used well, those numbers drive silo design, feeder selection, blender loading, packer settings and supplier specs. Used badly, they’re just another line in a COA that nobody trusts.

“If your whole flow strategy is based on ‘it seemed OK in the lab funnel,’ don’t be surprised when the 30-ton silo disagrees.”

1) What a Powder Flowability Index Is (and Isn’t)

In practice, “Powder Flowability Index” is not one single global standard. It usually refers to one or more quantitative metrics that correlate with flow behaviour, for example:

• Compressibility-based indices: Carr Index, Hausner Ratio derived from loose vs tapped bulk density.
• Shear-based indices: Flow Function Coefficient or flowability class from shear-cell tests.
• Simple physical tests: Angle of repose, flow through standard funnels or orifices, time to discharge.

A flowability index is useful if it:

• Is measured consistently with a defined method.
• Correlates with real plant behaviour (silo discharge, feeder stability, packer performance).
• Is integrated into design rules, specs and change control – not just printed on a certificate.

It is not a magic single number that replaces engineering judgement, shear testing or proper design. It’s a starting point – a way to rank powders and flag where deeper work is needed.

2) Why Flowability Indices Matter on the Shop Floor

Flowability is not an academic nice-to-have; it determines whether the plant runs or chokes. Flowability indices drive decisions like:

• Can this product safely go in a funnel-flow silo, or do we need mass flow and a bigger outlet?
• Will a basic screw feeder cope, or do we need agitation and LIW with a live hopper?
• Can we run this blend through a long vibratory conveyor, or will it compact and stop?
• Is a new supplier’s powder “equivalent” in flow behaviour, or will it rat-hole and cake in our existing equipment?

When sites don’t quantify flowability, all of these decisions are based on anecdotes and “tribal knowledge.” That works until something changes: PSD, moisture, fines content, supplier processing or weather. Then the same equipment suddenly “stops working,” and nobody can explain why in quantitative terms.

3) Common Flowability Indices – Carr, Hausner, Angle of Repose & Shear

The most widely used indices fall into three buckets:

• Carr Index (CI) and Hausner Ratio (HR): Derived from loose and tapped bulk densities. They measure compressibility – how much a powder packs down under tapping.
  • HR = ρ_tapped / ρ_loose
  • CI = (ρ_tapped − ρ_loose) / ρ_tapped × 100 %
• Angle of Repose (AoR): The angle of a free powder cone formed by letting powder flow through a funnel onto a surface. Larger angles generally indicate poorer flow / higher cohesiveness.
• Shear-cell indices: Flow Function Coefficient (FFC), unconfined yield strength vs consolidation stress. These underpin Jenike-style flow classifications (free-flowing, easy-flowing, cohesive, very cohesive).

CI/HR and AoR are popular because they are quick and cheap. Shear-cell indices are more informative for serious silo/hopper design and high-risk materials. A sensible “flowability index” strategy usually uses CI/HR as a screen and shear data where the risks or costs justify it.

4) Relationship to Bulk Density and Compressibility

Because CI and HR come directly from bulk density testing, they essentially answer two questions at once:

• How densely does this powder pack under gravity alone?
• How much denser does it get when subjected to vibration or tapping?

Powders with low CI/HR (loosely, <10–15 % CI / HR <1.15) are relatively incompressible and generally flow well. Powders with high CI/HR (>25–30 % CI / HR >1.25) are highly compressible and often exhibit:

• Bridging and rat-holing in silos.
• Unstable discharge rates from hoppers and feeders.
• Caking and “brick-in-bag” behaviour after storage and vibration.

For many sites, “Powder Flowability Index” in practice means “whatever CI/HR bucket this material belongs to,” used as a quick proxy for risk assessment before anyone pays for full shear testing.

5) Relationship to Cohesiveness and PSD Tails

Flowability is tightly linked to cohesiveness class and fines/coarse distribution:

• Fines: Increase surface area and interparticle forces, driving higher CI/HR and poorer flow.
• Broad PSD: Fines filling voids between coarser particles can either improve or worsen flow, depending on proportions and surface chemistry.
• Cohesiveness class: Flowability indices are often used to assign powders into classes (1–4) that then map to design rules for silos, feeders, conveyors and packers.

A “flowability index” that ignores PSD tails is misleading. Two powders with the same D50 but different fines fractions can share a nominal index but behave completely differently in silos and feeders. That’s why flowability work should always be interpreted alongside PSD and fines data, not in isolation.

6) Flowability Index vs Real Plant Behaviour

Lab-measured indices are only useful if they tell you something about plant reality. Good correlations include:

• High CI/HR, high FFC → requires mass-flow hoppers to avoid bridging and rat-holing.
• High CI/HR, high fines → chronic issues in LIW feeder refill and dosing stability.
• Poor AoR and cohesive shear behaviour → erratic flow on vibratory conveyors and chutes.

