BULKtalk, Equipment & Technology

BULKtalk: Selecting the right bulk silos

In his regular BULKtalk column, Steve Davis explores the different criteria for bulk silos and stockpile selection, and how to ensure it operates effectively.

Why use a silo instead of a stockpile? Silos typically occupy less real estate than an equivalent capacity stockpile. Silos typically have a much higher percentage live volume than a stockpile, assuming both are gravity reclaimed. Silos contain dust well. Silos are easier to weatherproof. 

In most cases a silo system will be cheaper than a stockpile having similar live volume. Silos have finite capacity, whereas stockpiles can be designed with expansion capacity via push out. Gravity reclaimed stockpiles are easier to clear than most silos. Stockpiles can contain much larger volumes of bulk materials. Both silos and stockpiles are suitable for batch or dynamic storage. Both stockpiles and silos can have mechanical reclaim for 100 per cent live capacity. The rest of this article looks at silos, but much is relevant to stockpiles.

Selecting the capacity of silos depends on use. Capacity for batch storage of a fixed quantity, which is loaded, held and then unloaded is straightforward. Capacity for dynamic storage, where the silo is filled and emptied at the same time is more complex as the maximum capacity and the operating capacity need to be determined to suit varying levels from the operation. Surge storage is the most common example of dynamic storage. This provides a buffer between two components in a system that have different feed rates but must operate together, such as a truck fed crusher and a mill.

Trucks loading crushers may arrive at random times allowing gaps in feed. Crushers are typically less reliable than mills and to maintain steady feed will be sized at a higher feed rate. The silo acts as a buffer or surge absorber in the system. A decision must be made on the time value of surge in the silo. How long can the crusher be offline before the mill is allowed to stop? How long can the mill be offline before the crusher is stopped? What catch up factor is to be used? If the silo has been emptied how quickly must it be refilled back to normal levels? After evaluation we should have maximum capacity and nominal silo operating level. We should also have sized the crusher and silo feed system for the agreed catch-up rate and assessed the reclaim system for mill feed catch up.

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Silos have a realistic maximum size, and it is unreasonable to exceed this. Depending on the bulk material the practical limit for a silo could be as low as 1000 tonnes or less. More than 5,000 tonnes may be impractical or require specialised design. Concrete dome and other silo types are capable of large volume storage of some materials with good flow properties, but they are still limited. Oil sands projects often use approximately 10,000 tonne capacity bins but these have many specific features. If very large storage is required, consider a stockpile. I was once introduced to a concept design that had a nominal 500,000 tonne silo with a single discharge. After discussion we changed to a stockpile.

As with most bulk materials handling, we design for tonnage throughput for the process but the system operates on a volumetric basis. This dictates we must have knowledge of bulk density and preferably of the material flow properties. The minimum properties that should be defined are lower and upper bulk densities, compressibility, repose and drawdown (rill) angles and flowability properties to define reclaim outlets and mechanical reclaim equipment. Ratholing is always a risk that must be identified and can often be avoided by selecting the reclaim outlet size and shape. Reclaim rate must be at least the design catch-up rate. You can look material properties up in various on-line and other sources, but it is far better to have the material tested at one of the specialist laboratories so that the data is specific to the material being stored.

We now look at the shape of the silo. If material properties allow choice, flow regime has most impact, whether funnel, mass or expanded flow. Typically, mass flow silos are taller and more expensive than funnel flow with expanded flow somewhere between. The type of flow selected is also dependent on process requirements; first in first out, segregation of size or properties of material of discharge, ratholing propensity and others. Silos with circular section are generally easier to fabricate and have symmetrical flow patterns but may not be the best for other reasons. If the silo has mechanical reclaim different considerations may also apply. The best sources of guidance on silo design are the bulk material testing laboratories who test bulk flow properties. They will also provide guidance on liner materials for flow and wear.

