Weigh batching directly from bulk bags.
The most common type of bag found in industry is constructed of woven polyethylene, although many other materials are available. The bags generally range from 30" to 48" sq. by 36" to 84" tall and hold anywhere from 500 lbs. to 4,000 lbs. of bulk material, although the most typical bag size is 36" sq. by 60" tall holding 2,000 lbs. of material. The most common bags include a lifting strap on each of the top four comers of the bag.
Bags are available with and without liners. In some cases the liner is fitted and attached to the inside comers of the bag. However, the most common liner is a long tube of thin sheet polyethylene. Although this type of liner is more economical than a fitted liner, it is very problematic due to its tendency to elongate during discharge. This can result in discharge of the liner into downstream equipment. Prior to filling the bag, the tube end is gathered closed and tied or sealed at the bottom. The closed end of the tube is then inserted into the bag through the top filling spout and drawn down through the bag and out the bottom discharge spout. As the bag is filled, material pressure seats the liner against the inside walls of the bag. When filled, the top of the tube is tied or sealed to prevent contamination, etc.
Most bag manufacturers will fabricate bags to your specification. However, there are certainly economical and functional advantages to using standard available bags.
Bag inlet and outlet spouts generally range from 12" to 22" in diameter. The appropriate diameter of the discharge spout for proper discharge of a particular material is dependent on the material's flow properties after transportation and storage. Many materials are free-flowing and easy to handle as the bag is filled only to become compacted and difficult to discharge after transportation and storage.
Just as with bin flow design, materials that tend to compact during storage will generally flow easier from a larger diameter bag outlet spout. Most commonly, the inlet spout is the same diameter as the discharge spout.
Uses and benefits of bulk bags in industry
Many companies have realized substantial benefits by designing systems to accommodate the use of bulk bags in new processes. Existing processes have also benefitted from modifications which accommodate receiving and using material(s) in bulk bags. Bulk bags provide a safe, economical means of handling and storing bulk materials. Since bulk bags are shipped and stored in a manner very similar to pallets of smaller paper bags, a move to bulk bags does not require any major changes in equipment or procedures used at the receiving dock or in the storage area.
Safe, easy, economical handling and storage
An important benefit which the use of bulk bags offers is increased operator safety. The chance of an operator injury is minimized since no manual lifting of material is required. All lifting and movement of the bag is accomplished using a fork track, a pallet jack, and/or a trolley and chain hoist. A healthier work force and fewer workman's compensation claims will lower insurance premiums and increase profits.
Particularly when handling hazardous materials, paper bag disposal costs can have a significant impact on the total cost of using a particular material. Also, it is not uncommon to dispose of as much as 1% to 5% of the material along with the bag. A proper bulk bag discharger can remove all but a dust coating on the interior of the bulk bag, minimizing material waste. Empty bags are normally returned to the supplier to be refilled. In many cases, bag disposal costs are eliminated.
Using bulk bags as opposed to paper bags is consistent with today's ecological concerns. It is doubtful that a method will ever be developed for reuse or recycling of paper bags. Therefore, nearly all contaminated paper waste eventually ends up in a landfill. In addition to the increased utility realized for each bag through multiple trips from the supplier to the user, a separate industry has developed which reconditions and recycles damaged or worn bags. This significantly reduces the amount of contaminated container waste that must be taken to a landfill, thereby reducing the risk of underground water and soil contamination.
Another benefit of the use of bulk bags is that suppliers often have lower costs in supplying material in bulk bags. Those savings may be passed along to the material users as reduced raw material prices.
Process challenges in using bulk bags
Discharging the material
There are common process related challenges associated with using bulk bags that are not encountered with paper bags. The first challenge is getting the material out of the bag properly. Each material may present unique challenges in this regard. Some materials may be very difficult to discharge, while others may create an unsafe, uncontrolled burst of material if the bag is simply opened without controlling material flow. A proper discharger should include a flow control valve and an effective means of dislodging compacted or difficult flowing material to promote discharge.
When a bulk bag is full it tends to bulge out on the sides, which creates a relatively lower bag height than when the bag is empty. Conversely, as the bag empties, the bag elongates and the height increases. If the discharger' s design does not compensate for the bag elongation, pockets of material will remain in the bag bottom and the bag will not empty without manual intervention.
Moving the material to the process
Although it is sometimes possible to locate the discharger above the point where the material is needed, space restrictions generally require that the material be conveyed and elevated to be introduced to the process. Also, many processes require that the material be fed at a controlled rate into the process so that it is properly absorbed, dissolved, etc. The best discharger designs are flexible to accommodate discharging into various types of mechanical conveyors and feeders, as well as pneumatic conveyors.