The right way to use a Powder Flowability Index is to:

• Measure it consistently.
• Log incidents (blockages, feeder instability, caking) against that index.
• Use correlations to refine thresholds and design rules.

Over time, you build plant-specific “flowability maps” – e.g. “anything with CI >25 % and fines >10 % <63 µm must not go in silo type X” – instead of generic statements that ignore your actual equipment and history.

7) Test Methods – Simple Screens vs Shear-Cell Characterisation

Flowability indices come from different levels of testing effort:

• Fast screens:
  • CI/HR from bulk density tests.
  • Angle of repose.
  • Flow/no-flow through a standard funnel or orifice.
• Intermediate tests:
  • Flow through a Hall, Carney or other standardised funnel (often used in metals and pharma).
  • Time to discharge a fixed volume from a standard hopper.
• Advanced characterisation:
  • Shear-cell measurements (ring shear, rotational shear) to generate flow functions and wall friction data for serious design work.
  • Consolidated-stress “funnel flow” tests for more realistic hopper conditions.

Most plants benefit from a layered approach: simple indices for screening and supplier control; shear data for critical materials and equipment design. Making “Powder Flowability Index” explicit in your test list and methods avoids the trap of having ad hoc, one-off flow tests that can’t be compared over time or between suppliers.

8) Using Flowability Index in Silo / Hopper Design

Flowability indices are inputs into silo/hopper modelling, particularly when combined with shear testing. They influence:

• Required hopper cone angle for mass flow vs acceptable funnel flow.
• Minimum outlet size to avoid stable arches and ratholes.
• Need for flow aids (aeration, vibration, agitation) and their sizing.

For example:

• Powders classed as easy-flowing by flowability index may work in properly sized funnel-flow hoppers.
• Cohesive powders (poor flow indices) typically require mass-flow design and, in some cases, mechanical reclaim systems.

Design standards can be tied to flowability classes: “Class 3 powders (poor flow index) must not be placed in hoppers with angles > θ and outlets < D without documented flow testing,” instead of leaving these decisions to vendor judgement or copy-paste of old designs.

9) Impact on Feeders, LIW Systems and Weigh & Dispense

Feeders care deeply about flowability. Poor indices manifest as:

• Erratic feed rates and large corrections in loss-in-weight feeders.
• Need for higher screw speeds and aggressive agitation in weigh & dispense automation.
• Higher variability and longer dosing times in gain-in-weight systems.

Practical uses of a flowability index here include:

• Pre-classification of materials into feeder “recipes” (agitation, speed ranges, refill logic) based on index class.
• Blocking certain powder classes from purely volumetric feeders for critical actives.
• Triggering IPV and closer monitoring for materials whose flowability index is drifting towards known trouble zones.

For automation engineers, tying control logic to flowability classes (stored in material master data) avoids reinventing feeder tuning every time a new SKU is introduced or a supplier changes their milling settings.

10) Environmental Effects – RH, Temperature and Conditioning

Flowability is highly sensitive to environment, especially for hygroscopic and fine powders; see Powder Conditioning. Flow indices can shift as:

• Relative humidity increases (more caking, higher cohesiveness, worse index).
• Temperature changes drive condensation or glass transitions.
• Powders become more or less aerated after pneumatic conveying.

For hygroscopic or temperature-sensitive materials, a “Powder Flowability Index” should be defined at specific RH/temperature conditions and, ideally, measured across a few key points (e.g. 30 % RH vs 60 % RH). Handling rules then reflect the “worst realistic” index the powder will experience, not just the nicest value measured in a lab at 21 °C / 40 % RH that the plant never sees in summer.

11) Flowability Index in Supplier Specs and Incoming QC

For critical powders, including a flowability index in supplier specs and incoming QC tightens control:

• Specs: “Powder Flowability Index class X: CI 10–18 %, AoR <35°” rather than “free-flowing” or “easy-flowing” as vague descriptors.
• Supplier changes: Milling, granulation or drying changes that alter flow indices become formal change-control topics.
• Incoming QC: Routine CI/HR (and, where justified, AoR or simple flow tests) to catch lot-to-lot variability before it hits silos and critical feeders.

This also gives procurement and QA a common language: not “Supplier B’s material is worse,” but “Supplier B’s material consistently comes in at CI ≈30 % vs our design basis of CI ≤20 % for this equipment.” That is an argument engineering and commercial teams can actually act on.

12) Digital Integration – Flowability as Master Data and Design Input

In an ERP/MES context, a Powder Flowability Index can be treated as part of the material’s master-data profile:

• Stored attributes: CI, HR, AoR, flowability class, plus test conditions.
• Routing rules: Certain materials auto-excluded from specific silos, feeders or lines based on flowability class.
• Recipe parameters: Flowability class used to select default feeder settings, refill strategies and allowable CIP/cleaning intervals.
• QRM linkage: Flowability classes surfaced in dashboards used during risk reviews, troubleshooting and SPC analysis.

