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Vibrating Screens for powder and bulk solids

How to chose or troubleshoot a vibrating sieve ?

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Section summary
1. Different vibrating sifters designs
2. Round vibrating sifter
3. Vibrating sieve wire mesh designs
4. Vibrating sieve cover design
5. Vibration of the sifter
6. Supporting of the sifter
7. ATEX
8. Common problems with vibrating sifters

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Vibrating screens allow to gently and easily remove foreign bodies from powders and granules by letting the good product flow through a wire mesh, vibrated to promote the flow of product, while contaminants (metal, wood...) are retained and then eliminated through the rejection.

1. Different vibratory sifters designs

What are the different types of vibrating sieves on the market ?

2. Round vibratory sifter

How is designed a round vibrating sieve ?

Round vibrating sieve are very widespread in process industries. They provide a compact solution for checking all types of bulk solids, from raw materials to finished product, regarding the presence of Foreign Bodies (contaminants).

Round sieves are generally made of the following parts :

• Cover, including inlet, inspection ports and rejection outlet
• Sieve deck, the part actually performing the sieving
• Cone / underpan, located right below the sieve and collecting the powder sieved to direct it to the next process step
• Frame and clamping system, allowing to keep firmly together the different components despite the vibration
• Unbalanced Motor (1 or 2), the driving force for the vibration
• Supporting and springs (sometimes rubber dampers) allowing to stabilize the sifter during the vibration

Figure 1 : Round vibratory sifter design

What is the working principle of a vibrating screen ?

Vibrating sifters are equipped with a sieving deck, made of a wire mesh or a perforated plate on which the bulk solids to check is loaded. An unbalanced motor (or 2 depending on the design), mounted on the chassis, is then transmitting a vibration to the sifting deck allowing the solids whose size is smaller than the mesh of the sieve to go through. This product is then channeled to the next unit operation. The parts which are larger than the sieve size don't go through and either stay on the sieve or are moved towards a rejection outlet thanks to the direction given to the vibration waves by the motor settings.

3. Sieve wire mesh designs

How are made wire mesh for vibrating sieves ?

The sieve is made in 2 parts for round vibrating sifters : a supporting ring and the actual sieve, which can take the form of a wire mesh or a perforated plate, welded or bonded on top of it.

Chosing the detail design of the sieve will depend on the application.

• For non hygienic applications (chemical...) : welding the wire mesh or perforated plate to the supporting ring is sufficient

• For hygienic applications (food, chemicals...) : a plastic bonding (epoxy most of the case) is generally more hygienic has it will avoid that material is trapped in between the sieve and the ring. However, the epoxy must be accepted by the local regulation (FDA approved...), should be durable and not be brittle.

Defining if a perforated plate or a wire mesh should be used will mainly be dependent on the throughput expected from the sieve. Indeed, perforated plates are much more durable and safer (less risk or breakage leading to a pollution of the product) but the reduced passing area vs a wire mesh means that at same throughput, the sieve has generally to be larger (costs, difficulties of implantation...). If wire mesh needs to be chosen, some mitigations to the risks of breakage can be done by choosing a durable material, magnetizable (Stainless Steel 430 or 318), so that it can be retrieved by a magnet positioned after the sieve in case of incident.

The sieve side is generally covered by a gasket which allowed to avoid that any foreign body bypass the sieve.

4. Vibratory sifter cover design

The cover of vibrating sifters play a bigger role than usually perceived. It ensures the dust tightness, but also helps to distribute the powder properly to the sieve decks. Inlet of product should not be close to the rejection, otherwise it will lead to the rejection of some acceptable material due to the fact that the actual sieving area will be reduced. Inlet should be positioned at the opposite of the rejection, or at the center of the sieve. If still not sufficient, it is possible to include a deflector welded to the cover that will direct the flux of material away from the rejection. The deflector can also have an effect to protect the sieve from the impact of the product and thus increase its life span.

Inspection ports should also be positioned on the cover. To have a good vision of the sieve from the inspection port, a minimum of 3 ports are necessary. They are closed by rubber pads when not in use.

