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After expansion in a turbine, the steam in a power plant goes to the condenser where it is condensed by cooling water. The steam thus condensed as water is cycled in the system through heaters, boiler, and turbine to come to the condenser again. Condensers are tubular heat transfer areas with steam on one side, maintained under vacuum to improve cycle efficiency, and cooling water on the other side. The cooling water is though the tube and steam over the tube.
The cooling water for a condenser can be sea water, river water, or tank or pond water. As the cooling water is brackish water with lot of contaminants, and since the steam side is under vacuum, this water can find a way into the steam condensed water. The cooling water mixing with condenser water leads to many problems on the boiler side. The main problem when operating the boiler with one or more condenser tube leaks is tube failure. The conductivity of the boiler water shoots up immediately and the pH level goes down, indicating the boiler water is tending to become acidic.
The decrease in the pH of the boiler water leads to acidic contact with tube material, and the interaction of the low pH water with the parent tube surface leads to generation of nascent hydrogen. If the hydrogen generation happens below a hard deposit, which is more probable in the high heat flux region, then the pressure within the deposit builds up the resulting hydrogen enough to penetrate the steel tube. This nascent hydrogen combines with carbon in the metal structure and forms methane. Methane molecules being too large to diffuse through the steel, excessive pressure develops within the metal structure, causing the metal to rupture along the crystalline boundaries where methane has formed. This splits the grain boundary along the crystalline boundaries and makes a window type opening. The tube cracks are primarily intercrystalline or intergranular; the metallic area so affected becomes decarburized in the process. This failure of the tube is also called hydrogen damage. The main problem with this failure is that it gets spread across the water wall tubes in totally an unpredictable way. Hence to find out the regions affected in the waterwall by condenser leakage, exhaustive special probe ultrasonic testing is required. This enables mapping of cracks in the tubes, and the area needing replacement is decided. Even after the replacement a few failures can still occur as it is very difficult to map each millimeter of the waterwall tubes.
Avoiding hydrogen embrittlement
To avoid failure of waterwall tubes due to condenser leakage:
- Constant surveillance of condensate purity is required, which can be done by online monitoring devices.
- It is preferable to shut down the unit immediately and attend to the condenser leakage.
- During the period of leakage starting and shutting down of the unit, maintain the boiler water pH by proper dosing of chemicals.
- Check the condenser tube for any possibility of leakage and to maintain the condenser tube internals. There are condenser tubes cleaning devises which can ensure fast cleaning of the tubes.
- Condenser tubes are more prone for leakages when sea water is used as cooling media, but in other cases this is not uncommon.
- This type of failure is prevalent in boilers with operating pressure greater than 100 to 105 kg/cm2 and so more care has to be exercised.
- Adapting to coordinated pH/phosphate control will reduce the possibility of hydrogen embrittlement, which is due primarily to the improved buffering of the boiler water with phosphate present.
The best way to avoid extensive damage and replacement of waterwall tubes in a high pressure boiler is to avoid condenser tube leakage, maintain a strict water chemistry regime, maintain drum water level within limits, and to check the internal deposits every five years of operation, then go for post operational acid cleaning if needed.
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About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.
Understanding Tube Failures in High Pressure Boilers - Boiler tube failures are inevitable. There are twenty-two primary reasons for tube failures in a boiler. It is true that being forewarned is being forearmed. Knowledge and good operating and maintenance practice reduce tube failures.
Water Quality in High Pressure Boilers - The process for water treatment is decided dependent on the source of water of the power plant and its quality. The levels of water quality to be maintained depend upon the drum operating pressure.
How Long Term Overheating Tube Failures Happen in Boilers - Long term overheating tube failures are due to operating metal temperature of the boiler tubes going above the allowable limit. These types of failure are seen in steam cooled tubes like superheaters and reheaters and in water cooled tubes of waterwalls.
Conductivity increase has been one of the prime tools for boiler operator to suspect condenser tube leakage and take corrective action. Conductivity at various locations in the plant has to be within specified limits.