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It is called the evaporation heating surface. A natural circulation loop is composed of a unheated downcomer, a heated upcomer, a steam drum, a lower water wall header, an upper water wall header, and steam-water outlet pipes. The drum has a large volume; its lower half is filled with water, while the upper half is the steam space. The boundary between the two is called the evaporation surface, and the water throughout the entire system is referred to as boiler water. In the rising tube, heating to the saturation temperature generates some steam, while in the descending tube it is saturated water or unsaturated water. Since the density of steam is lower than that of water, the average density of the steam-water mixture in the rising pipe is less than that of the water in the descending pipe. This density difference drives the steam-water mixture in the rising pipe upward toward the steam drum, where steam and water are separated. The separated steam is discharged from the steam drum, while the separated saturated water mixes with feed water and flows downward in the descending pipe, thus forming a steam-water cycle. The driving force for natural circulation is generated by the difference in density between the fluid in the downcomer and that in the upcomer; the driving force for a natural circulation loop is known as the motive head. The magnitude of the moving head depends on the saturation pressure of water vapor, the vapor content in the rising pipe, and the height of the circulation loop. As pressure increases, the density difference between saturated water and saturated vapor decreases, which in turn reduces the moving head. This makes it more difficult to maintain a stable water circulation. Therefore, as pressure rises, it is necessary to increase the vapor content in the rising pipe and the height of the circulation loop in order to maintain an adequate moving head. In boilers that use natural circulation, the maximum saturation vapor pressure is 19 MPa. The advantage of natural circulation is that it allows the quality of boiler water to be adjusted and maintained through continuous and periodic blowdown, thus requiring relatively low standards for the quality of feedwater. Natural circulation boilers have a large water volume and heat storage capacity, so they require a low level of automation. During boiler operation, there are many factors that affect the safe operation of the water wall, including influences from within the tubes as well as complex factors from outside the tubes. Factors inside the tubes include: (1) scaling and corrosion inside the water wall tubes caused by poor water quality ； (2) Stagnation or reverse flow of circulation in individual tubes due to the influence of heating unevenness in the water wall ； (3) Film boiling on the inner wall of the tubes caused by excessive heat load on the water wall ； (4) An excessively low drum water level leads to insufficient circulation flow in the water wall, and even more severe \"dry boiler\" conditions can occur. External factors affecting the pipe include: (1) high-temperature corrosion of the pipe wall by corrosive gases generated during combustion ； (2) Erosion of the tube wall caused by slagging and ash accumulation ； (3) Wear of the tube wall by coal powder gas flow or ash-containing gas flow. The flow of the steam-water mixture inside the water wall tubes is influenced by factors such as pressure, mass, flow velocity, mass vapor fraction, and the heat load outside the tubes. There are mainly four types of flow patterns for this steam-water mixture within the vertical tubes: bubble-shaped, slug-shaped, columnar-shaped, and mist-shaped. When the flow state of the steam-water mixture inside the water wall tubes is in the form of bubbles, vapor bubbles, or vapor columns, the heat transfer occurs in the nuclear heat transfer regime, and at this time the tube walls are safe ； In the water wall tubes in the high-heat-load areas of the furnace of high-parameter, large-capacity boilers, film boiling can sometimes occur. This happens when the steam inside the tubes comes into direct contact with the tube walls, resulting in poor heat transfer; as a consequence, the heat transfer coefficient of the fluid inside the tubes drops sharply, and the wall temperature rises suddenly, **exceeding the saturation temperature of the fluid inside the tubes, which can easily lead to tube failure. To prevent the water wall tubes from being damaged due to overheating during operation, it is necessary to avoid direct contact between steam and the inner wall of the tubes, as the heat transfer coefficient between steam and the tube interior is much lower than that between water and the tube interior. Stagnation, backflow, and film boiling will all affect the safe operation of the water wall. The cessation of water circulation actually leads to a deterioration in heat transfer in the water wall, and this phenomenon mainly occurs in the tubes that are less heated. The pressure difference in the reverse-flow tubes is greater than the average pressure difference in the same tube bank or the same circuit, which forces the working fluid to flow downward. In