In the article "High Speed Bulk Bag Filling - Preweighing", preweigh bulk bag filling systems, capable of achieving filling rates up to 40 bulk bags per hour, were described:

"Pre-weighing refers to weighing the payload of product to be placed in a bulk bag in a separate bin or hopper above the bulk bag filling machine.

Doing so eliminates the time required to precisely weigh the product as it flows into the bulk bag on the filling machine. Rather, the pre-weighed 'shot' can be dropped into the bulk bag as quickly as it can be made to flow through the pre-weigh hopper outlet."

In this article critical design considerations will be discussed.

Product Conveying Balance

A preweigh bulk bagging system capable of filling 40 bulk bag per hour must be 'fed' by the upstream production process and the system that conveys product to the bagging location, at the same rate - product in must equal product out.

This can be accomplished in two ways.

Often, the production line will continuously convey product to a large silo (50+ tons capacity).  The preweigh bulk bagging system is located underneath the silo and bulk bagging occurs either continuously, if production can keep up with the bulk bagging rate, or periodically when production has filled the silo to an appropriate level.

A less common arrangement is where the production line can continuously match the bagging rate and product is continuously conveyed to the bagging line where it is continuously filled into bulk bags.

Weighing

Designing the weighing sub-system is the most challenging aspect of implementing a preweigh bulk bagging system.

As was shown in the previous article, a 40 bag per hour system has a total cycle time of 90 seconds.  Of that, 15 seconds is allocated to emptying the preweigh hopper (the vessel wherein the product shot is weighed) into the bulk bag.  From the preweigh hopper's perspective, the remainder of the cycle time - 75 seconds - can be used to weigh the next shot.

Assuming a typical dribble feed duration of 15 seconds,  the fast fill portion of the weighing cycle is 60 seconds.

Typically, 50 - 100 lb of the total shot weight is dribble filled.  That means that, in the case of a 2,000 lb bag, 1,950 lb of product must be metered into the preweigh hopper in 60 seconds.  This equates to a very high instantaneous flow rate of approximately 117,000 lb/hr.

Two key factors must be considered to maintain accurate and consistent weighments:

1. Flow control.  The device used to meter the product into the preweigh hopper must be properly selected.
2. Flow rate.  The rate at which the product flows into the preweigh hopper must be consistent from bag to bag. 

There are a number of ways to design such a weighing system according to the desired weighing accuracy and capital budget.

Note that preweigh systems can be designed to consistently maintain accuracy of +/- 2 lb.

Bag Stability

The faster bulk bags are filled the less time is available for densification.  Further, bag stability becomes more of a concern as the bulk bag filling rate increases. 

Applications with bagging rates over 20 per hour must be carefully assessed to determine what amount of densification is required to produce safe and stable packages.

As the bagging rate approaches 25 bulk bags per hour and beyond, stability and densification become a major concern with almost any product - particularly if stacking the bulk bags is a requirement.

In a preweigh system capable of filling 40 bulk bags per hour, the filling machine must be carefully designed to ensure the filled bags are stable and safe.

In the next article we will look at when a preweigh bulk bagging system should be used.

Last week I had dialogue with a potential customer who was stumped with a bulk bag problem.  The solution had nothing to do with bulk bag handling equipment; rather it had everything to do with the flexibility of bulk bags themselves.

This customer was faced with having to fill bulk bags with a sludge like material that contained over 20% moisture.  Needless to say, it easily bridged and ratholed.

Specifically, the customer's system was being designed to mechanically convey the product to a small surge hopper, which would in turn empty into a bulk bag.

The customer was searching for a way to ensure that the product would flow easily from the surge hopper into the bulk bag.  However, they had been unable to find a suitable solution to getting the sludge through the bulk bag inlet spout sized at the typical 14" diameter.

The answer required a paradigm shift.  That was: bulk bags do not have to have a spouted top.

As can be seen in the following diagram, bulk bags can be manufactured with many different top and bottom configurations.

