A method for phased control of the cleanliness of the pipes of the secondary steam-water system during the construction phase of a nuclear power plant

Abstract

The present application belongs to the technical field of nuclear power plant construction, and particularly relates to a staged control method for the cleanliness of the secondary circuit steam-water system pipeline in the construction stage of a nuclear power plant. The method comprises the following steps: step 1: flushing of the condensate pipeline and the main feedwater pipeline; step 2: flushing of the main steam pipeline and the bypass discharge pipeline; step 3: flushing of the shaft seal steam pipeline and the auxiliary steam supply pipeline; and step 4: performing high-cleanliness control on the steam turbine extraction pipeline. The beneficial effect is that the differential cleaning strategy makes the cleaning action more targeted, significantly improving the overall cleaning efficiency. At the same time, the chemical cleaning is only focused on the necessary high-temperature pipeline, greatly reducing the total amount of chemical agents in the whole plant, not only reducing the cost and waste liquid treatment burden, but also reducing the potential impact of chemical residues on key equipment such as steam generators and steam turbines, providing a cleaner initial environment for their stable operation.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power plant construction technology, specifically relating to a phased control method for the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant. Background Technology

[0002] The secondary loop system of a nuclear power plant is an important thermodynamic cycle system connecting the steam generator, turbine, and condenser. The steam generator is the core hub of the primary and secondary loops of a nuclear power plant. It is responsible for transferring the heat of the primary loop coolant to the secondary loop feedwater to generate steam to drive the turbine generator set for power generation. It also serves as a key component of the primary loop pressure boundary, acting as a safety barrier to prevent the leakage of radioactive materials. The turbine is the core power equipment that converts the thermal energy of steam into mechanical energy, thereby driving the generator to generate electricity. Its operating status directly determines the power generation efficiency of the unit.

[0003] For this reason, steam generators have stringent requirements for the quality of feedwater in the secondary loop. If the feedwater carries impurities, they will accumulate and concentrate on the inner wall of the heat transfer tubes and in the gaps between the tube sheet and the support plate, damaging the passivation film on the surface of the heat transfer tubes and inducing corrosion and blockage. This not only reduces heat exchange efficiency but also threatens the integrity of the primary loop radioactive barrier. Furthermore, the efficient and stable operation of the steam turbine depends on high-quality, clean steam. If the steam contains hard particles such as oxide scale and welding slag, the high-speed flowing steam will carry these impurities, impacting and wearing down precision blades, and damaging the shaft seal structure. This leads to a decrease in unit vacuum, increased energy consumption, and in severe cases, equipment failure. Therefore, the cleanliness of the secondary loop piping directly affects the safety, stability, and economy of the unit's operation.

[0004] Secondary loop pipelines are susceptible to contamination from oxide scale, welding slag, grease, dust, and other pollutants during manufacturing, transportation, and installation. Currently, traditional cleaning methods for secondary loop pipelines mostly employ uniform flushing, purging processes and acceptance standards, failing to fully consider the differences in contamination types, adhesion strength, and operating environments among different types of pipelines (such as condensate pipelines, high-temperature steam pipelines, and extraction steam pipelines). This results in inconsistent cleaning effectiveness and exhibits the following main shortcomings: 1. The uniform cleaning process and acceptance standards are adopted, but the pollution characteristics of different types of pipelines (such as high-temperature oxide scale, welding oxide layer, installation dust, etc.) are not differentiated. The cleaning is not targeted enough and cannot accurately match the stringent requirements of steam generators and steam turbines for water quality and steam quality. 2. Conventional water flushing is ineffective at removing the dense oxide scale formed on the inner wall of high-temperature steam pipes, making it difficult to achieve deep cleaning requirements; 3. The acceptance criteria are singular, usually relying only on turbidity or visual inspection, which cannot comprehensively and quantitatively reflect the cleanliness of the inner wall of the pipeline, such as the distribution, hardness, and morphology of particulate matter, making it difficult to ensure the cleanliness of the medium entering the steam generator and turbine. 4. Each system (such as condensate system, water supply system, steam system) is flushed separately, which results in insufficient coordination, duplicated operations, significant waste of water resources and chemical agents, and long construction periods. 5. Due to the characteristics of the system, the steam extraction pipeline of the steam turbine cannot usually be cleaned, and the cleanliness of the pipeline cannot be verified, which poses a potential hazard to the safe operation of the steam turbine.

[0005] Practice has shown that secondary loop piping cleanliness control is one of the key paths in the later stages of nuclear power plant construction and commissioning. Its quality and progress directly affect not only the start time of hot commissioning and commercial operation milestones, but also play a decisive role in the service life and operational performance of steam generators and turbines. Scientific and effective cleanliness control can reduce the corrosion and wear of equipment by impurities at the source, ensure the heat transfer efficiency of steam generators and the power generation efficiency of turbines, reduce the frequency of unplanned shutdowns and maintenance costs, and is a core guarantee for supporting the long-term stable operation of nuclear power plants and improving overall economic benefits. Therefore, to improve the overall cleanliness quality of the secondary loop system of nuclear power plants, shorten the construction period, and reduce operational risks, it is urgent to develop a scientific, systematic, and differentiated phased control method for piping cleanliness. Summary of the Invention

[0006] The purpose of this invention is to provide a phased control method for the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant. This method can achieve precise control of pipeline cleanliness, improve cleaning efficiency, reduce construction costs, shorten the construction period, and prevent equipment failures caused by pipeline impurities.

[0007] The technical solution of the present invention is as follows: A method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, comprising the following steps: Step 1: Flush the condensate pipes and main water supply pipes; Step 2: Flushing of the main steam pipe and bypass discharge pipe; Step 3: Flushing of shaft seal steam pipes and auxiliary steam supply pipes; Step 4: Implement high cleanliness control on the steam turbine extraction pipeline.