the tubes where reverse flow occurs, water moves downward, while bubbles move upward due to buoyancy. When the backflow velocity is low and equal to the upward movement speed of the bubbles, the water flowing downward does not carry away the bubbles, resulting in a situation where the bubbles remain neither rising nor falling. This leads to bubble plugging, deterioration of heat transfer, and localized overheating of the pipe. If the heat load outside the tube is high, on the inner wall of the tube the rate at which bubbles are formed exceeds the rate at which they detach from the wall. As a result, these bubbles accumulate on the inner wall, forming a vapor film that separates the water inside the tube from the tube wall. This prevents the tube wall from being cooled by water, leading to deteriorated heat transfer at the tube wall. The safety performance of natural circulation refers to the ability of all the risers in the circulation loop to be cooled by a continuous water film during the operation of the boiler. The relevant indicators include: (1) no stagnant nuclear backflow occurring in the tubes that are most exposed to heat ； (2) The tube subjected to the greatest heat does not experience deterioration in heat transfer ； (3) The loop repetition ratio is greater than the threshold repetition ratio, giving the cycle compensatory properties ； (4) No vortex funnel forms at the inlet of the downcomer ； (5) The flow velocities in each circulation loop and the nuclear cycle multiplication factors are all within the recommended ranges. To improve cycle safety, the following measures can be taken: (1) Reduce uneven heating ； (2) Determine the appropriate heat absorption rate of the rising tube ； (3) Determine the appropriate rise tube height and pipe diameter ； (4) Determine the appropriate height and cross-sectional area of the steam conduit ； (5) Reduce the resistance of the cyclone separator ； (6) Reduce the downcomer resistance. To reduce uneven heating, the operational requirements are: (1) limit the minimum load, as a lower load leads to increased uneven heating ； (2) Do not let the flame deviate ； (3) Prevent slagging; if slag forms, it should be removed promptly. Introduction to the water wall of the B&W B-1025/17.2-M boiler used in the second phase of Leiyang Power Plant. The steam-water circulation system of the boiler includes a drum, large-diameter downcomers, water wall tubes, as well as steam and water inlet and outlet pipes. The unboiled water from the economizer enters the four large-diameter downcomers through feed distribution pipes arranged along the length of the drum. At the lower ends of the four downcomers, a distributor is connected to the water wall inlet pipes; these inlet pipes deliver water with insufficient enthalpy to the lower collection boxes surrounding the water wall. The water is divided into 26 circulation circuits based on the heating conditions. As the boiler water flows upward through the membrane water wall, it is continuously heated, gradually forming a steam-water mixture. This mixture is then introduced into the drum through steam-water outlet pipes. Inside the drum, an axial flow cyclone separator and vertical corrugated plates are used to effectively separate the steam from the water. The separated boiler water returns to the downcomers, while the dry steam is sent through 30 connection pipes to the inlet collection box of the furnace superheater. Heating devices adjacent to the furnace are installed in the lower collectors surrounding the water wall; before ignition, steam from adjacent furnaces is directed into these 26 lower collectors of the water wall in order to increase the boiler’s pneumatic speed. The part of the rear wall water-cooled wall that protrudes forward toward the front wall in the upper section of the furnace is called the flame deflection angle. This flame deflection angle serves several functions: (1) it increases the flow path of the flue gases, thereby prolonging the residence time of the fuel in the furnace, which facilitates complete combustion of the fuel ； (2) Ensure that the flue gas enters the superheater evenly in the vertical direction at the furnace outlet, thereby improving the heat transfer in the superheater ； (3) To allow the flue gas to fill the upper part of the furnace more thoroughly, thereby increasing the heat absorption by the front and side walls. The boiler’s evaporation heating surface refers to the surface where the working fluid absorbs heat and vaporizes. The water wall is where these evaporation heating surfaces are installed; it is usually arranged along the surrounding walls of the boiler furnace and constitutes one of the main pressure-bearing components of the boiler. The water wall serves the following main functions: (1) Protecting the furnace wall by reducing the damage caused by high temperatures and slag to it ； (2) Absorb the radiant heat from the high-temperature flames in the furnace, causing water to evaporate and turn into vapor ； The allowable flue gas temperature at the furnace outlet as specified by the flue gas cooling channel. The water wall used in the boiler of Phase II of Leiyang Power Plant is a fully welded membrane-type water wall.