Post by ND Engineering and Maintenance Services (Pty) Ltd
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Some of the equipment running in South Africa's pulp and paper industry has been in service for decades. The washer drums and filter drums that form part of the cellulose washing process are a good example :robust by design, but approaching end of life. Understanding what these units actually do helps explain why getting the refurbishment right matters. A washer drum works on the Archimedes principle. Submerged in a trough of cellulose fibre, the volume of water displaced creates a pressure differential between the inside of the drum and the liquid outside. That pressure forces water through the fine filter cloth wrapped around the drum's exterior, dewatering the fibres enough that the next wash stage can begin. This is repeated across multiple stations until the fibres are free of the chemicals used in bleaching. A filter drum does the same job, but goes further…The inner shell is completely sealed, and a vacuum is created by the water column draining down the outlet nozzles, amplifying the pressure differential. Now consider that these drums are up to 12 metres long and 4 metres in diameter — displacing more water than a large fishing vessel while rotating continuously on a central shaft. The forces acting on the end gables and shafts are substantial. Over decades, that takes its toll. For this unit, the refurbishment scope included centre hub redesign, replacement of hub bolting, new gland packing sleeves and sealing rings, and an upgrade to SAF 2205 duplex stainless steel, selected for its strength and corrosion resistance. The original material had reached the end of its service life, and the main failure risks (total shaft or hub and end gable failure) were real. The goal was not just to restore the unit to service, but to improve reliability and extend its operational life. When the work is done properly, refurbishment is a long-term investment. ND Engineering has the experience and the equipment to maintain, retrofit, and upgrade washer and filter drums of this type.
