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Reconditioning wire pressure screen cylinders.

Budgets for maintenance and spare parts are under attack every year. "Do more with less" is a dictum every mill has had to adopt to remain competitive. Frugal mill superintendents, faced with orders to decrease maintenance spending, are reconditioning and rebuilding their existing equipment rather than buying new replacements.

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Reconditioning is not just for tanks, pumps, dryers, and digesters. Now even pressure screens are repaired and reconditioned. While reconditioning is not a new idea (electric motors have been rebuilt for years), the reconditioning of slotted screen cylinders has been especially challenging. That is because of the difficulty in replacing the contour, the smallness of the slot size, and the tolerances required of the slots (plus or minus 0.025 mm, or less.) Reconditioning a used and worn wedge wire screen cylinder requires unique processes to ensure a quality outcome.

TWO TYPES OF WEAR

There are usually two aspects of wear on any slotted screen cylinder. The first type of wear results from an abrasive substance flowing through the slot, gradually increasing the slot size until the efficiency of debris removal is lost. The second type of wear occurs when there is sufficient abrasion to cause the contour to be worn away. Once the contour is worn down, the runnability and capacity of the cylinder are lost.

Other types of wear, such as pitting from cavitation and smearing from metal contact, occur less often. Cracks and missing wires pose a further problem. None of these wear phenomena can be corrected by a simple replating of the wear coating.

A special process was developed to handle this unique situation. The first step of all reconditioning and repair projects starts with a thorough inspection to evaluate the degree of wear or damage. As with any repair work, if the damage is too extensive or if there is not enough "base" to work with, then repairs and reconditioning may not be feasible.

The standard reconditioning process involves resizing small gaps caused by minor impacts. Old coatings are stripped away to expose the base metal. The desired contour--either a duplication of the original contour or slight modification to a new parameter--is ground into the old wire on the cylinder. Metal powder is applied to obtain the correct slot size, superior abrasion resistance, and proper contour parameters. Proper polishing and final slot sizing insures runnability and efficiency.

The finished product often performs better than a new screen cylinder. This is possible through a uniform application of a carbide-based wear coating. Hard chrome plating, by nature, leaves an uneven thickness that mainly protects the contour from wear. New coatings go right up to the slot and protect the wire from abrasive wear that opens the slot size.

Although it is impossible to guarantee the number of times a cylinder can be rebuilt, many users have had their worn slots resized several times, at a substantial savings to their screen cylinder replacement budgets. Mills can adopt this same type of process to rebuilding the rotor or foils of a screen. The abrasive nature of stock, coupled with today's higher consistencies and rotational velocities, are the prime ingredients in a recipe for high wear. A worn rotor can cause the capacity of a screen to decrease by half or more.

When weighing the benefits of reconditioning wedge wire screen cylinders, or other parts, consider the following guidelines:

* Conduct regular equipment inspections to determine the reconditioning schedule or cycle. Remove equipment from service before it is beyond reconditioning.

* Use a reputable contractor that can perform the service. Check their references.

* Work with your supplier to establish realistic and measurable results.

Recycling applies to equipment as well as to raw materials. The next time your equipment starts to wear out, investigate the possibility of reconditioning.

ABOUT THE AUTHOR

Dennis G. Purton is a sales representative for Euro Screen Norway AS. He is a graduate of the University of Maine and has worked for Georgia-Pacific Corp., Ahlstrom, and Beloit. He is based in Pewaukee, Wisconsin, USA. Contact him by telephone at +1 262 695-8219, or by email at purton@execpc.com

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Article Details
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Title Annotation:Four Minute Focus
Author:Purton, Dennis G.
Publication:Solutions - for People, Processes and Paper
Date:Mar 1, 2004
Words:683
Previous Article:Safety tips from the trade.
Next Article:Teaching paper science to a non-technical audience.


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