Selecting and Using Bulk Bags - Part 2

January 1, 2001

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Bulk bags, also known as flexible intermediate storage containers, easily, safely, and inexpensively move and store from 500 to 8,000 pounds of bulk materials. Selecting a bag that's right for your application and filling, handling discharging and disposing of it properly will make your operation more efficient. Part I of this article introduced bulk bag design. Part II discusses how to fill, handle, discharge, and dispose of bulk bags.

Using bulk bags to safely and easily handle bulk materials can improve your operating efficiency. To use bulk bags effectively, you should know how they're designed (Part I), as well as how they're filled, handled, discharged, and disposed of (Part II).

Filling bulk bags

Properly filling bulk bags produces tightly packed, stable bags that are easier and cheaper to ship. The following paragraphs explain how a filling machine works, how it densifies bagged material, and how filled bags are removed from a filling machine. Some factors to consider when choosing a filling machine are also discussed.

How a filling machine works. Empty bulk bags are hung by their lifting loops from hooks at the top of the filling machine. A filling device (such a screw conveyor) feeds material into the filling machine's surge hopper, which controls the flow through a flexible connection into the bag. The vibration table (which also supports the bag bottom and adjusts to different bag sizes) raises to support the bag bottom before filling starts. This causes material to flow into the sides of the bag bottom during filling and prevents the bag bottom from forming a rounded, unstable shape.

When the bag is about one-third full, the table begins to vibrate the bag and densify the material; this is repeated for several vibration/stretching cycles, depending on how densely packed the material must be (see the section Cyclical vibration and hang filling for more information). The vibration table lowers between cycles. When the bag is filled and densification is complete, the table lowers to disengage the bag. The self-releasing hooks then release the bag and a forklift truck or other device is used to remove the filled bag through the side of the filling machine's frame.

The platform at the rear of the machine enables the operator to easily perform various functions (such as attaching the bag's lifting loops to the filling machine's bag hooks) at chest or shoulder level.

How a filling machine densifies bagged material. A filling machine vibrates bagged material to densify the contents and remove entrained air, making the bag more compact and uniformly shaped, and thus, more stable. Materials with entrained air – particularly powders – that aren't compacted during or after filling make the bag unstable and dangerous and can raise handling costs. Improper filling, and improper handling after filling, can cause the bag to lean to one side or become banana-shaped, which can't be corrected once the bag is filled.

To understand how densification works, it may help to consider first how the bulk density of a bagged material is defined, then how entrained air is removed from different materials, and finally, how a filling machine uses cyclical vibration and hang filling to densify bagged material.

Bulk density. A bagged material's bulk density can be untamped or tamped. Untamped bulk density is the density of a material sample collected during its free fall through the filling spout; at this point, the material includes entrained air. Tamped bulk density is the density of a material sample collected after it's been vibrated for several minutes and the entrained air has been removed. A bagged material's bulk density should be as near as possible to its tamped bulk density. Table I provides volume ratios of untamped to tamped bulk densities for several materials.

Table 1 Volume ratios of untamped to tamped bulk density for several materials

Entrained air removal. Entrained air is removed from different materials at different rates, depending on particle shape and size. For example, removing entrained air from plate-shaped clay particles takes three or four times as much vibration as removing entrained air from round particles of similar size. Granular particles lose entrained air under their own weight, without vibration. Large granules reach tamped bulk density faster than small granules, although vibrating the bag as filling stops will make the bag even more stable.

Cyclical vibration and hang filling. A filling machine can perform vibration and hang filling (or stretching) in continuous cycles to densify the material in the bag and ensure that the bag bottom is stable.

In the vibration cycle, a coned-top vibration table engages the bag bottom; as the table vibrates, it forces material into the bag's corners and compacts it, while also deaerating and compacting the material throughout the bag. Because vibration works only on the weight of material above the vibration table, vibration usually starts when a minimum weight (about 400 pounds, or one-third of the bag's capacity) fills the bag. The vibration table moves down before bags are tied off and removed from the filling machine.

