Composite reagent chemical cleaning process for nuclear power station evaporator

Abstract

The invention belongs to the technical field of nuclear power station equipment maintenance, and particularly relates to a composite reagent chemical cleaning process for a nuclear power station evaporator. The composite agent is prepared from the following components in percentage by mass: 8% of EDTA (Ethylene Diamine Tetraacetic Acid), 1% of a cosolvent, 0.8% of a corrosion inhibitor, 0.5% of a reducing agent and ammonia water; a cleaning stage: injecting a composite reagent into the evaporator to ensure that scaling substances are effectively dissolved; in the rinsing stage, firstly, deionized water is used for conducting preliminary rinsing on the evaporator so as to rapidly reduce the concentration of the composite agent, then circulating rinsing is conducted, and in the circulating process, the pH value and the conductivity of rinsing water are continuously monitored till the pH value and the conductivity of the rinsing water are stable and reach the standard of the deionized water; in the passivation stage, sodium nitrate with the mass fraction of 3%, benzotriazole with the mass fraction of 0.5% and sodium carbonate with the mass fraction of 0.2% are selected and dissolved in deionized water with the corresponding amount, and the mixture is added into the evaporator for circulation; and evaluating the cleaning effect. The problem of scaling caused by corrosion products and other sediments carried by feed water in the long-term operation process of the nuclear power station evaporator is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear power plant equipment maintenance, and in particular relates to a composite agent chemical cleaning process for a nuclear power plant evaporator. Background Art

[0002] The evaporator of a nuclear power plant is one of the key equipment of a nuclear power plant. Its main function is to transfer the heat energy generated by the nuclear power reactor to the main steam circuit through the feed water of the conventional main feed water system to generate steam for transmission to the steam turbine for power generation. Over time, a large amount of dirt and sediment often accumulates inside the evaporator. These accumulations will significantly reduce the heat exchange efficiency of the equipment, increase energy consumption, reduce the output of electric power, and thus affect the economy and reliability of the entire production process. The formation of dirt and sediment is mainly due to the deposition of minerals, biomass and other impurities contained in the working medium during the evaporation process, especially in the gas-liquid two-phase area or throttling area of ​​a once-through steam generator or similar heat exchange tube. These deposits not only reduce the heat exchange efficiency, but also may cause blockage of the evaporator pipeline, or even damage the evaporator equipment, causing accidents. Therefore, it is necessary to implement regular cleaning of the evaporator of a nuclear power plant to ensure the normal operation of the nuclear power plant equipment and units.

[0003] Currently known cleaning methods mainly include physical cleaning and chemical cleaning. Although physical cleaning methods can remove some large-particle sediments, they are not effective in removing tiny, tightly attached sediments and corrosion products. Although traditional chemical cleaning methods can dissolve some sediments, they may cause secondary corrosion to heat exchange tubes and pipelines of specific materials, and some of the chemical agents used are potentially harmful to operators and the environment. In particular, common cleaning technologies use citric acid or other single cleaning agents for chemical cleaning. Although such technologies are effective to a certain extent, they have several technical defects: Low efficiency: Single cleaning agents are inefficient in dealing with complex corrosion products and stubborn scaling. Material damage: Some chemical agents may damage metal materials (such as T22 steel pipes and 321 stainless steel) in the evaporator, especially when the concentration of the agent and the cleaning conditions are not strictly controlled. Environmental and safety issues: Traditional chemical cleaning agents may pose potential safety risks to the environment and operators.