[0008] Step 1 includes a hot and cold water alternating circulation flushing method, with the hot water temperature controlled at 60°C. The flushing process includes cold water flushing, cleaning the condenser hot well and deaerator water tank and related pump inlet filters, hot water flushing, and cleaning the condenser hot well and deaerator water tank and related pump inlet filters again.

[0009] Step 1 includes the following: Preliminary preparations: Inspect the integrity of the piping system connections, remove precision instruments from the pipes and replace them with temporary short circuits; install temporary filters at critical pipe nodes; Hot and cold circulating flushing - cold water flushing: The cold water flushing is supplied by the demineralized water storage tank to the secondary circuit. The power station's own condensate pump and feed water pump provide power for the system flushing. The system forms the largest possible circulation loop through temporary pipeline connections. Pipes that can form circulation loops are flushed in a circulating manner, while sewage pipes or pipes that cannot form circulation loops are flushed in an open manner. During the flushing process, samples are taken at regular intervals to confirm that the water quality meets the acceptance standards. Hot and cold circulation flushing - hot water flushing: hot water flushing relies on auxiliary steam to heat the flushing water in the deaerator, control and maintain the flushing water temperature at 60℃, add hydrazine solution to the flushing water to adjust the pH value to a weakly alkaline range, and take samples every once in a while during the flushing process to confirm that the water quality meets the acceptance standards. System restoration: Remove temporary filters, sludge collection devices and short circuits, restore the installation of precision instruments, conduct a water pressure test on the piping system, and complete cleaning after ensuring there are no leaks.

[0010] Step 2 includes the following: Manual cleaning is carried out during the construction and installation phase; Steam purging was used during the hot test phase.

[0011] The construction and installation phase in step 2 includes the following: Horizontal pipe section: Enter the interior through the pre-reserved manhole of the pipe, remove the oxide scale and welding slag fragments on the inner wall, and use a clean sponge and lint-free cotton cloth to wipe the inner wall of the pipe repeatedly until the cotton cloth is free of obvious stains; collect all the cleaned impurities, used sponges and cotton cloths into a special sealed storage bag, and clean the pipe section by section in the order from upstream to downstream and from high to low. Vertical pipe sections are inspected from top to bottom using an endoscope. For pipes longer than 4.5m, the lower pipe opening is inspected visually upwards using a flashlight to ensure there are no blind spots. Any localized oxide scale buildup found during the inspection is cleaned specifically using a long-handled cleaning tool.

[0012] The hot test stage in step 2 includes the following: ① Open the bypass valve to full open; ② Check that the condenser water curtain spray valve is open; ③ Start purging and record the steam generator water level, pressure regulator liquid level, primary loop coolant pressure and temperature, main steam header pressure transmitter 1 / 2 / 3, main steam temperature, condenser vacuum, and condensate pump inlet filter differential pressure switch parameters during the purging process. ④ When the temperature of the primary circuit drops to 280℃ or the pressure of the secondary circuit drops to the limit, close the bypass valve; ⑤ Record the purging time and initial and final pressures; ⑥ Check that the condenser water curtain spray valve is closed; After three purgings, water quality samples are taken from the condensate polishing system inlet for analysis to determine whether the purging results meet the acceptance requirements. If not, the above purging steps are repeated.

[0013] Step 3 includes the following: ① Perform initial online valve setup; ②Start the boiler; ③ Open the steam shut-off valve to full opening; ④ Check that there is no steam leakage in the relevant pipelines and valves, and that the pipelines are not vibrating; ⑤ If present, shut down the boiler and inspect and eliminate the defects; if not, start purging for at least 20 minutes, recording the steam pressure and temperature parameters every 20 minutes, and close the steam isolation valve according to the purging situation and changes in steam pressure parameters. ⑥ Continue purging and acceptance testing until the steam is no longer black or brown. ⑦ Install and replace the target plate; ⑧ After the target plate is installed, open the steam isolation valve and purge for at least 15 minutes. Then record the steam parameters, close the steam isolation valve, check the target plate, and determine whether the purging results meet the acceptance requirements. ⑨ Repeat the above acceptance steps. If the purging results meet the acceptance criteria at least twice consecutively, the purging path is deemed qualified.

[0014] Step 4 includes the following: ① Equipment arrival acceptance and temporary storage: Verify the equipment nameplate, material certificate and non-destructive testing report; check whether the anti-corrosion coating on the outer surface of the equipment is intact, whether the visible parts are clean and dry, take photos for record, and record in the ledger; arrange storage conditions according to the characteristics of the equipment, and take measures to prevent moisture, sun and vibration. ② Internal protection and sealing: Use compressed air to purge the interior and spray temporary anti-rust oil on the inner wall of the carbon steel equipment; purge the equipment with nitrogen to maintain a slight positive pressure; seal all interfaces with flange blind plates and gaskets to prevent foreign objects from entering; ③ External protection: Repair damaged areas of the anti-corrosion coating; install rubber corner protectors at the edges of the equipment; use nylon slings during hoisting. ④ Protection during installation: Cover adjacent areas with fireproof cloth when welding to prevent spatter from damaging the equipment surface; seal installed pipe joints in a timely manner to prevent welding slag from entering; use an endoscope to check the internal cleanliness before installation, take photos for record-keeping, and keep a logbook; wipe the flange surface to ensure that there are no particulate residues. ⑤ Regular inspection and maintenance: Check the nitrogen pressure, internal humidity of the equipment, and whether the outer surface coating is peeling or corroded at least once or twice a month. Take photos for record-keeping and keep a logbook.

[0015] The beneficial effects of this invention are as follows: 1. By adopting a differentiated cleaning strategy, cleaning operations became more targeted, significantly improving overall cleaning efficiency. Meanwhile, chemical cleaning focused only on essential high-temperature pipelines, drastically reducing the total amount of chemicals used throughout the plant. This not only lowered costs and the burden of wastewater treatment but also reduced the potential impact of chemical residues on critical equipment such as steam generators and turbines, providing a cleaner initial environment for their stable operation.

