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The art of screen dynamics: lightweight designs vs momentum

 

For efficient classification, screens need to generate more acceleration than a rocket being launched, while also being lightweight and energy-conserving. Gary Styger and Steven Hunter, of Weir, explain how OEMs are successfully employing modern design tools to create economical, yet more effective screens.

The working principle of a screen is simple – you drop tonnes of material onto the screen from the feed end and through a combination of mechanical power and gravity, the screen separates it into oversize and undersize fractions.

More complicated however, are the screen’s dynamics – how all of its parts and forces affect each other and the feed. While engineers do their utmost to minimise the large accelerations screens produce, it takes a lot of power to achieve effective feed stratification and classification.

So, how much acceleration does a screen require?

Weir Minerals’ engineers design loaded screens to generate a minimum acceleration of four times’ gravity (Gs), whereas unloaded screens will operate at 4.5 to five Gs. Some of Weir’s triple-shaft horizontal screens can even run upwards of five to seven Gs.  

Between four and seven Gs of acceleration may be considered average by other screening manufacturers. However, Weir’s engineers consider these accelerations optimal. If you’re not familiar with Gs of acceleration, the following numbers may put things into perspective. During a rocket launch, astronauts experience around three Gs; when designing products capable of producing energy comparable to space rockets, overcompensation can have disastrous effects for producers.

The effect of such high accelerations on surrounding equipment and structures needs to be carefully considered.  Concrete is the preferred material for housing vibrating equipment and most larger sites use it to dampen a lot of the dynamic force generated by the screens.

Sometimes sites are forced to use steel structures – occasionally even free-standing ones. As you can imagine, the dynamic forces playing upon such an arrangement introduce significant structural and safety concerns if not accurately measured before installation and monitored throughout operation.

Although not ideal, many sites that require steel structures also lack a dedicated structural engineer to monitor them. Weir Minerals can assist producers in keeping personnel and equipment safe by evaluating the site’s structures and its equipment’s impact upon them.

There are several ways to manage a screen’s dynamic forces to ensure it operates efficiently without negatively affecting the surrounding environment. For example, Weir Minerals’ range of Trio and Enduron screens use optimum materials such as high quality springs and rubber buffers on support points to isolate live frame vibrating loads.

Screens and structures can also benefit from external supports, such as an isolation frame, a structure applied to the screen which contains a mass suspended in between springs. By working against the movement of the screen, the isolation frame can reduce the dynamic load on the structure by up to 90 per cent.

Although this method does increase the static mass of the structure, it is still a trade-off preferred by many structural engineers. An increased static mass is easier to calculate and address, whereas a high dynamic load progressively increases the structure’s fatigue, making the site less safe over time in unpredictable ways.

New Industrial Internet of Things monitoring systems such as Weir Minerals’ Synertrex technology allow screen operators to monitor several key factors, including the stroke and frequency, in real time.

Having access to live information on equipment, in addition to warning alerts when the screen or its environment changes, increases safety and allows operators to optimise equipment to levels that were practically impossible before.

Screen dynamics aren’t just important for new sites to consider.  Brownfield operations that install new equipment without considering the effects it could have on nearby structures and equipment can find themselves suffering from extended downtime and added costs.

While structural supports add costs to an upgrade, minor downtime and additional support costs at the project’s beginning are preferable to the significant downtime and costs associated with structural and equipment failure during operation. Not to mention the safety concerns associated with lack of support around dynamic equipment.

Weir’s expert team has years of experience commissioning screens and their supporting structures. Whether it is a site expansion or upgrading of existing machinery on existing structures, Weir Minerals can partner with quarrying producers to find the best screening solution for their operations. 

Trio screens use high quality springs and rubber buffers on support points to isolate live frame vibrating loads.

DO SCREENS NEED TO BE HEAVY?

With more safety features and higher quality materials, is a heavier screen always better? The answer may surprise you.

Traditionally engineers will want to incorporate additional, weight safety factors, together with the highest quality materials to guarantee a robust and long-lasting solution.

