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.
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:
- Flow control. The device used to meter the product into the preweigh hopper must be properly selected.
- 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.
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.
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:
|Rig bulk bag
|Remove filled bag
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.
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!
One of the most overlooked keys to bulk bag discharging success is training. This post reviews how properly trained operators facilitate a safer, cleaner and more productive bulk bag discharging operation.
Key bulk bag discharging tasks that require specific operator training:
- Preparing the bulk bag for unloading. In most cases this is a simple task requiring only the attachment of the bulk bag lifting loops to the lifting frame used to place the bag in the discharger. However, when certain types of liners are involved the operator must know how to safely use the bulk bag unloader's liner tensioner to hold or wind up the liner during discharge thus preventing it from becoming entangled in downstream equipment.
- Using bulk bag discharging equipment. The most common cause of bulk bag discharging problems is misuse of bulk bag discharging equipment. At best, not following the manufacturer's operating instructions results in poor ingredient yield, unecessary operator involvement to initiate or maintain ingredient flow and/or dusting. The worst scenario is when an operator's lack of knowledge creates a safety hazard. This is most often seen when operators reach under an unsupported, suspended bulk bag to untie the outlet and initiate flow.
- Removing the empty bulk bag. A bulk bag that has just been unloaded is full of dust laden air. How the operator removes the bag can either maintain a dust free environment or create a dust clogged mess that in some circumstances results in a safety hazard.
- Disposing of the empty bulk bag. Even if a bulk bag is removed from the discharging equipment and transported to the point of disposal properly, problems can arise if an operator folds the bulk bag or otherwise expels the dust laden air in an uncontrolled manner - causing a dust clogged mess.
Of course, the first step in sucessfully training operators is to develop well thought out Standard Operating Procedures (SOPs). With precisely defined operating objectives, tasks and expectations training success is greatly enhanced.
Often bulk bags require inner liners, usually made of polyethylene, to protect the ingredient. Care must be taken when discharging lined bulk bags to prevent entanglement with downstream equipment and to ensure 100% product yield.
Liners come in many forms: loose tubular, loose fitted, sewn-in, glued-in.
Loose means that the liner is not in any way attached to the outer bulk bag.
Tubular means that the liner is basically a cylinder of polyethylene film that is stuffed in the bag.
Form fit means that the liner is constructured with sides, top and bottom and usually inlet and outlet spouts - it looks just like a bulk bag only it is made from polyethylene film.
Loose Bulk Bag Liners
Whether tubular or form fit, a loose liner requires some sort of physical means to prevent it from discharging with the ingredient and becoming entangled in downstream equipment. A device called a liner tensioner is typically attached to the bulk bag lifting frame or rigging frame to prevent this from happening.
Liner tensioners vary in their operation, but the most basic simply hold the liner in place preventing it from moving out of the bag. More sophisticated tensioners wind up a portion or all of the liner as it is emptied.
Before the bulk bag is lifted into the discharger the inlet spout of the bag is untied to expose the tied liner inlet. The liner neck is then wound onto the liner tensioner spool piece, which is then either fixed in place or rotated to take up the slack in the liner by activating its actuator (typically pnemuatic).
Even though liners that are attached to the outer bulk bag are designed not to discharge with the ingredient, it sometimes happens anyway. Often a basic liner tensioner that holds the liner in place is used to guarantee that the liner cannot move.
Liner Tensioner Conisderations
- Liners 'grow'. Loose liners and liners attached to the top of the outer bulk bag will extend out of the bulk bag outlet spout during ingredient discharge. Particularly in the case of loose tubular liners, care must be taken to ensure that even if a liner tensioner is used, the liner cannot extend or grow out of the outlet spout enough to foul downstream equipment.
- Too much liner winding. If a liner tensioner that winds continuously is used typically the liner outlet is clamped to prevent it from winding all the way up onto the spool piece. If this is allowed to happen, and the ingredient is at all dusty, dust will be sprayed into the surrounding area when the empty liner is rapidly wound up through the empty bulk bag and then spins around the spool piece.
- Liner length. When a liner tensioner is used the liner inlet must be long enough to reach the tensioner spool piece.
- Ensuring 100% yield. A liner tensioner that winds up a loose liner - partially or completely - can assist with ensuring that all of the ingredient is dishcarged from the liner. The act of winding up the liner helps to get the last pounds of ingredient out of the bag.
