A manufacturer of sanitary paper uses aluminium sulphate, which is delivered in powder form, as an aid in production. The powder must first be dissolved in a medium and this is done in an open agitator tank filled with process water. Until now, the level in this tank was measured by a pressure transmitter with a metallic measuring diaphragm.
However, measurement of the density change that occurs during dissolution was never correct, which is why the shift personnel repeatedly had to check the concentration by hand. What is more, the metallic membrane was often damaged by high-pressure cleaning. The consequences for the user were high personnel costs, expenditures for a spare parts inventory and varying quality due to incorrect mixing.
The solution
As the first step to improve the situation, Vega installed its non-contact radar sensor Vegapuls 61 in the tank for level measurement. Mounted directly above the dissolution tank with a mounting bracket, it measures the change in level instantaneously during the pouring of aluminium sulphate. In addition, a pressure transmitter with ceramic measuring cell was mounted on the side of the tank.
Aluminium sulphate has a density almost three times that of water and the hydrostatic pressure changes considerably more than the level, so a Vegabar 52 was installed in the side of the tank. The physics of these properties are used by the control system to analyse the output signals of both sensors. Using a differential calculation, the density is determined to an accuracy of 1° Baume.
This solution has simplified and accelerated the production processes. The high overload resistance of the ceramic measuring cell also allows the vessel to be cleaned with high pressure. Damaged pressure membranes are now a thing of the past and the stock of spare parts is no longer needed. The redundant measurement has also increased operational reliability and savings have been made in raw material thanks to exact mixing ratios.
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