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|4. Important design points|
|6. Common problems with Big Bag
Big Bag tipping stations (also named Big Bag unloading station, Big Bag emptying station or Big Bag dumping station) constitute an important process equipment since they will allow to bring to the process the raw materials necessary to manufacture the desired product. All the subsequent process step depend on the feed of the raw materials, if those materials are brought by Big Bags, the tipping station must be well designed to allow a fast, clean and safe tipping.
Big Bag emptying stations allow to discharge bulk bags of powder or granules in many industries and for many applications :
- Construction industry : cements, mortars
- Food industry : flours, maltodextrins, sugar
- Chemicals industry
- Plastics : polymer pellets
The following items are to be taken into account when designing a Big Bag feeding station.
The flowability of the product to be discharged is a key parameter that will drive the design of the "upper" part of the tipping station. It is indeed of crucial importance that the product flows easily outside of the Big Bag, in order to make sure the capacity of the line is ensured, while ensuring that 100 percent of the material is discharged from the tipping station, to ensure 0 loss.
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A powder rheology analysis of the powder(s) to be discharged must be done. If the product is very free flowing, no specific aid will be required. If issues are expected, a vibrating table can be chosen, however this will fit only products that are not compacting under the vibration. For very difficult product, it will be necessary to foresee the installation of automatic pushers.
It is common to have rods or a grid at the tipping station inlet (to avoid foreign bodies or for safety reasons). This feature may cause issue with product presenting a difficult flow, or being lumpy. Installation of such grid should be well considered in the design
This looks obvious, but many time at the 1st use of the tipping station, the Big Bag is not fitting...
The min and max dimensions of the Big Bag must be well defined when ordering the tipping station. One particular dimension to check is the diameter and the length of the spout to be connected to the tipping station head.
The tipping head is the point where the Big Bag spout will be connected to the discharge tube.
The simplest form is just an open tube where the operator will introduce the discharge spout of the Big Bag. This is by no mean satisfactory since powder will leak, leading to Health and Satey issues. A clamping system must be foreseen. Commercial design can range from a simple manual mechanical clamp, to automatic clamping with pneumatic cylinder or inflatable seals.
Big Bag must be removed without spreading remaining powder to the environement. A system to collapse the Big Bag can be considered ; it is basically a fan located downstream the process, typically connected to the hopper to which the Big Bag discharges and that will induce a slight vacuum that will allow the Big Bag to collapse and be removed more hygienically.
Closing a Big Bag being discharged is possible - although not easy. Some suppliers are proposing arms, operated with pneumatic cylinders, that can squeeze the discharge spout of a Big Bag and interrupt the flow of product, allowing to re-tighten a Big Bag and remove it from the station.
This can only work if product is flowing out of the Big Bag during the closing process.
Figure 1 : Typical design of a Big Bag Tipping Station
The Big Bag dumping platform must be designed to unload a certain
number of Bulk Bags per hour. The cycle time achievable is made of
the following components :
- Bringing a Big Bag to the tipping station
- Connecting the Big Bag to the tipping station
- Discharging the Big Bag
- Disconnecting the Big Bag
- Removing the Big Bag from the tipping station
The duration of this activity will greatly depend on the setup and
the number of operators. It can range from 2 minutes to 10 minutes.
Shorter time will be achieved if a full Big Bag ready to be lifted is already in place
Longer time will be necessary if a full Big Bag needs to be brough to the tipping station, prepared
The time necessary to lift and bring, transfer and put the Big Bag in position is dependent on the hoist speed. Typical values range from 6 to 9 m/min
Opening the bottom of the Big Bag, adjusting the spout to the tipping point, untightening the spout and clamping it will take 2 to 3 minutes to a skilled operator.
The discharge of the Big Bag is dependent on the process downstream
Table 1 : Estimation of the time needed to tip a Big Bag
|Downstream||Time to tip|
|Direct discharge to a Buffer, no equipment in between||A big bag can be discharged in 2 to 5 minutes depending on the flowability of the powder|
|Discharge to a conveying line||It will depend on the conveying line throughput|
|Discharge to a vibrating sieve||The sieve must be designed to not impact the global process, the time should depend on the next process steps, typically a conveying line|
Stretching the Big Bag, disconnecting the spout and tightening it should take around 2 minutes to a skilled operator. Add 1 min if there is an option to collapse the Big Bag at the end of the discharge.
Bringing back the Big Bag at the lifting point - in order to remove it and hook a new one - will depend on the design of the hoist.
When reviewing the design of a Big Bag tipping station, the following points deserve full attention :
Table 2 : Key design points of a Big Bag Tipping Station
|Big Bag station access||The access to the Big Bag station should be done without having the need of a platform in order to prevent occupational safety risks|
|Powder Properties||Design the station when knowing about the flow properties of the product, conduct tests if necessary|
|Understand the impact of downstream process||The equipment located after the station may have an impact on its operation, for example by creating pressure effects that disturb the flow of product|
Big Bag tipping station have some inherent risks linked to the manipulation of large and heavy powder containers. It is necessary to manage those risks.
An incorrectly hooked Big Bag can fall during the transfer process. The whole path of the Big Bag must be closed by a safety cage that prevent any people to stand close or even below the Big Bag in movement. Specific features must be put in place to make sure no people is present in the cage of try to open it (interlock on door, sufficiently high fences, control fixed outsitde the station...).
It is possible that a rotating equipment is located right below the tipping point (star valve, rotating magnet...). An operator could reach those equipment while trying to unblock powder for example and be amputated. Measures to ensure 0 access to the rotating element must be considered : welded grid if possible, enough distance not to be able to reach the rotating equipment with the hand, light curtain
As every equipment processing powder, the ATEX risk needs to be considered when designing and sourcing a Big Bag station. The 1st condition is to prevent dust leaks, which means that the Big Bag station head clamping the spout must be well designed and tight. A system to collapse the Big Bag before removing can as well help reducing dust leaks. The training of the operator is key, procedures for tipping must be clear and applied. Finally, the ATEX rating of equipment must be performed and equipment sourced accordingly, with certification.
Big Bag tipping stations are usually reliable equipment, however a certain number of issues may require some corrections :
Table 3 : common problems with big bag tipping station
|Spillage of product||Make sure the bag is well emptied at the end of discharge, train the operator to pull a bit a the big bag to help the flow of product out. Tighten immediately the spout|
|Dust emission during tipping||Make sure that the clamping head is properly tight|
|Collapse of big bag spout during tipping||Look for an equipment creating vacuum, make sure the tipping hopper below the station has a filter properly sized|
|Liner of Big Bag is falling in the station head with the product||Make sure the liner is well tightened at the top of the big
Install a coarse grid to make sure no part of the bag can enter the process