Considerations to be made before designing a bag tipping system
October 9, 2018
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Bag tipping, the action of emptying bulk solids from bags, is at the basis of many bulk solids handling processes. However, this process step is far too often perceived as easy — and without added value — which can lead to a lack of focus during design, ultimately causing losses when the installation is operated.
Common issues with bag-tipping stations include leakage of material, a dusty environment, poor ergonomics and bad flow of material. The objective of this article is to detail the key design precautions that one must take to reach efficient material tipping.
Main types of bulk containers
Bulk solids can be transported in different ways in industrial environments, from a few liters to several dozen cubic meters depending on the need of the customer and what is offered by the supplier. Containers can take the shape of bags holding typically 25 kilograms (kg) of material, big bags of 1 to 2 cubic meters (m3) or truck trailers reaching around 35 m3. Other types of containers like bins (especially inside a factory) or octabins are commonly used.
This article focuses on bags, typically weighing 25 kg, which are a widespread bulk container format used in the bulk solids handling industry. Many notions described in this article can be applied to other types of containers as well.
Providing a good tipping position for the operator is a critical aspect of the design of bag-tipping stations. Organizations that invest in equipment that will be used for years should invest time in placing it properly to ease operator movement, which will lead to better safety, reduce work-related diseases and support profitability of a line that yields a regular tipping that will not delay the downstream process.
The first parameter to consider is the height of the tipping station support where the operator tips the bags. Generally, 0.9 m is accepted as a good value. The second parameter is the supply of the bags. Bags should be delivered at the same height of the tipping station so the operator does not have to lift or bend to pick them up. Belt conveyors or lifting tables are generally a good solution for this purpose, although their mechanism should be properly guarded to protect the operator’s hands and feet. The delivery of the bags should also be a design factor that enables the operator to make minimum-to-no twisting movements to pick up the bag and bring it to the station. A well-designed station should allow the operator to simply slide the bag before cutting it open.
Dust emissions are a common source of problems for plant owners. Dust emissions can present a dust explosion risk, can be harmful to the operator performing the task and represent a loss of product. Bag-tipping stations are particularly sensitive to dust emissions because they are at a point in the process which is open.
The most efficient design is to have a filter top-mounted on the station and equipped with a fan. The fan should be powerful enough to create sufficient air velocity through the large opening of the station and thus keep dust inside the equipment. Having a pulse-jet filter will allow the bag filter or cartridges to be unclogged easily and have the powder sucked back into the product stream without losses.
An alternative that can be more economical when having several tipping stations is to have centralized aspiration with a deported filter. However, great care must be given during the design to ensure that the fan will be powerful enough for the entire station. It must also be noted that the dust collected does not return directly to the process, which may lead to some losses in some industries where retipping the dust is not possible.
To cope with the manipulation of very sensitive powders that may be harmful to the operator, suppliers have developed glove boxes to allow the bag to be opened in a closed environment.
Foreign body prevention
Foreign bodies are a concern for all manufacturers because they can damage the process equipment, present a safety risk and, ultimately, cause consumer complaints. Many foreign bodies are actually entering the installation at the tipping stage: Materials tipped are, in most cases, coming from another supplier, which may not have the same quality requirements.
Moreover, tipping is done in open equipment in which a bag liner or a knife can fall by accident into the product stream. It is therefore critical to have a checking step located right after the tipping process unit. Material checking at this point should typically be performed by a sieve and a magnet. These pieces of equipment can, in some cases, be integrated in the tipping station when a compact design is required although they may reduce the tipping capacity because the operator has to wait for the sieve to empty before opening the next bag. In particularly sensitive operations, the magnet can be replaced by a metal detector.
To complement these pieces of equipment, some simple design steps can be taken such as using a chain to hold the cutting tool or placing a coarse grid at the tipping inlet that will stop large foreign bodies.
Flow of material
Having a good flow of material out of the tipping station is crucial for the process line. Any flow problem will stop the operator from tipping to try to make the product flow by different means, which may consequently stop the downstream equipment by lack of ingredient. The flow of material will be dependent on the type of station used.
For compact stations with an integrated sieve, a good flow of material will depend on the mesh opening of the sieve. If the opening is too small, the operator will have to wait in between two bags opening. Conducting trials with a supplier is the best way to select a mesh size as small as possible without hampering the product flow.
For other stations equipped with a buffer — which have higher capacity because the operator can tip directly in a hopper without bottlenecking equipment in the middle — the flow of material out of the hopper will be critical. The hopper must be designed with sufficiently steep walls and a large outlet so the product can flow; such characteristics can be calculated from the result of shear testing. For bad flowing ingredients, discharging aids such as a vibrator or fluidizing pads should be installed (pads should not be installed too close to the station opening to prevent dust emission).
A bag-tipping process can be greatly enhanced if a number of precautions are taken early in the design process. This includes especially making sure that the operator will have a safe workplace, which in return will guarantee reliability and high capacity. The design requirements in this article should be the basis of any specification for implementing a new tipping process.
Thomas Lamotte has 10 years of experience in process design and operation including more than six in dry-mixing and bulk solids handling. Currently a senior process engineer at Nestle Research Singapore, he holds two master’s degrees in process engineering from ENSIC Nancy and IFP School, France. Lamotte may be reached at firstname.lastname@example.org.