P13
Corrosion is an electrochemical reaction between oxygen, water and metal. The reaction can only happen if all three components are present. Therefore, make-up water should ideally be demineralized, deaerated and pH adjusted.
Salts and oxygen serve as catalysts for corrosion. Salts increase the conductivity in water, which is a prerequisite for corrosion, and the oxygen oxidises (combines with metal ions to form rust) and corrodes the steel in the DH pipe. Due to these processes, the use of chemicals in DH systems should be reduced as much as possible because they increase the salt content and thereby the conductivity of the water circulating in the system. Another issue with the chemicals used in DH systems is that they are often organic chemicals, which, if added to the water will serve as nutrients for bacteria growth and formation of biofilm, leading to biocorrosion below the film. They also form sludge and increase the risk of deposits, which typically occur in places where the water velocity is low, e.g. in heat and accumulation tanks. The bottoms of the tanks can suffer from severe corrosion due to deposits and sludge. Finally, it should be mentioned that the best oxygen scavenging agent in the system is the steel surface. If steel and oxygen scavenging chemicals compete for the oxygen, the steel surface always wins. If water contains oxygen the corrosion process will continue. If there is no oxygen the corrosion process will cease. At low pH values, the corrosion process increases, and at high pH values, the risk of corrosion in copper, copper alloys and metal parts of aluminum and galvanized parts also increases. The recommended pH of DH water is 9.8 ± 0.2, and this should be achieved by dosing sodium hydroxide (NaOH). When demineralized water is used in the system, the amount of NaOH will be reduced dramatically, whereas the use of softened water calls for high use of NaOH due to its content of carbonate. NEW MODERN WATER TREATMENT TECHNOLOGIES - WITHOUT USING CHEMICALS Natural drinking water is characterized by having a balanced content of cations and anions. All together these make up the total hardness, salts and gasses like oxygen (O2), carbon dioxide (CO2) and nitrogen (N2). For removal of hardness and salts the most common water treatment technologies are: • Water softening, which removes the total hardness from water. • Demineralization of water by use of reverse osmosis or ion exchange. • Production of ultra-pure water where the remaining salts are removed, sometimes referred to as polishing.
Example of water treatment steps – from ground water to pure water.
As mentioned, it is very important to remove salts and oxygen because they are prerequisites for corrosion. Furthermore carbon-dioxide should be removed since it increases the conductivity and the consumption of NaOH (lye). Therefore, gasses like oxygen and carbon dioxide should be removed before thewater is used asmake-upwater. In case oxygen enters the district heating system through leakages, the character of corrosion will change from aggressive local corrosion to general surface corrosion, which is not as critical as local corrosion. New efficient technologies have made degassing an attainable solution for all district heating companies. The recommended solution is a Membrane Degassing Unit (MDU). It should be said than an MDU can be used to degas softened water, but the superior result will be achieved using demineralized water.
Example of a MDU plant (Membrane Degassing Unit).
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