Boiler water treatment
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Boiler water treatment: What is the relationship between the forms in which acid compounds exist in water and the pH value of the water? Answer: Carbonates exist in water in several different forms: gases dissolved in water (i.e., so-called free CO2) ； Molecular carbonic acid H2CO3 ； Bicarbonate HCO3- and carbonate CO32-. There is the following equilibrium relationship among these four: CO2 + H2O ⇌ H2CO3; H2CO3 ⇌ H+ + HCO3-; HCO3- ⇌ H+ + CO32-. If these equilibrium equations are combined, they can be written as follows: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- ⇌ 2H+ + CO32-. In this series of equilibria, the equilibrium between CO2 and H2CO3 actually tends strongly toward the formation of CO2; the amount of H2CO3 present in water is very small (usually less than 1%), so the formation of H2CO3 can be ignored, and the equilibrium equation can be rewritten as follows: (At 25°C, K1 = 4.45×10-7); (At 25°C, K1 = 4.69×10-11). Based on this, it can be seen that certain factors play a decisive role in shifting the equilibrium. The relative values and concentration relationships of CO2, HCO32-, and CO32- in water are shown in Figure 4-3. As can be seen from Figure 4-3: (1) when pH < 4, only free CO2 is present in water. (2) As the pH value increases, the equilibrium shifts; it decreases or increases. When pH is between 8.3 and 8.4, over 98% of the carbonates exist in the HCO3- form. (3) As the pH value increases further (above 8.3), CO2 disappears; it decreases and then increases again. When pH = 12, carbonates in water exist almost entirely in the form of CO32-. What is the relationship between the form in which silicate compounds exist in water and the pH value of the water? Answer: Silicate compounds are one of the main impurities in natural water; they exist in various forms in water and are relatively complex compounds. The different forms of silicate compounds in water are closely related to the pH value of the water. When pH < 7, there are actually only silicic acid molecules in water, with no silicate ions present. Therefore, when the pH is low (in acidic solutions), the amount of colloidal silicic acid in water increases significantly ； When the pH value is >7, both H2SiO3 and HSiO3- are present in the water ； At pH=11, HSiO3- is the predominant form in water ； The SiO32- ion appears only in water with a high alkalinity (pH > 11). What are the requirements for the pH value of water in coagulation treatment? Answer: The pH value of water has a significant impact on the coagulation process, and different coagulants have varying requirements regarding the pH value of water. Therefore, it is essential to strictly control the pH value of water after adding the coagulants during coagulation treatment. The pH requirements for different coagulants are as follows: (1) Alum salts. Aluminum salts, upon ionization and hydrolysis in water, produce aluminum hydroxide colloids; the pH value has two effects on these colloids. Firstly, aluminum hydroxide is an amphoteric hydroxide; when the pH of water is below 5.5, it becomes basic and dissolves, as shown in the following reaction: Al(OH)3 + 3H+ → Al3+ + 3H2O. As a result of this reaction, the amount of aluminum remaining in the water increases. When the pH of water is higher than 7.5, aluminum hydroxide becomes acidic, and aluminate ions (AlO2-) appear in the water. The reaction is as follows: AI(OH)3 + OH– → AlO2– + 2H2O. As a result of this reaction, the amount of aluminum remaining in the water also increases. Therefore, it fails to produce AI(OH)2 flocs. Secondly, when the pH of water is between 5.5 and 8.8, aluminum hydroxide colloidal particles carry a positive charge. When the pH of water is <5, colloidal particles carry a negative charge ； Aluminum hydroxide dissolves when the pH of water is >8. Therefore, when the pH of water is higher than 8.0 or lower than 5.0, it affects the formation of positively charged aluminum hydroxide colloids; hence, when aluminum salts are used as coagulants, the pH of water should be between 6.5 and 7.5. (2) Iron salts. Iron salts ionize and hydrolyze in water to produce positively charged iron hydroxide colloids; the reaction is as follows: 4FeSC₄ + 10H₂O + O₂ → 4Fe(OH)₃ + 4H₂SO₄. During this reaction, Fe²⁺ is easily oxidized to Fe³⁺ at a pH greater than 8.5, thereby forming Fe(OH)₃ colloids ； At lower pH values, the rate of completing the aforementioned reaction is slow. Therefore, when using iron salts as coagulants for coagulation treatment, it is generally carried out together with lime treatment to maintain the water’s pH value between 8.5 and 10. Due to differences in raw water quality and the coagulants used, the most appropriate pH value should be determined through small-scale tests. What are the advantages of using polyaluminum as a coagulant? Answer: Polyaluminum coagulants have the following advantages: (1) Wide range of applications. It exhibits excellent coagulation effects on water with low turbidity, high turbidity, color, and certain industrial wastewater. (2) Low dosage (based on Al2O3). For water with low turbidity, the dosage is equivalent to half that of aluminum sulfate ； For water with high turbidity, the dosage can be reduced to 1/3 to 1/4 of that of aluminum sulfate. (3) Simple to operate. After chemical addition, the alkalinity of the water decreases only slightly; as a result, the pH value of the water also drops only modestly. The optimal pH range for coagulation is wide, and good results can generally be achieved at pH values between 7 and 