MIC mitigation in a 100 MW district heating peak load unit

During inspection of AISI316 stainless steel plate heat exchangers in a district heating peak load unit, localised corrosion attacks along with indications of microbiological activity were found on the boiler side beneath patches of sturdy black deposits. Bacteria and sulphide were detected within black deposits. Thorough investigation of the boiler system revealed several incidents of localised corrosion on low alloy steel along with deposits of organic matter and bacteria primarily in places with stagnant water or places operating at a low flow rate. A relatively large amount of bacteria was detected within the system, primarily in deposits and around corrosion sites. The observations suggested the combination of deposits and bacterial activity, being the major reason for the observed corrosion. Prior to the investigation, the boiler system had operated with cat-/anion-exchanged, de-aerated water for 3 years, during which the water fulfilled strict chemical limits set to minimise corrosion. Based on these findings, the system has been modified in order to minimise the risk of microbiologically influenced corrosion and a monitoring program for fouling and corrosion has been established.