In March 2017, the co-writer of Danish District Heating Associations water treatment recommendations, Karsten Thomsen, specialist in water chemistry from COWI A/S – issued an article in the association’s magazine dealing with the comments received since the recommendations were released late 2015. The recommendations have been very positively received by the district heating companies in Denmark.
Corrosion at the bottom of district heating pipe caused by particles, sludge and oxygen in the water.
Factors causing corrosion - issues that can be experienced through inadequate water treatment in district heating: • Aggressive ion content, especially presence of chloride. High salt content promotes damages by local corrosion, low salt content leads to widespread general corrosion that is far less damaging. • Oxygen content above a few micrograms per litre. • Incorrect pH-value of the water. pH above 9 maintains the protective oxide layer on steel surfaces. • Biological activity: Demineralized water adjusted to pH 9.8 with sodium hydroxide and low content of organic substances presents the least attractive conditions for growth and thus stresses the bacteria maximally.
Gothenburg Energy’s membrane degasser installation.
The general picture, however, is that chemicals historically have been added to try to help a problem of more fundamental character with inadequate water chemistry. The water treatment recommended by the Danish District Heating Association takes the basic conditions of corrosion into account and avoids the problem without the use of chemicals. This may be seen as a professional way to handle the real issue before it becomes a problem. Historically, use of demineralized water and removal of oxygen without chemicals were often not economically feasible – now this has changed due to new technologies available on the market. On top of that, chemicals are expensive so investments in new technology to remove oxygen without chemicals represents a short return of investment. Therefore, the recommendations from the Danish District Heating Association can be implemented as the modern, correct way to secure optimum water chemistry, and in the long run reduce maintenance costs of the district heating system. The German water treatment standard for district heating, AGFW FW510 to a large extend shares conclusions with the Danish recommendations. The Danish guidelines are, however, more comprehensive and detailed i.e. concerning recommended pH-range as well as oxygen levels in the water. The recommendations for water treatment in district heating do not apply to water and steam circuits in combined heat and power plants, nor to water treatment for steam installations. In such cases, we refer to separate recommendations for relevant technologies, e.g. “The International Association for the Properties of Water and Steam”, SIAPWS.
Furthermore, correct water treatment helps minimize formation of deposits.
Biofilm in district heating systems can cause much technical and environmental damage: • Microbial corrosion.
• Reduced heat transfer. • Increased pump costs. • Increased chemical consumption.
Illustration showing comparisons on different technologies to remove oxygen from district heating water.
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