Sifters / Vibrating Sieves - Removing Foreign Bodies

Section summary
1. Types of Sieves
2. Design and Feeding
3. Sizing
4. Troubleshooting

Types of sieves

- Static grid

The simplest way to check a material to verify it does not contain big foreign particles or pieces is to have it go through a static grid. This method is simple but does not prove to be always efficient. The solids (powder or larger particles like pellets) has to be very free flowing to be able to pass through the grid without further action. Thus, these grids are usually quite coarse, from 10 to 40 mm. Such grids are particularly interesting when tipping bags of 25 kg. They allow the operator to support the bag and avoid that big parts of the bag fall in the product stream. They can also be located at the discharge of Big Bags where they will prevent the liner (if any) to be sucked to the product stream. For these 2 applications however, it is important to make sure that the materials don't lump in the bags, otherwise, it will be particularly difficult to the make them pass through the grid, which may endanger the performance of the process.

- Round vibrating sifters

In contrary to the static grid, the vibrating sieves are equipped with a motor allowing to vibrate the sieve. The vibration action allows the material to go through the sieve. As a consequence, vibrating sieve can achieve higher throughput, at smaller mesh size. They allow to optimize process safety and process performance. Their design is however not trivial and many examples have shown that vibrating sieves not carefully designed, specified or maintained can actually bring or release foreign body to the good product. See below the parts on vibrating sieve mechanical design to know how to properly specify such equipment.

- Rectangular vibrating sifters

Rectangular vibrating sieves have similar capabilities as the round ones. Vibrations allow the product to go through a sieve in order to check it regarding the presence of foreign bodies. However, it brings an additional function : the possibility to move the powder laterally from 1 point to another. The vibrations are indeed designed to apply an horizontal movement to the particles. Such rectangular vibrating sieve can therefore be used, in some situation, as a feeder. The design of the machine is very important, especially its supply, since a proper control of the flow is also necessary to avoid a "flooding" of the sieve.

Note : classifying

Classifying is another form of product sieving that is not detailed here. Multi deck round vibrating sifters can be used for this purpose. This can be used for classifying pellets, beans or smaller powders.

Vibrating sieve mechanical design

Among design parameters for vibrating sifters, important ones are the following :
- Sieve design
- Sifter opening
- Foreign Body rejection
- Flexible connections
- Vibrating sifter support structure

Table 1 : The different components of a vibrating sieve

Part Design
Sieve Sieve support is important. Sieve plates cannot be used as such, they need a support in order to be properly clamped. Sieves can be welded or glued to their supporting ring

Welding should be avoided for hygienic application. Epoxy gluing offers generally a good and durable solution although the quality of application is very variable depending on suppliers.

Using a perforated plate insteand of a woven wire mesh offers a 3rd option which is to have the sieve plate formed in a certain way to be autosupported, non need of supporting ring, this appears to be a very hygienic and robust solution
Sifter opening Sieve plates must be regularly inspected in order to verify that the sieve plate is not damaged, to change the sieve size...etc...

The simplest design is to have clamping rings (similar to Jacobs clamps for pipes) but it is not very convenient. Quick release systems are preferable.
Rejection If a Foreign Body is carried with the product, it should be stopped by the sieve plate. Some sifters are equipped with an outlet called rejection. The Foreign Body will be rejected through this outlet.

In order to avoid dust emissions, the rejection outlet should be connected to a closed bucket that will collect the rejected powder and Foreign Bodies. The level should be viewable in order to inspect the content of the bucket when product has been rejected.

In order to cope with ATEX risks, the sifter as well as any pieces in the rejection part must be earthed.
Flexible connections Since the sifter is vibrating, flexible connections are located up and downstream, as well as on the rejection outlet.

The size of the flexible is important, it should not be too tight to avoid disturbing the vibrating movement. Quick release connections like BFM are to be preferred on sifters opened often, provided it does not cause safety concerns (access to rotating equipment around the sifter...)
Support The supporting structure of the sieve is of primary importance to ensure an optimal vibration. The structure must be stiff enough to not vibrate, otherwise all the energy of the sifter will be lost to moving the structure, not the sieve plate.

Feeding

The feeding of a vibrating sieve must be controlled. If uncontrolled, there may be an overflow of the sieve deck, with too much powder on it. If it happens, the performances of the sieve will dramatically be reduced and one common problem appearing can be the reject of good product. This generates waste where good product should actually have been valued.

To avoid this, a dosing equipment must be installed prior to the sieve. Common elements used are :

Table 2 : Powder supply to vibrating sieve

Equipment Description Sketch
Feeding tube A flexible tube is installed at the sieve entrance. The height in between the bottom of the tube and the sieve deck will determine the flow of product admitted
Screw feeder The screw controls the feeding to the sieve thanks to the speed of rotation of the screw flight
Rotary valve An airlock rotary valve can be used but it is an expensive and fragile equipment, to be considered only if the sieve is below a hopper under pressure or vacuum. Other possibility is to use a pneumatic rotarz valve, but the throughput may be limited
Vibrating valve Some suppliers can actually propose a butterfly equipped with a special actuator that controls the flux of product by vibrating
Butterfly valve Due to its "On/Off" nature, this type of feeding is not recommended
Operator control Vibrating sieves can be installed at a tipping station where bags are tipped by operators. Due to the limited tipping speed (max 2 t/h/operator), the feeding of the sieve is controlled. It is however recommended to have a sieve without rejection to avoid overflow at each bag tipping

Vibrating sieve sizing

The sizing of a vibrating sieve is difficult to model and is most of the times based experience : through past references or tests in a pilot plant. It is however possible, with a reference, to calculate the scale up of a certain sieve.>br/> The scale up is based on the open area of the sieve. If the throughput reached is 1000 kg/h for a passing area x, you can expect 2000 kg/h for a passing area 2x, provided the powder distribution is correct

To be noted that following general rules :
- The smaller the mesh, the smaller the passing area.
- A perforated plate will have a smaller open area than a wire mesh
- A vibrating sieve located in-line (= in a pneumatic conveying line) will have a higher throughput than a sieve fed by gravity

Troubleshooting

The following table summarizes actions that can be taken in front of specific problems

Table 3 : Vibrating sieve operation troubleshooting

Issue Possible action
Too low throughput Check feeder is giving right throughput

Check that vibration is correct (as per supplier manual) - especially for rectangular sieves

Check that nothing prevents vibrating sieve to vibrate properly

Check that support is properly designed and does not vibrate

Use larger sieve or switch from perforated plate to wire mesh

Install larger sieve
Screw feeder The screw controls the feeding to the sieve thanks to the speed of rotation of the screw flight
Rotary valve An airlock rotary valve can be used but it is an expensive and fragile equipment, to be considered only if the sieve is below a hopper under pressure or vacuum. Other possibility is to use a pneumatic rotarz valve, but the throughput may be limited
Vibrating valve Some suppliers can actually propose a butterfly equipped with a special actuator that controls the flux of product by vibrating
Butterfly valve Due to its "On/Off" nature, this type of feeding is not recommended
Operator control Vibrating sieves can be installed at a tipping station where bags are tipped by operators. Due to the limited tipping speed (max 2 t/h/operator), the feeding of the sieve is controlled. It is however recommended to have a sieve without rejection to avoid overflow at each bag tipping

Other related sections

Metal Detectors

Magnets




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