Online monitoring method for detecting coating wear of screen cylinders

Screen cylinders are used to separate unacceptable fibres and sand, grit, pins, glass and other contaminants from pulp. In order to improve wear resistance, the feed surfaces of the screen cylinder wires are hard chrome plated. Coating failure results in rapid wear of the substrate and the cylinder needs to be replaced. If the wear of the chrome plating could be monitored and the coating failure detected early enough to prevent wear of the substrate material, recoating of the cylinder would be possible. Experimental research was carried out to study the possibilities of various methods such as electrochemical measurements, resistance measurements and fibre optical sensors for monitoring the wear of chrome coating. A method was developed for embedding a conductive wire for resistance measurements or the optical fibre underneath the chrome plating. It was demonstrated that both methods could be used to detect the coating failure, whereas the electrochemical measurements were not found suitable in this application. Further development work is required to adapt the methods into industrially applicable form. The methods could be utilised also in other applications than the screen cylinder. The benefits will be highest in applications where coating failure results in rapid failure or quality loss and where recoating, instead of component replacement, would give significant cost savings. In the case of screen cylinders, the costs can be reduced even to one third of the cost of replacement by recoating.