[0016] 2. The innovative multi-system joint flushing strategy optimizes the commissioning logic and sequence, avoiding the repeated isolation, connection, and system recovery work required by traditional subsystem flushing. This effectively shortens the overall cleaning and commissioning period of the nuclear power plant's secondary loop system, improves economic efficiency, and lays the time foundation for the power plant to be put into commercial operation on schedule and achieve safe and stable operation.

[0017] 3. By implementing enhanced source control and high-standard acceptance of key components such as steam turbine extraction pipes and auxiliary steam pipes, the sources of solid particles and oxide scale entering the steam turbine flow path are fundamentally reduced. This reduces wear on the steam turbine and its auxiliary system equipment and pipes caused by impurities in the steam, extends the service life of the steam turbine and its auxiliary system equipment, and provides a solid guarantee for the long-term safe and efficient operation of the steam turbine.

[0018] 4. Comprehensive, phased cleaning control effectively prevents the risk of localized blockage and corrosion in the steam generator heat transfer tubes due to foreign object accumulation, reduces abnormal wear on rotating equipment such as the main feedwater pump and condensate pump, and lowers the initial operating resistance of the system. This not only directly improves the reliability of equipment such as the steam generator, but also provides a key guarantee for the long-term safe and stable operation and high availability of the nuclear power plant as a whole. Attached Figure Description

[0019] Figure 1 Flowchart for cold water rinsing; Figure 2 Main process flow diagram for hot water rinsing; Figure 3 Schematic diagram of the side steam purging pipeline; Figure 4 Schematic diagram of the combined purging of auxiliary steam and turbine shaft seal steam supply pipelines; Figure 5 A schematic diagram of the steam pipeline purging path is provided. Figure 6 A schematic diagram of the steam pipeline purging path for the turbine shaft seal; Figure 7This is a schematic diagram of the overall purging process. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] This invention provides a phased control method for the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant. Its core lies in abandoning the traditional single-treatment model. Based on the characteristics of the medium within the pipelines, the pipeline structure, and cleanliness constraints, the secondary loop pipeline system is divided into four categories, and differentiated cleanliness processes, process controls, and acceptance standards are formulated and implemented for each category. The specific classification and control methods are as follows: Condensate pipes, main water supply pipes, etc. This pipeline system transports water media ranging from ambient temperature to 180°C. The main contaminants are silt, rust, welding slag, oxides, and silicates. Traditional flushing methods often use ambient or cold water for continuous unidirectional flushing, relying on the shear force of the water flow to remove loose impurities. However, for deposits that are tightly bonded to the substrate, simple cold water flushing has limited effectiveness. The main reasons are: the difference in thermal expansion coefficients between impurities and the metal substrate means that at ambient temperature, the two are tightly bonded, and shear force is insufficient to overcome the adhesion; cold water has a higher viscosity, resulting in a thicker flow boundary layer, which weakens the near-wall flushing effect.

[0022] This invention employs a hot and cold water alternating circulation flushing method, with the hot water temperature controlled at around 60℃. Through periodic temperature changes, the pipes and impurities undergo differentiated micro-expansion and micro-contraction, reducing the adhesion of impurities. Combined with the optimized water flow characteristics under variable temperature conditions, efficient cleaning is achieved.

[0023] Overall flushing process: Cold water flushing → Cleaning condenser hot well, deaerator water tank and related pump inlet filters, etc. → Hot water flushing → Cleaning condenser hot well, deaerator water tank and related pump inlet filters again.

[0024] Theoretical basis: Thermal stress peeling effect The difference in thermal expansion coefficients between the metal pipe and the impurities can be used to estimate the interfacial stress using the following formula: , Where E is the elastic modulus (for pipe materials). , These are the coefficients of thermal expansion of the metal and the impurity, respectively. This represents the change in temperature.

[0025] Dynamic viscosity of water ( μ ) Dynamic viscosity of water ( μ( ) is a key physical parameter affecting the flow pattern and scouring capacity during the flushing process. It changes significantly with temperature, and its core simplified expression can be approximated as follows in the range of 0-100℃: , in, T Temperature is measured in Celsius (°C). , A , B, C This is an empirical constant.

[0026] Reynolds number ( Re ) Reynolds number ( Re ( ) is a dimensionless number that can be used to characterize fluid flow, and its formula is as follows: , in: It is fluid density. It is the fluid velocity. It is the equivalent diameter of the pipe. It is the viscosity coefficient.

[0027] The effects of different hot water temperatures are shown in Table 1: Table 1 Comparison of effects of different hot water temperatures Choosing 60℃ as the hot water flushing temperature is the optimal solution based on a comprehensive consideration of thermodynamics, fluid mechanics, and engineering safety. ① Thermal stress effect: Taking 316L stainless steel pipes ( ≈17×10 -6 / ℃) and typical silicate deposition ( ≈8×10 -6 For example, when alternating between 60℃ hot water and 20℃ cold water (°C) = At 40℃, the interfacial thermal stress is approximately 72 MPa. This stress level far exceeds the adhesion of most deposits (typically <10 MPa), effectively inducing interfacial microcracks and promoting delamination. Simultaneously, this temperature difference avoids microscopic plastic deformation or thermal fatigue damage to the base material that could result from excessive thermal stress.

[0028] ② Fluid performance optimization: The dynamic viscosity of water at 60℃ is approximately 0.47 mPa·s, a 53% decrease compared to cold water at 20℃ (1.0 mPa·s). According to boundary layer theory, the reduced viscosity significantly thins the flow boundary layer, increases the near-wall velocity gradient, enhances the shear stress of the water flow, and strengthens the scouring capacity. Simultaneously, the Reynolds number... Re At the same flow velocity, the turbulent core region expands and the turbulence intensity increases, which is beneficial for impurity transport.