[0004] Therefore, there is an urgent need for a new chemical cleaning process that can efficiently and safely solve the corrosion and scaling problems in the evaporator. Summary of the invention

[0005] The purpose of the present invention is to provide a composite chemical cleaning process for nuclear power plant evaporators, aiming to solve the scaling problem caused by corrosion products and other sediments carried by feed water during long-term operation of nuclear power plant evaporators. In particular, the composite chemical cleaning process is aimed at T22 material heat exchange tubes, 321 stainless steel throttle valves, WB36 feed water pipes and P91 steam pipes in nuclear power plant evaporators. The process achieves good cleaning effects on the above materials while controlling the corrosion rate, protecting the material surface, improving the heat exchange efficiency of the evaporator and extending the service life of the equipment through precise selection and proportioning of chemical agents.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A composite chemical cleaning process for nuclear power plant evaporators.

[0008] Step 1: Selection and mass ratio of compound reagent: 8% EDTA, 1% co-solvent, 0.8% corrosion inhibitor, 0.5% reducing agent, and ammonia water;

[0009] Step 2: Implementation of cleaning process:

[0010] Cleaning stage: inject the compound reagent into the evaporator to ensure that the scaling substances are effectively dissolved;

[0011] Rinsing stage: First, use deionized water to preliminarily rinse the evaporator to quickly reduce the concentration of the compound reagent, and then perform a cyclic rinse. During the circulation process, continuously monitor the pH value and conductivity of the rinse water until both are stable and meet the deionized water standard;

[0012] Passivation stage: 3% sodium nitrate by mass, 0.5% benzotriazole by mass, and 0.2% sodium carbonate by mass are dissolved in corresponding amounts of deionized water and added to the evaporator for circulation;

[0013] Step 3: Cleaning effect evaluation:

[0014] After cleaning, the cleaning effect is evaluated by comparing the heat transfer efficiency, corrosion rate and material surface condition before and after cleaning.

[0015] The cosolvent is polycarboxylic acid.

[0016] The corrosion inhibitor is a mixture of 75% by mass of benzoic acid and 25% by mass of cinnamic acid.

[0017] The reducing agent is an equal mass mixture of glucose and ascorbic acid.

[0018] Ammonia water controls the pH value of the compound agent in the range of 7-9.

[0019] Ammonia water controls the pH value of the compound agent at 8.7.

[0020] The flow rate of the composite agent is controlled to be no less than 0.3m / s, and the cleaning temperature is controlled at 120°C to 140°C.

[0021] The cleaning time is controlled within 24 hours.

[0022] The initial rinsing flow rate is controlled at above 0.5m / s and the time is 30 minutes.

[0023] During the cyclic rinsing step, a neutralizer is added to neutralize the residual acidic substances. The criteria for judging the end of rinsing include the total iron content of the rinsing water being less than 1 mg / L, and the pH value and conductivity reaching the tap water standard. The passivation process is carried out at 50°C to 70°C to promote the formation of the passivation film. The cycle time is 2 to 4 hours. The quality and uniformity of the passivation film are detected by sampling and surface analysis methods to ensure the passivation effect.

[0024] The beneficial effects achieved by the present invention are:

[0025] The process is used to improve and restore the operating performance of the evaporator of a nuclear power plant, and in particular, provides an effective solution to the corrosion and scaling problems generated during the operation of the nuclear power plant. The present invention not only greatly improves the cleaning efficiency and effect of the evaporator of a nuclear power plant, but also protects the metal materials in the evaporator during the cleaning process, avoids secondary corrosion, and ensures environmental safety, reflecting the comprehensive advantages and significant effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a process flow chart of chemical cleaning with composite reagents for evaporators in nuclear power plants. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] The core of the present invention is to use a specific composite agent, including EDTA, cosolvent, corrosion inhibitor Y, reducing agent A and ammonia water, and through the precise proportion and use of these components, effectively clean various materials (such as T22, 321 stainless steel, WB36 and P91, etc.) in the evaporator of the nuclear power plant. The composite agent not only has a strong dissolving ability, and can specifically dissolve the main scaling components such as Fe3O4 and Fe2O3 in the evaporator, but also can effectively control the corrosion rate in the cleaning process by adjusting the pH value and adding corrosion inhibitors, and protect the evaporator material.

