Load & Haul

Selecting the right screen for the job

Many articles have been written about screen selection over the years. Much has been made about the need for more efficiency and bigger throughputs. A lot of screens, however, fail to live up to the expectations of their operators.
For the sake of this article, we shall consider an aggregate screen typically found in a hard rock quarry anywhere in the world. 
For any application, we need to determine the type of screen required. There is very little difference in products produced by a circular motion screen versus a linear motion screen. The area determination is the same for standard linear or circular motion screens. The transport speed of material on the deck is a function of the screen dynamics, so it is possible to have a linear motion screen transport speed higher than that of a circular motion screen and vice versa.
Quoted feed rates can be misleading; you should visually check for feed stability. At the extreme, if a screen averages 250 tonnes per hour (tph), but only operates for 15 minutes in the hour, your instantaneous tonnage could be as high as 1000 tph! This can often happen on batch processes where they consider the average daily plant throughput.
A contentious issue amongst screen manufacturers over the years was the method for determining the screen size for a given duty. Although the methodology was similar from one to the other, most made subtle ?tweaks? to the parameters. The end result was 10 screen suppliers all quoting different screen sizes for the same job.
In the USA, screen manufacturers were losing face with confused customers; hence the need for a common methodology. Following the formation of the UK Vibrating Screen Manufacturers Association, a standardised system of area determination was developed to eliminate such issues.
Today we use a modified version of this methodology which ostensibly runs to using metric instead of empirical units. It will not guarantee all people quoting the same job will quote identical size units because some manufacturers will still interpret the data differently, but they will be a lot closer to each other than in years gone by. 
The screen width may be determined by considering the relationship between the depth of bed (DOB) of material and the average particle size. A good rule of thumb for dry screening is the DOB should not be more than 2.5 to three times the average particle size of material passing over the deck. After determining the figure for the overall area requirement, you can divide the area requirement by the screen width to equal the screen length, eg 9m? ? 1.8m = 5m. 
Once the screen size and type have been determined, we turn to the dynamics for efficient screening. Studies have shown that the correct stroke selection, screen speed and inclination will produce the correct particle trajectory, such that a particle resting on a blind area of media will land in an aperture and pass or be rejected. Optimising this process is fundamental to efficient screening but while it is fine on a single separation machine, it will always be a compromise on a multi-separation screen.
The screen is now optimised from a theoretical perspective, assuming a number of conditions will be met, eg to utilise the entire screen surface area, the screen has to be fed, full width, into a feed box or feed tray ideally. Feed boxes are useful because they take the impact of the material falling on to the screen and start the distribution process, utilising all available screen surface. The best way to feed a screen is with a full width feeder/belt/screen/chute, so that the material cascades onto the feed tray. Ideally drop heights of material should be no higher than 300mm to 400mm to reduce damage to the screen and/or media.
There should be a minimum 75mm clearance between static chute work and the screen in the direction of rotation or line of action of the screen and a minimum 25mm clearance side to side. A vibrating screen should not impact on a static structure while in operation. Serious structural damage to the screen live frame is likely, which is why it is important to check for broken springs in daily inspections. A broken spring in one corner produces a twisting load in the live frame, due to the different spring reaction in that corner. If ignored, it can result in cross-member and/or screen sideplate damage.
The addition of water to the screening process can be a double-edged sword. While correctly positioned and orientated spray nozzles can improve the passage of material through the screen media, it can impact the working environment, screen longevity, screen media wear rate and site cleanliness. Rules of thumb have been applied to screening aggregate, but the performance will be determined by the water quantity and quality available to the screen. 
Ideally, between 2.8 and 3.2gpm/tph at 2.0 bar is required. Water delivered to the screen through fantail nozzles helps wash and turn and the passage of material through the media in an efficient manner reduces the area requirement by a significant amount. However, a damaged or missing nozzle can result in a hole in the media.
Over the years, the requirement for screen manufacturers to ensure ease of access to and from their screens has increased. In the past, many screen manufacturers supplied screens with increasing numbers of decks while minimising sideplate height, machine weight, installed power and cost.
The result was machines that were awkward to access for media changes and inspections. Subsequently, screens suffered due to poor maintenance, products went out of spec and machines were disliked. Many plants often kept on employees because they were very slim or small!
Now it is very different, as most screen inquiries stipulate a requirement for ease of access and egress. This really benefits all, with the exception of plant designers wishing to minimise plant height; the screen benefits from deeper sideplates, providing a robust live-frame, and there will be better accessibility between decks.
Some modular screen systems are available so that when considering a multi-deck machine, the upper deck panels can be wider than the lower deck panels. This makes it difficult to pass the lower deck, but easy to get through the upper media surface to access the space between. It also prevents workers from falling into empty bins.
There are many types of screen media available to screen manufacturers and end users alike, but in the last 20 years the biggest growth has been in the synthetic media market, with a decline in the woven wire market. The popularity of synthetics is due to its ability to provide a longer service life when compared to wire or steel modular polyurethane (PU) panels, including a significant reduction in noise nuisance and manual handling issues of big wire or perforated plate.
There are many old screens still providing good service and for a lot of operators the thought of converting their machines to a modular PU system fills many with dread. This is a shame; while woven wire may have a marginally better open area at bigger apertures (+20mm), at the smaller end that advantage disappears to virtually none. The loss in performance or throughput is either nil or marginal and in the case of dynamic membranes much better. When you offset that against the vast increase in service life, ease of use, fitting, manual handling, performance and process flexibility, the argument for steel media is very weak.
Some older machines may not be suited to conversion for a number of reasons, eg poor structural integrity, or weakened cross-members, or if you replaced the support frames with heavier items, the additional weight could reduce the machine?s stroke or affect bearing life. These issues need consideration ahead of conversion.
There are many methods of screen construction and design. A given manufacturing method was and is largely determined by the manufacturer?s available facilities or personal preference for a specific technique. Some prefer a fully welded method or a bolted construction or a combination of many methods. 
There are, however, only two methods of lubrication: oil or grease. On some machines, oil is the only option. Both operators and manufacturers often prefer grease lubrication.
Machine cross-members come in as many shapes and sizes as screen types, the most popular being circular, square and rectangular hollow sections along with I beams and channels. The sectional depth of the cross-member needs to be calculated to provide sufficient strength and to withstand the weight of material on the screen surface plus the media at the operational acceleration. A dewatering screen will carry more weight than a standard sizing screen, so the cross-member depth must reflect this.
Thermal stress relieving of the support frame is desirable. Some machines use a one-piece assembly support frame, others use individual cross-members. If the screen?s design is to a high standard and manufactured with quality materials, diagonal cross-bracing should not be necessary. 
So we now have the right screen for the job. Provided you adhere to much of the advice offered, your screen should meet and exceed expectations in the above areas. Optimising screen performance will save money in the long term. It is worth spending a little more and ensuring that most points raised in this article are designed into your next screen.
Source: CMB International Ltd.
This article first appeared in the October 2010 issue of Quarry Management (UK) and is reprinted with kind permission.

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