Sappi increases digester capacity with flexible radiation-based level detector.
Founded in 1938, the Sappi mill, south-east of Johannesburg is one of the oldest mills in South Africa. It has a production capacity of 115 000 tons of pulp and 200 000 tons of fine paper per annum.
There are seven paper pulp digesters in operation at the plant, running at six to nine cycles per day; an extremely abrasive and corrosive process involving high temperatures.
One problem that faced the Sappi engineers was that they were unable to tell exactly how much yield they were getting from the digesters during this process. They were running blind, without an efficient level detection device. Point level switches had been tried but this method of level measurement was affected by the foam generated during the boiling process, and proved to be unreliable.
The solution proposed by Clint Viviers of Vega Instruments SA, was to use the Fibreflex continuous radiation-based flexible level detector from Ohmart, The Ohmart nuclear level detectors form part of Vega’s range of level measurement products and are sold and installed worldwide.
Nuclear continuous level measurement works by directing a narrow fan of radiation through a vessel towards a detector; as the product level rises, it shields larger amounts of radiation from the detector. The more radiation the detector (scintillator) sees, the lower the product level, discernable to 1% of span. The less radiation detected, the higher the product level.
The Fibreflex’s lightweight scintillation fibres are encased in a galvanised flexible conduit, designed to be wrapped around the vessel under measurement; giving excellent strength and environmental resistance and eliminating the need for multiple detectors on the vessel wall. The conduit is available in lengths of up to 7 metres, making the whole level detector simple and safe to install.
With the first Fibreflex installed the digester could be run at optimal yield. Result – the system paid for itself within four weeks through production gains of four tons per day.
Plans are in place to install the next level detector within a few months and eventually fit all seven digesters with Ohmart radiation-based flexible level detectors. This is estimated to give an increase of about 28 tons in total daily production.
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