High Speed Bulk Bag Filling - Preweighing Design Considerations

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.

High Speed Bulk Bag Filling - Preweighing

Maximum bulk bagging rates are achieved when using a pre-weigh system. 

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.

So, instead of a fast feed and dribble feed cycle with weighing conducted on the filler that, at its fastest consumes 50-55 seconds, a pre-weigh system can fill a bulk bag in approximately 15 seconds.

Removing 40 seconds from the bulk bag filling cycle time dramatically increases the bulk bagging rate.

Bulk Bag Filler vs. Pre-weighing

Let's examine the numbers in more detail:

Task	Filler Weighing (seconds)	Pre-Weigh (seconds)
Rig bulk bag	30	30
Tare	5	5
Fast Fill	40	15
Dribble Fill	15	NA
Final densification	15	25
Remove filled bag	15	15
	120 seconds	90 seconds

Note that the Filler Weighing cycle time is only 120 seconds - that equates to 30 bags per hour.

That is a very high rate and is possible with many products using high speed bulk bag filling equipment.  However, as can be seen, adding a pre-weigh system to this type of bulk bag filler increases the bagging rate considerably.

A 90 second cycle time means that a properly designed pre-weigh system is capable of filling 40 bulk bags per hour!

Also note that the final densification time has been increased.  This compensates somewhat for the reduced amount of total densification time compared to the Filler Weighing cycle where the bag would be densified for most of the Fast Fill task.

In our next article we will dive into the design issues that must be considered to properly implement a bulk bag pre-weigh system.

Bulk Bag Flexibility

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!

Bulk Bag Weighing Accuracy - 4 Critical Success Factors

Bulk bags can and should be weighed to within +/- 2 to 5 lbs accuracy - at any bagging rate with any product.

Following are the four critical factors that must be considered to achieve acceptable bulk bag weighing accuracy:

1. Consistent product flow.  Any batch weighing application's accuracy increases as the consistency of the product flow rate increases.  This refers to the flow rate during a batch and from batch to batch.  Bulk bag fillers typically have a weighing algorithm that includes an "in-flight offsett" program that 'learns' when to stop product flow ahead of reaching the weight target to compensate for product that is falling between the metering device and the bulk bag.  The more consistent the flow the better the algorithm is able to fine tune the system.
2. Metering device. Weighing accuracy is critically dependent on how well suited the metering device is to the application.  A simple, two position gate valve may provide adequate accuracy when filling bulk bags with reasonably low bulk density product at a slow rate.  However, when filling a high bulk density product at a high rate a conveyor/feeder may be required to achieve the desired accuracy.  Note that the ultimate accuracy of a positive displacement type metering device (e.g. screw conveyor, screw feeder, rotary airlock, etc.) is limited to +/- the weight of product in one flight or pocket.  Therefore, high bagging rate applications require great care in selecting the best metering system: a conveyor sized to provide a high flow rate necessary to achieve the desired bagging rate may not be able to produce adequate weighing accuracy because it is too large  In that case, a different metering device, a second dribble feed device or a surge hopper with fast acting gate valve may be required.
3. Dead load.  The weight that the weighing system sees before product is added to the bulk bag is called the dead load.  Gross weighing systems - those that weigh the entire bulk bag filler along with the product in the bag - can see a dead load of up to 2,000+ lb depending on the filler and its configuration.  Bulk baggers that use hang weighing see a dead load equal to a few hundred pounds.  This means the hang weighing filler can use smaller load cells or that the portion of the load cell span taken up by the dead load is much smaller.  Either way, higher weighing resolution and more accurate weighing is the result.
4. Weighing system minutiae.  When filling bulk bags the specific type of load cell, controller/PLC resolution and scan time and the like are really not significant factors in achieving weighing accuracy.  In my experience any weight controller and PLC implimentation has more than enough resolution to achieve the desired bulk bag weighing accuracy.

Handling Hazardous Ingredients In Bulk Bags

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.

