Principles of steam system control

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Principles of steam system control: Boiler feedwater treatment: To ensure the safe and reliable operation of the hydrogen production facility, it is necessary to maintain the appropriate quantity and quality of boiler feedwater. To ensure a continuous and reliable supply of boiler feedwater to the steam system and prevent interruptions in the boiler water supply, boiler feedwater pumps are installed in the feedwater system, with one in operation and one as a backup, plus an emergency boiler feedwater pump. 1. Quality of boiler feedwater: It must meet the standards specified in the regulations; otherwise, the following adverse consequences will occur: 1) Scale and sludge will form in the boiler feedwater pipes, boiler feedwater heaters, and within the boiler itself. 2) Salt scale forms in the steam passage of the superheater, reducing the plant’s production capacity and even damaging the equipment. 3) Metal corrosion occurs in water supply pipes, boilers, steam pipeline networks, condensate pipes, and other auxiliary equipment. 2. Deoxidation of boiler feed water: When thermal equipment is corroded, oxygen plays a key role in the depolarization of the cathodic part of the corrosion cell. The progress of this process is determined both by the dissociation rate of oxygen at the cathode and by the diffusion of oxygen toward the cathode. When boiler water comes into contact with iron, trace amounts of iron dissolve in the water, forming ferrous oxide. If there is no oxygen in the boiler water, the reaction quickly reaches equilibrium, preventing further corrosion of the iron. However, if there is oxygen in the water, an insoluble iron oxide is formed, and the iron in this portion of the system is corroded to form a precipitate. If oxygen continues to be supplied, the pipes and boilers will continue to erode. Therefore, the requirements for oxygen in boiler water are also quite strict; the oxygen content in the feed water after deoxygenation must be less than 0.015 ppm. 1) Deoxygenation principle: The concentration of any gas dissolved in water is proportional to the equilibrium pressure of that gas at the water surface and the absorption coefficient of the gas at the water temperature. According to Dalton’s law, this concentration can be determined using the following formula: Gas content in water = absorption coefficient × gas pressure (total pressure in the gas phase). If the concentration of a gas in the solution is higher than the concentration corresponding to its partial pressure above the solution, then the gas will gradually be released from the solution ； Conversely, the solution will continue to absorb gas. Under normal circumstances. The higher the solution temperature, the lower the solubility of the gas. The working principle of a deaerator is to heat the feed water to boiling using heat, while simultaneously allowing a certain amount of steam to be continuously vented to the atmosphere, thereby reducing the partial pressure of oxygen above the water surface to near zero and allowing the oxygen in the feed water to escape from it. Even in the best-operating deaerators, trace amounts of oxygen remain in the treated water; therefore, in addition to thermal deaeration, chemical deaeration must also be employed. That is, hydrazine is added to the water to further remove trace amounts of oxygen from it; the reaction between hydrazine and oxygen is as follows: N2H4 + O2 = 2H2O + N2. 2) Factors affecting oxygen removal: A) Temperature. As mentioned above, the feed water must be heated to boiling point and sufficient evaporation must occur in order to ensure that the oxygen partial pressure at the water surface is zero. According to Dalton’s law, since the partial pressure of oxygen is zero, the oxygen content in water is also zero; as a result, oxygen escapes from the water. The deaerator of this device operates at a pressure of 0.12 MPaA; at this pressure, the saturated temperature of water is 104°C. B) Steam-water contact surface: To improve deoxygenation efficiency, it is necessary to ensure good mixing between steam and water, thereby increasing the steam-water contact area. To this end, in the deaerator, water is evenly sprayed onto the distribution plate through nozzles located in the deaeration chamber; it is then heated by steam to undergo primary deoxygenation. Then, the deoxygenated high-temperature water is sent to the packing layer, where it is directly heated by steam from the deaerator to undergo deoxygenation. During this process, the water is thoroughly heated and stirred, and oxygen is essentially removed. Oxygen must be continuously removed from the deaerator; to this end, an excess of steam is added so that oxygen-containing steam can be continuously expelled from the upper part of the deaerator. C) The water volume fed into the deaerator must be stable. It is necessary to continuously add a certain amount of hydrazine to the water, so as to maintain a sufficient level of it in the water; the hydrazine content in the boiler feedwater should be between 0.01 and 0.05 ppm. 3. Control of the pH value in boiler feed water: The level of the pH value has a significant impact on equipment corrosion. When water comes into contact with metal, trace amounts of iron dissolve in the water to form ferrous hydroxide, and its solubility depends on the pH value of the water. The lower the pH value of water, the faster ferrous hydroxide dissolves, and the faster corrosion occurs. To maintain a certain pH level in the water, a certain amount of ammonia must be continuously added to the water supply system (at approximately 0.3 ppm) to keep the pH of the water between 8.8 and 9.2. 