In the case of this application the solution was to use the 'skirt top' design shown above - also commonly known as a duffle top bag.

This allowed the customer to design the surge hopper with an outlet large enough to prevent bridging and ratholing while still being able to get the product into the bag. 

All they had to do was open the duffle top to completely expose the inside of the bulk bag and let the sludge fall from the large opening in the bottom of the surge hopper.

Because the product was so heavily moisture laden dusting was not an issue. Therefore, there was no need to have the bulk bag inlet sealed to the surge hopper outlet.

Plus, the duffle top allows the bag top to be tied and closed securely preventing comtamination of the product and spills.

The customer was able to simply, easily and inexpensively solve their problem once they learned that - true to their name - Flexible Intermediate Bulk Containers or bulk bags can be designed to suit almost any bulk solids handling task - flexible indeed!

Increasingly, bulk bags are used to ship 'hazardous' dry bulk solid materials.  This post investigates what you need to know to handle them.  Note that we will address the topic of how bulk bag manufacturers make bags to meet relevant regulations related to robustness and package integrity in a subsequent post.

First, let's discuss what hazardous means.  The United Nations defines hazardous goods as follows:  "Definition of 'dangerous goods' covers articles or materials capable of posing significant risk to people, health, property, or environment when transported in quantity."

That's a broad definition, but for the purpose of this discussion we'll focus on three potential hazards that can arise during bulk bag handling:

1. Static discharge.  Bulk solids that within a bulk bag may generate enough static charge to iginite flammable vapors, gases or dust.
2. Explosive dust.  Bulk solids whose dust, if allowed to accumulate in a high enough concentration, may explode if exposed to a suitable ignition source.
3. Health hazard.  Bulk solids that present a health hazard to humans via direct contact.

Dissipate Static

There are two ways to mitigate the risk of static discharge: bag and equipment design.

Of the four types of bulk bag (Type A, B, C and D), typically only types C and D are used to prevent static discharge leading to explosions. 

Type C bulk bags feature conductive fabric or conductive threads or filaments woven into non-conductive fabric.  The key factor is that Type C bags must have a grounding point to which all fabric, threads or filaments are connected.  The grounding point must then be connected to an external ground point duing filling and discharging of the bulk bag.

Type D bulk bags are constructed from fabric that allows static charge to be dissipated without being connected to ground.  The advantage of Type D bags is that operators are not required to making a ground connection with the bag prior to filling and unloading - a task that can be forgotten sometimes resulting in disaster.

Equipment used in situations where static discharge could ignite flamable substances must, as a minimum, have ground lugs.  Depending on the Area Classification electrical components may have to be explosion rated.

Contain Dust

If the product you are filling or discharging can cause an explosion if a high enough concentration of dust is exposed to an ignition source, dust containment is critical.

Bulk bag fillers must have 'twin-tube' fill heads and provide a tight seal with the bulk bag inlet spout.  The outer tube of the fill head must then be connected to dust collection system so that dust laden air that is displaced during the filling process is safely extracted.

Bulk bag dischargers must have dust containment features suitable for the hazard level.  There are different levels of dust containment available (discussed in another blog post).

All equipment must feature electric components rated for the Area Classification.

Isolate Human Contact

Bulk solids that are health hazards include various refined metals that are carcinogens, chemical compounds that cause respiratory problems and so on.  When dealing with these products maximum dust containment is required.

Specialized bulk bag discharging equipment is available for this application.  It is critical that such equipment contain dust at the following critical steps in the bulk bag unloading process:

• Initial onset of product discharge.  The point in time when product begins to flow from the bulk bag can create a large amount of dust.
• Throughout bulk bag emptying.  Of course, dust must be contained as the bulk bag is emptying.
  
• End of discharge. Near the end of the discharge cycle the bulk bag will loose its shape.  Equipment must be designed to maintain dust containment integrity at this crucial stage.
  
