Fluidisation in silos: loads and discharge
A bulk solid is fluidised when the weight of the particles is carried wholly or partly by air. It then behaves much like a liquid: the horizontal load on the silo wall rises more steeply with depth, and during discharge the bulk solid can flood out uncontrollably. The most common causes are filling, during which the bulk solid mixes with air, and collapsing ratholes in silos without mass flow.
For silos in which fluidisation cannot be ruled out, we calculate the horizontal loads to EN 1991-4 in the fluidised and the non-fluidised condition.
When fluidisation must be considered in design
The standard considers powdery bulk solids with a mean particle size smaller than 0.05 mm. Fluidisation is to be assumed if the surface of the stored solid rises faster than 10 m/h during filling, and in homogenising silos in circulation operation. Such silos are to be designed for the fluidised and the non-fluidised condition.
In silos storing powders, it should be assumed that the stored solid can fluidise if the velocity of the rising surface of the solid exceeds 10 m/h.
Clause 5.5.2 (1)
Wherever there is doubt about possible fluidisation of the solid, consulting appropriately specialised experts (e.g. in bulk solids mechanics) is recommended.
Clause 5.5.2, Note
Horizontal loads with fluidised bulk solid
Fluidised condition
The horizontal load is calculated hydrostatically with the fluidised unit weight γ1 – without wall friction and without lateral pressure ratio:
ph = γ1 · z (Eq. 5.99)
γ1 = 0.8 · γ (Eq. 5.100)
In the hopper the same relationship applies without wall frictional loads (6.5).
Comparison with the non-fluidised condition
In the non-fluidised condition the wall carries part of the load through the wall friction coefficient μ; the horizontal load starts with the gradient K · γ and tends towards a limit value with depth. The fluidised load, by contrast, continues to rise linearly with 0.8 · γ. For the horizontal load the fluidised case therefore governs over the whole height, and the difference grows with depth.
The non-fluidised case is still required: no wall friction is applied in the fluidised condition, so the wall frictional loads follow only from the non-fluidised condition.
The calculation tool compares both cases over depth and shows the governing value. For the non-fluidised case the regular filling and discharge loads from the measured parameters are used.
Main causes: filling and collapsing ratholes
In practice fluidisation arises mainly in two places: during filling, when the bulk solid mixes with air, and when ratholes collapse in silos that are not designed for mass flow.
Filling
- Pneumatic filling and free fall bring air into the bulk solid
- Fine bulk solids deaerate only slowly
- The faster the filling, the less time remains for deaeration – the standard sets the limit at a rise of the surface of 10 m/h
- Discharge shortly after filling with still aerated material
Collapsing ratholes
- In funnel flow a stable rathole remains standing
- When it collapses, bulk solid falls into the channel and takes up air
- Similarly when an arch collapses and during free fall in the flow channel
- The cause is a design without mass flow
Consequences
- Uncontrolled flooding through feeder and shut-off device
- Fluctuating mass flow rate that cannot be metered
- Dust generation
- Increased loads on the silo wall and feeder
Measurements for fluidisable bulk solids
| Measurement | Parameters | Use |
|---|---|---|
| Bulk and compression density | bulk density ρb0 to EN ISO 60, density function ρb(σ) | unit weight γ and fluidised unit weight γ1 |
| Sieve analysis | particle size distribution to DIN 66165, fines < 63 µm | proportion of fine particles |
| Yield locus | all parameters to EN 1991-4, flow function | non-fluidised load case, arching and ratholing |
| Wall friction | wall friction coefficient μ | non-fluidised load case, mass or funnel flow |
| Deaeration behaviour | deaeration time in an aerated tube with a porous base + on request | time that freshly filled bulk solid needs to deaerate |
Measures against fluidisation
- FillingLimit the filling rate so that the surface of the bulk solid rises more slowly than 10 m/h.
- Mass flowDesign for mass flow: no ratholes that can collapse, no free fall in the flow channel.
- Residence timeGive freshly filled material time to deaerate before it is discharged; provide venting of the silo.
- DischargeA metering feeder or shut-off device that also holds back fluidised material; use aeration only in a controlled way.
- DesignDesign the silo for the fluidised and the non-fluidised condition if fluidisation cannot be ruled out.
Frequently asked questions about fluidisation
- When is a bulk solid considered powdery?
- EN 1991-4 treats a material as a powder if its mean particle size is smaller than 0.05 mm. For such bulk solids fluidisation is to be assumed if the surface of the stored solid rises faster than 10 m/h during filling.
- Must the silo be designed for both cases?
- Yes. Homogenising silos with fluidised solids and silos with a high filling velocity are to be designed for the fluidised and the non-fluidised condition under 5.5.1(2)P. The larger value governs in each case.
- Why are the loads higher in the fluidised condition?
- In the fluidised condition the wall does not carry any share of the load through friction. The horizontal load grows linearly with 0.8 · γ as in a liquid, whereas in the non-fluidised condition it starts with K · γ and tends towards a limit value. The fluidised horizontal load is therefore larger over the whole height, and the difference grows with depth.
- How long does a bulk solid take to deaerate?
- This depends strongly on the bulk solid. On request we measure the deaeration time in an aerated tube with a porous base: the sample is loosened with air, then the time until it has settled is determined.
- Which measurements are needed?
- For the loads, the unit weight γ from bulk and compression density; for the non-fluidised comparison case, all parameters to EN 1991-4. Sieve analysis shows the fines content; yield locus and wall friction assess arching, ratholing and the flow pattern, which can lead to fluidisation during discharge.
Have the loads under fluidisation calculated
Describe your bulk solid, your silo and the filling rate – we put together the right measurements and prepare a non-binding quote for you.
- Unit weight and all parameters to EN 1991-4
- Horizontal loads fluidised and not fluidised
- Arching and ratholing as causes during discharge