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Maintenance
Microbiological-influenced corrosion in main steam condensers
Often in power generation , the maintenance practice motto of the heat exchangers is “ don ’ t worry about it until it leaks .” Only then do plant personnel open the heat exchanger just to plug the leak and then close it back up and keep running . They never get to the root cause of the leak . Due to this practice , the main steam condenser is one of the most overlooked exchangers in the plant . This can lead to catastrophic results . One of the most misunderstood and detrimental defect mechanisms that a condenser can experience is microbiological-influenced corrosion ( MIC ).
Michael Davis , Managing Partner , ProQual Inspections
Recently , a two-unit ( Unit 1 665 MW and Unit 2 700 MW ) coal fired power plant in the Midwest of the United States was experiencing leaks in their high pressure feed water heaters on unit 1 . The tube specifications are . 625 ” X . 072 ” 304 SS . During their next outage , a 100 % eddy current exam was conducted on the heater with the leaks . Eddy current testing showed very deep , small volume indications throughout the outlet ( upper ) section of the heater with all the signs of being chloride-induced stress corrosion cracking . The SCC found in the HPFWH is a classic example of chloride induces SCC . So , the question is , where are the chlorides coming from ?
Small volume stress corrosion cracking ( SCC ) indication .
Figure 1 . OD Indication
Figure 2 . Multiple pits with some undercutting or tunneling ( typical of MIC )
Initial investigation After discussions with the plant engineers , there were a few theories for the origins of chlorides . During these discussions it was discovered that the main condensers have never been eddy current tested . In the past whenever they had a leak , they just brought the unit down , plugged the tube ( s ) and brough the unit back online . In addition to finding out the condenser has never been tested , we found out the water used for cooling came from the local reservoir . Like many reservoirs , it is used for recreational use as well as agricultural use . As the water comes into the intake for the plant , it is chlorinated to kill off any bacterial growth that can cause other issues . Once we discovered this , we were convinced that tube leaks in the condenser were the source of chlorides causing the tube leaks in the HP feedwater heaters . It was highly recommended that an eddy current exam be conducted on the condenser to see if the cause of the leaks could be discovered .
Pinpointing the source On the next outage , a 25 % ( 2,827 each ) exam was conducted on the east and west water boxes concentrating on 100 % of the tubes in the first ten rows , then random tubes throughout the bundle to make up the remaining 25 %. The tube specification for the condenser is . 875 ” X . 028 ” 304 SS . The results of the exam were quite revealing . Several tubes had inside diameter ( ID ) indications with some of them very deep and at or very near through wall . There were also several with outside diameter indications ( see Figure 1 ). So now that the source of the chlorides was found ( condenser tube leaks ), we now need to know what the caused these indications . Tube samples were extracted from both water boxes to include both ID and OD indications . Metallurgical analysis was performed on these samples . We finally found the cause .
26 Heat Exchanger World October 2024 www . heat-exchanger-world . com