condenser

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Corrosion on the water side of the condenser cooling tube has always been a focus of urgent attention and high prevention for designers, manufacturers and operators. However, the corrosion problem on the steam side of the cooling tube is often ignored or left behind due to various reasons. If this problem is fully grasped and solved, corrosion on the steam side of the condenser cooling tube can be reduced to a minimum or not occur during normal operation of the steam turbine unit. 1 Corrosion on the steam side of the condenser cooling tube Corrosion on the steam side of the cooling tube is different from corrosion on the water side. Usually, as long as certain attention is paid to the structural design of the condenser, better results can be obtained. Common steam side corrosion is described below. 1.1 Stress corrosion cracking Stress corrosion cracking that occurs on the water side of the condenser can also occur on the steam side. It is a form of crack that slowly forms in a sensitive alloy due to increased tensile stress in a special corrosive medium. 　　Only damage to the copper alloy due to stress corrosion cracking was found in the condenser. Generally, stainless steel tubes and titanium tubes are not affected by stress corrosion cracking under normal operating conditions of the condenser. Generally, most stress corrosion cracking damage starts from the steam side of the cooling tube. In each case, stress corrosion cracking damage occurs only where tensile stresses (residual or applied stresses) are high enough to cause stress corrosion cracking. 　　Assuming that there is a sufficiently high stress in some parts of the copper alloy pipe, the second prerequisite for stress corrosion cracking is a medium that can cause cracks. For example, copper-based alloys are prone to occur in ammonia-containing media. 　　The media conditions that lead to stress corrosion cracking on the steam side of copper alloy pipes are very clear. The vast majority of stress corrosion cracks here (although not all) are caused by ammonia solutions containing dissolved oxygen. In particular, the concentration of ammonia in the air cooling zone is particularly high, so stress corrosion cracking often occurs here. 　　Among the commonly used copper alloys for cooling tubes, brass tubes are most prone to stress corrosion cracking in ammonia-containing media, and navy brass (with or without corrosion inhibition treatment) is more prone to stress corrosion cracking. In addition, if iron precipitation in the copper-nickel alloy is avoided during processing, the copper-nickel alloy is essentially unaffected by ammonia-induced stress corrosion cracking. For example, BFe30-1-1 is often used in air cooling areas. 　　Regardless of which side of the cooling tube the crack begins to develop, there are some effective ways to prevent stress corrosion cracking in the cooling tube. Therefore, during the installation process of cooling pipes, care should be taken to prevent the expansion pipe from exceeding the thickness of the tube plate. ； Fully stress-relieved cooling tubes should be provided to avoid dents caused by bending, impact and mechanical impact after final stress relief ； Another way to prevent steam-side stress corrosion cracking is to use alloys that can withstand stress corrosion cracking, such as copper-nickel alloy tubes, stainless steel tubes, and titanium tubes. 　　Any leaks in the condenser should be quickly located and resolved immediately, because a tight condenser will not only * * Improving the resistance of copper alloys to steam-side stress corrosion cracking can also * * Increased resistance to condensation corrosion. 1.2 The type of media conditions that lead to ammonia-induced stress corrosion cracking on the steam side of copper alloy cooling pipes due to condensate corrosion is also the type of media conditions that lead to condensate corrosion. Condensate corrosion, also known as ammonia corrosion, is mostly limited to air cooling areas because the concentration of ammonia and oxygen here is particularly high. 　　In addition, in the presence of oxygen, these ammonia solutions can produce circular deep corrosion grooves (condensate corrosion grooves) on the adjacent copper alloy pipes in contact. 　　The role of oxygen in the corrosion mechanism of condensate is no less than that of ammonia. 　　The air leaking into the condenser not only provides oxygen, which plays a key role in condensate corrosion, but also causes CO2 to leak into the condenser. CO2 in ammonia-containing condensate significantly accelerates the corrosion rate of copper alloys. 　　Many of the methods used to prevent ammonia-induced stress corrosion cracking can also be used to prevent condensate corrosion. For example, BFe30-1-1 copper alloy tubes are used in the air cooling area, and it is better to use stainless steel tubes and titanium tubes. 1.3 Erosion corrosion Erosion corrosion, also known as steam side corrosion, is one of the main forms of damage to cooling tubes. It mainly occurs on the peripheral cooling tubes that are directly washed by the turbine exhaust steam and other high-energy fluids discharged into the condenser. 　　The corrosion resistance of the steam side is related to the corrosion resistance, fatigue strength, elastic modulus, hardness and ultimate strength of the cooling tube, and stainless steel tubes and titanium tubes are better than copper alloy tubes. 　　