Scale Preventions

The buildup of scale deposits is a common and costly problem in many industrial processes using natural water supplies. In Britain alone the formation of scales in industrial process plant where water is heated or used as a coolant is estimated to cost £1 billion per year. Such costs can be attributed to cleaning (i.e. decaling) or the poor thermal conductivity of scaled surfaces; heat transfer is decreased by 95% by a CaCO3 scale layer 25 mm thick whereas a SiO2 scale layer 0.5 mm thick results in a 90 % decrease in heat transfer. The principle scale-forming ingredient in cooling water is calcium carbonate, which has a solubility of about 15 PPM and is formed by the decomposition of calcium bicarbonate. The maximum amount of calcium bicarbonate that can be held in a solution depends upon the temperature and the free carbon dioxide content of the water. Raising the temperature or decreasing the free carbon dioxide, at the point of equilibrium, will result in the deposition of scale. If agents (such as sulfuric acid) are added to convert a portion of the calcium bicarbonate to calcium sulfate, the resultant concentration of calcium sulfate should not exceed 1200 PPM (expressed as CaCO3). Otherwise, sulphate scale may begin to form, which is very dense and quite difficult to remove. Treatment companies may also advise the use of selected compounds.
 

Since the solubility of calcium carbonate is dependent on temperature, the water in a cooling system will have a different index for each temperature encountered. In practice, the Indices are used to arrive at a calculated method of treatment Removable lengths of pipe or metal coupons, should be inspected periodically to confirm that the treatment is in balance. If objectionable scaling is occurring, an increase in acid feed may be required. If corrosion is evident, a reduction of acid feed or the introduction of a lime or soda ash solution may be needed. For a system in which a considerable temperature spread occurs and treatment is set to control scale lay down, it is frequently necessary to use an inhibitor to prevent corrosion in the low temperature areas. It is desirable to calculate both the Saturation Index and the Stability Index in order to most accurately predict the scaling or corrosive tendencies of water.