Equipment managers have done a good job designing bin level indicators resilient to common storage issues. Although these instruments provide operators with accurate contents readings, they can do little to prevent ratholing and bridging inside containers.
Let’s take a look at how operators can reduce these issues.
Also known as arching, bridging transpires when a physical layer of material develops at the opening of a hopper, thus preventing the funnel from functioning properly. A variety of factors can cause this problem, including particle shapes, temperature, the concentration of fines and moisture.
According to bin, feeder and chute manufacturer Jenike & Johanson, there are two types of arching:
- Interlocking happens when the products push against each other. When downward pressure is applied, they press harder, creating a strong bridge. Think of gravel locking against each other at a small opening.
- Cohesion is the result of wet or sticky material packing together around an opening. The same force that induces interlocking also leads to cohesion.
To resolve interlocking, the outlet diameter must be six to eight times larger than the biggest piece of material. So, if the greatest bit of gravel is 5 centimetres, the outlet diameter would, ideally, be 40 centimetres.
Tackling cohesion requires a bit more attention. The exact dimensions of the hopper may need to change depending on the density, friction and cumulative strength of the product causing the issue. High accuracy precision instruments can provide assistance in this regard.
Plugging the ratholes
Ratholing transpires when material discharges straight downwards through the hopper outlet. This usually happens when the material is cohesive, so, it doesn’t cascade in a cone shape through the bottom.
The key to resolving this issue lies in establishing consistent, downward pressure. This can be done by ensuring the grade of the hopper is steep enough to prevent vertical force to become isolated. In truth, creating an ideal design that prevents ratholes requires operators to gather the following materials information:
- Frictional properties
- Flow function
- total density or permeability
Industrial measuring instruments can enlighten operators to how temperature, climate and other factors impact these characteristics. Once this analysis is complete, engineers should have a strong idea of what sort of hoppers (conical, chisel, etc.) will best mitigate ratholes.
Supporting mass flow
Mass flow occurs when all products within a hopper flow out consistently, without disruption. Ratholing and bridging and other issues do not occur whatsoever. It’s the ideal operation.
Maintaining mass flow requires two factors to be addressed. First, the slopping hopper walls need to be steep enough with modest friction, allowing particles to slide along them. Second, the hopper outlet must be wide enough to prevent arching.
As has been alluded to in this article, there is no one-size-fits-all design. The hopper configuration of one operation will not be compatible with another. The following video from Jenike & Johanson delivers a good visual of the issues discussed throughout:
Maintaining optimal procedure
Why take the time to discuss these problems? They disrupt operations. Jenike noted one instance when a South African iron ore mine had to suspend the workflow several times per shift to unclog its shoots, which suffered from plugging.
The bottom line: engineers design extractive processes to run within budget. Expenses associated with those operations are impacted by time, resources, labour and other factors. Think of a mining operation as a formula. If one element of the formula deviates from its predetermined course, it affects the formula’s outcome.
So, if a shoot is crafted to funnel 10,000 total petroleum hydrocarbons (TPH) of iron ore every 10 minutes, but does so every 25 minutes, the cost-effectiveness of the operation will suffer. If a machine is calibrated to behave a certain way, removing just one gear will change this behaviour.