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Coal Preparation

Screen Aperture Wear and Performance and CFD Modeling of Screen Wear

Coal Preparation » General

Published: May 07Project Number: C13057

Get ReportAuthor: Anita Hill, Michael O'Brien, A Taylor, Bruce Firth, JI Mardel, MR Hill, TJ Bastow, RHJ Hannink, L Graham, J Wu, A Bui | CSIRO Energy Technology

In ACARP C11006 (O’Brien et al. 2003), screen aperture dimensions and open area inclusive of pegging were measured using a new image analysis system. In ACARP C11012 (Hill et al. 2004), the material performance and the critical failure mechanisms of screen panels were identified by post service failure analysis.  Both projects provided independent data to assist plant managers in making choices to increase screen panel wear life, reduce their screen panel maintenance costs and costs associated with unscheduled downtime, as well as reduce misplaced material.  Both projects also provided feedback to screen panel manufacturers regarding common manufacturing defects and their effect on wear rate and aperture size distribution with recommendations for production methods to eliminate manufacturing defects.

This project, ACARP C13057, uses the outcomes from those two projects to link measurable screen wear parameters with screen panel failure potential and screen performance whilst the screen panels are still in service.  The project gives details of screen aperture studies of the wear and performance of screen panels in operation at plants in the Hunter Valley and the Bowen Basin.  The results from this project offer plant managers quantitative measure of the wear and performance as a function of tonnage. 

In the Bowen Basin, desliming screens with polyurethane screen panels 60 cm by 30 cm with nominal aperture of 0.4 mm were monitored as a function of time and tonnage over a period of three and a half months. Maintenance protocol meant that panels were changed when the measured aperture size reach 0.9 mm.  The average aperture size for the screen and the average open area (inclusive of pegging) increased with tonnage.  The wear rate at the feed end, as measured by aperture change and open area change, is linear and can vary by a factor of two depending on feed and flow conditions.

In the Hunter Valley, desliming screens with stainless steel wedgewire screen panels 60 cm by 30 cm with nominal aperture of 1.4 mm were monitored as a function of time and tonnage over a period of three months.  Maintenance protocol meant that panels were changed when aperture size reached 1.9 mm. The average aperture size for the overall screen increased with tonnage.

Preliminary computational fluid dynamics simulations indicate that with a linear wear rate for the poly panels the variation in wear rate over the screen is due to velocity and impact angle.  These two parameters can be modified by minor alteration in the screen such as reducing inlet flow velocity, using a coarse particle bypass in the feedbox, and reducing the stroke of the screen, each of which is shown to result in lower wear rates for the simulated screen.

Premature and catastrophic failures cause many tens of thousands of dollars of lost revenue production time for each plant.  It has been shown that through a combination of maintenance practice, manufacturing quality and monitoring, and by readily achievable design modifications, that the conditions for premature and catastrophic failure can be avoided and the operational life of the panels extended.

If properly manufactured, screen panels can be ranked based on wear rate as defined by change in aperture size and open area as a function of tonnage.  The image analysis apparatus allows these critical parameters to be monitored during normal maintenance shut downs.  Change out should be based on efficiency and average aperture at which the probability of panel failure exceeds a certain percentage. This percentage, and hence the associated degree of risk of failure, should be decided by the plant operator.  Further data must be collected in order to develop a process control model for screens. Inputs to this model include tonnage, slurry density, nominal aperture size, top size, downstream processing and hardness of seams.

A purchasing decision based on whole of life costing as directly related to screen panel performance can then be made by the plant manager.

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