In the hang filling or stretching cycle, the coned-top vibration table moves down and causes the center column of material in the bag to collapse, flattening the material's surface. This ensures that the material in the bag corners is the most densely packed, so the filled bag will sit flat, not on a rounded bag bottom. During the final 10 percent of filling, the bag remains in the stretch cycle to ensure that the bag is accurately weighed to within 0.1 percent of the scale's capacity.

Some bag types work better than others for hang filling. For instance, all woven polypropylene bags – the most common type of bag – stretch when hung. However, straight, compacted, unwrinkled polypropylene bags will remain more stable after hang filling.

How bags are removed from a filling machine. A forklift or a motorized roller conveyor may be used to remove bags on pallets from a filling machine. Using a forklift involves the following steps (Fig. 3): Once the bag is filled, the filling machine's vibration table moves down and the operator detaches the bag inlet from the filling head. A forklift is used to move a pallet under the bag. The machine's bag hooks automatically release the bag's lifting loops as the bag is placed on the pallet, and the forklift removes both the bag and the pallet (Fig. 3b).

Using a motorized roller conveyor involves these steps: At the final filling stages, the vibration table still engages the bag; the movable conveyor section – carrying a pallet – is ready to move under the bag being filled. A previously filled bag waits on the conveyor next to the filling machine, ready to be removed. When the bag is filled, the vibration table moves down and movable conveyor section and pallet move into place below the bag. The operator detaches the bag inlet from the filing head, and a forklift removes the previously filled bag. The movable conveyor section raises until the newly filled bag rests on it and the lifting loops disengage. The bag then moves along the conveyor to wait for removal by the forklift.

Some factors to consider when choosing a filling machine.

When selecting a filling machine for your application, consider the following factors

Strength. Choose a machine that's strongly built and will withstand damage from forklifts.

Easy operator access. Make sure the machine has a platform or other support structure for the operator to stand on that is safe, protects the operator, and enables the operator to easily attach the lifting loops to the filling machine's bag hooks, engage the bag's inlet spout to the filling machine's spout, disengage the inlet spout after filling and tie off the inlet spout. Keep in mind that these duties are easiest to perform at chest or shoulder level.

Bag bottom support. Choose a machine with a platform or table that supports the bottom of the bag as it's filled (a vibration table can service this purpose). This creates a stable bag by supporting the bag's bottom center when filling starts, forcing the material to flow into the sides of the bag near the bottom. This side material will then have the highest bulk density, so the bag will rest on its outside bottom edges instead of a rounded bottom center.

Bag height adjustment. While a filling machine is usually designed to fit one bag height and has fixed hooks to keep the bag's lifting loops vertical, bag heights can vary with different materials and different shipment weights. Thus, choose a filling machine that is big enough to accommodate the tallest bag the machine will fill, but will adjust to the shortest bag. Also ensure that operator duties can be performed for each bag size at chest or shoulder level.

Liner attachment. Slip-seal (rather than clamp) liners onto the filling spout to prevent dust from escaping. Slip-sealing causes the liner to slip into the bag's inside dimensions, rather than stretch and weaken, as the liner is inflated and filled.

Feed rate control and displaced air exhaust. Choose a filling machine that will control the feed so the material flows into the bag at the required rate. Make sure air displaced by the material is able to exit the bag.

Bag filling rate. Choose a filling machine that can fill bags at the rate your application requires. Filling rates for most filling machines can range from 8 to 15 bags per hour, depending on how easily the operator can access the bag hooks and filling head and how many automated filling controls the machine uses. The rate also depends on the filling feeder's feed capacity, the required fill weight, the required fill accuracy, the ratio of untamped to tamped bulk density, and the time it takes two-thirds of the bag's contents to reach tamped bulk density. Bag filling rates are also affected by how easy it is to remove filled bags and what type of pallet- or skid-sheet-handling/removal system is used.

High-capacity filling machines can fill more than sixty 2,000-pound bulk bags per hour and typically include three filling stations mounted on a carousel below a hopper, which preweighs the material for each bag. Forklifts can be used to remove filled bags from high-capacity machines by inserting the forklift tines directly into the bag lifting loops; this eliminates the need for pallets or skid sheets.

Controller. Choose a filling machine with a controller that allows the operator to perform all bag filling functions manually and that operates automatically once the filling starts. Also make sure the controller is compact – preferably microprocessor-based – and easily accessible.

