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Monitoring corrosion in district heating systems

  • Published 25/02/2004
  • Last updated 20/05/2011
Corrosion problems in district heating systems in the Nordic countries are rare. Nonetheless, corrosion is occasionally still responsible for leak problems and an important factor for process control and asset management. Thus, corrosion should be monitored as other important process parameters such as pH, conductivity and dissolved oxygen.

Frontpage report

A research project with the objective to implement an online, real-time corrosion monitoring method for quality control in district heating systems has been carried out in the last three years. 

It has prior been difficult or almost impossible to obtain reliable online real-time corrosion data for some industrial systems, including district heating systems, due to the low conductivity, presence of sulphur in some cases and low uniform corrosion rates. This project has resulted in directions for choices of reliable methods for online real-time monitoring of corrosion rates in district heating systems. 

Recommended to use two methods
It is recommended to use two methods simultaneously for continuous corrosion control in district heating systems, for uniform corrosion rates and for localized attacks, respectively. The high sensitive ER-technique developed by MetriCorr ApS in Denmark provided the most reliable uniform corrosion rates. The LPR method did also result in promising data in Icelandic and Finnish systems, but care has to be taken in choice of polarization rates and the algorithm to calculate the polarization resistance. The LPR method was not successful in the Danish systems. For detection of risk of localized corrosion, either ZRA or electrochemical noise can be suggested. These methods respond promptly to changes in the water chemistry, e.g. to oxygen ingress and thus will alarm if unfavourable changes occur that may result in localized attacks. 

More experience is though needed with both methods for more accurate data analysis in district heating systems. The application of the EIS technique can not be recommended since consistent results were not obtained. The evaluation of the results is complicated and prone to miscalculations.

Recommend a continuous real-time monitoring
Researchers and representatives from the industry in Denmark, Finland, Iceland, Norway and Sweden cooperated in the project. The cooperation with the district heating companies has been successful and synergistic effects have been obtained by the collaboration between the Nordic participants. The final conclusion is to recommend a continuous real-time monitoring of corrosion in district heating systems as a quality measure, to act as an early warning system if abnormalities occur. The results show that two simultaneous methods, for uniform and local corrosion attacks, respectively, should be applied. The corrosion data can easily be used by plant operators for surveillance of the system, in the same manner as other important process parameters are followed, such as oxygen ingress, pH and conductivity. The results are to be found in a Nordic Innovation Centre report from May 2004, Monitoring corrosion in district heating systems.

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