Some bulk materials will require assistance for reclaim as gravity flow is not practical or feasible. There are few bulk materials that have not been successfully handled in silos, and many examples where the silo design has been incorrectly chosen requiring modification or addition of flow promoters. The best result will be achieved through consideration of a proven system and understanding the bulk handling properties. Assuming or guessing the design of a bulk storage silo is risky. 

I once assessed a biomass silo with a 45-degree conical discharge into a 200mm pipe with short radius bends and gate valve isolation. There was no flow under gravity. The silo had no material flow consideration in its design The cone was removed, and a live floor reclaim installed. I have seen multiple outlet silos where only one outlet works and silos with eccentric outlets that resulted in structural failure.

With silo size and shape determined it is time to design the shell and structure so that the carefully selected shape will stand up to all forces generated internally and externally. Silos can be constructed from several materials. Concrete is common and can be slip formed or cast conventionally. Steel can be assembled or welded on site or in some cases prefabrication and transport in one piece. Selection will depend on location, available resources, and other considerations such as cost. There are many codes and standards in use for guidance on the design process. In Australia we must consider State regulations and codes and sign off by Registered Engineers where applicable. 

We now have a design for the correct size silo, with shape and / or facilities that permit reclaim at the required rate and that should not fall over in operation. We need to look at some other issues relating to operation and maintenance. 

When bulk solids are poured into the top of a silo the stream will separate as it falls, and any dust will be released. The air displaced will find its way through the openings in the top of the silo and a dust cloud will follow. We must consider dust collection that will have capacity for the maximum air displacement and prevent discharge. There are several options available, and the best result will be found by modelling the airflow and working with a filter system supplier to develop the best solution. For many applications a drop in dust collector comprising a baghouse with reverse pulse cleaning will be sufficient. We will need to get to the top of the silo to maintain the system.

If the silo discharge has no material in it, the falling stream will damage the silo and cause significant wear, so we operate the silo with a minimum material level and the material impacts itself. One silo that I assessed went from a five-week liner cycle to over six months by always leaving  two metres in the silo. We would also like to assure that the silo is never overfilled, as this can cause significant damage and any spill over can be a safety hazard. I was on a site looking at silo discharge when it overflowed – an unpleasant experience. 

This leads to the requirement for level measurement to ensure we know at least a safe minimum and maximum shut off level reclaiming before the silo is empty under normal usage. Better still, we are able continuously monitor level either on a single point basis or over the surface of the stored material. Different types of level sensor are available for every requirement but take guidance from previous experience and suppliers who can offer multiple options and advise the best one for the application. If level monitoring is to work correctly the sensors must be installed correctly with supplier recommended support and with access to align, calibrate, inspect, and maintain. If the instrument just hangs on its cable is unlikely to function correctly.

Silos generally discharge onto a feeder and conveyor or similar device that has the potential to fail. We should consider how to isolate flow at the discharge to permit repair without having to manually discharge the silo. It has been common practise to use isolation or spile bars, however in most cases these are now unacceptable for manual activation. Bars may not provide sufficient safety under current legislation. Consider a methodology for mechanically pushing the bars in and pulling them out, and whether sealing will be adequate, or look to an actuated slide gate with proven credentials.

Fatalities and injuries from operators entering a silo are not uncommon, so the design must minimise the potential for well-meaning people to be able to access the silo for any reason. Access doors and hatches into the silo are acceptable if there are good reasons, but make sure they are impossible to open easily. The interior of a silo is a dangerous enclosed space and should be treated as such.

As a final consideration, silos with poor flow may be difficult to modify even with assistance from flow specialists and the wide range of experiences and flow assisting equipment available. A solution will generally be available at a cost. If, however the silo fails structurally there is little chance of repair in most cases and real risk of injury or worse.

If, however the silo fails structurally there is little chance of repair in most cases and real risk of injury or worse. I refer to an excellent paper reflecting some of the causes and effects written by Tracy Holmes of Jenike & Johanson for Powder and Bulk Solids, Preventing Silo Failures. 

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