Suspension weighing. Use a filling machine with a suspended scale, because the scale will be isolated from vibration and make weighing more accurate.

Handling filled bulk bags

Proper handling will speed and simplify the transportation of filled bags, as well as protect the bagged material. The following paragraphs detail methods for transporting filled bags, restacking filled bags, and preventing contamination of bagged material.

Transporting filled bags. Bulk bags can be shipped on pallets and skid sheets both over the road and by rail; bulk bags can also be shipped by sea. Each requires different bag handling methods.

Shipping bags on pallets. The easiest way to move bulk bags is on pallets, which can be lifted by forklifts. To efficiently load bags on pallets, follow these steps.

First, use a pallet size that allows the rounded section of each bag to overlap the pallet edges, so the bags on adjacent pallets (rather than the pallets) touch each other. This not only minimizes the risk that pallets will damage the bags, but ensures that bags stacked on pallets will be more stable.

Second, position each bag on the pallet's center. While the rectangular shape of most pallets can make centering difficult, most forklifts can either side-ship pallets or use other devices to precisely place loads.

Third, use close-slatted pallets, which have top and bottom surfaces with tightly fitting slats; the small amount of space between slats will prevent bags from protruding through and being pinched and damaged either by the slats of other pallets when the pallets are stacked or by forklift tines during unloading. Using close-slatted pallets can also improve your load's stability, which will reduce bag damage, unloading time and work for the forklift driver.

However, close-slatted pallets are costly, so only use them in-house unless your customer will return the pallets. Instead, us nonreturnable pallets for stacked loads, or stack two bags on one pallet (as long as the filled bag height is under 42 inches). Though these methods are labor-intensive, they may still be cheaper than shipping close-slatted pallets. For nonstacking loads, use standard single-trip pallets, but be aware that forklift tines can damage the bags and poorly constructed pallets.

Shipping bags on skid sheets. If you have equipment for handling skid sheets, you can use the sheets instead of pallets, but your loads won't be as stable. On skid sheets, stack bags only two high if they're filled to heights over 40 inches; don't stack bags filled to heights over 54 inches. If the filled heights of your bags exceed these heights, consider shipping the bags on racks.

As with pallets, use a skid sheet size that allows filled bags to overlap the sheet's sides, so that the bags on adjacent sheets (rather than the sheets) touch. Center the bags on the sheets.

Shipping bags by sea. Bulk bags are frequently used to ship minerals, chemicals, grains, and other dry solids by sea. Using bulk bags for sea shipment lowers shipping costs.

Consider the following factors before choosing to ship by sea. Bulk bags must be loaded into and unloaded from ships by crane-mounted hooks that pick up bags by their lifting loops. Thus, ship your bags in vessels with wide hatches and small wing spaces so cranes can move the bags directly into place without requiring additional shifting by forklifts. When bags must be stacked, make sure that the crane's spreader bars are large enough to keep the lifting loops vertical as the bags are lifted over the ship's side. Use elastic to tie together loop extenders; this enables the four loops to meet in the center and be picked up by the crane hook, which saves loading and unloading costs. Also use bags with slightly better quality and higher weight cloth to reduce the damage caused by rough handling at shipping docks; remember that some ports reject poor-quality or unsafely filled bags.

Restacking filled bags. Bags must often be restacked during handling. Restack your bags in the same way they were originally stacked to keep the bags stable. For example, reposition the more compressed bags from the bottom of the old stack at the new stack's bottom or, in a truck trailer or International Standards Organization (ISO) container, in the lower tier of a stack.

Preventing contamination of bagged material. Handling your bags carefully can prevent foreign materials and dirt from contaminating the bagged material. Avoid puncturing the bag during handling, because this can degrade and waste the material. Also clean the external bag bottom while the bag is suspended and before it's opened to avoid contaminating the material during discharge.

Discharging filled bulk bags

Properly discharging bulk bags saves time, material, and money. Understanding how bulk bag dischargers work, which dischargers empty bags of different types, and how to prevent material contamination at discharge will help you select the right discharger for your operation.

