Maintenance Screening Efficiency

November/December 2004

Poor Screening Can Rob Production
A key to improving overall efficiency can be as simple  as having the savvy to look at the screen as the culprit.

by Gary Pederson

A common goal for most aggregate producers is to put as many tons per hour of in-spec product on the ground as possible, but oftentimes, when production fails to meet this expectation, producers typically focus on their crushing operation. However, frequently the culprit is in their screening operation and its performance efficiency, not the overall crushing operation itself.   

Inefficient screening costs millions

Inefficient screening or poorly maintained screens can severely affect the success of any operation and its costs — no matter how much material is produced per hour. Lower production rates, poor product quality, complicated process add-ons, increased wear on other equipment, and higher labor costs are all outcomes of poorly performing screens. Frequent shutdowns to clean or replace worn or broken screen cloth; reduced screen cloth capacity due to blinding, pegging, or clogging; or having in-spec product trapped re-circulating in closed-circuit loop systems all greatly affect production and cost producers millions of dollars in lost revenue each year.

For example, a blinded screen can reduce screen efficiency by 70% or more, effectively making an 8- by 20-foot screen deck produce at the level of a 4-foot by 12-foot screen deck. The rule of thumb is that fully loaded screen decks that are even partially blinded will result in a production loss equal to the percentage of a screen that’s blinded, pegged, or clogged. Even a single screen deck blinded just 5% on a 500 TPH plant that produces 960,000 tons per year will lose about 48,000 tons of that total production in spec product. At an $8-per-ton selling price, this operation could easily lose $384,000 per year in potential revenue from this one screen-deck problem.

A common scenario in many pits across North America is inefficient screening that substantially reduces revenue. However, the cost of inefficient screening doesn’t end there. Poor screening often leads to low product quality or out-of-spec product and both can have multiple repercussions. On the plant side, these include the additional cost and time to re-screen the material, additional wear and tear on the equipment, lower prices, and even lost customers. 

For the quarry’s customers, out-of-spec product could cause them to face stiff penalties and fines for using inferior stone, as well as delayed project costs, while the stone is replaced. In addition, inconsistent gradations can cause asphalt and concrete mix design complications, often requiring completed jobs or portions of jobs to be replaced at the contractor’s cost. In the highly competitive road business, which ultimately affects the aggregates industry, many bids today represent zero profit — just the opportunity for the contractor to generate income based on early completion bonuses. Poor quality, slow delivery, or out-of-spec product can cripple a contractor’s opportunity to make money when using this approach.

The cost of inefficient screening can also severely affect a portable contract crushing operation as well. For example, for a 350 TPH portable crushing and screening plant to fulfill a 500,000-ton contract, it would typically require 3,150 tons of production per day within a nine-hour day. It would then take approximately 159 working days to complete the contract working five days per week for a total of 31 weeks and four days. 

If the plant had even a 5% reduction in screening efficiency due to blinding or pegging, it could cost the contractor eight more days to complete the contract. Eight more days of electric power, diesel fuel consumption, wear on the equipment, and employee salaries. What’s more, is that there have now been eight days lost that could have been used to start another contract. Since income for portable crushing operations is often based on the number of contracts that can be completed each year, every day lost directly affects the income potential.

Screening efficiency is not rocket science

Maximizing screening efficiency is not rocket science, although many producers believe the opposite. Simple, consistent screen box maintenance and taking advantage of the more recent screen media solutions available today can all but guarantee a higher level of in-spec product production. 

Although there are many different types of screen media — from polyurethane to punch plate and rubber — the vast majority of screen cloth used today remains woven wire. Therefore, the focus here is on woven wire problems and solutions. 

When screens fail — the term producers use when a screen has become unexpectedly non-operational — the problem often results from broken woven-wire screen cloth. Wire breakage typically occurs for any one or more of the following reasons:

Poorly tensioned, loose screens that beat themselves against the crown bars at a high frequency.

Inconsistent crown bar heights preventing a proper arc for the screen cloth to tighten and rest against.

Worn out, damaged, or missing clamp bars and rubber channel that allow material to wedge between the screen cloth and screen box rails.

Wash plant spray bars forcing a steady stream of water directly onto screen cloth wires from a close distance or running when there isn’t any aggregate on the screen deck leads to cutting holes in the wire.

Too heavy of an impact on the feed end of the screen from material falling too great a distance onto the screen or very large material falling on small wire diameters causing premature wear and breakage.

Off calibration of a screen’s eccentric throw, damaged springs or rubber mounts, or damaged decks creating a poorly vibrating screen prevent an even flow and spread of material across the screen cloth for maximum throughput.

All of these problems are relatively easy to spot with frequent visual inspection of the screen boxes and screens while they are in operation and when shut down. A formal inspection schedule will allow producers to make pre-emptive repairs and adjustments before they begin affecting production, a practice they more often do today with their crushing operations. Frequent inspections also allow a producer to make repairs or replace parts when it’s convenient for his schedule and not during the height of the production day.

