Recently, we engaged with a customer that a few years ago had spent over $2 million with another vendor on an 18 station, automated bulk bag batching system. Depending on the recipe some or all of the 18 different ingredients are simultaneously metered onto a common belt conveyor for transfer to one of two mixers. Each station dispenses its ingredient in specific batch weights according to the recipe.
Unfortunately, the system doesn't work.
Individual ingredient batch weights are inaccurate and vary both above and below the weight setpoint and the variations are random. Therefore, the total recipe weights are inconsistent and are randomly under or over weight.
As a result, they have had to go back to adding most of the ingrdients by hand and use the automated system for only four or five ingredients.
The customer asked Control and Metering if we had any ideas that might help them. Based on a very brief overview of the system and how it operates we identified two key factors that we believe may be root causes of the weighing problem. These factors are critical to achieving success with any batching system.
- Dry bulk solid metering device selection. The 18 ingredients vary dramatically in their bulk density, particle size distribution and flow characteristics. Each station must be optimized for its ingredient so that the bulk bag discharger maintains a steady flood feed condition to the metering device and so that the metering device is properly selected to maintain a constant - and therefore accurate - feed rate when it is dispensing its batch. In this case, vibratory feeders were supplied at every station, which appear to be inneffective with many of the ingredients.
- Batch weighing structure integrity. Each of the 18 bulk bag batching stations are constructed of 3" steel tubing and the metering devices - vibratory feeders - are suspended below each frame on chains. Load cells are located under each of the bulk bag discharger legs. We suspect that the overal structure lacks integrity and may well exhibit hysteresis - the frame subtly shifts each batch run or series of batch runs and the weighing system physically alters its orientation - perhaps by only millimeters - between batches. Further, it may be that frame flexing occurs during batch operation. Both of these phenomenon, if they are occuring, would lead to random batch inaccuracies. The weigh frame or weighed structure must be designed to avoid frame flexing and deformation so that the weighing system remains structurally rigid from batch to batch.
As can be seen, a properly designed bulk bag batching system must be designed specifically for each ingredient. Unless multiple ingredients are similar in bulk density and flow characteristics, it is highly likely that each station will have its own optimum design solution.
As was pointed out in an earlier blog post "How Big is A Bulk Bag", there is no such thing as a 'standard' sized bulk bag. The base or bottom seam dimensions are generally sized to fit within the mode of shipping to be used (shipping container, truck trailer, etc.) and/or to fit properly on the selected pallet.
Depending on the bulk density of the product and its characteristics, the bulk bag's height can be significantly affected by the bulk bag filling technology employed. And, this can be a source of lucrative operating cost savings!
If your product's packed bulk density is around 35 lb/cu ft or less and its loose bulk density is more than 10-15% less than that, chances are reasonable that you are not able to achieve bulk bag weights of 2,000 to 2,205 lb using bulk bags sized to fit within a shipping container or a truck trailer.
If that's the case, maximum densification during filling can in most cases put more weight into the same sized bulk bag.
When that happens operating cost savings pile up quickly:
- Since you can put more weight in each bag, you can buy fewer bags. A 10% increase in payload weight equals a 10% reduction in the number of bags you have to buy!
- Since you can put more weight in a container or trailer there are fewer shipments that must be made and each shipment carries more weight per container. Once again, a 10% increase in payload weight reduces the number of containers thereby reducing shipping costs. This can be a HUGE cost savings!
So, if you are unable to currently achieve one ton/tonne payload weights in your bulk bags investigate using bulk bag filling technology that will mazimize densification. You may be able to put tens and even hundreds of thousands of dollars onto your plant's profit line by increasing the payload weight of your current sized bulk bags !
Learn about Control and Metering's cone table bulk bag filling technology that provides maximum densification.
Do you receive ingredient in bulk bags with loose liners? Do you have problems getting the ingredient to flow from the bulk bag? Here's some tips on how to overcome bulk bag discharging flow problems.
Focus on how the bags are filled.
Flow problems often are the result of the bulk bag liner not being inflated prior to filling. If this crucial step is not taken, the liner can fold over itself in the bottom of the bag. If that happens and a ton of ingredient is filled into the bag on top of the folds, flow problems are sure to occur.
The only way to prevent this is to properly inflate the liner just prior to filling so that it takes the shape of the bulk bag without folding over on itself. Talk to your supplier or, if you fill bulk bags for in-house or intra-company use, take a close look at your bulk bag filling equipment and standard operating procedures.
Optmize liner handling during discharge.
Liner tensioners are devices that wind up loose liners during discharge to prevent them from 'growing' too far out of the bottom of the bulk bag and becoming entangled in downstream equipment. They also improve product flow and ensure that all of the ingredient is discharged from the liner thereby providing 100% yield.
Using one of a number of different styles of liner tensioner can prevent flow problems from bulk bags with loose liners.
- Hold in place. The simplest type of liner tensioner simply holds the liner in place to prevent it from completely exiting the bottom of the bag during discharge. Make sure that there is sufficient space between the bottom of the bag and the downstream equipment so that when the liner grows out of the bag during dicsharger it doesn't become entangled in downstream equipment. Also note that this style tensioner does not pull up on the liner and therefore does not do an effective job of ensuring complete discharge.
- Partial wind up. A pneumatically actuated tensioner winds up a predetermined amount of the liner during discharge - say 24" - 36". This reduces or eliminates liner growth out of the bag bottom and ensures complete discharge.
- Complete wind up (see photo above). If you use a discharger that clamps the liner outlet you must use a tensioner that continuously winds the liner. Otherwise, the liner will gather at the clamp and almost always prohibit ingredient flow. This type of discharger and tensioner are typically used when dust containment is critical.