How dischargers work. Bulk bag discharges empty bulk bags easily and cleanly with minimum dust and maximum safety. First, a filled bag that is to be discharged is fitted with a rigging frame, which is a support frame placed over the top of the bag that engages the bag's lifting loops. Second, a forklift truck or hoist lifts the rigging frame and bag onto the discharger hopper, where the bag top continues to be supported by the rigging frame. The material bridge, or set, in the bag is broken as the bag is placed in the discharger hopper because the unsupported bag bottom sags.

Third, the operator unties the bag outlet (and liner, if used) through a door that provides safe access to the outlet. Fourth, the discharger starts to vibrate the bag to induce the flow of material out of the bag through the cone-shaped discharger hopper to a central outlet.

A discharger for lined bags. Liners tend to slip out of the bag outlet during discharging. A liner tensioner and retracting system, which form part of the rigging frame, will prevent the liner from slipping out by pulling the liner over the liner tensioner's drum as the material discharges. The discharger's drop chute is also longer than the liner's outlet extension below the bag outlet spout to prevent the liner from entangling the transfer equipment or process below the discharger.

A discharger for single-trip, flat-bottom bags. A discharger for single-trip, flat-bottom bags features a knife that slits the entire bag bottom. A side-of-bag diaphragm seals the bag bottom to contain the dust as the bag is slit.

A discharger for hygiene bags. A discharger for hygiene bags features a violator knife that pierces only the liner – not the bag – at discharge.

How to prevent contamination during discharging. If the bag has a bottom flap, fold the flap back while the bag is suspended and before it's placed over the discharger. Also remove frayed ends or nonsecured cords from the bag and liner so they don't flow into the process through the discharger.

How to prevent contamination during discharging. If the bag has a bottom flap, fold the flap back while the bag is suspended and before it's placed over the discharger. Also remove frayed ends or nonsecured cords from the bag and liner so they don't flow into the process through the discharger.

Some factors to consider when selecting a discharger. Look at the following factors when selecting a discharger for your operation.

Bag Loading. Because the bag must be lifted onto the discharger, ensure that the discharger's hoist or forklift can reach the bag. Also make sure that the rigging frame fits your bag size and supports the bag so that it doesn't collapse and restrict the flow.

Operator access and safety. Choose a discharger that allows the operator easy access at chest or shoulder level so he or she can quickly untie the bag's outlet before discharging and quickly remove the bag after discharging. Also make sure your discharger doesn't require the operator to stand under a bag during either hoisting or discharging; standing to the side of the bag is safest.

Dust containment. Choose a discharger that will capture dust created when the bag enters the discharger or is being discharged. For some applications, you may need a discharger that is equipped with dust-collection equipment, such as a side-of-bag diaphragm seal, exhaust ventilation at the door where the operator unties the bag outlet, and a liner sealing system.

Outlet. Make sure the discharger's outlet is long enough to allow the bag to elongate as it empties, without letting the bag's outlet spout or liner entangle the feed or weighing system below.

Alarm. Select a discharger with an alarm that signals when the bag is empty; this speeds the process of removing empty bags and loading filled bags onto the discharger.

Processing or feeding devices for your discharger. Many devices for processing or feeding discharged material fit under dischargers, such as vacuum systems, blowers, weighing devices, screw feeders, loss-in-weight feeders, and vibratory tray feeders. Select the device you need by considering the material being discharged, the process you'll use, and the feed method you require.

Disposing of empty bulk bags

Although a good discharger may leave as little as 2 to 3 ounces of material in a bag, some material will remain. This causes problems – particularly when handling powders – in collapsing bags for disposal or folding them for reuse.

If possible, drop empty bags onto a pallet without collapsing them so they can be moved and easily handled elsewhere without adding dust to the work environment; folding empty bags where they're discharged not only creates dust, but requires a lot of floor space. Instead of folding empty single-trip bulk bags, compact them to save disposal costs, but use equipment that can extract dust. Fold multi-trip bags carefully, making sure that the outlet and inlet are open so air can escape without raising excessive dust.

Conclusion

While understanding how bulk bags are designed, filled, handled, discharged, and disposed of its important, consult both your bag supplier and your customers to determine which bulk bag and type of equipment are right for your operation. No matter which type of bag or equipment you choose, operating efficiency and safety should guide your choice.

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