Producers need to also check out the performance of their woven wire screens. One of the easiest and quickest ways to achieve this is to check the pile of discarded screen cloth for recurring problems. Problems become quite evident, such as broken wires, wires pulled out of the hooks, blinding and/or pegging, or unusual wear patterns. 

Advanced solutions further maximize screen efficiency

Maximizing screen efficiency has typically taken a backseat to wear life as it relates to woven-wire screen cloth. Since woven-wire screen cloth has been a low-cost commodity for many years, the focus has been on price and long life versus maximizing efficiency performance. The increased use of polyurethane, rubber, and punch plate to extend the life of screen cloth has done little to increase efficiency. In fact, they often reduce efficiency or throughput in exchange for significantly extended wear life while costing considerably more than woven wire.

Throughout the years, woven-wire manufacturers have developed various versions of woven-wire screen cloth to try to eliminate or reduce the common production-robbing challenges created by material blinding, pegging, and clogging. These solutions have typically involved various designs of slotted screens — still woven but with various combinations of crimp designs, rectangular slots, and special-weave patterns — versus traditional, uniform, square openings. Throughput was improved somewhat, but not significantly for many producers.

Within the past 10 years, screen media suppliers have pioneered what some screen manufacturers have called a breakthrough in screening efficiency with the development of non-woven, self-cleaning wire screen cloth that employs single wires running horizontally from hook to hook and bonded in place with vertical polyurethane or rubber strips extending the length of the screen panel. 

When properly sized for the application, the combination of individual wires vibrating independently and bonded in place by polyurethane strips used in this new technology can increase screen production 40% or more, according to a manufacturer. This new category of flexible, self-cleaning wire screen cloth is said to greatly reduce, if not eliminate, many severe blinding, pegging, and clogging problems on screen boxes of all types. Additionally, wear life tends to be longer than traditional woven wire — up to five times longer has been cited — because there aren’t any cross wires with high wear spots as with woven wire.

Because the self-cleaning cloth can vibrate, unlike rigid woven wire, it prevents blinding, pegging, and clogging — while still producing spec product. The screen cloth also generates far more throughput at a faster pace than woven wire. The design allows for far more open area per square foot than conventional woven wire. The more open area per screen available, the more capacity a screen has to size more product. For this reason, many producers are also using these self-cleaning screens just to produce more product per hour, even if they don’t have a blinding, pegging, or clogging problem.

However, woven-wire screen cloth has been around for many years, and under the right circumstances, it can do the job it’s intended for extremely well — and at a very low cost. The newer self-cleaning screen cloth costs more than woven wire, but the results typically outweigh the added cost. It all comes down to what your facility needs, how adamant you are with maintenance of your screens, and whether, after doing a cost-analysis, the traditional versions or the newer screen cloths fit your budget and will ultimately help your operation become more efficient.

Six Tips For Screen Maintenance

Inspect all clamping bars for corrosion and wear. Make sure bars are not warped or worn to the point they cannot provide even, tight tension.

Inspect all nuts and bolts. Replace all worn or damaged parts in the screen assembly. All hole positions on the clamping bars must be utilized.

Maintain the support deck. Remove channel rubber to inspect steel bars for wear, high and low spots, or bar breakage. The support area must be uniform to ensure an even and tight screen fit.

Change your channel rubber frequently. Remember that channel rubber will wear from the bottom up as well as from the top down. Check both sides. Never mix channel rubber sizes or styles on the same deck.

Check cushion and spread of material feed. Cushioning of feed to the screen deck is essential to long screen life. Use feed plates or stone boxes so that material does not hit wire cloth directly. Materials should be spread out to feed evenly over the entire screening surface for maximum screen life and production.

Inspect equipment for balance and excessive vibration. An unbalanced machine can cause premature wire cloth failure. Equipment with a violent, uneven vibration motion will cause stresses in the screen and the cloth may actually break apart. Correct the unbalanced condition before continuing screening operation.

Screening and the Bottom Line

Let’s see how inefficient screening can affect the bottom line. Here’s a plant that averages 500 tons per hour throughout a 2,000-hour period, annually, with an average product selling price of $7.50 per ton. 

Each production interruption of one hour to clean screens results in a production loss of 500 tons a week — a revenue loss of $3,750 per week.

If closed-circuit screen efficiencies drop from 90% to just 80%, production could drop 13% or 130,000 tons during the year — a loss of more than $975,000 in potential product value.

Screen decks that are fully loaded and partially blinded will result in product loss equal to the blinded percentage. Screens that are 20% blinded will decrease capacity by 20%, or 200,000 tons during the year — $1.5 million or more in lost potential sales each year.

Reprinted from Aggregates Manager Magazine
November/December 2004