[0029] The composite agent used in the invention is mainly composed of EDTA, a solvent, a corrosion inhibitor Y and a reducing agent A, and a detailed chemical formula design is carried out according to different pollutants and corrosion conditions inside the evaporator.

[0030] As a further improvement of the present invention, the cleaning agent is prepared by mixing 8% by mass of EDTA, 1% of a co-solvent, 0.8% of a corrosion inhibitor Y, 0.5% of a reducing agent A, and an appropriate amount of ammonia water.

[0031] As a further improvement of the present invention, the co-solvent is polycarboxylic acid, which is used to enhance the dissolution ability of EDTA on scaling substances and improve the cleaning efficiency.

[0032] As a further improvement of the present invention, the corrosion inhibitor is a mixture of 75% by mass of benzoic acid and 25% by mass of cinnamic acid, which is selected as the corrosion inhibitor due to its good metal corrosion inhibition performance, especially its ability to protect iron and copper, to reduce metal corrosion during the cleaning process.

[0033] As a further improvement of the present invention, the reducing agent A is a mixture of glucose and ascorbic acid in equal proportions, which is selected due to its stability at high temperatures and its ability to reduce oxides, so as to reduce oxidative corrosion.

[0034] As a further improvement of the present invention, the chemical cleaning agent uses ammonia water to control the pH value in the range of 7-9.

[0035] The chemical cleaning liquid is obtained by mixing in proportion, and the cleaning temperature is controlled at 120℃ to 140℃. The high temperature is used to enhance the complexing ability of EDTA and accelerate the dissolution process of scale. The flow rate of the cleaning medium is not less than 0.3m / s.

[0036] The chemical cleaning circuit of the evaporator includes a cleaning tank, a dosing tank, a cleaning pump, a temporary discharge and water inlet connection, a cleaning platform, an evaporator, a circulation and filtration system, a liquid level indicator and a variety of control valves.

[0037] The key steps of the cleaning process include: 1. On-site preparation of the cleaning agent, preparing an appropriate amount of composite cleaning agent according to the actual contamination situation and water volume of the evaporator; 2. During the cleaning process, by controlling the flow rate, temperature and pH value of the cleaning agent, the inside of the evaporator can be effectively cleaned while protecting the material surface from damage; 3. Post-cleaning flushing and corrosion control, using appropriate flushing strategies and corrosion inhibition measures to ensure the complete removal of cleaning agent residues and prevent possible subsequent corrosion problems.

[0038] The present invention particularly emphasizes the applicability and corrosion control ability of the cleaning agent for different materials, and verifies the cleaning effect of the cleaning process on T22, 321 stainless steel, WB36 and P91 materials and its control of material corrosion. The results show that the cleaning process of the present invention can not only effectively remove the dirt and corrosion products inside the evaporator, but also protect the materials inside the evaporator during the cleaning process, avoiding shortening the service life of the equipment due to corrosion during the cleaning process. In short, the cleaning process provided by the present invention has good cleaning and corrosion control effects on various materials in the evaporator of a nuclear power plant, and is a safe, effective and environmentally friendly chemical cleaning technology.

[0039] The precisely proportioned composite cleaning agent, especially the specific ratio of EDTA, co-solvent, corrosion inhibitor Y and reducing agent A, is designed to effectively remove deposits while protecting the metal materials in the evaporator. The pH adjustment mechanism of the cleaning medium optimizes the complexing effect of EDTA and controls the corrosion rate of the metal surface by using ammonia water. The post-cleaning rinse strategy uses deionized water for batch rinsing to ensure that there is no residual cleaning agent and minimize the impact on the environment. The selection and application of co-solvents and corrosion inhibitors not only enhance the cleaning efficiency, but also effectively control the corrosion rate. The selection of reducing agent A highlights the ability to remove deposits such as iron oxide and prevent secondary corrosion at high temperatures.

