Best practices: Screening

By |  July 17, 2015

PQ1507_schlabachRDeister’s Joe Schlabach discusses the benefits of different screening options – and more – with Pit & Quarry.

Joe Schlabach, vice president of marketing and sales with Deister Machine Co., led an educational session on the basics and misconceptions of screening at the 2015 AGG1 Aggregates Academy & Expo. Schlabach, a 36-year Deister veteran who focuses on applications and applying screens in his position, says working with equipment firsthand helped him gain expertise on screening and screen media.

Pit & Quarry connected with Schlabach to discuss screening further.

P&Q: At AGG1, you said screening combines art and science. What do you mean by this?

Schlabach: There is no literary source – science – that can tell you everything you need to know about how to properly select and operate a vibrating screen. There is a great deal of experience – art – that is a valuable help in selecting and operating vibrating screens. Every deposit is unique and may require some special consideration to get the best results. This is where experience can be a big help in pointing in the right direction to get the best results.

Even then, the final “tweaking” often is a matter of trial and error or different “tests” to see what combination of operating parameters and screening media will produce best results.

P&Q: In the beginning of your AGG1 session, you reviewed horizontal screens, triple-deck horizontal screens and inclined screens. Between these, which would you say is most cost effective? When is each screen useful?

Schlabach: The type of screen is dependent on the application. That said, if any of the three types would work, then the inclined screen is the most cost effective for a number of reasons. Because it is inclined, it uses gravity to convey the material, allowing for a circular stroke rather than a linear or elliptical stroke required with the horizontal screens. The amount of stroke or amplitude of the stroke is typically less than what is required on the horizontal screens. The end result is that the inclined screen operates at a lower g-force and requires less horsepower to operate.

P&Q: You also mentioned multiple times that no screen operates at 100 percent efficiency, as opposed to what a lot of companies advertise. If that’s the case, what is a good efficiency percentage producers can expect to get from their screens?

Schlabach: I believe that if a screen is at 90 percent efficiency or better, it is usually very acceptable. Maximum efficiency is probably around 95 percent. Depending on the application, screens can operate at less than 90 percent efficiency, but as the efficiency is reduced, there is more and more potential for the screen to operate less consistently on a day-to-day basis.
In other words, as time goes on with more deposit and weather to the screen, there is more potential for the screening results to change, which could affect the specifications of the material being produced.

P&Q: How does an operator determine 
efficiency of the screen?

Schlabach: Efficiency equals the percentage (or tons) of undersize that actually pass through the openings on a deck divided by the percentage (or tons) of undersize in the feed to that deck. For example, if there is 100 tph in the feed available to pass the openings and 90 tph actually passed the openings, then 90 divided by 100 equals 90 percent efficiency.

Operators should know how their screens are performing from an efficiency standpoint if they are concerned about performance of the plant. If the efficiency is low, then operators should use their knowledge and experience to improve the screening efficiency. Or, they can ask for outside help from experts such as the screen manufacturer or screening media personnel.

P&Q: When problems arise to the screen or screen media, you said it’s best to only make one change at a time in an effort to fix the problem. Why is that the best course of action?

Schlabach: I always maintain that only one change should be made at a time and that the streams of material affected by that change need to be measured before and after. This way, the cause and effect can be established. If a combination of changes is made prior to taking measurements, then the cause and effect cannot be established, so this would not be useful for other applications, should similar problems occur. If measurements are made after each change, then it could be useful for resolving similar problems if they arise again.

About the Author:

Megan Smalley is the associate editor of Pit & Quarry. Contact her at msmalley@northcoastmedia.net or 216-363-7930.

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