Preventing Problems with Fly Ash Silos
It's important when taking on a new fly ash silo project to be aware of all the potential problems with the material before making any decisions. Here we discuss what to be thinking about and why it's important to get an experienced engineer to assist in addressing these problems before you ever decide on a solution.
When transferring newly collected fly ash from a flue or nag house to a fly ash silo, there will need to be some consideration for where the fly ash is going from there. Are you going to be trucking out the material? Sending off to landfills or recycling for portland cement? Maybe even loading it into rail cars? Can your fly ash silos discharge the fly ash at a fast enough rate for rail? What problems arise when trying to discharge fly ash faster?
Here are some of the typical challenges we must address in a standard fly ash silo or dome application:
Ratholing & Bridging
Flow Rate Limitations
Ratholing & Bridging in the hopper of your fly ash silo
It is common to see the material in a silo start to rathole or bridge if not appropriately managed.
Ratholing occurs in a fly ash silo when the product has an inability to flow freely down the side walls of the hopper and creates a point of feed above the intended spot in the hopper. In layman terms, a rathole in a fly ash silo or any silo for that matter happens when the material on the walls of the hopper is not moving and the material in the center is.
Bridging occurs in a fly ash silo when the product has enough friction with itself to create an archway that touches the side walls of the hopper. Similar to ratholing, this will happen in the hopper of the fly ash silo when the friction between the material and side walls is too great. In layman terms, a bridge happens when the material in your fly ash silo packs down on itself and doesn't let any other material flow out.
When a product beings to experience these flow issues inside your fly ash silo a dynamic load is exerted on the walls of the silo from the switching between free flow, bridging, and ratholing. This can cause structural damage to the walls of your fly ash silo and shorten the life of your equipment. The other and more obvious problem with the ratholing and bridging is the fly ash silo not emptying quickly enough to accept newly collected fly ash, thus creating a bottle neck in your entire process.
Flow Rate Limitations at the input and discharge for fly ash silos
Another less common issue you can run into with your material in silos is flow rate limitation due to the low permeability of the fly ash. Because of its fine particle size, dehydrated fly ash has a significant resistance to flow. This results in systems relying on gravity to discharge the bulk material from the silo to have an inherent flow rate limitation.
Flood flow out of a fly ash silo
On the opposite end of flow rate limitations, when silo have a large enough discharge and the fly-ash has the opportunity to be fluidized, proper measure must be taken to ensure flow rate is metered and controlled.
Abrasiveness of fly ash on silo walls
Due to the high concentration of silicon dioxide that can be present, fly ash is generally regarded as a highly abrasive material and proper consideration needs to be made to prevent excessive wear on silo walls.