Steam velocity is the main parameter that affects the erosion and corrosion rate. In the structural design of the condenser, if the steam flow can be properly dispersed to reduce the steam velocity near the exhaust, hydrophobic and exhaust ports, it can * * Reduce or eliminate erosion corrosion ； For rapid discharge of large amounts of soda water, a large-capacity external hydrophobic expansion container can be used. 　　Another common method to control erosion corrosion is to install steam deflectors or sheathing pipes made of materials with better corrosion resistance. 1.4 Vibration damage When discussing the corrosion damage on the steam side of the condenser, the vibration damage of the cooling pipe is often included and discussed together. 　　Vibration occurs in the condenser cooling tubes. When the amplitude is high, the cooling tubes may be damaged by one or more mechanisms. High wear rates can also promote fatigue damage or corrosion fatigue. Even some cooling tubes with small amplitudes will produce fatigue damage or fretting wear near the middle tube sheet and tube sheet. 　　Fluid-induced vibrations are generally limited to the peripheral tubes of the condenser tube bundle or to tubes on either side of narrow steam passages caused by improper design. Whether it is fretting wear, collision damage, or fatigue damage, a large number of cooling tubes may be affected. As a result, the three rows of tubes on the windward side of each condenser now use thick-walled tubes. 　　The reason for the vibration of the cooling tube is that the span of the middle tube plate is too large due to design errors or the guide plate at the inlet of the high-energy fluid cannot effectively disperse the fluid. 1.5 The relationship between steam side corrosion and condenser load and structure. In most or many cases, the steam side of the cooling tube will be corroded by the high-speed wet steam flow from the exhaust steam of the low-pressure cylinder of the turbine. However, in general, the severity of corrosion on the steam side of the cooling tube is related to the load of the condenser. 　　The occurrence of the above-mentioned accidents will encounter this problem when considering the size and arrangement of the exhaust part of the low-pressure cylinder of the steam turbine and the location of the condenser. Basically, the corrosion of the cooling tube and the structural part of the condenser has a great relationship with the steam speed discharged from the low-pressure cylinder and the moisture content of the steam. Of course, this is also related to the surface hardness and heat treatment of the material of the impacted part. However, the excessive local steam velocity caused by the condenser structure and the number of local high-speed flowing water droplets are particularly important. 　　Since steam velocity is inversely proportional to absolute pressure, the magnitude of the pressure is very important. It can be seen that when the inlet temperature of the condenser circulating cooling water is low, it will cause a lower pressure on the steam side of the condenser and a higher exhaust speed. 　　The condenser throat between the exhaust part of the low-pressure cylinder of the steam turbine and the top row of cooling tubes above the tube bundle should preferably be as diffuse as possible. In areas where the exhaust steam expands rapidly, the high-speed steam flow cannot be deflected and should fully fill the cross-section of the entire flow channel. The top row cooling pipe of the condenser is the most severely corroded part on the steam side. 2 The development trend of modern technology must be studied to provide necessary layout plans and structures to adapt to and meet the development of high-power steam turbine units. 　　Some influencing factors related to changes in low-pressure cylinder exhaust speed can be summarized as: 　　The appropriate gradually expanding steam inlet (throat) of the condenser, the distance between the steam turbine exhaust port of the low-pressure cylinder and the top row of cooling tubes on the top of the condenser. 　　The arrangement shape and size of the low-pressure cylinder exhaust port and the condenser tube bundle, the arrangement and relative position of the condenser tube bundle and the turbine low-pressure cylinder, and the structure of the expansion part of the low-pressure cylinder, etc. This creates an unavoidable negative impact on the high-speed steam flow distribution entering the condenser, increases the pressure loss of the exhaust steam flow, and also aggravates the corrosion on the steam side of the cooling tube. 　　Another issue worth noting is that according to the "Recommended Design Guidelines for Discharging High-Energy Fluids into Surface Condensers" (U.S. Electric Power Research Institute), a large amount of hydrophobic and exhaust steam pours into the condenser, making the condenser inevitably a gathering point for various water and steam. They ultimately affect the flow shape and speed of the low-pressure cylinder exhaust steam. The increased number of water droplets will inevitably accelerate the erosion of the steam side cooling pipe. 3. Discharge of additional fluids to the condenser. In order to meet the needs of the thermal cycle system of the power plant and save working fluids, in addition to receiving the normal exhaust steam from the steam turbine, the condenser also receives various additional fluids (hydrophobic, water make-up, recirculated water and steam-water mixture). For a long time in the past, due to people's lack of understanding of the serious problem of additional fluid discharge into the condenser and negligence in design and operation, the condenser cooling tube and some parts were seriously impacted and corroded, resulting in excessive thermal deformation. Even the high-speed steam flow and water droplets in the cooling tube caused a series of accidents such as vibration damage and corrosion leakage. 　　