By David Boger, Flexicon Corporation
Manual weigh batching, a feature of many plants that batch-blend bulk products, involves the individual weighing of ingredients before they are charged into a blender or other process vessel. An automated weigh batching system, providing a more-accurate and consistent mixture, may be a viable alternative.
In many plants that use manual methods, it is common practice to work with pre-weighed bags. There are problems associated with this method, such as each bag may not contain the specified amount of material or the worker may not empty the bag completely. Inaccuracies are compounded as more bags are used. Additionally, if an operator is required to count bags in order to achieve proper weight, there may be an increased risk of human error.
There are two automated weigh batching methods: gain-in-weight and loss-of-weight. In the first arrangement, batch ingredients are generally conveyed in sequence into a 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 the 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 vessel.
When to specify a gain-in-weight versus a loss-of-weight system
The most suitable weigh batching method for your plant will depend on how and where bulk material is received and stored. If the material is delivered in rail cars or bulk trucks and stored in silos, which are impractical to mount on load cells needed for a loss-of-weight system, a gain-in-weight system is appropriate. Conversely, if material is received in bulk bags, a loss-of-weight system integrating bulk bag unloaders mounted on load cells may be the solution for you.
Integrating a weigh batching system into an existing process
An automated weigh batching system is often integrated with the plant's bulk handling system, and generally incorporates pneumatic and/or mechanical conveyors. All upstream and downstream equipment, from receiving to processing or packaging, can be sealed to eliminate contamination of the product and plant environment.
The entire operation can be automatically controlled. One option is to use a scale interface card to link up with an existing plant-wide PLC system; a second is to use a stand-alone PLC, and a third is to employ a sequential batch controller. The latter is a programmable device that will run all the equipment associated with the weigh batching system and store the product recipes. It can operate in stand-alone mode or be integrated to varying degrees with the plant PLC.
A weigh batching system should be engineered according to the number, volume, flowability and location of materials to be weighed, the respective strengths of loss-of-weight and gain-in-weight batching methods, and the new or existing conveyors, control systems and processes with which it will be integrated. The availability of numerous weigh batching components and many ways in which they can be configured, enable the plant engineer to design a system that functions reliably, improves product quality and boosts output while cutting cost.