[0029] ③ Balance between energy consumption and safety: A 60°C heat source is readily available in nuclear power plants through low-pressure heating systems or waste heat recovery, eliminating the need for additional high-temperature heat sources and resulting in economical energy consumption. This temperature is far below the boiling point of water, making system pressure easy to control and ensuring high operational safety. Compared to higher temperature solutions, it has less impact on the aging of pipe sealing materials and support components.

[0030] Flushing and acceptance criteria: Domestic power plants often use filters or gauze installed at the discharge end of the cleaning water, with the absence of foreign matter on the filter or gauze being the main control indicator during the cleaning process. This invention, however, primarily uses turbidity (kaolin), oil concentration, and Cl... - Concentration is used as a standard for judging water quality.

[0031] The acceptance criteria are shown in Table 2: Table 2 Acceptance Criteria ① Acceptance criteria for cold water circulation rinsing: Kaolin turbidity ≤ 5 mg / L Traditional visual filter inspection methods (such as using 100-mesh sieves) can only trap particles larger than approximately 150 μm. A large number of micron- and submicron-sized fine particles and colloidal substances, which pose a more significant threat to system operation, will penetrate the filter and cannot be effectively detected and assessed. In contrast, kaolin turbidity detection technology possesses extremely high detection sensitivity for particles ranging from nanometer to micron in size, enabling digital, high-precision online monitoring and closed-loop control of water clarity. Setting a limit of "≤ 5 mg / L" constitutes a strict quantitative control threshold. This threshold means that the turbidity level of the flushing water is close to the optical characteristics of high-purity water, objectively confirming that particulate matter has been effectively removed. This ensures that the flushing depth and cleanliness level significantly exceed traditional qualitative methods, providing a low-particulate-concentration initial environment for the subsequent hot water flushing stage.

[0032] ② Acceptance criteria for hot water circulation flushing: oil concentration ≤ 1 mg / L, Cl- ≤ 0.1 mg / L The oil concentration index targets potential organic contaminants such as lubricating oil and sealing grease within the system. Hot water can promote the emulsification and peeling of organic oil films from the pipe walls. The presence of oily substances significantly reduces the heat transfer efficiency of heat exchange surfaces and may act as nutrients to promote the growth of microorganisms; therefore, they must be completely removed. The concentration limit of "≤1 mg / L" is a key control threshold to ensure that the metal surface regains good hydrophobicity, optimizes heat transfer performance, and inhibits microbial growth.

[0033] Cl -Chloride ion content is a critical corrosion indicator related to material integrity in nuclear-grade water chemistry control. Chloride ions are one of the main environmental factors inducing pitting corrosion and stress corrosion cracking in austenitic stainless steel (such as 316L). Controlling chloride ion content is particularly critical under the cyclic micro-stress environment generated by alternating hot and cold processes. "≤ 0.1 mg / L" is a strict limit widely adopted in the international industry for preventing chloride stress corrosion cracking.

[0034] Main steam pipes, bypass discharge pipes, etc. This pipeline operates under high temperature and pressure, and the main contaminants are oxide scale, hard welding slag, and high-temperature sintered materials. Due to its large diameter, manual entry is permitted. Therefore, a cleaning method of manual cleaning during the construction and installation phase and steam purging during the hot commissioning phase was adopted.

[0035] During the construction and installation phase, before and after pipeline installation, thorough manual cleaning is performed using an endoscope and tools such as a small hammer, steel brush, clean sponge, and lint-free cotton cloth, focusing on removing large foreign objects and visible debris. During the hot commissioning phase, high-temperature steam purging is used. The thermal stress of the steam causes the oxide scale to crack, and the kinetic energy of the high-speed steam flow is used to peel it off and remove it. Steam purging removes rust, welding slag, and other foreign objects remaining in the main steam and bypass piping systems during construction and installation.

[0036] The purging acceptance criteria are as follows: ① During and after the purging of the bypass valve, the differential pressure of the condensate pump inlet filter screen should be ensured to meet the operating requirements of the condensate pump.

[0037] ② After the bypass valve is purged, the condensate water quality should meet the commissioning requirements of the CPS (Condensate Polishing System). The inlet water quality requirements of the condensate polishing device are shown in Table 3.

[0038] Table 3. Inlet Water Quality Requirements for Condensate Polishing Units ① During and after the bypass valve purging process, ensure that the differential pressure of the condensate pump inlet filter meets the operating requirements of the condensate pump. Steam purging aims to remove residual welding slag, rust, and other debris from pipelines during installation and welding using high-speed steam. These blown-out impurities will eventually enter the condenser hot well and be transported by the condensate pump. The condensate pump inlet filter is the first line of defense for protecting the pump. If the pressure difference across the filter is too large, it means there are many impurities and the filter is severely clogged, which will directly lead to excessively low pump inlet pressure, potentially causing pump cavitation, impeller damage, insufficient flow or flow interruption, affecting the normal operation of the entire condensate system, and even causing the condensate pump to trip.

[0039] ② After the bypass valve is purged, the condensate water quality meets the commissioning requirements of the CPS (Condensate Polishing System).

[0040] The condensate polishing system is a core component used to remove dissolved salts and corrosion products from condensate to ensure the quality of the secondary circuit water. During the initial startup of the unit or when a minor leak occurs in the condenser, it serves as the last line of defense for ensuring feedwater quality. Steam purging is a vigorous physical process that flushes out a large amount of metal oxides from inside the pipes, leading to excessive impurities and turbidity in the condensate during the initial purging phase. Acceptance testing requires qualified water quality, indicating that large, one-time-use impurities and corrosion products have been largely removed from the system. Only when the inlet water quality meets the design limits of the CPS (Condensate Purification System) can the CPS be successfully put into operation and begin its refined water purification process, providing a qualified working fluid for unit startup and grid connection.

