Behavior of condenser tubes in thermal power plants

In power generation plants using a water-steam cycle, the condenser is the cold source of the thermodynamic cycle. After working in the turbine, the steam is condensed by heat exchange in contact with a bundle of tubes through which cooling water circulates [B 1 540] . The cooling water is taken from the river or the sea (open circuit) or is itself cooled by atmospheric air in an air cooler before being reused (closed circuit).

Secondary circuit chemistry helps to control all corrosion phenomena and their consequences (in particular their impact on steam generator cleanliness).

The condenser must therefore limit the risk of pollution by condensed water, either through the introduction of water from the cooling circuit, or through the entry of air leading to the dissolution of oxygen in the condensed water. The integrity of the condenser tube bundle therefore plays a key role in the chemical conditioning of the secondary circuit.

During operation, continuous monitoring of the secondary circuit's chemical parameters enables the entry of pollutants to be detected and helps identify their origin. The aim of this monitoring is to detect any drift in these parameters as early as possible, in order to initiate a search for the source of the pollution.

When the source of pollution is a leak in one of the bundle's tubes, the first step is to identify the condenser module in which the pierced tube is located. Various methods can be used: sodium meter scanning, resin concentration method or helium global leak test.

Once the offending module has been identified, it is isolated, depending on the condenser configuration, either by stopping the circulation pump supplying it with cooling water, or by closing the isolation valves located upstream and downstream of the module. In the first case, it is necessary to reduce the unit's load to between 50 and 60% of its rated load in order to limit the risk of tube vibration (unless this vibration risk is taken into account). In the second case, this mode of operation (with an isolated module) results in a loss of efficiency of the order of one percent of the power produced.

The isolated module is drained and opened. The tubes are cleaned and dried so that one or more leak detection methods can be used. Among these methods, those requiring the condenser to be evacuated (and therefore keeping the unit running) are distinguished from those implemented when the plant is shut down for maintenance:

• installation in operation: helium leak test, soap or foam test, candle or smoke test, deformable plugs ;

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