• Bag removal.  Removing the empty bulk bag - still full of dust laden air - must be done safely without exposing operators to dust.

Unfortunately, Dusting is a Common Problem

While bulk bags can be the most economical package for shipping semi-bulk quantities of dry bulk solids, they often get a bad rap from end-users because the process of unloading them can be dusty.

Excessive dusting during discharge obviously causes a mess requiring additional clean up labor.  However, it can cause significant profit reduction because every pound of product that lands on the floor reduces ingredient yield and increases input costs.

Dusting Can Be Costly!

We know of a food manufacturer whose bulk bag discharging stations create a lot of dusting during unloading.  A plant visit confirmed the mess made during bulk bag discharging, but curiously their floors are always spotless!  

The plant has hose bibs EVERYWHERE, which encourages cleaning.  Seemingly a good thing.  

However, a thorough analysis of their ingredient yield showed that they were losing approximately $300,000.00 annually due to poor yield: TONS of product - along with a good chunk of profit - were being washed down the drain!

While it's true that not all of that waste was directly attributable to dusting from bulk bag dischargers, most of it was.

So, not only is dusting during bulk bag unloading a nuisance and housekeeping issue, it can quickly affect - negatively - the bottom line.

What can be done?

1. Training.  The biggest issue we see with the use of bulk bag dischargers is that operators are not following the manufacturer's operating instructions.  Regardless of the equipment used to unload bulk bags, if it is not used properly optimal dust containment cannot be achieved.  Left to their own devices operators will quickly figure out the easiest way to get product out of the bag - and that often leads to unecessary dust emissions.  Easy operation sounds like a good thing, but the reality of discharging bulk bags is that they require some effort to use properly.  Work with your bulk bag discharger vendor to fully understand how their equipment should be used to best effect and then train and supervise your operators accordingly.
2. Bulk bag design.  We often see situations where fine product sifts through the fabric of bulk bags.  If that sounds familiar your ingredient vendor should be willing to work with you to provide a bulk bag that eliminates this problem.  The easiest solution is to used coated bulk bags that feature a film of polypropylene bonded to the inner surface of the bag fabric.  This should cure most sifting problems.  However, if a coated bag isn't enough your vendor should be using a separate polyethylene liner.
3. Bulk Bag Discharging Equipment.  Bulk bag dischargers come in many configurations.  Some are better than others when it comes to dust containment.  Look for designs that allow easy  and safe access to the bag outlet spout while keeping it enclosed in a hopper that can be connected to a vacuum dust collection system.  However, not all of these designs are created equal!  If the hopper that is under negative pressure is filled with product during discharging, dusting will still be a problem when the hopper door is opened and product that has adhered to the inside of the door drops on the floor.  Ideally, the outlet spout of the bulk bag should protrude out of the hopper during discharge so the only time the hopper sees dust is during the initial stage of product flow and perhaps at the end of the discharge cycle.  Ergonomics are also a key criteria when selecting a bulk bag discharger.  As mentioned above, you don't want your operators circumventing Standard Operating Procedures and creating excessive dusting because the bulk bag discharger is too difficult to use properly.  Carefully evaluate dust containment features and ergonomic design when selecting a bulk bag discharger.

Before we discuss all of the considerations necessary to design a successful high-speed bulk bag filling line, let's first agree on what "high-speed" means!

If you are currently filling four bulk totes per hour, 10 bags per hour may seem to be high-speed.  After all, that's a 250% rate increase!

If you're filling 10 bags every hour, an automated bulk bag filling system that produces 15 - 20 FIBCs in the same time it takes you to fill 10 is a big deal.

However, at least for the purpose of this article, we're going to define high-speed bulk bag filling as producing 30 or more filled bulk bags per hour.

Yes, that's a lot, but it can be done consistently, with excellent weighing accuracy and such a rate can be achieved while producing stable - safe - bulk bags.

Sequence of Events

What happens when filling bags at a total rate of over 30 tons - 60,000 lb - per hour?  Well, exactly the same things happen as when filling 3 bags per hour, except they happen in a much shorter period of time.