[0040] The composite chemical cleaning process for a nuclear power plant evaporator of the present invention is aimed at the specific cleaning requirements of T22 heat exchange tubes, 321 stainless steel throttle valves, WB36 water supply pipes and P91 steam pipes. By comprehensively analyzing the corrosion sensitivity and scaling characteristics of different materials, a suitable cleaning medium is selected and suitable cleaning conditions are controlled, thereby achieving an efficient and safe cleaning effect.

[0041] 1. Selection and ratio of cleaning media

[0042] The present invention adopts a high-temperature EDTA cleaning process. Compared with traditional citric acid cleaning, EDTA has stronger complexing ability and can dissolve scale more effectively, especially for main scaling components such as Fe3O4 and Fe2O3. In order to optimize the cleaning effect and reduce the corrosion to the material, a co-solvent with optimized dissolution ability, a corrosion inhibitor Y with corrosion mitigation effect and a reducing agent A with reducing performance are introduced to form a composite EDTA cleaning agent.

[0043] The ratio of composite EDTA is 8% EDTA, 1% cosolvent, 0.8% corrosion inhibitor Y, 0.5% reducing agent A, and an appropriate amount of ammonia water to adjust the pH value to about 8.7, ensuring that the corrosion to the material is reduced while optimizing the cleaning effect. The flow rate of the cleaning medium is controlled to be no less than 0.3m / s to ensure that the cleaning agent can fully contact the scale and improve the cleaning efficiency. The cleaning temperature is controlled at 120℃ to 140℃, and high temperature is used to enhance the complexing ability of EDTA and accelerate the dissolution process of scale.

[0044] Polycarboxylic acid is selected as a co-solvent based on its excellent water solubility and enhancement of EDTA dissolution ability, which can effectively improve the cleaning ability of the cleaning agent for stubborn scale.

[0045] A mixture of 75% benzoic acid and 25% cinnamic acid is used as a corrosion inhibitor. This mixture has good corrosion inhibition properties for a variety of metals and is particularly suitable for corrosion control in high temperature and acidic environments.

[0046] A mixture of glucose and vitamin C (ascorbic acid) in equal proportions is selected as the reducing agent. Glucose can provide a mild reducing environment during the cleaning process, which helps to remove the oxide layer on the metal surface without introducing side effects, while vitamin C can enhance the reducing effect of glucose and help protect the metal surface from oxidation.

[0047] 2. Implementation of cleaning process

[0048] The cleaning process mainly includes the steps of preparing the cleaning agent, cleaning, rinsing and passivation.

[0049] In the process of preparing the cleaning agent, the required amount of EDTA and other additives is first calculated based on the water volume of the evaporator and the estimated amount of scaling to ensure that the agent concentration and pH value achieve the best cleaning effect.

[0050] Cleaning stage: Inject the prepared composite EDTA cleaning agent into the evaporator, control the cleaning temperature and flow rate. Based on experimental and field application experience, the cleaning time is generally controlled at about 24 hours to ensure that the scaling substances are effectively dissolved.

[0051] Rinsing stage: First, use a large amount of deionized water to rinse the evaporator initially, and control the flow rate at more than 0.5m / s for about 30 minutes to quickly reduce the concentration of the cleaning agent. Then, deionized water is injected into the system for circulating rinsing. During the circulation process, the pH value and conductivity of the rinse water are continuously monitored until both are stable and close to the deionized water standard. In this step, a small amount of neutralizer (such as sodium carbonate) can be added to neutralize any residual acidic substances. The criteria for the end of rinsing include the total iron content of the rinse water being less than 1mg / L, and the pH value and conductivity being close to the tap water standard.

[0052] Passivation stage: 3% sodium nitrate, 0.5% benzotriazole (BTA), and 0.2% sodium carbonate are dissolved in the corresponding amount of deionized water and added to the evaporator for circulation. The passivation process should be carried out at 50°C to 70°C to promote the formation of the passivation film. The recommended circulation time of the passivation solution in the system is 2 to 4 hours, and the specific time is adjusted according to the system's response and test results. The quality and uniformity of the passivation film are tested by sampling and surface analysis methods (such as X-ray photoelectron spectroscopy XPS) to ensure the passivation effect.

