When a fine powder traps air between particles, it may exit a bin like it is in a pneumatic conveyor system, shooting through any crack or crevice, including poorly fitting or worn rotary valve vanes, screws, feeders, bolt holes, vibratory feeder pans or belt feeders, until the powder is depressurized. Yet once the powder settles and air is squeezed from voids between powder particles, it can plug feeders, arch over hopper outlets or cling to hopper walls and form a stable rathole, which creates equally unstable process conditions downstream.
Talc, lime, powder cleansers, fly ash, pulverized coal, resin, plastic powders, iron ore and cement are most susceptible to flooding and flushing.
The interaction of powder and air cause flooding and flushing problems they always require a source of pressurized air. Some of these sources include collapsing ratholes, back-pressure from baghouses, free-falling powder, air pads, improperly set air permeation units, rotary valves that feed pressurized pneumatic conveying systems (especially those that are not vented), and gas/solids units with gas counterflow. The three most common sources, however, are collapsing ratholes, free-falling powder and uncontrolled air injection.
Collapsing ratholes. The most frequent source of powder flushing is a flow pattern within a hopper that results in a rathole (Figure 1).
Usually this happens in conical hoppers when hopper walls are not steep enough to cause flow along the walls. As a result, flow is directed to a narrow central flow region while non-flowing material accumulates at the sides and eventually becomes an integral part of the hopper structure. If the rathole collapses or a high rate of powder is then introduced into the hopper, the incoming powder can entrain pressurized air, which will fluidize the powder and result in flushing through screw, belt and vibratory feeders, and even in rotary valves with an entrained air pressure of several psi.
Depending upon the hoppers outlet diameter and the strength of the material, a rathole may become destabilized and can peel off and collapse into the hopper, essentially flushing a large quantity of powder through the outlet and into any feeder below
Free-falling powder. Flushing also results when powder free falls into a hopper in a stream and traps air from the surrounding environment between particles (Figure 2). The faster the powder flows, the larger the void or space between particles, and the greater the entrained air. As the powder accelerates, so does the air. When the powder strikes the top level of material in the bin, some air is dispersed into the atmosphere, creating dust. The rest of the air is trapped within the powder. If the hopper is small, relative to the flow rate out, and if the material within the hopper has a short retention time that doesnt allow the trapped air to escape, fluidized powder can flow at an uncontrolled rate through the outlet.
Uncontrolled air injection. A third cause of flushing is uncontrolled air injection or a chemical reaction within the powder that produces gas (Figure 3). Both can fluidize powder. The only circumstance in which uncontrolled air injection should be considered is in a pneumatic conveying system that transports powder subject to wide fluctuations in feed rate, or into a closed container such as a blow tank that may require a high charge rate to reduce the cycle time.
Tackling the problem
Properly addressing fine powder flooding and flushing starts with testing the materials flow properties to understand how the particular powder interacts with air and what its potential is for flooding and flushing. Often, this indicates that only minimal adjustments are required.
However, the typical approach for controlling or eliminating excess air that leads to flushing is to place a rotary valve at the hopper outlet. Unfortunately, the valves rotating vanes actually pump air into the hopper outlet; so, instead of reducing excess air, the rotary valve supplies additional air. Eventually, the close tolerances between vanes wear down and flushing can occur.
Other ways plants deal with powder flushing are by building a settling chamber at each feeder, increasing a dust collectors capacity, over-filling packages to compensate for occasional underflow, and putting in automatically closing shut-off gates that activate whenever a paddle switch senses flushing.
Fortunately, there are better methods for preventing powder flushing. Potential solutions include: an air permeation system, a sloping let-down chute, a vertical pipe below the hopper, a deaeration cylinder, and hopper retrofits to correct flow patterns. Their costs can vary considerably.
Air permeation system. Powder will not flow at consistently high rates without some entrained air. When totally deaerated, flow rates may fall significantly below the rate required. When totally fluidized with indiscriminate air injection, as may be the case with air pads or pulse jets, the flow rate may increase, but so will the potential for sudden uncontrolled flushing.
An air permeation system with injection nozzles placed at critical positions in the hopper can control both the air pressure and rate of injection, and provide consistently high powder flow rates without the potential for severe flushing. Such a system can replace air lost when a powder compresses and thereby prevent an inrush of air from the outlet that could impede flow. The small amount of air needed to fill voids and increase the flow rate 10 or more times is not enough to create dust or fluidization.