Handling and storing the bulk bags
Full bulk bags of material are received and stored in much the same way as paper bags. However, care must be taken to ensure that the materials and the type of bulk bags used are suited to the atmosphere in which they will be stored. For example, a heat sensitive material that tends to absorb moisture should be stored in a lined bag in a cool, dry area. Although the same concerns may apply to paper bags, a solidified 50 lb. or 100 lb. bag normally is less of a problem to handle than a 2,000 lb. block of material would be.
Protecting the operator and the atmosphere
When properly designed and operated, a bulk bag discharge station can significantly reduce the risks associated with operator and atmospheric exposure to the material. By enclosing the bag discharge spout and controlling the rate at which material is allowed to flow from the bag, the chance of operator and/or plant contamination is minimized. We have found that an iris type valve with a flexible, impermeable diaphragm is ideally suited to this purpose. The closed bag spout can be held closed by the iris valve when untying the spout's draw strings through a bag spout access door on the discharger's spout intake chute. The access door can then be closed prior to allowing material to flow from the bag. Once the access door is closed, the iris valve can be gradually opened, creating minimum displaced air velocity and therefore less chance of dust escaping. The iris valve also provides a means for tying off partially empty bags.
Application considerations for dischargers
Materials to be discharged
One of the most important considerations in applying a bulk bag discharger to your process is the type of material to be discharged. Obviously, discharging plastic pellets from a bulk bag is a much easier task than discharging packed, smearable sub-micron sized zinc oxide. Therefore, the first step in applying a bulk bag discharger is to understand how the material behaves in various forms and under various conditions. For example, many antioxidants are very easy to discharge in a pelletized form. However, those same materials can be very difficult to handle as a fine powder. Also, variations in temperature and humidity can cause major changes in a material's flow properties.
Particle size distribution and shape
Wide particle size distributions and irregular shapes can lead to knitting and solidification of the material within the bulk bag, making discharge difficult. Pressure, vibration and heat cycling of the material during storage transportation can contribute to material packing and worsen the effects of these tendencies.
Prior to purchasing discharge equipment, an actual bulk bag of each material in the state it will be received by the user should be tested by the manufacturer under realistic conditions. Working with a manufacturer who has successful experience with your material can often provide you with performance guarantees without the time and effort required for testing. However, beware of taking inappropriate short cuts at this point. The extra time and effort may be well worth it in the long run.
Mode of discharge required
The mode of discharge required is very important in deciding what features will be needed in a discharge system. The three basic types of discharge possible are batch, intermittent or continuous.
The most common mode of discharge encountered in the rubber industry is the batch mode. Batch discharging simply requires that a given amount of material be discharged within a maximum time frame. In batch mode, the actual delivery rate is usually not critical, but the amount of material delivered is.
Intermittent discharge is normally required when a vessel or surge bin must be refilled periodically. A use bin which initiates refill based on level control(s) or timer(s) is a good example of a requirement for intermittent discharge.
Continuous discharge is appropriate when the process requires material delivered at a specified rate. This type of discharger must include a closed loop control system which monitors the discharge rate and continually makes adjustments to meet the rate required by the process.
Required batch/flow rate accuracy
There are several parameters which dictate whether weigh batching from a bulk bag is appropriate and, if so, what type of weighing system should be used. The most important of these are the batch size and discharge time allowed, the gross weight of the bag and discharging equipment (i.e. if the system is intended to be supported by the scale), and the allowable deviation from the batch target weight.
Since most processes express the allowable deviation from the target batch weight as a percent of the total batch, by weight, a larger batch is more forgiving of inaccuracies. However, if the larger batch must be delivered in a short time period, a larger capacity conveyor/feeder will be required which may affect the accuracy that can be obtained.
Distance/path from discharger to destination
Where the material must be delivered relative to the discharger's position is one of the most important parameters in choosing the discharger's feed device. If the discharger is to be located directly above the delivery point, a rotary feeder or a short screw feeder may be appropriate. However, if the material must be conveyed horizontally and/or elevated, a flexible screw conveyor or another type of mechanical or pneumatic conveyor may be used. The discharge conveyor should have the flexibility to accommodate physical obstructions and traffic isles. Available headroom at the discharger location and delivery point can also impact the selection of the discharge device.