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.
- 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.
- 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.
- 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.
- 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.
- 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.
ISPM 15 - the international regulation stipulating global wood pallet use - has caused a significant change in wood pallet usage.
Wikipedia describes ISPM 15 as the:
"International Phytosanitary Measure developed by the IPPC that directly addresses the need to treat wood materials of a thickness greater than 6mm, used to ship products between countries. It affects all wood packaging material (pallets, crates, dunnage, etc) requiring that they be treated with heat or fumigated with methyl bromide and marked, often branded, with a seal of compliance. "
Wood Pallet Cost Rising
As a result, the total cost of wood pallet usage has escalated dramatically. Not only has the cost of ISPM 15 compliant pallets increased, shipping documentation preparation and possible border crossing delays consume additional time and resources.
That coupled with historic problems related to mold growth, damage to shipped product from pallet defects (splinters, nails, etc.) and the outright refusal of some countries and/or customers to accept wood pallets has caused a strong desire to find a wood pallet alternative.
Bulk Bag Solution
If you ship your product in bulk bags there are two solutions that Control and Metering can recommend:
- Plastic pallet replacement channels.
- Shipping bulk bags without any kind of pallet-like support.
Plastic Pallet Replacement Channels
We have had extensive experience with BagPal channels. These are designed to slip into sleeves attached to the bottom of specially made bulk bags allowing the bulk bag to be handled by a forklift just like a bulk bag on a wooden pallet.
BagPal channels are robust, compare favorably price-wise with treated wood pallets and can be reused and recycled.
Also, they provide a traditional means of moving bulk bags with a forklift - any forklift with standard tines can be used to move a BagPal equipped bulk bag.
Plus, a pair of BagPal channels weighs less and is much smaller than a wood pallet. These features provide two significant financial benefits:
- Reduce shipping costs by increasing your bulk bag payload by the difference in weight between a pallet and the channels - roughly 30+ lb. Higher payload per bag equals more product in a truck or container.
- Increase warehouse efficiency by using MUCH less space to store empty channels vs. empty pallets.
Bulk Bags Minus Bottom Support
Bulk bags can be shipped without bottom handling support of any kind. This has been done in a number of industries for both domestic and export delivery to customers.
Without bottom handling support bulk bags must be lifted using their loops. Preventing damage and contamination of the bag bottom is also required.
Specially designed bulk bags with unique lifting loops and protective bottom panel barriers solve these issues.
Shipping bulk bags without bottom handling support is a topic by itself and will be the topic of next week's blog post.
Ever heard of a 'porthole' style bulk bag? If you haven't we'll fill you in. If you have, and you're having problems with them, we'll list what you need to know.
A porthole bulk bag has a porthole in its bottom. A flat bottom liner is inserted into the bag and filled.
Porthole syle bags are discharged using a bulk bag unloader that features a 'knife' to cut the liner through the porthole.
Stuff the liner properly. One of the key benefits of using porthole style bags for in-house use (you fill and empty the bags within your plant) is that the outer bulk bag can be reused by simply discarding the old liner and stuffing a new one into the bag. However, unless the liner is properly reinserted, twists or folds can occur that will prevent the liner from being easily cut by the discharger. This can lead to shredding of the liner - resulting in downstream product contamination - or a badly twisted or folded liner can seriously impede or even prevent easy product discharge. Work with your bag/liner supplier to develop a Standard Operating Procedure to ensure the liner is properly inserted.
Keep the knife sharp. This advice might seem counter intuitive from a safety perspective. However, unless the bulk bag discharger knife is sharp you run the risk of tearing the liner instead of cutting it cleanly. Tearing through the liner can sometimes result in shredding, which in turn can deposit liner material in the product flow. Product contaminated with liner shreds is a serious problem to be avoided at all costs.
- Use the bulk bag discharging equipment properly. A knife style discharger will have a particular procedure that needs to be followed to ensure trouble free cutting of the liner and complete discharge of the product. Make sure you confer with your equipment supplier to develop a proper procedure for unloading porthole 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!
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 Duration (seconds)
| Bag rigging
| System start
| Bag Fill
| Final Densification
| Remove filled bag
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!
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.