Chemicals clearly add to the salt content in district heating water and thereby cause risks for corrosion. Chemicals also add to the operational costs (OPEX); they are a health and safety issue for storage and handling – and finally, it makes sense that a sustainable heating system is constructed from sustainable components/technologies. 25 years ago, most Danish district heating companies produced make-up water from municipal water by softening to remove hardness, then added chemicals for pH adjustment and oxygen removal. Today, in addition to softening the recommendation calls for removal of salts through membrane technology (RO) or ion exchange technology, in order to produce demineralized water with low conductivity. These steps can then be followed by oxygen removal, e.g. by a membrane technique. Use of chemicals for that purpose is viewed as the least attractive alternative. The last step is adjustment of pH by means of sodium hydroxide. Reasons for using demineralized water include: • Salts in district heating water promote local corrosion in which oxygen is consumed. Local corrosion is damaging for the components. • Salt load in district heating water provides a nutrient supply for bacterial growth and thus promotes microbial corrosion. • Salt load is crucial for how much NaOH (Sodium hydroxide) to use to raise the pH to 9.8 (> 90% reduction of chemical consumption for pH adjustment when switching from softened to demineralized water). • Demineralized water makes it easier to detect intrusion of raw water. Reasons to remove oxygen from the water: • Oxygen is necessary for corrosion to proceed. • In district heating systems, no chemicals react more quickly with oxygen than the steel. • Remove the oxygen before adding make-up water to the district heating network. In most cases, this is the necessary and sufficient means to control corrosion, i.e. there is no reason to add oxygen binding agents also. • The Danish District Heating Association does not, in general, recommend chemical oxygen binding agents, as they contribute to the salt load and promotes bacterial growth in district heating water.
Black untreated steel and low-alloy steel are the most common materials in district heating systems. Water treatment and corrosion prevention primarily aim at protecting these materials against corrosion considering that other materials could be present. The main elements of corrosion prevention pertaining to non- and low-alloyed steel are: • Oxygen-free environment with an oxygen content < 0.02 mg/l. • Alkaline environment with pH = 9.8 ± 0.2. • Minimization of salts, formation of deposits, and precipitations. On clean and untreated steel surfaces, a thin magnetite layer will form at pH-values over approx. 9.5, which passivates the steel surface. The magnetite layer is soluble at lower pH-values and can be liberated by mechanical influence or resulting from thermal and dynamic movement in the pipe system. In case of new plants, it is important to clean the surfaces before commissioning, i.e. iron scales, paint residue, and oil stains are removed by sand blasting and degreasing. SUCCESS CRITERIA OF WATER TREATMENT A success criteria of good water treatment in district heating systems is optimum technical and economical operation both in the short and in the long run. Likewise, low chemical consumption is a success criteria, since the environment will be the least stressed by leakage of district heating water and at the same time, the risk of corrosion will be reduced. This means: • Minimization of corrosion and formation of deposits. • Optimum plant and operating economy. • Minimum maintenance costs. • Minimum internal and external environmental impact.
Front page of Danish District Heating Association latest recommendations for water treatment and corrosion prevention.
The water treatment recommendations from the Danish District Heating Association include a message to minimize (and where possible fully avoid) the consumption of chemicals – especially for removal of oxygen in the water, where new technologies have been developed.
Oxygen corrosion due to high oxygen content and low pH-value in district heating make-up water.
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