Further, bulk bags must be designed to prevent the escape of product particles during handling - otherwise referred to as 'sifting'.  Coated or lined bags can be specifically designed to suit the product and the hazard.

This is only a brief overview of handling hazardous goods in bulk bags.  Users and bulk bag and equipment manufacturers must coordinate closely to ensure safety.

5 Ways to Reduce Bulk Bag Packaging Cost

In previous posts we have discussed reducing shipping costs and the benefits of replacing or eliminating pallets.

In this week's post let's discuss all of the ways that the cost of bulk bag packaging can be reduced.

1. Reduce bulk bag size.  Reducing the height of a bulk bag by 10% decreases its price by roughly 5%.  Maximum densification, for example using the cone table densification system, can almost always decrease bag height by 10% - sometimes by as much as 40% or more.
2. Eliminate baffles.  If your bulk bags have internal baffles and they are being used to enhance stability, it is highly likely you can eliminate them by using a densification system designed for maximum results.  Getting rid of baffles can save as much as $5 per bag.
3. Use coated bulk bags.  If you use liners in your bulk bags you may be able to eliminate them and use coated bulk bags instead.  See the blog post "Bulk Bag Liners - When To Use Them... or Not!" for an excellent review on when to use liners and when not to.  Switching from a liner to a coated bulk bag can save as much as $2 -$3 per bag.
4. Replace wooden pallets with plastic channels.  Using plastic pallet replacement channels can reduce direct packaging cost and logistics related costs too.  See the blog post "Eliminate Wood Pallets - Ship Bulk Bags Without Wood Pallets" for details.
5. Eliminate wooden pallets.  Properly designed and filled bulk bags can be handled only by using their lifting loops thereby eliminating pallets - and their cost - completely.  See the blog post "Pallet Free Bulk Bag Handling" for details.

As you can see from there are numerous ways to significantly reduce the cost of your bulk bag packaging.

Reduce Bulk Bag Shipping Cost

How can bulk bag users reduce shipping costs?

The bulk density of your product determines the opportunity for shipping cost savings.

If your product is heavy enough to weigh-out a shipping container/trailer then, as we have seen in the last two posts, replacing wood pallets with a plastic channel replacement system or simply eliminating bottom support altogether can significantly reduce shipping costs.  Reducing the weight of packaging per shipment means you can replace the weight reduction with product thereby shipping out more product per container and reducing shipping cost per pound.

However, if your product's bulk density is such that you can't weigh-out a container/trailer there is an additional step you can take to minimize shipping cost.

Bulk Bag Densification Increases Payload

Light products such as wood flour, fluff carbon black, fumed silica and the like react favorably to certain types of densification.  This means that the payload of a given size bulk bag can be increased.  Increasing the bulk bag payload increases the amount of product that be shipped per container thereby reducing shipping cost per pound.

Low bulk density bulk solids tend to trap air between their particles. Depending on the particle shape it can be very difficult to remove the air while the bulk bag is being filled. Bulk bag filler densification systems are designed to remove the trapped air thereby densifying the product.

Some densification systems are more effective than others. Carefully matching the method of densification with the product characteristics can result in a significant improvement in packed bulk density thereby increasing payload.

Bulk Bag Densification Reduces Shipping Cost

As a starting point it is reasonable to assume that a densification system capable of maximum compaction should be able to increase payload weight by a minimum of 10%.

Control and Metering's experience is that payload increases of 20-30% are often achievable and in some cases as much as 40%+ more weight can put in a bulk bag that has been previously filled without densification or with ineffective densification systems.

Pallet Free Bulk Bag Handling

Our last article discussed alternatives to using wood pallets when shipping bulk bags. This article focused on handling bulk bags without a pallet or pallet replacement device.

Are Pallets Necessary?

Pallets are used to move heavy objects. Forklifts or pallet trucks are used to move them. In the case of bulk bags however, there is a built-in alternative to using a pallet or bottom located lifting device to move them: bulk bag lifting loops.