Selecting a bulk bag discharger
Simple loading/unloading of bulk bags
There are two basic methods of loading/unloading the bulk bags into the discharger. The first method pertains to the fork lift truck loaded type of discharger and utilizes a fork lift track to deliver and load the bulk bag into the discharger. The second method utilizes a fork lift track or possibly a pallet jack to deliver and stage the bag on the floor at the discharger. A powered trolley and chain hoist are then used to elevate and position the bag in the discharger.
Promoting flow from the bag
Once the bag is in place and opened, many materials will not flow freely from the outlet spout. The common problem is that a bridge forms above the outlet spout and only the material below the bridge is discharged.
Although many dischargers include vibratory and/or gyratory flow promotion, our research and field experience have shown that mechanically massaging the bag is the most effective means of discharging difficult materials from a bulk bag. When it is simply required to discharge the entire contents of the bag by gravity into a bin or vessel, vibratory/ gyratory flow promotion may be effective. However, when material flow from the bag is restricted, as is often the case when the bag is being discharged through a feeder or conveyor, material may pack in the bag, preventing discharge. Also, frequent starting and stopping of vibratory/gyratory devices is not recommended.
Flow-flexers are pneumatically operated massaging paddles which effectively promote discharge of even the most difficult, packed materials. Flow-flexers can be cycled as frequently as necessary without damage. Since the massaging action does not tend to pack the material in the bag, they are well suited to applications where a feeder or a conveyor is receiving the material at the bag outlet.
When flow-flexers are employed, large pneumatically driven pivoting plates are used to massage the bottom of the bag. When activated, the plates raise opposite bottom comers of the bag to loosen and discharge stubborn material. As the bag empties and resistance decreases, the stroke of the air cylinders increases to raise the bag bottom into a steep "V" shape for complete discharge of material.
Compensating for bag elongation
As a full bulk bag empties, the bag tends to elongate and the overall length of the bag increases and causes slack in the bag. If provisions are not taken to compensate for this elongation, the bag bottom tends to remain flat and material will not flow to the discharge spout. Pop-top spring loaded bulk bag support posts raise the top of the bag as it empties and provide constant tension to remove slack. This tends to create a cone shaped bag bottom which eliminates the dead material zones in the bottom of the bag in order to completely discharge the bag contents without manual intervention.
Flexibility for future changes
In today's environment of ever changing processes and processing equipment layouts, care must be taken to choose systems that are flexible in design. The interface between the bag outlet and the process should be such that the equipment is not rendered useless by process changes. For example, if it becomes undesirable to use a fork lift truck to load the bags, modular designs provide for a simple retrofit to a powered trolley chain hoist. Also, care must be taken to choose controls that allow for expansion to suit future management information systems needs without becoming obsolete.
Measurement of material discharged
Since bulk bags contain anywhere from 500 lbs. to 4,000 lbs., and it is very difficult to design processes so that a batch requires materials in whole bag increments, it is usually necessary to discharge smaller portions of the bag contents consistently and reliably. Although some materials and processes lend themselves to volumetric dischargers using batch timers and calibrated feed rates to measure material discharged, much more consistent, reliable results are obtained with weigh batching dischargers.
There are two basic types of weigh batching bulk bag dischargers available. The first type is referred to as a gain-in-weight batcher. The second type is known as a loss-in-weight batcher.
A gain-in-weight batching system uses an integral conveyor/feeder to discharge material into a hopper or vessel which is mounted on a scale. The scale and the vessel can be mounted on the bulk bag discharger or independently. A batch target weight is selected through the batch controller keypad and software. When a batch is initiated, the conveyor/feeder starts to discharge material into the weighed vessel. As the weight of material in the vessel approaches the target batch weight, the conveyor/feeder's discharge slows to a preselected dribble speed to accurately complete the batch.
An advantage to gain-in-weight batching is that the required scale capacity can generally be reduced with a proportional increase in scale resolution. Also, since the batch can often be created while the process is consuming the previous batch, it is often possible to use a smaller capacity conveyor/feeder which can increase batch accuracy. A disadvantage to gain-in-weight batching is that multiple ingredient batches require that ingredients be discharged and weighed sequentially, which requires more time to create the batch. Also, batch sizes are often limited by the amount of space available for the weighing vessel above the material use point.
In a loss-in-weight batching system, the bulk bag discharger and often the entire conveyor/feeder is mounted on a scale. Once a batch target weight is selected and the batch is initiated, the conveyor/feeder starts to discharge material from the system. As the weight of material removed from the system approaches the selected target batch weight, the conveyor/feeder's discharge slows to dribble speed to accurately complete the batch.