[0041] Auxiliary steam supply pipelines, shaft seal steam pipelines, etc. The pressure and temperature of this group of pipelines are relatively low, and the pipe diameter is small, making manual internal cleaning impossible. Cleaning relies entirely on the dynamic effect of steam purging. The control strategy focuses on the effectiveness of the purging process and the objective verification of the results. The steam source for purging the auxiliary steam pipelines and turbine shaft seal steam pipelines is the extraction steam from the power plant's auxiliary boiler or the main steam pipelines of adjacent operational units. If the steam source is the auxiliary boiler, purging will significantly increase boiler fuel or power consumption; if the steam source is the extraction steam from the main steam pipelines of adjacent operational units, it will reduce the power generation of those units. Therefore, minimizing the steam purging time will significantly save on the power plant's economic losses and improve economic efficiency. Purging both the auxiliary steam pipelines and shaft seal steam pipelines requires the addition of numerous temporary measures such as pipes and valves. The installation and removal of these temporary measures require significant manpower and time. Furthermore, the presence of these temporary measures occupies a large amount of space within the power plant building, and the purging pipelines are all high-temperature and high-pressure pipelines, causing considerable inconvenience to other operations within the power plant. Therefore, in order to efficiently and reliably improve the purging quality of auxiliary steam and turbine shaft seal steam supply pipelines in nuclear power plants, shorten purging time, and reduce purging costs, this invention adopts a combined purging method for auxiliary steam and turbine shaft seal steam supply pipelines.

[0042] The general principle of the combined purging method is: to jointly design and install temporary measures for purging auxiliary steam and shaft seal steam supply pipelines; to plan the purging path according to the steam application; and, provided the purging coefficient meets the requirements, to purge the main pipe first and then the branch pipes, performing purging work on one or more paths each time. Different purging paths can also be flexibly switched according to the actual situation. The purging coefficient k = [(purging steam flow rate)]. 2 * Specific volume of the purged steam / [(Rated load steam flow rate)] 2 * Steam specific volume at rated parameters, i.e. The principle for selecting steam purging parameters is to ensure that the steam momentum inside the pipe is greater than the steam momentum under rated load during purging. That is, the purging coefficient at any point in the purged system should be greater than 1, usually taken as 1 to 1.5.

[0043] The final acceptance test was conducted by installing an aluminum target plate at the purge pipe inlet. The acceptance criteria were divided into two aspects: quality and quantity. The detailed acceptance criteria are as follows: Quality standards: ① The effective range is the area from the center of the pipe to 0.85 times the radius. Areas outside this range are not considered for judgment. The surface of the target plate should not be blackened due to rust.

[0044] ②The target plate surface is free of foreign objects.

[0045] Standard of quantity: ① The diameter of the dent on the target plate should be less than 0.5 mm, and the number of dents per square inch (25.4 mm x 25.4 mm) (between 0.25 mm and 0.5 mm) should be less than 1.

[0046] Steam turbine extraction pipelines, etc. Due to the limited installation location of this section of pipeline and its direct interface with the steam turbine, flushing could easily lead to water ingress into the turbine. The large diameter of the pipelines makes water flushing difficult in terms of power source and temporary measures. The pipelines are mostly installed vertically, making manual cleaning challenging. There is no steam medium available for purging when the unit is not running, and this section of pipeline requires extremely high cleanliness. Any residual hard particles could cause severe erosion of the turbine blades under steam drive. The main contaminants are internal rust and corrosion, iron filings from mechanical drilling, welding slag from pipe joint welding, and loose soil and debris left from on-site pipe storage. Therefore, this invention employs a high-cleanliness control method, which relies primarily on strict pre-installation cleaning, storage protection, installation process protection, and finished product protection.

[0047] Pre-installation cleaning: Use a high-pressure air gun to blow out the inside of the pipe. Only after the pipe is clean can it be connected. The cleanliness before connection needs to be witnessed by relevant personnel, and photos of each section of pipe before connection should be taken.

[0048] Storage and Protection: Pipes and fittings should be stored separately, with stainless steel and alloy steel materials isolated to avoid electrochemical corrosion; the storage site should be flat and dry, and the pipes should be isolated from the ground using wooden or steel supports; all pipe openings and joints should be sealed with special pipe caps and end caps, and covered with rainproof cloth. The integrity of the sealing should be checked regularly. If any damage or detachment is found, the inside of the joint should be checked for cleanliness. If it is not clean, it should be cleaned and resealed in time.

[0049] Installation process protection: Pipeline installation follows the principle of "from top to bottom". When installing vertical pipes, the lower pipe section should be temporarily sealed and removed before connecting with the upper pipe section. Before installation, all valves and fittings should be internally cleaned and inspected, and the flange sealing surfaces should be wiped clean with anhydrous ethanol. During construction, it is strictly forbidden to leave tools, welding rods, gloves, etc. in the pipes. All pipe openings should be sealed immediately during non-construction periods. Pipeline opening operations must be completed before cleaning, and drilling and welding on cleaned pipes are prohibited.

[0050] Finished product protection: After the pipeline is installed, timely protection of the finished product should be carried out, and identification signs and item maintenance cards should be hung. Regular inspections should be conducted. It is forbidden to use the pipeline as a scaffolding support point or a platform for stacking heavy objects. During pipeline transportation and hoisting, protective measures should be taken to avoid collisions that could cause the seals to fall off or the pipeline to be damaged.

[0051] A phased control method for the cleanliness of the secondary loop steam-water system piping during the construction phase of a nuclear power plant includes the following steps: Step 1: Flushing of condensate pipes and main water supply pipes This pipeline system transports water media ranging from ambient temperature to 180°C. The main contaminants are silt, rust, welding slag, oxides, and silicates. Traditional flushing methods often use ambient or cold water for continuous unidirectional flushing, relying on the shear force of the water flow to remove loose impurities. However, for deposits that are tightly bonded to the substrate, simple cold water flushing has limited effectiveness. The main reasons are: the difference in thermal expansion coefficients between impurities and the metal substrate means that at ambient temperature, the two are tightly bonded, and shear force is insufficient to overcome the adhesion; cold water has a higher viscosity, resulting in a thicker flow boundary layer, which weakens the near-wall flushing effect.

