Position: Excerpt from Article:
Automating Your Weigh Batching System
Reductions in dust, lower costs aud improved measurement accuracy for your ingredients are just some of the advantages
David Boger Flexicon Corp. n many plants tbat batch-blend bulk products, weigh batching is a manual, time-consuming operation in which ingredients arc weighed individually before being discharged to a blender or some other process vessel. A significant number of such plants could benefit from tbe installation of an automatic weighing and batching system. For small as well as large operations, an automated weigh-batching system can pay for itself fairly rapidly through increased productivity, increased accuracy for the measurement of ingredients (resulting in better product quality and lower feedstock costs!, minimization of product losses and dust, and reductions in the cost of materials now purchasable in larger containers or volumes. One of the strongest cases for an automated weigh-batching system is that it improves product quality by providing a more accurate and consistent mixture. In many plants that use manual methods, a common practice is to work with pre-weighed bags -- for example, dumping ten 50Ib (23-kg) bags to add 500 Ib (227 kg) of an ingredient. Problems associated with this method are that each bag may not contain exactly 50 lb of material, and that tbe worker may not empty the bag completely. Inaccuracies are compounded as more bags
Mechanical coiiveyots slow leedrales luf dccurate ingredient measurement to within 1% of recipe targets, and isolate the powders from ambient moisture. System software compensates for "material-in-flight" once the batch weight is gained by stopping the conveyor remotely
are used. Additionally, if an operator is required to manually count bags in order to achieve the proper weight, yet another chance for human error is present.
Gain-in-weight vs. loss-of-weight
Tbere are two automated weigh batching methods: Gain-in-weigbt and lossof-weight. In the first arrangement, batch ingredients are generally conveyed in sequence into a weigh hopper located above a process vessel, typically a blender or storage vessel. The hopper is set on load cells that transmit weight-gain data to a programmable logic controller (PLC) that starts tbe conveyor for each ingredient and then stops it when the preset weight for that ingredient is reached. Finally, the controller automatically charges the batch to the process weighing vessel. In a lossof-weight system, the source of each ingredient (such as, a bulk bag unloader or preloaded hopper) is mounted on load cells that transmit weight-loss data to a controller that starts and stops each conveyor (or rotary airlock valve) to weigh each ingredient. Determining the most suitable weigh-batching method can depend on how and where bulk material is received and stored. If. for example, the material is delivered in rai I cars or bulk trucks for storage in silos -- which are impractical to mount on the load cells required of loss-of-weight systems -- a gain-in-weight system would be more appropriate. Conversely, if the material is received in bulk bags, a loss-ofweight system integrating bulk bag unloaders mounted on load cells may offer the simplest solution.
Accounting for material-in-flight.
When operating in a gain-in-weight mode, an additional variable that must be accounted for in the design of tbe system is the "material-in-flight".
Solids Processing which is defined as the amoimt of material still on its way to the scale after the batching controller has deactivated the material-feed device. Although the material-in-flight variable can be minimized by proper control sequencing, it is important to be aware of this source of potential inaccuracy. In general, greater volumes of material-inflight and greater variations in material flow to the batch scale will result in higher potential for inaccuracies. For example, if a mechanical conveyor (Figure 1) is discharging by gravity directly into a gain-in-weight hopper, the amount of material-in-flight will be proportional to the vertical distance from the conveyor discharge to the gain-in-weight hopper, If the discharge of the mechanical conveyor is immediately above the top of the gain-in-weight hopper, there will be little material-in-flight. Hence, even if there is a good deal of variation in the flowrate, which can be caused by a poorly flowing material being inconsistently introduced to the mechanical conveyor, the material-in-night will be relatively constant from batch to batch. Since a constant amount of material-in-flight is predictable, it is easily compensated for by the weigh-batching controller by stopping the feed device at a point prior to actually achieving the desired batch weight. An example of a gain-in-weight batching system with a significant amount of material-in-flight would be a relatively long pneumatic conveyor with a rotary valve functioning as the material feed device. In this case, when the rotary valve at the material in-feed point is stopped by the batching controller, the pneumatic conveying line will be full of material that i.s already in the process of being conveyed to the batching scale. Although trial-and-error tests can approximate how much sooner the conveyor should be stopped to compensate, the increased volume of material-inflight can result in a higher degree of variability from batch to batch. In this situation, it may be prudent to utilize alternate modes of operation, or a different equipment configuration, such as using a scale valve. A scale valve is a type of diverter valve mounted in a pneumatic conveying line that passes above a gain76
FIGURE 2. Mechanical (above) and pneumatic (below) gain-in-weight batching systems can transport material from silos, manual dumping stations, process equipment, bulk bags or any other source to a hopper, a biender, or other downstream equipment mounted on load celts in weigh hopper. It allows material to either pass through the valve body or, directs material downward into the gain-in-weight hopper below. When the target weight is reached, the valve redirects any remaining material in the conveying line past the scale, eliminating or minimizing the aforementioned problem of excessive material-in-flight. Because the material continuing downstream ("carry-over") must be accumulated, and air vented fi'om the system, the pneumatic line downstream of the scale valve is normally routed back toward the material source (such as a bulk bag unloader) into a filter receiver with a rotary airlock valve that reintroduces the material into the pneumatic line immediately downstream of the material source. If the material-in-flight is to be reused in the process, a separate filter receiver is needed for each batch ingredient. Accordingly, scale valves are generally suited for the weigh batching of only one or two ingredients having single or multiple destinations, particularly if headroom above the gain-in-weight hopper(s) or process batching equipment is limited. Scale valves are unsuitable and unnecessary for loss-of-weight systems because the rotary valve feeding material into a conveyor line stops …