An advantage to loss-in-weight batching is that multiple ingredients can be simultaneously discharged into the process, reducing batch creation time. Also, since an intermediate vessel may be eliminated, space requirements are reduced and maximum batch size is virtually unlimited. A disadvantage of a loss-in-weight discharger system is that required scale capacity is greater. This will generally lead to a proportional decrease in scale resolution.
Durability to withstand plant environment
A bulk bag discharger's structure should be strong and rigid to withstand typical harsh plant environments. The support frame should be constructed of heavy steel members to resist damage from occasional bumps and scratches with fork lift trucks and pallet jacks. In the case of loss-in-weight dischargers, all plant personnel should be informed that the discharger is mounted on a sensitive scaling system and care should be taken not to shock the scale system, particularly as the batch is in the dribble mode of batch completion.
Controls and instrumentation considerations
Number of materials and recipes required
The number of materials to be batched, as well as the number of recipes that must be stored, will impact the type of controls that are appropriate for a particular application. Also, the complexity of the batching routine required will affect the associated software and hardware requirements. If it is intended to interface with plant process controls and auxiliary devices such as gate valves or related equipment drive motors, presently or in the future, necessary software and hardware modifications should be discussed with your equipment supplier at the design stage. This will avoid the possibility of purchasing controls that are not able to meet your present or future needs or require costly redesign and modification to do so. Also, your supplier should be aware of the capabilities and capacity of your plant systems. It is possible that some functions of the batching process, such as recipe storage, can be accomplished without additional hardware or modifications.
Resolution of weigh system
Resolution of the weigh system is defined as the smallest increment of weight that the system can "see." Resolution is strictly a function of total scale capacity and the number of graduations that the scale can be divided into. For example, if the total scale capacity is 4,000 lbs. and the scale rating allows for 10,000 divisions, the scale resolution would be [+ or -] 4,000 lbs./10,000 divisions = [+ or -] 0.4 lbs. Scale resolution should not be confused with system accuracy, which takes into account all sources of system inaccuracies, such as ambient vibration, electrical and mechanical noise, and variations in material flow properties. Typical accuracies obtained for loss-in-weight bulk bag dischargers are approximately [+ or -] 1 lb. If your process requires batching accuracy better than [+ or -] 1 lb., gain-in-weight batching should be considered.
Batch continuation after bag change
As discussed earlier, required batch weights sizes are most commonly inconsistent with total bulk bag weight. This inevitably leads to an incomplete batch and an empty bulk bag. The loss-in-weight discharger's software must be capable of storing the incomplete batch so that the operator can initiate batch completion after the empty bag is replaced with a full bag.
Typically, a bag change will take approximately 5 to 10 minutes. In critical applications, it is prudent to create a surge of material in the system with adequate capacity to accommodate at least one batch after the bulk bag is empty. Some applications may require additional surge capacity or redundant dischargers in order to meet process material requirements during bag changeover.
Present and future technical support
Due to continual advances in electronics and related computer hardware and software, it is not uncommon for new superior technology to emerge no sooner than a given control and instrumentation system is installed. For this reason, a supplier with an eye on the future should be selected. Controls and instrumentation should be flexible enough to accept field modifications in the future to take advantage of subsequent developments. Also, it is important that the manufacturer maintain ongoing technical support and parts for control systems that become obsolete. Be sure to discuss your supplier's policy for dealing with obsolete systems to be sure that you do not end up without a source of support and parts for your system prematurely.
Summary and conclusions
The use of bulk bags can offer significant process advantages, as well as reduced costs and increased profits. Since manual lifting of material is virtually eliminated, bulk bag systems can help you to maintain a healthier work force and reduce workmen's compensation insurance premiums. Using bulk bags reduces the amount of contaminated waste that is sent to landfill, which reduces associated environmental and health risks.
Selection of suitable bulk bag discharging equipment can lead to higher, more consistent end product quality at reduced production costs. Since each batch is individually weighed and verified by the system, the chance of producing off-spec end products due to operator error is minimized or eliminated.
If you are unsure as to whether a move to bulk bags is appropriate for your particular plant and process, you should contact a reputable bulk bag discharger manufacturer for recommendations. Be sure to inquire about prior similar systems which have been successfully designed, fabricated and installed in the past. If in doubt, get a second opinion.
"Weigh batching directly from bulk bags" is based on a paper given at the May, 1997 Rubber Division meeting. "Understanding the influence of polymer and compounding variations on EPDM extrusions" is based on a paper given at the October, 1997 Rubber Division meeting. "On-line scanning of extrusions and tires" is based on a paper given at the October, 1997 Rubber Division meeting.
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|Date:||Jul 1, 1998|
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