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|4. Important design points|
|6. Common problems with bag tipping stations|
Bag tipping stations, also named bag dumping station, bag dumping platform or sacks tipping stations are often an indispensable process step for manufacturers to open and discharge bags of powder inside a process. Indeed, many ingredients are still delivered in 25 and 50 kg sacks all over the industry :
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Bag unloading stations are used to open and empty bags efficiently, which means quickly and without dust emission, towards dry processing equipment. Tipping stations are especially useful to load hoppers, conveying lines or mixers. They are positioned right above the equipment they need to feed and product flowing from the opened bags is then falling by gravity to the next process equipment.
Although the function appears very simple, there are many different design parameters to take into consideration in order to have an efficient bag emptying station. This page is detailing the key design aspects to be addressed by manufacturers and plant operators.
The capacity (= number of sacks to be tipped / h) will be highly dependent on the workforce skills, strength and motivation, manipulating and opening sacks being a particularly demanding task. It will also be dependent on the surroundings of the platform, especially if the bags are provided directly to the operator on an ergonomic way. However, as a rule of thumb, one can consider that the capacity per station is around 2000 kg/h (which represents in between 1 to 2 bags tipped / min).
Some stations have 2 lids which allows to have 2 operators tipping at the same time and therefore increase the tipping throughput to around 4000 kg/h.
Semi automatic bag tipping stations have recently appeared on the market. Bags are dropped and opened automatically thanks to knifes on the tipping station, the operator having then only to shake and remove the empty bag. Fully automatic stations are also in development.
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The following items are to be taken into account when designing a sack tip station.
Sacks have to be opened while the station is opened to the environment, therefore the risks associated with the product must be evaluated and protective equipment provided to the operator when necessary (dust mask...). For very hazardous products, it is advisable to use glove boxes which are a specific type of dumping station where the operator can manipulate and open the sack without being in contact with the product, the station ensuring full containment.
Associated to this risk, the sack feeding station must be
equipped with an aspiration system that avoids to have dust
getting out of the station. The aspiration system is made of
and a fan. The fan must be sized according to the opening of the
station in order to provide sufficient suction power, which means
sufficient air velocity - speed - at the opening of the tipping
station. To be noted that it is not recommended to have deported filters (although
it could look economic for several stations) but rather have Filters
top mounted on the station so that dust can fall back directly on the product, than collected elsewhere and maybe wasted.
The station must be designed to ensure that the operator will have a position as comfortable as possible to empty bags. This is related to the station height which would be around 900 mm and the size of the support, or table, the operator can use to put the bag prior to opening which should be long and wide enough. It may be necessary to have the operator on a platform so that the tipping table is not too high, but this may be related to safety risks for the operator (stairs to get on the platform...).
As the station is opened during the tipping, it presents an increased risk for the entry of foreign body in the process. The station should be equipped with the following equipment to prevent the entry of foreign bodies, from the environment or from the materials :
The flowability of the product to be discharged is a key parameter that will drive the design of the "bottom" part of the tipping station, i.e. the buffer hopper if it is required. It is indeed of crucial importance that the product flows easily outside of the dumping station, 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.
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 the hopper of the dumping station must be designed accordingly, with steep walls and possibly vibrator or fluidizing pads. A knocker can be interesting to empty the station at the end of a shift for example.
Once emptied, sacks can be a source of dust. A bag collector must be positionned on the side of the station to avoid dust is spilled around the station or when removing the empty sacks. Some manufacturers can also propose a bag compactor to reduce the volume of the waste.
Figure 2 : Typical design of Bag Tipping Stations
When reviewing the design of a manual bag dump station, the following points deserve full attention :
Table 2 : Key design points of a Bag / Sacks Tipping Station
|Bag station access||The access to the Bag station should be done without having the need of a platform in order to prevent occupational safety risks.|
|Station height||The station must be around 900 mm from the floor in order to allow for a comfortable manipulation of the bag|
|Delivery of bags||Sacks should be delivered by a conveyor directly at tipping
height in order to reduce operator efforts and maximize
productivity. The bag supply must also be such that the
operator does not need to twist to grab the bags and tip them
One alternative is to have a pallet lifter if bags are not delivered through a conveyor.
|Dust aspiration||A filter, equipped with a suction fan, should be installed on top of the sacks feeding station in order to capture dust during tipping.|
|Foreign Bodies||Compact stations can integrate a Vibrating Sieve. All stations should include a grid. The cutting tool should be held by a chain in order to avoid to fall in the station.|
|Understand the impact of downstream process||Capacity and design will not be the same if the station is
equipped with a buffer hopper. Thanks to a buffer hopper,
tipping capacity can be higher as there is only 1 grid to go
through, but the buffer must be designed as to ensuring a good
flow out of the station. From the buffer hopper, the product
is typically fed to a product checking step (sieve, Magnets
) before being transferred to downstream processing
As the station is open, risks related to rotating equipment that may be present below the station must be taken into consideration. Such equipment should be kept far away from the operator, or the grid in the station should in any circumstances prevent the operator to access.
As every equipment processing powder, the ATEX risk needs to be considered when designing and sourcing a Bag dumping station. The 1st condition is to prevent dust emissions, which means that the Bag tipping platform must be equipped with an aspiration system. All equipment in the station and around the station must be rated according to the ATEX zoning assessment.
Bag tipping stations are usually reliable equipment, however a certain number of issues may require some corrections :
Table 3 : common problems with bag dumping station
|Spillage of product during tipping||Review the size of the supporting table and the position of the grid to support the bag during slitting|
|Dust emission during tipping||Review the strength of the fan providing the dust suction.
Make sure filters are clean
Prefer top mounted filter and fan vs deported and centralized aspiration
|Bad flow of product out of the tipping hopper||Check hopper slope, add discharging aids to promote the flow|
|Bad flow of product through sifter||Review vibration strength
Adjust mesh size
Adjust feed to sieve
There are numerous bag dumping station suppliers. However the user
must be careful when reviewing the design and check the criteria
above in order to select the best supplier.
Some sack tipping stations may also be available second hand on the
market. It can be a good option for manufacturers but an expert view
is required in order to assess if the 2nd hand proposal will fit the
need in terms of ergonomics, hygiene and possibility to implement in
the client process environment.