8, with stable performance even at low temperatures. (4) The flocculation formation speed is fast. Since this agent forms flocs quickly, it allows for a reduction in the size of the clarification equipment. (5) Adding too much chemical is also harmless and will not deteriorate water quality. What is the coefficient of uniformity of the filter media? How does its value affect the operation of the filter? Answer: The coefficient of uniformity of the filter media is usually expressed as KB. It refers to the ratio of the sieve pore size d80, through which 80% (by mass) of the filter media can pass, to the sieve pore size d10, through which 10% of the filter media can pass. In other words, the uneven size of the filter media particles has two negative consequences: first, backwashing becomes difficult, as excessive backwashing intensity can carry away the smaller filter media particles from the upper layer ； And if the backwashing intensity is too low, it cannot loosen the lower filter layer. Secondly, the filtration performance deteriorates as fine filter media particles accumulate on the surface of the filter layer, causing suspended solids in the water to be trapped and piled up there, forming a thick, solid film. As a result, the head loss in the filter increases rapidly, and the filtration cycle is shortened. What are the reasons for the difficulty in adding acid and alkali during the regeneration of a counter-current regeneration ion exchanger? How should this be addressed? Answer: The reasons for the difficulty in adding acid and alkali during the regeneration of a counter-current regeneration ion exchanger may be: (1) Excessively high back pressure inside the ion exchanger. (2) The acid and alkali discharge devices are blocked. (3) During the regeneration of the ion exchanger, the valve inside the unit fails, causing the regeneration fluid to flow into another exchanger. (4) The ejector for adding acid or alkali is damaged, or the water pressure at the inlet is too low while the water pressure at the outlet is low. The treatment method is as follows: (l) The ion exchanger maintains a certain back pressure (>0.05 MPa). (2) When the acid and alkali discharge devices as well as the acid and alkali injection nozzles are damaged, they should be repaired or replaced promptly. (3) During the regeneration process, carefully check the opening and closing status of the valves in each ion exchanger to prevent malfunctions and improper sealing. (4) Regularly clean the nylon mesh sleeves of the acid and alkali discharge devices. What are the main reasons for the decrease in the operating exchange capacity of ion exchangers during operation? Answer: The reduction in the operating exchange capacity of ion exchangers over time can be attributed to the following factors: (1) When new resin is first put into use, its operating exchange capacity is high; as operation continues, this capacity gradually decreases, and after some time it stabilizes. (2) The surface of the exchanger particles is contaminated by suspended solids, and even bonding may occur. (3) The raw water contains ions such as Fe2+, Fe3+, and Mn2+, which poison the exchanger and cause its color to darken. (4) The dose of the regenerant is low, resulting in insufficient regeneration. (5) The operating flow rate is too high. (6) The salt content and hardness in the raw water are too high during the dry season. (7) The resin layer is too low or the resin is gradually decreasing. (8) The quality of the regenerant is poor, with too many impurities. (9) Clogging or damage to the water distribution device, drainage device, and regeneration liquid distribution device causes flow deviation. (10) During the backwashing of the ion exchanger, if the backwashing intensity is insufficient, a large amount of suspended solids accumulate in the resin layer and bind together with the resin to form mud balls or mud cakes, causing water to flow in a deviated direction. Why do counter-current regeneration ion exchangers fail to function properly shortly after being put into operation following regeneration? How should this be addressed? Answer: The reasons for this phenomenon may include: (1) Problems with the regeneration process, such as insufficient backpressure leading to disordered layering of the resin. (2) The flow rate of the regeneration liquid is too high, causing disorder in the resin layers. (3) The lipid compression layer thins, causing deviation in the regeneration fluid and the backpressure fluid. The handling methods are as follows: (1) Strengthen training in regeneration operations to master the relevant techniques correctly and proficiently. (2) Adjust the flow rate of the regeneration liquid. (3) Resin (or white ball) to supplement the fat layer. (4) Perform a heavy backwash. Why does the water quality of the effluent from a counter-current regeneration ion exchanger deteriorate, or why is its operating cycle significantly shortened? How should this be addressed? Answer: The reasons for this phenomenon may be: (l) During the regeneration process, demineralized water (or softened water) is not used as the water for displacement or backwashing, which results in the lower resin layer becoming ineffective; as a result, Na+ (or hardness ions, or HSiO3- ions) continue to leak out at the start of operation. (2) The pressure of the backpressure fluid is too high, affecting the amount of regenerating fluid that can enter. The treatment method is as follows: (1) It is necessary to use demineralized water (or softened water) for backwashing or flushing. (2) Adjust the top pressure device and