Every bulk bag has lifting loops that are designed to safely lift the bag and its contents. They can be used to move the bag from the filling station to storage into and out of a shipping container and everywhere in between.

So, technically, a bulk bag does not require a pallet or equivalent.

Checklist for Handling Bulk Bags Without Pallets

Bulk bag filling equipment. Before you can safely handle bulk bags without pallets you must ensure that your equipment is designed to allow the bag to be removed from it without a pallet. Bulk bag fillers can be designed to release the lifting loops while still supporting the bulk bag thereby allowing the bag to be removed by threading the fork tines through the loops. Note that you must have a forklift to remove the bulk bag from the filler - a pallet truck won't work.

Bulk bag design. The bulk bag loops must be 'cross corner' design and the must be designed to stand up right on their own. Both of these features make it easy to thread the fork lift tines through the loops without assistance. Cross corner refers to loops that are sewn on side of the bag and looped over the corner and sewn on the adjacent side. This provides a 'tunnel' for the forklift driver to aim for. Of course, the loops must stand up on their own to make the tunnel. This is done by sewing a strip of plastic within the loop to give it rigidity.

Sanitary considerations. Since the bottom of the bulk bag comes in direct contact with the floor - of your plant or warehouse and the shipping trailer/container - it is often advisable to add a protective flap across the entire bottom of the bag to minimize contamination and provide an extra layer of fabric for added robustness. The flap is sewn along one edge, drawn across the bottom of the bag and attached to the opposite side using Velcro or loops and ties.

End user capabilities. While shipping bulk bags without bottom support has many benefits for the producer, it is a catastrophe for an end user without a forklift. Your customers have to be prepared, willing and able to work with your product in bulk bags without pallets.

Shipping Bulk Bags without Pallets - Is It Really Feasible?

Short answer: yes. Although a small percentage overall, there are many bulk bags being shipped without pallets. For example, export shipments of various products that travel via shipping containers are often shipped without any kind of bottom support. In North America, one of the resin industries converted from boxes to bulk bags now all domestic production of this type of resin is shipped in bulk bags without bottom support.

Palletless Bulk Bags - Benefits

Shipping without a pallet or any bottom support is a lucrative proposition. A number of operatic cost saving opportunities immediately occur:

1. Eliminate pallet cost.
2. Reduce shipping cost. Removing the weight of the pallet from each shipped package means you can increase the payload weight of each bulk bag. This can significantly reduce the shipping cost per pound of product.
3. Reduce handling labor. Not having to manage the handling of heavy pallets increase productivity and reduces packaging labor cost per pound.
4. Increase warehouse efficiency. Not having to store pallets can significantly increase available warehouse space.
5. Eliminate border crossing delays. Borders crossing delays often occur because palleted shipments may be inspected. Additional time (money) can be lost if documentation has to be resubmitted or modified.

Shipping bulk bags without bottom support is feasible, is already being done and can dramatically reduce operating costs.

Sustainable Packaging - Reduce The Size of Your Bulk Bags

Looking for ways to improve the sustainability of your bulk bags?

How about reducing their size?

How do you do that?

Do a better job of densifying your product so that it takes less volume for a given weight... and use a shorter bag!

Ideally, the product you put in your bulk bag should be at its maximum packed bulk density when it leaves your bulk bag filler.  If it's not you are using a larger bulk bag than necessary and, more importantly, it may not be safe.

Not all bulk bag fillers do the same job when it comes to compacting or densifying your product.

At Control and Metering we use a 'rule of thumb' that for every 10% you can reduce the height of your bulk bag, the cost reduces by 5%.

So if you can do a better job of densifying your product while it's being filled you're not only lowering your packaging costs you're also improving your packaging sustainability.

A VERY nice win-win!

High Speed Bulk Bag Filling - Lots To Consider

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:

Task	Task Duration (seconds)
Bag rigging	30
System start	5
Bag Fill	55
Final Densification	15
Remove filled bag	15
	120

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.