since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blowdown and leakages have to be made up to maintain a desired water level in the boiler steam drum. for this, continuous make-up water is added to the boiler water system. impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. thus, the salts have to be removed from the water, and that is done by a water demineralising treatment plant (dm). a dm plant generally consists of cation, anion, and mixed bed exchangers. any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. very pure dm water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.