The last fitting important on the cover is the rejection, which can be more or less obstructed by a wear plate. The wear plate leaves a little space only for foreign bodies to exit to the rejection. Wear plate are used mainly in case the sieve is not properly sized and product accumulates on the sieve desk. The wear plate then avoids to have too much rejection. But it can only be viewed as a mitigation measure and design should done considering enough sieving area to let all the product go through without flooding. The rejection should lead the overs to a tight container, a BFM connection is a good tool for this purpose.

The cover must be clamped strongly to the sifter in order to avoid any friction that could lead to heating, damage, or leakage of product. Different designs exist, depending on the application of the sifter : quick clamp can be used when cleaning requirement are high, otherwise, a simple ring clamp will be efficient.

5. Vibration

How the vibrating impacts the performance of a vibrating sifter ?

The vibration is performed by an unbalanced motor (sometimes 2). Changing the unbalance will have an effect on the amplitude and direction (when 2 motors) of the vibration. Unbalance should only be changed under the direction of the manufacturer to ensure that the sieving is optimized.

6. Supporting frame

Vibrating sieves' vibrations is strong, and amplitude is particularly important when the sifter stops (although some system now exist to reduce such vibrations). The support frame must be designed to handle such vibration and avoid to transmit it to the rest of the installation.

7. ATEX

Due to vibrations, static electricity can build up. As a consequence, the sieve must be well grounded (manufacturer provide grounding cables). Usually metallic clips are added on 3-4 points of the gasket wrapped around the sieve deck in order to ensure this electrical continuity. Having those clips in place is very important and operators must have a checklist to make sure they have been mounted.

8. Common problems with vibrating screens

How to solve sieving problems ?

Vibrating sieves are usually reliable equipment, however a certain number of issues may require some corrections :

Table 2 : common problems with vibrating sieves

Issue	Root cause and action
Too much vibration	Check the setting of the unbalance motor Make sure the supporting is correct Check that flexible connections before and after are not too stiff
Top cover is moving	Check the strength applied by the clamps
Wire mesh breakage	Change regularly the sieve Make sure that solids are gently poured to the sieve and do not impact it Change material of sieve for longer lifespan
Too much rejection	Control the feed to the sifter, do not overfeed Check the adequacy of opening size of mesh Verify position of wear plate Check vibration direction Use a bigger diameter sieve
Too low capacity	Check vibration Check the adequacy of opening size of mesh Use a bigger diameter sieve

Vibrating screen buying guide - How to select a vibrating sieve

When sourcing a new vibrating sieve for your factory, the following questions need to be asked in order to buy the right specifications :

• What is the expected throughput the sieve ? What is the material particle size and product density to be sieved ? This will give the size of the vibrating sieve to buy.
  
• How fine should be the screening ? This will allow to determine the mesh size of the vibrating sieve machine. Depending on the finess, the supplier will be able to advise if some deblinding features (ultrasonic...) are required
  
• Does the manufacturer has good references for the application ? if not, maybe another supplier is more relevant or trials should be organized
  
• Can the supplier guarantee the throughput and a maximum rejection rate of good material ? (in case a rejection is installed)
• If the supplier able to offer a sieve that can be quickly opened to reduce potential downtime during cleaning and inspection ?
• What is the material of the wire mesh ? Can the supplier offer a resistant material with a long lifespan, is the material magnetisable ?
• Is it in ATEX area ? If yes, the vibrating screen must be certified and the supplier must be able provide those certifications
• Is it used in food / pharma application ? if yes, all materials must be certified for product contact and the supplier must be able to give every certificate.

Vibrating screens manufacturers

Note that PowderProcess.net has no commercial link with those companies.

Among the largest sifter manufacturers, an interested reader could contact :

• Russell Finex
• Sinex / Chauvin
• Vibra Schultheis
• Farley Greene
• Kason
• Sweco
• Virto / Cuccolini

Many other smaller vibrating sieve machine manufacturers are offering quality products and could be worth contacting, trade fairs such as Powtech are a good tool for Process Engineers wishing to check actual design of companies.

To know more about vibrating sieves

Vibrating sieves are a key components in a powder process to ensure reliability, safety of the installation and safety of the product.

Please follow the link to get access to vibrating sieves design details : All you need to know on industrial vibrating sieves for powder checking