When flowability data live in the same system as equipment capabilities, routing and recipes, it is much harder for obviously incompatible combinations (e.g. very cohesive powder in a tiny funnel-flow hopper) to slip through unnoticed until start-up day.

13) Limits and Misuse of Flowability Indices

Flowability indices are powerful but easy to misuse:

• Over-reliance on a single index: CI alone does not capture wall friction, arching in real hoppers or dynamic flow in feeders.
• Ignoring test conditions: A CI measured at one RH/temperature being applied at vastly different plant conditions.
• Assuming universality: Flowability classes defined in one plant or industry used blindly in another with different equipment and risk profiles.
• No re-testing: Treating an old flowability value as eternal despite changes in PSD, fines, moisture, supplier or process.

Used correctly, a Powder Flowability Index is the front end of a flow design and troubleshooting toolkit – not the entire toolkit. It should always be interpreted with PSD, moisture, equipment design and real incident history in mind, and refreshed when the underlying material or process changes significantly.

14) Implementation Roadmap – Making Flowability Indices Work for You

A pragmatic roadmap for an Ingredients & Dry Mixes site might be:

• Step 1 – Pick priority materials: Start with powders that cause the most flow issues, silo hammering, feeder instability or seasonal behaviour.
• Step 2 – Standardise tests: Define simple, repeatable methods for CI/HR, AoR and basic flow tests; add shear testing for the worst offenders.
• Step 3 – Characterise and classify: Measure indices for priority materials and group them into flowability classes aligned with cohesiveness classes.
• Step 4 – Link to behaviour: Correlate indices with real flow incidents, LIW performance and packer behaviour; adjust class thresholds to match reality.
• Step 5 – Embed in design & systems: Update silo, hopper and feeder standards; load flowability classes into ERP/MES and routing rules; add indices to RM specs and change control.
• Step 6 – Monitor and refine: Re-test when PSD, moisture, suppliers or processes change; use SPC on flow metrics to detect drift and feed back into testing and design rules.

The outcome is a plant where “flowability” is not a vague complaint but a quantified, managed property with clear implications for design, operations and quality – and where unexpected flow behaviour triggers structured investigation instead of shrugged shoulders and another vibratory hammer bolted to the cone.

15) FAQ

Q1. Is Carr Index or Hausner Ratio on their own a good “Powder Flowability Index”?
They’re a solid start but not sufficient for high-risk or critical designs. CI and HR are easy to measure and useful for screening and supplier comparison, but they don’t capture wall friction, arching behaviour or dynamic effects in real hoppers and feeders. For serious silo/hopper work or difficult powders, you still need shear-cell data and flow-function analysis alongside CI/HR.

Q2. Do we need to define a different flowability index for every product?
Not at first. A practical approach is to classify powders into a handful of flowability classes based on indices (e.g. “good”, “moderate”, “poor”, “very poor”) and link those classes to design and operating rules. As you refine data, some high-value or high-risk products may justify product-specific indices and design assumptions, but broad classes are usually enough to avoid the worst mismatches between materials and equipment.

Q3. How often should we re-measure flowability indices?
At minimum, whenever PSD, milling screens, drying conditions, suppliers or key environmental conditions change. For critical powders, periodic spot checks (e.g. quarterly, seasonally or per supplier lot) help detect drift before it becomes a plant-wide flow problem. If flow incidents increase, re-checking flowability indices should be one of the first diagnostic steps.

Q4. Does improving flowability always mean making powders less cohesive?
Improved flowability often correlates with reduced cohesiveness, but you can overshoot: extremely free-flowing, low-cohesion powders can be very dusty, highly segregating and problematic for weighing and safety. The goal is a controlled flowability window appropriate for your equipment and product – not maximally free-flowing at any cost. That’s why indices must be interpreted alongside segregation risk, dust behaviour and safety drivers like Dust Explosion Hazard.

Q5. What’s a practical first step if our plant has lots of flow complaints but no formal flowability data?
Start small: pick 5–10 problematic powders, define a simple CI/HR and angle-of-repose method, and measure them under representative conditions. Rank materials by these indices and compare ranking with where you see silo, hopper and feeder issues. That alone usually reveals patterns (“all our worst offenders are CI >25 % and fines >10 %”) and gives you enough evidence to justify integrating flowability indices into specifications, design standards and future supplier discussions.

Related Reading
• Flow & Cohesion: Bulk Density Testing | Powder Cohesiveness Classification | Fines & Coarse Particle Distribution | Powder Conditioning
• Equipment & Handling: Silo Rat-Holing & Bridging | Vibratory Conveying Dynamics | Air Fluidization & Powder Aeration | Loss-in-Weight Feeder Calibration | Weigh & Dispense Automation
• Systems & Governance: Quality Risk Management (QRM) | Quality Management System (QMS) | Critical Process Parameters (CPPs) | Statistical Process Control (SPC)