Bulk Handling Global - Silos, hoppers and bulk storage
Without proper consultation and design reviews, common industry practices such as:-
Silo design is taken as per previous models, however, minor changes in silo height or diameter are made to obtain additional capacity.
Changing product specification.
Changing of silo construction material to that specified.
The assumption that structural design verification ensures proper product flow.
Installation of low friction liners to assist in reducing wall friction values.
Installation of flow promotion devices such as external vibrators or air blast devices.
“Standard off the shelf” silos are suitable different products - arching, bridging and design pressure relations.
- can often lead to safety issues and unreliable product flow.
Typically, there are two modes of product flow to consider for silo and hopper design – mass flow or funnel flow. In mass flow silos, during discharge, the product is in motion over the entire area of the silo or hopper if the side walls are steep enough. Funnel flow is where the product discharges through a flow channel which forms within the material in the bin. The funnel flow channel may not necessarily be central to the silo, and flow can be erratic, leading to segregation, degradation as well as increasing risk to dust explosions.
The design of silos and hoppers is dependant on a number of factors. These include:
|
Product characteristics |
Yield Loci, unconfined compressive stress, major principle stress internal friction angle, Flow function, wall friction static and wall friction dynamic, particle size, angle of repose, moisture balance, as well as bulk densities. |
Required capacity of the silo.
|
Required storage time |
Instantaneous and time consolidation. |
Materials of silo construction.
|
Process requirement |
Required discharge rates, method of feeding into the silo, and any particular requirement such as heating, cooling, mixing or treatment. |
Once the product flow characteristics and design parameters are established, the efficient design of a silo or can be undertaken. Typical, the calculations include dimensional analysis for critical bin opening dimensions, bridging, arching, ratholing, consolidated bin opening pressures, vertical pressures and horizontal pressures in the silo.
Most importantly, the silo pressures and product pressure relations are reviewed, to ensure that ratholing does not occur, checked for a cohesive arch and also, that the product is not sufficiently strong to support the silo pressures, and that the pressures do not compress the product to form ratholing.
The silo pressure reviews are considered for initial and flow conditions. The following graph 1 illustrates typical silo pressure distribution for a cylindrical silo with conical converging wall hopper.
Graph 1 - Typical silo pressure distribution per AS 3774-1996 Loads on bulk solids containers
Silos and hopper geometry is designed based on the effects of gravity. The primary design objective when determining the bin geometry is to ensure that bulk solid or powder will flow with the effects of gravity, and without flow obstructions occurring.
If the silo to product pressure relations are not properly considered, then silo flow obstructions will occur. The typical problems encountered are either a cohesive arch or ratholing. Flow obstructions occur when the forces opposing motion exceed the forces promoting gravity flow. These are shown in the following sketch 1. These flow problems of silo bridging or ratholes lead to safety issues (when operators to clear or break the bridges or ratholes), loss of silo capacity, poor plant reliability and quality control issues.
Sketch 1 - Bin flow obstructions
It is important to note that standards such as AS3774-1996, and other silo design standards or codes whilst do provide “generic” type properties of bulk solids for certain products, reliance on the generic such as this does not assure correct silo design for flow purposes. Any extrapolation of generic data for any other products can lead to potential silo safety issues as well as product flow - silo flow problems, and is not recommended.
The outlet dimension as determined for gravity flow must not be obstructed. Proper discharger and feeder design techniques and interfacing and proper silo design practices can easily be maintained to provide reliable discharge to concentrate the flow stream into downstream equipment.
The converging walls of a pyramid shaped hopper form a valley angle where the side walls meet. It is important that the hopper valley angle is not less than the wall friction angle as measured for the particular product. It is also important to radii the corners of a hopper as fines tend to settle in corners, which can eventually lead to flow problems within the hopper. We have a handy hopper valley angle calculator in our free design section for your use.
The relevant Australian design codes that apply to Silos and hoppers include:-
AS3774-1996 Loads on Bulk Solids Containers
AS1210 Pressure vessels
AS 1250 SAA Steel Structures Code
AS 1538 Cold Formed Steel Structures Code
AS 1657 Fixed platforms, walkways, stairways and ladders
AS 1170.2 and AS 1170.4 Wind and Earthquake design
In addition to the silo design reviews, more often than not, the effects of:-
Feeders that side or end discharge,
Bin activators that channel and vibrate the product through a narrow annulus,
Fluidising pads,
Dischargers that rely on compaction to discharge product,
Flow aids such as air cannons or skimmer type valves,
Internal ledges, fasteners or even incorrectly placed level probes within the silo,
- can easily cause additional consolidation loading above the normal design pressures, creating serious product flow problems.
If you are about to design, purchase a silo, require assistance in solving flow problems, wanting to minimise your silo cleaning costs, contact us and we will be happy to offer assistance in your silo requirements and solve any flow problem issues.