[0053] 3. Cleaning effect evaluation and optimization

[0054] After cleaning, the effect of cleaning is evaluated by comparing the heat exchange efficiency, corrosion rate and material surface state before and after cleaning. Problems found, such as galvanic corrosion and cleaning agent residue, are optimized by adjusting the cleaning agent ratio, cleaning conditions or strengthening the rinsing steps. Ultimately, the scaling and corrosion problems are solved, the heat exchange efficiency of the evaporator is restored or improved, and the service life of the equipment is extended, which is of great significance to ensure the safe and stable operation of nuclear power plants.

[0055] 4. Verification of cleaning effect

[0056] The following results were achieved by implementing the composite EDTA cleaning process on site: The uniform corrosion rate of T22 pipe was controlled below 20g / m 2 , no galvanic corrosion risk. 321 stainless steel throttle valve has no risk of intergranular corrosion, and the deposits at the throttle valve change hole can be completely removed. It is recommended that the cleaning temperature be controlled at 120-130℃, and the cleaning time be determined by on-site analysis of test data.

Claims

1. A composite agent chemical cleaning process for nuclear power plant evaporators, characterized in that: Step 1: Selection and mass ratio of compound reagent: 8% EDTA, 1% co-solvent, 0.8% corrosion inhibitor, 0.5% reducing agent, and ammonia water; Step 2: Implementation of cleaning process: Cleaning stage: inject the compound reagent into the evaporator to ensure that the scaling substances are effectively dissolved; Rinsing stage: First, use deionized water to preliminarily rinse the evaporator to quickly reduce the concentration of the compound reagent, and then perform a cyclic rinse. During the circulation process, continuously monitor the pH value and conductivity of the rinse water until both are stable and meet the deionized water standard; Passivation stage: 3% sodium nitrate by mass, 0.5% benzotriazole by mass, and 0.2% sodium carbonate by mass are dissolved in corresponding amounts of deionized water and added to the evaporator for circulation; Step 3: Cleaning effect evaluation: After cleaning, the cleaning effect is evaluated by comparing the heat transfer efficiency, corrosion rate and material surface condition before and after cleaning.

2. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1 is characterized in that: The cosolvent is polycarboxylic acid.

3. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1 is characterized in that: The corrosion inhibitor is a mixture of 75% by mass of benzoic acid and 25% by mass of cinnamic acid.

4. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1, characterized in that: The reducing agent is an equal mass mixture of glucose and ascorbic acid.

5. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1 is characterized in that: Ammonia water controls the pH value of the compound agent in the range of 7-9.

6. The composite chemical cleaning process for nuclear power plant evaporators according to claim 5, characterized in that: Ammonia water controls the pH value of the compound agent at 8.

7.

7. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1, characterized in that: The flow rate of the composite agent is controlled to be no less than 0.3m / s, and the cleaning temperature is controlled at 120°C to 140°C.

8. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1, characterized in that: The cleaning time is controlled within 24 hours.

9. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1, characterized in that: The initial rinsing flow rate is controlled at above 0.5m / s and the time is 30 minutes.

10. The composite agent chemical cleaning process for nuclear power plant evaporator according to claim 1, characterized in that: During the cyclic rinsing step, a neutralizer is added to neutralize the residual acidic substances. The criteria for judging the end of rinsing include the total iron content of the rinsing water being less than 1 mg / L, and the pH value and conductivity reaching the tap water standard. The passivation process is carried out at 50°C to 70°C to promote the formation of the passivation film. The cycle time is 2 to 4 hours. The quality and uniformity of the passivation film are detected by sampling and surface analysis methods to ensure the passivation effect.

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