1. Overview Currently, most power plants in China use traditional deaerators to deoxygenate feedwater. Various textbooks and materials primarily introduce the principles, usage, and maintenance of traditional deaerators. With the emergence of some drawbacks in traditional deaerators, researchers have developed a new type of built-in deaerator and applied it in power plants. Although some problems still exist, this deaerator has a novel structure, fast heating speed, and good deoxygenation effect. With proper use and maintenance, it remains an excellent deaerator. 2. Principle of Built-in Deaerator 2.1 Problems with Traditional Deaerators The so-called traditional deaerator is the commonly used high-pressure spray packing (or water film) deaerator, generally including vertical single-head deaerators, vertical double-head deaerators, and horizontal double-head deaerators (see Figure 1). These types of deaerators require holes with a diameter of 1600–2400 mm to be drilled in the feedwater tank, which is 40%–80% of the feedwater tank diameter. This exceeds the requirements of GB150-1998 "Steel Pressure Vessels," weakening the strength and rigidity of the feedwater tank. This results in high local stress and deformation at the connection between the deaerator head and the feedwater tank, causing cracks inside the tank, especially in the weld area, threatening the safe operation of the deaerator. Although these cracks are related to many factors, large-diameter openings are a significant cause of cracking in deaerators. 2.2 Structural Characteristics of Built-in Deaerators Built-in deaerators are a new type of deaerator that abandons the deaerator head of traditional deaerators, retaining only the water tank section. The deaeration function of the original deaerator tower is transferred to the deaerator's water tank, integrating deaeration and water storage functions within the tank. Its advantages, besides eliminating the large-diameter opening of traditional vertical deaerators, reducing local stress and improving the safety factor of the deaerator, also include the use of new nozzles, which improves the deoxygenation effect. 2.3 Principle of Built-in Deaerator The deoxygenation principle of built-in deaerators still adopts the thermal deoxygenation principle. According to Henry's Law and Dalton's Law, the water to be deoxygenated is heated to the saturation temperature corresponding to its pressure, and the oxygen, other gases, and some vapor separated from the water are discharged from the exhaust port. 3 Structural Features of Built-in Deaerator 3.1 Use of Steam Jet Nozzle Traditional deaerators use separate nozzles to spray gas and water, and through methods such as counter-flow heating and delayed heating of packing, the water to be deoxygenated is fully heated to achieve the purpose of deoxygenation. The built-in deaerator uses a new type of composite jet nozzle (see Figure 2). As shown in the diagram, the jet nozzle consists of a shell, a jet nozzle pipe, and a nozzle head. Water and gas enter from different positions within the same nozzle. Several tangential water inlet grooves are formed on the circumferential wall of the shell. Water enters from the outside of the shell through these grooves, forming several rotating water streams. The jet nozzle pipe converts the pressure energy of the steam entering from the outside of the shell into velocity energy, resulting in a high velocity at the nozzle outlet, forming a high-speed jet stream. This high-speed jet stream, on the one hand, drives the rotating water stream forward within the shell and impacts it at the nozzle outlet, increasing the atomization power; on the other hand, this high-speed jet stream contacts the rotating water stream within the shell, advancing the gas-water heat exchange time. [ALIGN=CENTER] [/ALIGN] After leaving the nozzle, this steam diffuses outward from the center of the atomizing cone, ensuring uniform heating of the atomized water droplets. This demonstrates that initial heat exchange occurs between the air and water in the nozzle, and the atomized air and water further enhance the heat exchange effect after exiting the nozzle. Multiple sets of nozzles are arranged axially along the water tank, ensuring that the deoxygenated water receives adequate heating. Therefore, this type of jet nozzle differs significantly from the heating method of traditional deaerators. 