In the design and operation of large condensers, a series of typical accidents are caused by the discharge of additional fluids. The main reasons are as follows. The condenser is limited to a limited space, and many devices of different sizes and sizes must be arranged in the throat. Therefore, the remaining space in the throat is generally difficult to meet the reasonable requirements for arranging additional fluid discharge devices and safe diffusion of additional fluids. 　　The correct calculation method was not used or reasonable design principles were not followed, and the discharge parameters of additional fluid were not accurately grasped or underestimated. 　　Not enough attention is given when water and steam are mixed and discharged. The high-speed two-phase flow carrying numerous water droplets is extremely harmful to the impact corrosion of cooling tubes and related components. 　　The discharge position is not correctly selected based on the energy level of the additional fluid. The main manifestations are as follows:: 　　The diffusion direction of the discharged fluid is facing the cooling pipe, and there is no or no effective baffle to install. 　　The diffusion distance of the discharged fluid is not enough, so the energy when it hits the components in the condenser is still too large. ； 　　Additional fluid discharged into the system controls part of the imbalance. 　　The main principle for dealing with the discharge of additional fluids into the condenser should be to minimize or even avoid the damaging effects of impact corrosion on components (including cooling tubes) of additional fluids (cold water, saturated water or superheated water, flash mixture, saturated steam or superheated steam), especially when high-energy level additional fluids are discharged into the condenser. In some specific cases, a flash box must be specially set up independent of the condenser or attached to the wall of the condenser shell. 　　In addition, when dealing with the issue of additional fluid discharge, it is necessary to make the heat load of the additional fluid discharged as evenly distributed as possible on the cooling tube bundle. 　　In particular, nuclear power plant steam turbines require 100% bypass of all steam from the desuperheater and pressure reducer to be discharged into the throat of the condenser, and all water drains during startup, operation and shutdown of the steam turbine must also be discharged into the condenser (or through a water drain expansion vessel). 　　In order to ensure the safety and reliability of additional fluid discharged into the condenser, the kinetic energy of the discharged fluid needs to be limited. The operating mode of fluid discharge should also be considered, the most important of which is to distinguish whether additional fluid is discharged into the condenser continuously or intermittently or briefly. 4. Preventive measures for corrosion on the steam side of the condenser. In order to prevent the high-speed exhaust flow of the low-pressure cylinder from carrying high-speed water droplets to impact and corrode the top row of the tube bundle (usually three rows), the cooling tubes can be thick-walled tubes with thickened tube walls. This also brings corresponding benefits to the top row cooling tubes to avoid induced vibration. As we all know, the wall thickness of the condenser cooling tube is determined by the corrosion life of the cooling tube material, not by the strength condition. 　　Regarding the corrosion of the shell part on the steam side, a baffle can usually be installed at the throat of the condenser to rearrange the shape of the high-speed steam flow. In particular, the problem of higher steam velocity caused by the lower cooling water temperature in winter cannot be ignored. 　　The impact corrosion caused by the discharge of additional fluid into the condenser is mainly through the arrangement of baffles or diverter headers. Correct design of the position and size of the baffle can avoid damage to the cooling tube after additional fluid rushes out. The maximum airflow velocity entering the baffle area should be calculated correctly to check the accuracy of the baffle design. 　　Correctly calculating the formula for avoiding vibration of cooling tubes can determine the minimum spacing between the condenser middle tube plates. However, in the design of the condenser, great attention should be paid to the arrangement of the cooling tube bundle. The distribution of water droplets in the steam flow is very important for the corrosion of the cooling pipe, but for the forced vibration of the cooling pipe, the steam flow distribution of the steam flow is more important. 5 Conclusion In summary, to prevent and reduce corrosion on the steam side of the cooling pipe, the throat structure and space permitting, the low-pressure cylinder exhaust steam, especially the flow rate at the cooling pipe on the windward side of the top row and the flow rate at the narrowest parts of the middle and side steam channels should be limited to the allowable range. Special attention should be paid to avoid the occurrence of local high speeds. At the same time, great attention is paid to the discharge of additional fluid in the throat. It is particularly important to correctly calculate and rationally adopt the baffle structure to prevent high-speed steam flow and water droplets from directly shooting into the cooling tube. It is essential to eliminate excessive cooling pipe stress, rationally organize the flow of condensate water and accurately calculate cooling pipe vibration.