Here is the timing chart for a bulk bag filling system designed to achieve a rate of 30 bags per hour:

Those are the same five tasks that are required to fill virtually any bulk bag at any rate.  However, as you can see and as we predicted above, the duration of each task is very short if we want to achieve a total cycle time of only 120 seconds!

High Speed Bulk Bag Filling - Critical System Considerations

Bulk Bag Design

The design of your bulk bag must support the overall system required to achieve a high speed bagging rate. 

For example, is the design of the loops compatible with the equipment such that they will be easily released and not hang up when the bag is automatically removed from the filling machine? 

Is the diameter of the inlet spout big enough to accommodate the filling machine fill head - that has been designed to achieve the instantaneous flow rate necessary to fill the bag in the required time?

Bulk Solid Handling System

The system that moves or conveys the dry bulk solid powder, flake, granules, pellets, etc. to the bulk bag filling system must be sized to maintain the gross throughput of the system.  In the case of 30 - one ton bags per hour, that is 60,000 lb per hour.

Instantaneous Bulk Solid Flow Rate

Ensuring that the bulk material handling system upstream of the bulk bag filler can maintain a continuous rate of 30 tons per hour does not necessarily guarantee that bulk bags can be filled to match that rate.

As can be seen in the timing chart above, over half of the cycle time is consumed by tasks unassociated with actually getting the bulk solid product into the bag.

In our example there is only 55 seconds allowed to deposit 2,000 lb of product into the bulk bag.  That means the required instantaneous bulk solid flow rate is roughly 130,000 lb per hour!

If the bulk bag filling machine performs weighing during this time, a 'dribble feed' task is required.  That means that part of the 55 seconds alloted to filling the bulk bag with product is used to dribble the last, say, 50 lb of product into the bulk bag to maximize weighing accuracy. 

Typically, for it to be effective, the dribble feed time is 10-15 seconds in duration.  That leaves only 40 seconds to fill 1,950 lb into the bag.  In that case the instantaneous fill rate is over 175,000 lb per hour!

Surge Capacity

As described in the above section, the bulk bag is actually being filled for only a portion of the total bag fill cycle time.  The rest of the time - called overhead time - is taken up by other tasks. 

During the overhead time bulk solid product cannot flow into the bag.  However, since our bulk material handling system continuously feeds 60,000 lb per hour to the bulk bag filling system, the product conveyed during the overhead time must be stored above the bulk bag filler. 

Typically, a surge hopper or storage vessel of some kind is used to hold the product conveyed to the bulk bag filler during the overhead time.  Calculations can be made to determine the vessel volume necessary to hold the product - commonly called surge capacity.

Bulk Bag Filling System

The bulk bag filling machine must be designed to accomplish the tasks in the time alloted - while maintaining acceptable weighing accuracy and producing stable and safe bulk bags.

Features such as automatic loop release, automatically movable loop attachments, automatic bag removal, etc. are required.

However, the most critical consideration is that because each task duration is so short, each aspect of the bulk bag filler's design must be optimized to not only operate quickly, but to perform its function properly in so short a time.  Weighing accuracy of +/- 1-2 lb and bags that have been filled to their maximum bulk density can be achieved when filling bulk bags in excess of 30 per hour.  However, specialized designs are required.

Filled Bulk Bag Takeaway

Removing one filled bulk bag every two minutes from the bulk bag filling system is no easy task.  Forklift traffic patterns must be considered as well as the round trip distance from the filling system to/from the warehouse storage point or loading dock.

Pushing The Envelope

Bulk bag filling rates in excess of 30 tons per hour can be achieved.  Filling machines that perform weighing can be made to fill up to 35 bags per hour.  Pre-weighing - weighing the product in a vessel above the filler and dropping the 'shot' into the bag as quickly as possible - can reduce the bulk bag filling cycle to the point where it is possible to produce over 40 bags per hour.