3.2 Purging Pipe Installation The purging pipe is positioned on the water surface. Numerous purging ports are installed in the purging pipe, utilizing heated steam to disperse the oxygen layer accumulated on the water surface, increasing the oxygen concentration difference between the surface and the bottom, thus facilitating oxygen diffusion. Simultaneously, the purging steam breaks the water surface, reducing surface tension and allowing oxygen to diffuse towards the surface. The steam also flows upwards after purging, heating the spray water, the water film in the packing layer, and the atomized water sprayed from the nozzles, fully utilizing residual heat. 3.3 Foam Generator (Reboiling Pipe) A boiling main pipe and several boiling branch pipes are installed at the bottom of the deaerator, and numerous foam generators are installed on the boiling main pipe and boiling branch pipes. The foam generator has many staggered injection holes on its four walls. Heating steam is ejected from these holes and mixes with the surrounding water to form numerous bubbles, enhancing heat and mass transfer between the gas and water (see Figure 3). As can be seen from the figure, the principle of the foam generator is similar to the reboiling principle of the traditional deaerator, and the function is the same. However, due to the different internal structure, the new deaerator produces more foam, heats up faster, and has a better effect. 4. Deaerator Safety Issues Analysis and Countermeasures 4.1 Shaft Seal Steam Carryover Since the deaerator head has been eliminated, the primary and secondary deaeration processes of the deaerator are carried out in the deaerator's water tank. In particular, the injection distance of the jet nozzle is relatively long, while the air for the shaft seal is directly drawn from the top of the deaerator's water tank. If the jet nozzle is improperly arranged, too close to the shaft seal steam port, or if the gas and water coordination is abnormal during operation, resulting in poor atomization, it is very easy for the shaft seal air to carry over water (see Figure 4). Water carried over to the shaft seal poses a significant safety hazard to the normal operation of the steam turbine. Solutions include: during design and installation, positioning the nozzle assembly far from the shaft seal air supply pipe inlet to ensure it remains outside the nozzle's range; alternatively, installing a baffle plate at a certain angle between the shaft seal air supply pipe and the injection nozzle inside the deaerator (as shown in Figure 4). Even if the nozzle ejects a high-velocity working fluid, travels a long distance, or the water is not fully atomized, resulting in larger droplet diameters, the baffle plate will prevent water from entering the shaft seal air supply pipe. However, this baffle plate installation must be done during unit downtime. Another approach is to install a condensate drain bag on the shaft seal system (see Figure 5) to continuously drain the air supplied to the shaft seal, ensuring normal air supply. For some units (with shaft seal system isolation valves and switchable air sources), this allows operation without shutdown, ensuring safe unit operation. The disadvantage is that the condensate drain system requires continuous operation, resulting in significant steam and water loss. 4.2 Backflow of Air (Water) into the Extraction Pipeline Due to the unique structure of the jet nozzle, when the load is low, the extraction pressure into the deaerator is too low. Since the high-pressure heater drain and the continuous exhaust gas, used as a heating source for the deaerator, still enter, the deaerator pressure may exceed its extraction pressure. This causes cold air and water to flow back along the extraction pipeline. If the backflow valve is not tight or jammed, cold air (water) can easily enter the cylinder, seriously threatening the safe operation of the unit. Therefore, built-in deaerators have strict requirements for their sliding pressure operation range, generally within 30% to 100%. When the unit's sliding pressure operation is lower than 30% of the rated load, the deaerator's gas source must be switched in a timely manner. Monitoring the deaerator pressure, shaft seal pressure, and temperature is particularly important when the unit is stopped. If possible, the shaft seal gas source can also be switched when the unit is stopped at a constant load, simultaneously switching the corresponding gas source for the deaerator. 4.3 Increased oxygen content: Nozzle clogging, poor atomization, and high deaerator water level are the main reasons for increased oxygen content in built-in deaerators. Nozzle clogging mainly occurs after unit overhaul or condensate system maintenance. Due to rough maintenance procedures, metal or mechanical impurities enter the air-water pipelines, clogging the nozzles. Therefore, before starting after an overhaul, the nozzles should be removed and the system flushed. During condensate system maintenance, maintenance procedures must be strictly followed to prevent impurities from entering the system. Poor atomization mainly occurs during variable operating conditions when the deaerator fails to promptly stop and start the nozzle groups