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When powdered materials are discharged from bulk containers, a controlled, efficient powder flow can be crucial to ensuring efficiency in the rest of the production process. Traditional butterfly, slide, gate and iris valves are often used in this operation. The valves are placed at the bottom of a rigid intermediate bulk container (RIBC), bin, hopper or silo and have some or all of their working components immersed in the discharging powder. However, powders with inherently difficult characteristics—such as alumina, mica and zirconia—often experience bridging, ratholing or flushing with these types of valves. Bridging occurs when the powder compacts itself over the valve and will not flow. Ratholing produces a small inner core of powder that will readily flow, leaving an immovable larger outer core of powder resistant to being discharged. The opposite of these actions is a total “flushing” of powder, characterized by an uncontrollable discharge of powder.
To overcome bridging and ratholing, vibrators or air cannons are commonly fixed to the wall of the bin. In some cases, an operator is assigned to beat the side of the bin with a sledgehammer to prevent these problems. Though these methods can aid product flow, they often exacerbate the problem by creating compaction, consolidation or flushing. Discharge aids also create noise pollution and can damage the hopper.
Another common tool to combat these flow problems is the bin activator, an inverted cone that is fixed to an assembly and flexibly supported to the bin outlet. The bin activator is externally vibrated to promote positive discharge. However, this device does not feature any valves to stop the product flow and does not have metering capabilities to control the flow. A properly designed system with a bin activator should also include a positive shut-off valve and a metering device or feeder, and this type of system requires more headroom, more maintenance and more cost.
Cone Valve TechnologyIn the early 1980s, a new cone valve technology that combined the flow promotion of a bin discharger with the controlling characteristics of a traditional valve hit the market. The device consisted of an air-actuated inner cone that acted as bridge breaking device when lifted and as a valve when the cone was lowered and seated next to the RIBC body. Some amount of flow control was achieved by controlling the frequency of the cone. The design spawned numerous competitors and various copycat pieces of equipment that fed off the initial interest.
Although it was a happy medium between bin dischargers and traditional valves, some inherent problems existed with the early equipment. Ideally, a valve should maintain its simplicity with a minimal number of wear parts, and it should also be isolated from the powder flow to prevent equipment wear and extensive maintenance. However, the initial designs were constructed of almost 100 parts that were mostly in contact with the powder flow, potentially causing product contamination and cross-contamination. Even general maintenance with the early complex designs was extremely involved, requiring extended downtime.
Another major problem with these cone valves involved the tipping of the valve in-transit, which caused powder to leak. Even the slightest movement often broke the seal between the valve and the RIBC spout.
New TechnologyIn 2002, a new company* was started to research the intrinsic problems of earlier cone valves. The company spent two years developing a new design that directly challenged each of the past problems with cone valves.
The resulting technology includes a cone valve and transfer hopper. The valve is placed in the RIBC or bin and seals with a hygienic gasket at the RIBC or bin spout. At the same time, a transfer hopper consisting of an inflation device and probe is part of the discharge station below.
Once the RIBC or bin is lowered, the inflation device lifts the probe vertically to raise the cone valve. The powder is free to flow through the transfer hopper tube without coming into contact with any valve parts that could contaminate the material.
The new cone valve is composed of only six parts, which significantly reduces maintenance requirements. One maintenance worker can totally dismantle and clean the valve unit in minutes compared to the hours required to clean the older cone valve technology. Additionally, no mechanisms are immersed in the powder flow, no leakage occurs due to tilting and no degradation of fragile powder occurs.
The new device also provides variable flow metering and a failsafe closing capability. Conventional butterfly, slide, gate and iris valves are very difficult to close against the large force of powder flow. If air or power supply is lost while the valve is open, the powder will continue to flow. Cone valves meet this demand by using gravity. When de-energized, the valve closes with the powder flow and not against it. However, older cone valves faced a problem of trickle leaking around the outer base where the cone meets the seal.
The new cone valve design lowers the cone valve to approximately 15 mm from its fully closed position, thus allowing an upper seal to close while any residual powder is emptied. The valve is then fully closed, without any possibility of leakage. The valve uses a simple external sensor and control logic to vary the lift height of the cone, and it can be set, reset and/or continuously adjusted for accurate batch and rate capabilities. The cone can be statically adjusted to a certain height or can be pulsed to act as a discharge aid for difficult, cohesive powders.
ApplicationsCone valves have predominantly been used in RIBC systems for the pharmaceutical, food and chemical industries. However, they are also gaining popularity in the ceramic industry. Cone valve systems have been used in a number of European ceramic companies with great success, and they have also recently been installed in a few East Coast ceramic facilities.
In addition to being used on process hoppers and silos, the new cone valves have also recently been used in flexible intermediate bulk containers (FIBCs), also known as bulk bags. Both new facilities and current old cone valve users can reap the benefits of this new technology—cone valve upgrade units are available that require very little retrofitting and work as well as new installations.
With this advance in cone valve technology, today’s ceramic manufacturers now have a new tool to ensure problem-free batching and efficient manufacturing.