As can be seen there are many considerations that must be addressed when implementing a high speed bulk bag filling system.  Careful work conducted in partnership with the bulk bag and bulk bag equipment vendors is a must to ensure success.

Many producers of dry bulk solids, who have traditionally shipped their products in drums, boxes or rigid totes, want to take advantage of the financial benfits of converting to bulk bags. 

However, it is often difficult to convert every customer at once.  That means that for at least some period of time bulk bags and the legacy package(s) must be filled.

The problem is how do you handle having to fill multiple packages with multiple packaging machines? 

Changeover can be a significant challenge particularly if your current packaging machine is not portable.  Production scheduling and the time required to simply change from the existing packaging machine to the bulk bag filler can be barriers that prevent companies from benefiting from bulk bags.

What's the answer?

A multiple-package filling machine.  Purchasing a bulk bag filler that also fills drums or boxes or rigid totes - or ALL of these packages - is feasible and economically very attractive.

Filling multiple packages on one machine requires more up front capital as compared to a machine designed to fill only bulk bags.  However, the operational benefits of using a single machine that automatically changes its configuration to fill up to four different packages are significant.

Just think, no delays for changeovers AND you can satisfy those customers who have not yet converted to bulk bags.  A good deal I think.  Plus, there's the financial benefits that immediately hit your bottom line from converting to bulk bags... but that's another story! 

We often run across bulk bag users who subscribe to the idea that bulk totes MUST be inflated before filling.

Not true!

It's understandable why this common practice exists and why it persists: through-pallet densification FIBC fillers.

A through-pallet bulk bag filler is characterised by the bag resting on a pallet throughout the fill cycle.  As you can imagine, if a bulk bag is attached to the filler and left to drape randomly on the pallet prior to filling, it is possible - perhaps likely - that the bag will become twisted or folded on itself and not fill properly.  Hence, bags are often inflated prior to filling when a through-pallet filler is used.

While inflating a big bag is generally a good idea with this kind of bulk bag handling equipment, it takes time during the filling cycle and adds capital cost.  However, there is an alternative.

If you use a filler that features hang filling - the bag hangs from its loops for about half the time it is being filled - you don't have to inflate your bulk bags.

A key characteristic of this type of filler is that a pallet is NOT in the machine while it is filling bulk bags.  Because the bag is not in constant contact with a pallet and because the bag is hung from its loops, you don't have to worry about poor performance from not using inflation.

With a hang filling bulk bag loader the ONLY time you need to use inflation is if your bags contain loose tubular liners.

Stacking bulk bags CAN be done safely.  There are a few things to carefully consider before attempting to stack FIBCs, but if you do your homework and if testing conducted with your product and bulk bags proves they can be safely stacked, the increase in warehouse efficiency can be a remarkable financial benefit.

Here are the things to consider before attempting to stack bulk bags:

1. Is the nature of your product conducive to stacking?  Some dry bulk solids are easily fluidized and behave like a liquid, others densify like a brick.  Where does your product fit within this spectrum?
2. Are your bulk bags properly designed to facilitate stacking?  Unique bulk bag designs exist with features specifically aimed at aiding stacking stability such as stand up loops, strategically placed gussets, etc.  Do you need these features?
3. Is your filling equipment up to the task?  Does your filling equipment densify your product enough to produce a safe and stable FIBC capable of being stacked?

The best way to determine the answers to the above questions is to work with your big bag and filling equipment vendors.  They should be able to conduct tests that will prove that your product can be stacked safely in bulk bags.

Regarding benefits, double stacking doubles your storage space utilization.  Triple stacking improves warehouse efficiency by another third.  And, quadruple stacking - yes it can be done! - reduces your bulk bag storage footprint by a factor of four compared to not stacking at all.

As you can see stacking bulk bags can provide a huge improvement in warehouse efficiency and profit improvement.  If you're interested contact your bulk bag and equipment suppliers and get stacking!