4. Influence of feedwater temperature on the boiler’s steam production: In addition to ensuring the reliability and quality of the feedwater, the feedwater system must also maintain the feedwater temperature within specified limits, as this temperature has a significant impact on the boiler’s steam production. Increasing the feedwater temperature will increase the steam production of the boiler. It is generally recommended that the water in the boiler feedwater preheater operate at a temperature about 20°C below the boiling point. 5. Steam purification: In addition to maintaining the specified steam parameters, this device also requires that the quality of the steam be ensured, so as to minimize or completely eliminate salt deposits on the surface of the steam superheater. Causes of steam contamination: 1) Water droplet carryover – When the vapor film breaks down during steam formation, water droplets are ejected into the steam, carrying impurities with them. 2) Foaming in the boiler water: An increase in the salt content in the boiler water causes it to foam, which in turn increases the humidity and salt content of the steam. Therefore, the salt content in the boiler water must be strictly controlled. 3) Selective carryover: Steam selective carryover is mainly caused by the solubility of impurities in the boiler water. As the vapor pressure increases, the content of non-volatile substances in the vapor increases. It should be specifically noted here regarding the presence of silicic acid in steam: silicon exists in steam in the form of silicic acid, but in furnace water it is usually present as sodium silicate, with only a portion being in the form of silicic acid. Therefore, the less silicon in water exists in the form of silicic acid and the more it exists as silicate compounds, the smaller the transport coefficient is. Therefore, the selective carryover in the steam is due to the dissolution of silicic acid in the steam. Its content increases as the steam pressure rises, and it is also related to the pH value of the boiler water, as well as the composition of the boiler water and its silicon content. Under normal conditions, as the pH value in the furnace water increases, the silica carrying coefficient decreases. When the pH value in the boiler water exceeds 13, the carryover coefficient of silicic acid is zero. Therefore, during normal operation, it is essential to pay close attention to controlling the pH value of the boiler water; otherwise, the SiO2 content in the steam will increase. The pH value of the boiler water should be strictly maintained between 9.6 and 10.3. 4) Steam separation device: The medium-pressure steam separation in this device is accomplished through a \"cyclone separator with corrugated plate box\" installed in the conversion drum, in order to obtain as pure steam as possible. 5) Water circulation issue: Water tube boilers must rely on natural circulation or forced circulation to ensure safe operation of the boiler. In a boiler, the steam-water mixture formed inside the rising tubes due to heating has a lower specific gravity than that of the water in the unheated descending tubes. As a result, water flows downward from the drum through the unheated descending tubes and then returns to the drum via the rising tubes, thus forming a circulation loop. Since the density of the water in the descending tubes is greater than that of the mixture in the rising tubes, this type of circulation is known as a positive circulation. However, sometimes for various reasons, the flow in the cycle does not proceed in the aforementioned direction, but instead flows in the opposite direction, resulting in a reverse cycle – that is, the fluid density in the rising pipe is greater than that in the descending pipe. When the fluid density in the rising pipe is equal to the water density in the descending pipe, no circulation occurs in the entire system; this is a state of circulation stagnation. When a reverse cycle or cycle stagnation occurs within the system, it affects the safety of the furnace tubes; the heating surfaces are quickly damaged, and this also has a severe impact on the quality of the steam. Therefore, it is essential to avoid the aforementioned phenomena in both boiler design and operation. 6) Addition of descaling agents: The amount of Na3PO4 added to the water in the boiler must be strictly controlled. The purpose of adding Na3PO4 to the boiler water is to prevent scaling inside the boiler. When the boiler water is at boiling point and has a high alkalinity, calcium ions in the water react with phosphate ions as follows: Ca2+ + 6PO43- + 2OH- = Ca(OH)2(PO4)6. The calcium hypophosphate that is formed is a soft type of sludge that can be easily removed through the boiler’s blowdown process, thus preventing it from accumulating inside the boiler and forming secondary scale. However, the level of PO4-3 in the boiler water should not be too high either. If it is too high, not only will more chemicals be discharged along with the wastewater, increasing the consumption of these chemicals, but it will also lead to many adverse consequences: A) It increases the salt content in the boiler feedwater, thereby affecting the quality of the steam. B) There is a possibility of forming Ca3(PO4)2. C) If the iron content in the boiler water is high, there is a possibility of the formation of iron phosphate scale. Therefore, as long as the purpose of scale prevention is achieved, it is better to keep the PO43- concentration in the boiler water low