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  • Maintenance technology of cooling tower outer wall
  • Author / arrangement: YueShun chimney corrosion protection Co., Ltd. release time: May 23, 2020
  • Based on the experience of domestic wet cooling tower anti-corrosion design in recent years, combined with the old chimney desulfurization reconstruction project, this paper puts forward the technical route of desulfurization transformation of old chimney in coal-fired power plant, hoping to provide reference for the anti-corrosion design of old cooling tower in coal-fired power plant.

    First, the main factors to be considered in the reconstruction of cooling tower. If the wet flue gas is discharged to the chimney without adding GGH, the wet flue gas will condense on the inner wall of the chimney to form strong corrosion. If the anti-corrosion design structure of the wet chimney is not considered in the design of the original chimney, the anti-corrosion transformation of the original chimney must be carried out at the same time of adding desulfurization process. The basic requirement of the old chimney reconstruction is that when the condensate appears in the inner wall of the wet chimney, the lining of the smoke exhaust tube should not be corroded within the design service life and lose the structural safety. Moreover, the condensate should not seep into the reinforced concrete outer wall after the lining is damaged due to the corrosion, which will endanger the bearing capacity of the chimney structure. When the outer wall of reinforced concrete cylinder is corroded, the problem of structural safety is directly affected, and the corrosion of inner lining of smoke exhaust tube is a long-term durability problem.

     Maintenance process of cooling tower outer wall (Fig. 1)

    Structure analysis of cooling tower

    1. Carbonation depth

    Carbonation of concrete is a kind of chemical corrosion to concrete. Carbon dioxide in the air penetrates into the concrete and reacts with its basic substances to form carbonate and water. The process of reducing the alkalinity of concrete is called concrete carbonation, also known as neutralization. The chemical reaction is: Ca (OH) 2 + CO2 = CaCO3 + H2O.

    In the hydration process of cement, a large amount of calcium hydroxide is generated, which makes the voids of concrete full of saturated calcium hydroxide solution. The alkaline medium has a good protective effect on the reinforcement, and forms insoluble ferric oxide and ferric oxide on the surface of steel bar, which is called passive film. When the carbonation of concrete begins, the degree of protection of the concrete will be reduced after carbonation.

    The carbonation depth of concrete is the main index that affects the durability of reinforced concrete structure. The carbonation depth of concrete is measured by dropping 1% alcohol phenolphthalein solution through drilling hole. The results show that there are different degrees of carbonation depth in concrete, and the average carbonation depth is close to the thickness of concrete cover.

    There is a lot of dust in the surrounding environment of the project. In the cooling tower and surrounding areas, the water vapor and dust in the air form weak acid or weak alkali and adhere to the concrete surface, causing corrosion on the concrete surface.

    2. Current situation investigation

    The cooling tower was inspected in detail on site. It was found that after nearly 20 years of use, some concrete components were seriously corroded by the environment, and the following defects appeared, such as concrete falling off and reinforcement corrosion. The damage parts and photos are shown in Table 1

    1) The reinforcement of herringbone column is corroded, the concrete falls off and the stirrup is exposed

    2) The protective layer at the bottom of ring beam falls off, and the main reinforcement and stirrup are exposed

    3) There are cracks along the reinforcement on the tower wall, the internal reinforcement is corroded and the protective layer falls off.

    4) The frame column has the defect of missing edge and corner

    3. Identification analysis

    It can be seen from the on-site inspection that the concrete pillars, inclined columns, ring beams and tower walls are damaged to varying degrees, among which the ring beam and tower wall concrete are most seriously damaged.

    A small number of concrete column concrete fell off along the direction of stirrup. Although some of the column surface concrete did not fall off, there were slight expansion marks on the column surface, which indicated that the reinforcement inside the column had been corroded. With the volume expansion of steel corrosion, it was only a matter of time before the protective layer fell off. From the photos of ring beam and tower wall, it can be seen that the protective layer at the bottom of ring beam and tower wall has fallen off in large area, and the reinforcement is seriously corroded.

    Due to the corrosion of reinforcement and the falling off of protective layer, the effective cross-sectional area of beams, columns and tower walls becomes smaller, resulting in the decline of the overall bearing capacity of the structure, which directly affects the mechanical performance of the structure. The stirrup is located on the outside of the main reinforcement. After the reinforcement corrosion, it not only affects the bearing capacity of the inclined section of the structure, but also reduces the binding force on the concrete, and indirectly affects the bending capacity of the structure. On the other hand, steel corrosion also reduces the bond strength between reinforcement and concrete.

    According to the above inspection results, it can be determined that the damage of the components has seriously affected the bearing capacity of the main structure. The bearing capacity of the main structure of the building has not met the requirements of the current code and is in a dangerous state.

    4. Usability rating based on cracks, deformation, defects and damage, corrosion

    It can be seen from the inspection results that there are some defects in the column, such as the corrosion of the column and the concrete corner. According to the provisions of article 6.2.5-6.2.8 of "standard for reliability appraisal of industrial buildings" (GB 50144-2008), when the support system is evaluated according to cracks, defects and corrosion, it can be evaluated as grade C.

    According to the investigation results of the concrete tower wall, there are horizontal cracks along the reinforcement of the tower body. In some areas, the protective layer has fallen off and the reinforcement is exposed. According to the provisions of article 6.2.5-6.2.8 of the standard for reliability appraisal of industrial buildings (GB 50144-2008), the tower wall can be evaluated as grade C when evaluated according to cracks, defects and corrosion.

    According to article 7.3.8 of "standard for reliability appraisal of industrial buildings" (GB 50144-2008), the serviceability of tower wall and support system can be evaluated as grade C.

    5. Conclusions and suggestions

    5.1 identification conclusion

    1) The cooling tower support system and tower wall steel corrosion, concrete cover fall off, concrete carbonation depth is close to the thickness of the protective layer, cooling tower durability failure.

    2) Because the tower wall is thin (120mm at the thinnest part), the effective cross-sectional area is reduced and the structural stiffness is reduced under the condition of corrosion, falling off of protective layer and corrosion of reinforcement.

    3) The stress on the lower part of the cooling tower is the largest, and the bottom ring beam and tower wall are damaged most seriously, which seriously affects the overall bearing capacity, and the overall bearing capacity is insufficient.

    In conclusion, according to the provisions of article 3.3.1 and article 8.0.2 of "standard for reliability appraisal of industrial buildings" (GB 50144-2008), the comprehensive reliability assessment of cooling tower in thermal power plant is grade IV, which has seriously affected the overall safety, and reinforcement measures or demolition and reconstruction must be taken immediately.

    5.2 maintenance suggestions

    5.2.1 the above identification is the preliminary identification conclusion, and the inner wall and internal frame structure of the cooling tower shall be inspected in detail during the maintenance period.

    5.2.2 for damaged herringbone column and ring beam, loose concrete on the surface shall be removed, rust of corroded reinforcement shall be removed, and high-strength grouting material shall be used for repair. If the reinforcement is seriously rusted, reinforcement shall be added and welded with the original reinforcement, and then carbon fiber shall be used for reinforcement;

    5.2.3 the loose concrete on the surface of damaged tower wall and other damaged components shall be removed, and the rusted reinforcement shall be removed and repaired with repair mortar.

    5.2.4 in order to ensure the durability of concrete, after the above repair, the construction waterproof coating and polymer mortar shall be applied on the concrete surface to block the direct contact between moisture and concrete.

    5.2.5 the reinforcement scheme shall be designed in detail by the unit with corresponding qualification.

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