While this line of thinking is more acceptable in certain types of equipment like pumps and valves, when applied to screens, it often results in exceptionally heavy screens over-engineered in all the wrong places.

New methods are radically changing the way screens are designed. Digital calculations, simulations, modelling and drafting allow designers more freedom to challenge traditional designs and make use of material advancements to build screens that are more reliable, safer and efficient without the need for unnecessary weight increases.

When a product’s primary function is to move, the product’s mass is always a drawback. The airline industry is a prime example; a lighter plane will always outperform the heavier model.

Considerable effort is made on every new iteration of design to ensure that a vehicle is made as efficiently as possible by removing unnecessary weight.

A lot of similarities can be drawn from the airline industry to screening but until this point screening has been unable to capitalise from the best practices shown in other industries, due to the difficulties and inertia manual design entailed.

As computer-aided design has become more advanced, screen designers are now empowered to make wholesale changes to their product. The introduction of computer-simulated finite element analysis (FEA) has enabled design engineers to instantly validate any design changes made, a process that wasn’t previously possible.

The result of such progression and innovation within the field of screening is often a differently shaped and weighted product.

However, such screen products should not be mistaken for being lightweight. While these screens may be lighter compared with previous models, Weir’s engineers ensure that they are only “as heavy as they’re required to be”.

To that end, Weir’s range of “engineered to order” Enduron screens, which are built specifically to meet quarrying producers’ requirements, with its engineers using the customer’s requirements, flowsheet and on-site data as the design’s foundation.

Without modern design tools, it would be practically impossible to customise the equipment so fundamentally to meet the needs of a specific application on a specific site.

By identifying the specific features, size and wear materials required to excel in their intended application, Weir’s design engineers are to free remove weight where the screen assembly is experiencing low stresses and thus decrease both operational and manufacturing costs.

SAFETY IN PROCESS

In the old days, screens were built big, heavy and padded out to try to ensure that if anything went wrong, it wouldn’t go catastrophically wrong. It was a brute force approach to safety, which if used today, produces an unnecessarily heavy screen.

Today, there are a number of tools available to ensure there are no surprises when the time finally comes to physically test the equipment. Engineers run a natural frequency simulation to check that the screen will not run at its natural frequency when operated, which is vital to avoiding resonance which can cause premature failure in the equipment.

A frequency response simulation is also used which enables technicians to analyse each design using the actual forces the screen will encounter on-site (loaded and unloaded). This allows them to evaluate each part of the product for high stress points and reinforce the screen where it is actually useful.

Weir’s design and testing processes are extensive and carried out by a team of experts, who combine experience with “outside of the box” innovation. They test new ideas and design philosophy against an extensive bank of successful, industry-leading designs and data gathered from real applications in mines and quarries around the world.

Using these tools, this expert team can build screens that are optimised around a specific application’s requirements, rather than against any possible problem a screen could possibly face. This allows the team to reduce the screen’s weight and complexity, benefits that are passed on to Weir’s customers.

This reduction in material also has knock on savings in terms of manufacturing time and transport costs, as well as lowering the weight required to lift and rig on-site, which makes installation and maintenance safer.

However, this modern approach to screen design goes beyond material reduction. The added data and evaluation enables Weir to supply products that will be:

  • Easier to maintain.
  • Easier to operate.
  • More efficient.
  • Less power-intensive.
  • Accessible to alternative exciter and vibrating motor options.

The benefits of this approach are clear. Using modern design tools, expertly customised screens can provide robust construction and vital safety features without the excess baggage.

Drawing on decades of working with screens, Weir’s designers pick and choose features useful for applications from a rich library of drawings which have proven themselves in real applications, all using Weir Minerals’ industry-leading wear materials, such as Linatex premium rubber. 

So, the next time you’re comparing heavy-duty screens for a job, ask yourself: Are they heavy because they need to be? Or are they just heavy because no one’s put in the effort to optimise them?  

Gary Styger is a senior engineer and Steven Hunter the leader in separation in the Weir Minerals Separation Technology Group.

 

This article appeared in the June issue of Quarry Magazine.

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