according to load changes, or when the air source is not switched in time. During unit start-up with sliding parameters, nozzle groups should be activated one by one as the unit load increases. During sliding shutdown, nozzle groups should be deactivated one by one according to the load to ensure normal air and water pressure and ratio, and to ensure good nozzle atomization. When the deaerator water level is too high, especially when the purge pipe is submerged, the purge efficiency decreases or fails, the oxygen concentration on the water surface increases, and oxygen escapes from the water becomes difficult. When the jet nozzle is submerged, the gas and water atomization heating fails, all of which lead to an increase in the oxygen content in the water. Therefore, the water level requirements for built-in deaerators are quite strict during operation. This is not only to consider the problem of water carryover to the shaft seal, but more importantly, to consider the normal operation of the purge pipe and nozzle. Therefore, the water level protection of the built-in deaerator should be ensured to be intact and activated in a timely manner. 4.4 Feedwater Pump Cavitation Due to the significant changes in the structure of the foam generator in the built-in deaerator, it produces far more foam than traditional reboiling. When the feedwater pump is running, if the foam generator is activated, it should not be turned on too high to prevent air bubbles from entering the feedwater pump inlet through the drain pipe, causing cavitation in the feedwater pump. A small amount of air bubbles may not be easily detected in time, but since the feedwater pumps currently in use are generally high-speed centrifugal pumps (around 5000 r/min for 200 MW units and around 6000 r/min for 300 MW units), over time this will cause erosion of the pump blades, reducing pump efficiency and shortening pump lifespan. Therefore, when starting the unit, the foam generator should ideally be used before the feedwater pump starts to quickly raise the water temperature. When using the foam generator while the feedwater pump is running, the opening of the foam generator should be appropriately controlled to prevent excessive foam production, and the operation of the feedwater pump should be monitored to ensure it is operating normally. 4.5 Whistling and Vibration: Due to the installation of the purge pipe, whistling sounds may occur inside the deaerator, which is normal. When the whistling sound is excessive, it may be due to the purge pipe inlet opening being too large, and it should be adjusted promptly. When a large amount of heat exchange occurs in the deaerator in a short period of time, it may cause vibration. Therefore, it is important to avoid excessively low inlet water temperature and excessive water flow, especially when using a water supply pump to feed water into the deaerator. The inlet water rate should be appropriately controlled, and the operation of the deaerator should be closely monitored. 5. Energy Saving Analysis of Deaerators 1) For large thermal power plants, the oxygen content of feedwater required for natural circulation boilers during normal operation is less than 7 ug/L. For once-through boilers, the quality requirements for feedwater are even higher. This necessitates that the deaerator's deoxygenation effect be more sufficient to meet the boiler feedwater requirements. Since large thermal power plants generally use thermal deaeration, from an energy-saving perspective, it is necessary to reduce the amount of heat source used to heat the deaerated water while achieving optimal deaeration effect, further reducing the amount of discharged steam, and minimizing working fluid waste to truly achieve energy saving. 2) Traditional deaerators, limited by heat transfer efficiency, require increased air intake and widened exhaust valves when the oxygen content of the deaerated water increases to maintain feedwater quality. This results in the inadvertent discharge of large amounts of steam along with the removed gases, leading to significant resource waste. Built-in deaerators, however, offer superior heat transfer and significantly improved deaeration capacity. Tests show that even with condensate (deaerated water) oxygen levels as high as 700 ug/L, feedwater oxygen levels can be maintained below 5 ug/L without increasing heating air volume or widening exhaust valves. This ensures boiler feedwater quality even under low loads and abnormally high condensate oxygen levels, achieving energy savings. This solution has been successfully applied to the 135 MW unit at Huaibei Power Plant. This article is of practical significance for operation and maintenance personnel to learn and master the principles of built-in deaerators, improve operational skills, and enhance unit safety performance.