[0052] This invention employs a hot and cold water alternating circulation flushing method, with the hot water temperature controlled at around 60℃. Through periodic temperature changes, the pipes and impurities undergo differentiated micro-expansion and micro-contraction, reducing the adhesion of impurities. Combined with the optimized water flow characteristics under variable temperature conditions, efficient cleaning is achieved.

[0053] Overall flushing process: Cold water flushing → Cleaning condenser hot well, deaerator water tank and related pump inlet filters, etc. → Hot water flushing → Cleaning condenser hot well, deaerator water tank and related pump inlet filters again.

[0054] The rinsing method follows the principle of rinsing from small to large range, rinsing each system one by one, and alternating hot and cold cycles.

[0055] Overall flushing process: Cold water flushing → Cleaning condenser hot well, deaerator water tank and related pump inlet filters, etc. → Hot water flushing → Cleaning condenser hot well, deaerator water tank and related pump inlet filters again → Cold water flushing → Hot water flushing.

[0056] Preliminary preparations: Check the integrity of the pipeline system connections, remove precision instruments such as flow meters and pressure gauges from the pipeline and replace them with temporary short circuits; install temporary filters at key pipeline nodes (such as elbows and tees).

[0057] Hot and cold circulating flushing - cold water flushing: Cold water flushing uses a demineralized water storage tank to supply water to the secondary loop. The power plant's own condensate pumps and feedwater pumps provide power for system flushing. Temporary pipelines are used to connect and form the largest possible circulation loop. Pipes that can form a circulation loop are flushed, while sewage pipes or pipes that cannot form a circulation loop are flushed openly. Samples are taken periodically during the flushing process to confirm that the water quality meets the acceptance standards, as shown in Table 2. The cold water flushing process is as follows: Figure 1 As shown.

[0058] Hot and cold circulation flushing - hot water flushing: Hot water flushing mainly relies on auxiliary steam to heat the flushing water in the deaerator, controlling and maintaining the flushing water temperature at 60℃. During this stage, hydrazine solution is added to the flushing water to adjust the pH to a slightly alkaline range, forming a passivation protective film throughout the carbon steel pipe network to prevent secondary pollution from corrosion products during flushing. Samples are taken periodically during the flushing process to confirm that the water quality meets the acceptance standards, as shown in Table 2. The main hot water flushing process is as follows: Figure 2 As shown.

[0059] System restoration: Remove temporary filters, sludge collection devices and short circuits, restore the installation of precision instruments, conduct a water pressure test on the piping system, and complete cleaning after ensuring there are no leaks.

[0060] Step 2: Flushing of main steam pipes and bypass discharge pipes Construction phase: Manual cleaning ① Horizontal pipe section (diameter ≥ 700mm): Enter the interior through the pre-drilled manhole. Use a small hammer and steel brush to remove oxide scale and welding slag from the inner wall. Use a clean sponge and lint-free cotton cloth to repeatedly wipe the inner wall of the pipe until the cloth is free of obvious stains. Collect all the removed impurities (oxide scale, welding slag, etc.), used sponges, and cotton cloths into a special sealed storage bag. Manual cleaning follows the principle of cleaning from upstream to downstream and from high to low, cleaning each section of the pipe one by one.

[0061] ② Vertical pipe sections: For lengths ≤4.5m, use an endoscope to conduct a comprehensive inspection from top to bottom. For lengths >4.5m, use a flashlight to visually inspect upwards through the lower pipe opening to ensure there are no blind spots. For any localized oxide scale buildup found during the inspection, use a long-handled cleaning tool for targeted cleaning.

[0062] 2) Hot test stage: Steam purging like Figure 3As shown, the bypass steam pipeline is connected from the main steam header, and then splits into three lines to the condenser accessories. It is then connected to the condenser through six bypass valves. The condensate pump draws condensate from the condenser hot well. During and after the bypass valve purging, the differential pressure of the condensate pump inlet filter screen should be ensured to meet the condensate pump operation requirements.

[0063] Before starting the purging, ensure that all auxiliary systems are in normal operating condition, the condensate pump header pressure is normal (greater than 2.2MPa), the main pump speed is reduced to 88%, and the corresponding metal temperature on the surface of the steam trap reaches above 200℃.

[0064] The steps for each purging are as follows: ① Slowly open the bypass valve to full opening; ② Check that the condenser water curtain spray valve is open; ③ Start purging and record parameters such as steam generator water level, pressure regulator liquid level, primary loop coolant pressure and temperature, main steam header pressure transmitter 1 / 2 / 3, main steam temperature, condenser vacuum, and condensate pump inlet filter differential pressure switch during the purging process. ④ When the temperature of the primary circuit drops to 280℃ or the pressure of the secondary circuit drops to the limit, close the bypass valve; ⑤ Record the purging time and initial and final pressures; ⑥ Check that the condenser water curtain spray valve is closed.

[0065] After three purgings, water quality samples of the condensate are taken from the CPS (condensate polishing system) inlet for analysis to determine whether the purging results meet the acceptance requirements. If not, the above purging steps are repeated.

[0066] The purging acceptance criteria are as follows: ① During and after the purging of the bypass valve, the differential pressure of the condensate pump inlet filter screen should be ensured to meet the operating requirements of the condensate pump.

[0067] ② After the bypass valve is purged, the condensate water quality should meet the commissioning requirements of the CPS (Condensate Polishing System). The inlet water quality requirements of the condensate polishing device are shown in Table 3.

