Measured material data are the key to safe silo design
EN 1991-4 expressly bases the characteristic values on laboratory tests – for silos in Action Assessment Class 3 they should be determined by testing. Even small differences in particle shape, particle size, moisture or composition change the flow behaviour considerably.
You receive all the parameters needed to characterise your bulk solid and for further technological calculations.
Characteristic values of the material properties are to be selected on the basis of values determined in laboratory tests, taking into account well-established experience.
Clause 4.2.1 (7)P
For silos in Action Assessment Class 3, the bulk solid properties should be determined by tests according to 4.3.
Clause 4.2.2 (3)
Material parameters for silo calculation to EN 1991-4
For the actions on silos you receive the mean values of the parameters and the corresponding conversion factors (scatter factors), from which the upper and lower characteristic values follow.
Notation as in EN 1991-4. The angle of internal friction φi of the standard corresponds to our effective angle of internal friction φe; φe is used for the calculation to the standard.
For silo loads to EN 1991-4
| Parameter | Symbol | Conversion factor (scatter factor) | From the measurement |
|---|---|---|---|
| Unit weight | γ | aγ | Bulk and compression density |
| Angle of internal friction | φi | aφ | Yield locus – corresponds to our effective angle of internal friction φe |
| Lateral pressure ratio | K | aK | Yield locus |
| Wall friction coefficient | μ | aμ | Wall friction |
| Cohesion | c | – | Yield locus |
| Patch load solid reference factor | Cop | – | from aφ and aK (Eq. 4.8) |
| Angle of repose | φr | – | Angle of repose |
For technological design
| Parameter | Symbol | From the measurement |
|---|---|---|
| Effective angle of internal friction | φe | Yield locus |
| Flow function, unconfined compressive strength | σc(σ1) | Yield locus |
| Flowability | ffc | Yield locus |
| Wall friction angle | φw | Wall friction |
| Density function | ρb(σ) | Compression density |
| Time consolidation | σc,t | Time yield locus (optional) |
Calculations for technological silo design
With the measured parameters you calculate the actions on silos to EN 1991-4 and the flow behaviour in the silo using the methods of bulk solids technology – for designing new silos as well as for checking existing silos with a new product.
Actions on silos to EN 1991-4
- Filling and discharge loads in the vertical sectionHorizontal loads, wall frictional loads and vertical loads, with classification into the Action Assessment Class.
- Patch loads and hopper loadsPatch loads during filling and discharge, hopper loads in the filling and discharge condition – including the vertical pressure at the outlet as the load on the feeder.
- Special casesLarge eccentricities during filling and discharge, and fluidisation.
Because of internal friction, the vertical stress is only partly converted into horizontal stress (lateral pressure ratio K, typically about 0.3 to 0.6); through the wall friction coefficient μ part of the load is transferred into the silo wall. The pressure therefore tends towards a limit value with depth. During discharge the loads are higher than during filling, and a load peak occurs at the transition from the vertical section to the hopper.
Flow behaviour in the silo
- Ensuring mass flowLimit criterion for mass and funnel flow from hopper inclination, effective angle of internal friction φe and wall friction angle φw.
- Preventing archingCritical outlet diameter from the flow function σc(σ1): above it, no stable arch can form over the outlet.
- Preventing ratholingMinimum shaft diameter or maximum fill height up to which no stable rathole forms in funnel flow.
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Mass flow The entire bulk solid is in motion – what goes in first comes out first.
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Funnel flow Only a core flows; dead zones remain at the hopper walls – segregation and ageing.
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Ratholing Only a stable channel above the outlet empties; the rest stays in the silo.
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Arching A stable arch above the outlet stops discharge completely.
Calculation tool in the customer portal
Once the measurements are complete, the calculation tool is available to you. You define the silo geometry and the basis of calculation yourself.
- Create, save and compare several silo configurations
- Retrieve results at any time, process them further and use them in your own calculations
- PDF report to your own specification – chapters freely selectable
- Test report with an assessment of the flow behaviour; where needed, flow problems of the existing silo geometry and specific suggestions for improvement
Measurements for technological silo design
Yield loci at a minimum of three different consolidation stresses, wall friction and bulk and compression density together provide the parameters for the actions on silos to EN 1991-4 and for technological design. For squat and intermediate-slenderness silos (hc/dc < 2) we also determine the angle of repose. Testing is carried out on standardised shear testers to ASTM D6682.
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Test YL Yield locus at a minimum of 3 consolidation stresses φi, φe, K, c, σc, ffc + optional time consolidationFlow function, angle of internal friction and lateral pressure ratio across the stress range
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Density function and unit weight γ
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Wall friction coefficient μ across the stress range
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Test AR Angle of repose DIN ISO 4324 or ASTM D6393, recommended for squat and intermediate-slenderness silos φrAngle of repose for the surface cone and dischargeability
- Silo loads to EN 1991-4
- Mass flow or funnel flow
- Arching
- Ratholing
Additional measurements
- Time consolidationTest series from a few hours to several days, matched to the storage and standstill times of your plant.
- Moisture contentEven small deviations in moisture affect flowability. For a comparison at a different moisture content we adjust a target moisture in the laboratory or measure your prepared second sample.
Frequently asked questions about silo design
- Which EN 1991-4 parameters do I receive?
- The mean values of the unit weight γ, the angle of internal friction φi, the lateral pressure ratio K and the wall friction coefficient μ, with the conversion factors (scatter factors) aγ, aφ, aK and aμ for the upper and lower characteristic values, plus the cohesion c, the patch load solid reference factor Cop and the angle of repose φr.
- Which measurements does a silo calculation need?
- Yield loci at a minimum of three different consolidation stresses, wall friction on your wall material, and bulk and compression density. For squat and intermediate-slenderness silos (hc/dc < 2) the angle of repose is added, and time consolidation on request.
- Are table values sufficient for silo design?
- EN 1991-4 bases the characteristic values on laboratory tests; for silos in Action Assessment Class 3 they should be determined by testing. There are no table values for mass flow, arching and ratholing – they depend directly on the measured flow behaviour.
- Are there reference data for particular bulk solids?
- Measured values of other samples can only be transferred to a limited extent. Even bulk solids with the same name can have very different flow properties – depending on production and processing, particle size distribution and particle shape, moisture, temperature and storage time. For a reliable design we therefore measure a sample of the bulk solid that is actually to be stored in the silo.
- How much sample material is needed?
- About 1 to 2 litres per sample for a complete bulk solids analysis for silo calculation. The material should be representative and not exceed a particle size of 2 mm. For wall friction we also need a wall sample made to our drawing; we keep standard materials in stock.
- Which silo details are needed?
- Approximate figures are sufficient: silo shape (circular, rectangular) and dimensions, height of the vertical section, hopper inclination relative to the vertical, hopper height and/or outlet diameter, wall material (polished, smooth, raw, rough) and approximate bulk density. We use them to match the consolidation stresses to the stresses that actually occur in your silo.
- How long do the measurements take?
- Results are usually available about two weeks after sample receipt – depending on laboratory capacity and the number of samples. Time consolidation tests can extend the standard lead time accordingly.
Measured data for your silo design
Describe your bulk solid and your silo – we put together the right measurements and prepare a non-binding quote for you.
- All parameters for silo calculation to EN 1991-4
- Mass flow, arching and ratholing in the calculation tool
- Test report with an assessment of the flow behaviour
- Confidential, under NDA on request