[0068] Step 3: Flushing of shaft seal steam pipes and auxiliary steam supply pipes like Figure 4As shown, the auxiliary steam has two sources: the auxiliary boiler and the main steam. The users of the auxiliary steam include: supplying steam to the deaerator for deaerator maintenance, supplying steam to the deaerator water tank for feedwater heating, providing steam for the turbine shaft seals, and supplying steam to hot water systems such as laundry rooms. The turbine shaft seal steam also has two sources: auxiliary steam and the main steam. The shaft seal steam is mainly sent to the shaft ends of one high-pressure cylinder and three low-pressure cylinders. To increase the feasibility of purging and reduce the possibility of damage to precision equipment caused by purging steam, the combined purging method requires removing the filter elements of the auxiliary steam system and the shaft seal steam system filters, as well as the valve cores of the check valves. Simultaneously, the regulating valves in the system are temporarily not installed during purging. Based on the actual pipeline layout and purging requirements, a total of three purging outlets are set up. Each outlet is equipped with a silencer to reduce the impact of purging noise. A dedicated location is provided upstream of the silencer for installing a target plate during the purging acceptance phase.

[0069] To make the explanation clearer, Figure 4 Split into Figure 5 and Figure 6 .Depend on Figure 5 and Figure 6 As shown, there are 10 purging paths. Figure 5 Path 1 is the auxiliary steam connection header to the main steam header; Path 2 is the auxiliary steam supply header to the shaft seal steam header; Path 3 is the auxiliary steam supply pipeline to the deaerator maintenance pipeline; Path 4 is the auxiliary steam supply pipeline to the deaerator water tank heating pipeline; and Path 5 is the auxiliary steam supply pipeline to the laundry room hot water heating pipeline. Figure 6 Path 6 is the main steam to shaft seal steam connection header, Path 7 is the high-pressure cylinder shaft seal steam supply pipeline, Path 8 is the No. 1 low-pressure cylinder shaft seal steam supply pipeline, Path 9 is the No. 2 low-pressure cylinder shaft seal steam supply pipeline, and Path 10 is the No. 3 low-pressure cylinder shaft seal steam supply pipeline.

[0070] The purging is powered by auxiliary boilers, with two auxiliary electric boilers, each with a power of 31MW. The purging starts from the auxiliary steam header. Each time, the purging coefficient is calculated based on the pipe diameter and steam parameters of the pipe to be purged. Provided that the purging coefficient meets the requirements, one or more paths are purged each time.

[0071] The overall process for each purging is as follows: Figure 7 As shown, the purging steps are as follows: ① Perform initial online valve setup.

[0072] ②Start the boiler.

[0073] ③ Slowly open the steam shut-off valve until it is fully open.

[0074] ④ Check that there is no steam leakage in the relevant pipelines and valves, and that there is no vibration in the pipelines.

[0075] ⑤ If present, shut down the boiler and inspect and eliminate the defects; if not, start purging for at least 20 minutes, recording steam pressure, temperature and other parameters every 20 minutes, and close the steam isolation valve according to the purging situation and changes in steam pressure and other parameters.

[0076] ⑥ Continue until the steam is no longer black or brown, indicating that a large amount of loose impurities have been discharged, and formal purging and acceptance can begin.

[0077] ⑦ Install (replace) the target plate.

[0078] ⑧ After the target plate is installed, gradually open the steam isolation valve and purge for at least 15 minutes. Then record the steam parameters, close the steam isolation valve, check the target plate, and determine whether the purging results meet the acceptance requirements.

[0079] ⑨ Repeat the above acceptance steps. Only when the purging results meet the acceptance criteria at least twice consecutively can the purging path be determined to be qualified.

[0080] The purging of auxiliary steam and turbine shaft seal steam supply pipelines shall be strictly inspected and accepted in accordance with the following standards: Quality standards: ① The effective range is the area from the center of the pipe to 0.85 times the radius. Areas outside this range are not considered for judgment. The surface of the target plate should not be blackened due to rust.

[0081] ②The target plate surface is free of foreign objects.

[0082] Standard of quantity: ① The diameter of the dent on the target plate should be less than 0.5 mm, and the number of dents per square inch (25.4 mm x 25.4 mm) (between 0.25 mm and 0.5 mm) should be less than 1.

[0083] Step 3: Implement high cleanliness control on the steam turbine extraction pipeline; Implement high cleanliness control, intervene in the installation of equipment and temporary pipelines in advance during the construction and installation phase, and after the pipelines are cleaned, witness the process, conduct an internal inspection to ensure there are no impurities, take photos for record-keeping, and record the equipment maintenance issues found during the inspection process to form a maintenance log.

[0084] The high cleanliness control steps are as follows: ① Equipment arrival acceptance and temporary storage: Verify the equipment nameplate, material certificate and non-destructive testing report; check whether the anti-corrosion coating on the outer surface of the equipment is intact, whether the visible parts are clean and dry, take photos for record, and record in the ledger; arrange storage conditions according to the characteristics of the equipment, and take measures to prevent moisture, sun and vibration.

[0085] ② Internal protection and sealing: Purge the interior with compressed air, and wipe with acetone or alcohol if necessary. Spray temporary anti-rust oil on the inner wall of carbon steel equipment. Purge the equipment with nitrogen to maintain a slight positive pressure. Seal all interfaces with flange blind plates and gaskets to prevent foreign objects from entering.

[0086] ③ External protection: Repair damaged areas of the anti-corrosion coating; install rubber corner protectors at the edges of the equipment; and use nylon slings during hoisting.

[0087] ④ Protection during installation: Cover adjacent areas with fireproof cloth when welding to prevent spatter from damaging the equipment surface; seal installed pipe joints in a timely manner to prevent welding slag from entering; use an endoscope to check the internal cleanliness before installation, take photos for record-keeping, and perform secondary rinsing if necessary; wipe the flange surface to ensure that there are no particulate residues.

[0088] ⑤ Regular inspection and maintenance: Check the nitrogen pressure, internal humidity of the equipment, and whether the outer surface coating is peeling or corroded at least once or twice a month. Take photos for record-keeping and keep a logbook.

Claims

1. A method for staged control of the cleanliness of the piping of the secondary water-steam system during the construction phase of a nuclear power plant, characterized in that, Includes the following steps: Step 1: Flush the condensate pipes and main water supply pipes; Step 2: Flushing of the main steam pipe and bypass discharge pipe; Step 3: Flushing of shaft seal steam pipes and auxiliary steam supply pipes; Step 4: Implement high cleanliness control on the steam turbine extraction pipeline.

2. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 1, is characterized in that: Step 1 includes a hot and cold water alternating circulation flushing method, with the hot water temperature controlled at 60°C. The flushing process includes cold water flushing, cleaning the condenser hot well and deaerator water tank and related pump inlet filters, hot water flushing, and cleaning the condenser hot well and deaerator water tank and related pump inlet filters again.

3. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 2, is characterized in that... Step 1 includes the following: Preliminary preparations: Inspect the integrity of the piping system connections, remove precision instruments from the pipes and replace them with temporary short circuits; install temporary filters at critical pipe nodes; Hot and cold circulating flushing - cold water flushing: The cold water flushing is supplied by the demineralized water storage tank to the secondary circuit. The power station's own condensate pump and feed water pump provide power for the system flushing. The system forms the largest possible circulation loop through temporary pipeline connections. Pipes that can form circulation loops are flushed in a circulating manner, while sewage pipes or pipes that cannot form circulation loops are flushed in an open manner. During the flushing process, samples are taken at regular intervals to confirm that the water quality meets the acceptance standards. Hot and cold circulation flushing - hot water flushing: hot water flushing relies on auxiliary steam to heat the flushing water in the deaerator, control and maintain the flushing water temperature at 60℃, add hydrazine solution to the flushing water to adjust the pH value to a weakly alkaline range, and take samples every once in a while during the flushing process to confirm that the water quality meets the acceptance standards. System restoration: Remove temporary filters, sludge collection devices and short circuits, restore the installation of precision instruments, conduct a water pressure test on the piping system, and complete cleaning after ensuring there are no leaks.

4. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 1, is characterized in that... Step 2 includes the following: Manual cleaning is carried out during the construction and installation phase; Steam purging was used during the hot test phase.

5. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 4, is characterized in that... The construction and installation phase in step 2 includes the following: Horizontal pipe section: Enter the interior through the pre-reserved manhole of the pipe, remove the oxide scale and welding slag fragments on the inner wall, and use a clean sponge and lint-free cotton cloth to wipe the inner wall of the pipe repeatedly until the cotton cloth is free of obvious stains; collect all the cleaned impurities, used sponges and cotton cloths into a special sealed storage bag, and clean the pipe section by section in the order from upstream to downstream and from high to low. Vertical pipe sections are inspected from top to bottom using an endoscope. For pipes longer than 4.5m, the lower pipe opening is inspected visually upwards using a flashlight to ensure there are no blind spots. Any localized oxide scale buildup found during the inspection is cleaned specifically using a long-handled cleaning tool.

6. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 4, is characterized in that... The hot test stage in step 2 includes the following: ① Open the bypass valve to full open; ② Check that the condenser water curtain spray valve is open; ③ Start purging and record the steam generator water level, pressure regulator liquid level, primary loop coolant pressure and temperature, main steam header pressure transmitter 1 / 2 / 3, main steam temperature, condenser vacuum, and condensate pump inlet filter differential pressure switch parameters during the purging process. ④ When the temperature of the primary circuit drops to 280℃ or the pressure of the secondary circuit drops to the limit, close the bypass valve; ⑤ Record the purging time and initial and final pressures; ⑥ Check that the condenser water curtain spray valve is closed; After three purgings, water quality samples are taken from the condensate polishing system inlet for analysis to determine whether the purging results meet the acceptance requirements. If not, the above purging steps are repeated.

7. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 1, is characterized in that... Step 3 includes the following: ① Perform initial online valve setup; ②Start the boiler; ③ Open the steam shut-off valve to full opening; ④ Check that there is no steam leakage in the relevant pipelines and valves, and that the pipelines are not vibrating; ⑤ If present, shut down the boiler and inspect and eliminate the defects; if not, start purging for at least 20 minutes, recording the steam pressure and temperature parameters every 20 minutes, and close the steam isolation valve according to the purging situation and changes in steam pressure parameters. ⑥ Continue purging and acceptance testing until the steam is no longer black or brown. ⑦ Install and replace the target plate; ⑧ After the target plate is installed, open the steam isolation valve and purge for at least 15 minutes. Then record the steam parameters, close the steam isolation valve, check the target plate, and determine whether the purging results meet the acceptance requirements. ⑨ Repeat the above acceptance steps. If the purging results meet the acceptance criteria at least twice consecutively, the purging path is deemed qualified.

8. The method for phased control of the cleanliness of the secondary loop steam-water system pipelines during the construction phase of a nuclear power plant, as described in claim 1, is characterized in that... Step 4 includes the following: ① Equipment arrival acceptance and temporary storage: Verify the equipment nameplate, material certificate and non-destructive testing report; check whether the anti-corrosion coating on the outer surface of the equipment is intact, whether the visible parts are clean and dry, take photos for record, and record in the ledger; arrange storage conditions according to the characteristics of the equipment, and take measures to prevent moisture, sun and vibration. ② Internal protection and sealing: Use compressed air to purge the interior and spray temporary anti-rust oil on the inner wall of the carbon steel equipment; purge the equipment with nitrogen to maintain a slight positive pressure; seal all interfaces with flange blind plates and gaskets to prevent foreign objects from entering; ③ External protection: Repair damaged areas of the anti-corrosion coating; install rubber corner protectors at the edges of the equipment; use nylon slings during hoisting. ④ Protection during installation: Cover adjacent areas with fireproof cloth when welding to prevent spatter from damaging the equipment surface; seal installed pipe joints in a timely manner to prevent welding slag from entering; use an endoscope to check the internal cleanliness before installation, take photos for record-keeping, and keep a logbook; wipe the flange surface to ensure that there are no particulate residues. ⑤ Regular inspection and maintenance: Check the nitrogen pressure, internal humidity of the equipment, and whether the outer surface coating is peeling or corroded at least once or twice a month. Take photos for record-keeping and keep a logbook.

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