check the top pressure gauge. What are the reasons for the decreased exchange capacity of floating-bed ion exchangers? How to address it? Answer: The reasons for this phenomenon in floating beds may be: (1) During regeneration, the resin at the top of the exchanger is exposed to air, which affects the effectiveness of regeneration. (2) When the water outlet device and the regenerating liquid inlet device are shared, the nylon mesh covering the surface becomes partially blocked by broken resin, resulting in uneven distribution of the regenerating liquid. The treatment method is as follows: (l) Modify the discharge pipe for recycled waste liquid into an inverted U-shaped tube. (2) Remove the resin from inside the system, perform external backwashing, and inspect and maintain the water outlet device. Why does the outlet flow resistance of a floating-bed ion exchanger increase, or even no water flow occur? How should it be dealt with? Answer: The reasons for this phenomenon in a floating bed may be: (1) An increase in broken resin and suspended particles within the resin layer. (2) The nylon mesh of the water outlet device is damaged, causing a large amount of resin to accumulate in the resin trap and obstructing the water flow. The treatment method is as follows: (1) Remove the resin from within the body and perform external backwashing. (2) Service the water outlet device. (3) Drain the resin from the catcher. What are the reasons why a floating-bed ion exchanger fails to function shortly after being restarted after regeneration? How should it be dealt with? Answer: The possible reasons for this phenomenon in the floating bed are: (1) At the start of operation, the water inlet pressure is low, resulting in the resin not forming a proper layer and becoming disordered. (2) The resin in the exchanger was not properly packed naturally, the water cushion layer was too high, and the resin layers were disordered. The handling method is as follows: (l) Increase the flow rate at startup. (2) Fill the resin to reduce the height of the water cushion. What are the advantages and disadvantages of using hydrochloric acid and sulfuric acid as regenerants for H-type ion exchangers? Answer: The choice of regenerant is a crucial aspect in water treatment processes, as it directly affects the exchange capacity of the exchange resin and the quality of the treated water. Hydrochloric acid, used as a regenerant for H-type ion exchangers, offers advantages such as simple operation, good quality of the effluent, high exchange capacity of the exchange resin, and no precipitation during regeneration. This is because hydrochloric acid is a monoprotic acid that dissociates easily, and a higher concentration of regeneration solution can be used, resulting in good regeneration effects. Therefore, the exchange capacity of the resin regenerated with hydrochloric acid can be nearly doubled compared to that regenerated with sulfuric acid, thereby extending the operating cycle of the exchanger, reducing the number of regeneration cycles, and saving water consumption. However, hydrochloric acid is expensive, the cost of water production is higher compared to regeneration using sulfuric acid, acid storage equipment and related systems require corrosion protection, which involves significant costs, and measures must also be taken to prevent acid mist from contaminating the environment. The water production cost is low when using sulfuric acid to regenerate H-type exchangers; concentrated sulfuric acid does not corrode steel, so ordinary steel containers can be used for storage, which saves on investment costs. However, sulfuric acid is a dibasic acid with low activity and incomplete ionization, resulting in poor performance of the regenerated resin. The exchange capacity of the regenerated resin is only half that of regeneration with hydrochloric acid; it consumes a large amount of water and results in poor quality effluent. Furthermore, when regenerating sulfuric acid resin, precipitates such as calcium sulfate are easily formed, making the regeneration process cumbersome (step-by-step regeneration is required). To ensure the safety of equipment and personnel, safety measures are in place to prevent water from flowing back into the equipment containing concentrated sulfuric acid. The nozzles and mixing tubes of the acid ejectors must be made of materials resistant to high temperatures and corrosion, such as polytetrafluoroethylene and aluminum-antimony alloys. Currently, power plants mostly use hydrochloric acid to regenerate H-type ion exchangers; based on a technical and economic comparison, hydrochloric acid is considered more advantageous than sulfuric acid. In water chemical desalination systems, why are anion exchangers placed after cation exchangers? Answer: In chemical desalination systems, anion exchangers are placed after cation exchangers for the following reasons: (1) After passing through the cation exchanger, the effluent becomes acidic, which facilitates the exchange reaction in the anion exchanger and results in high efficiency in silicon removal. (2) When raw water enters the anion exchanger directly for exchange, insoluble compounds are formed, which block the cross-linked pores within the exchange resin and reduce its exchange capacity. (3) There is a large amount of carbonate in the raw water; it can be decomposed into H2O and CO2 through an cation exchanger. By removing CO2 with a decarburizer, the total amount of anions entering the anion exchanger is reduced, thereby extending the operating cycle of the anion exchanger and lowering the consumption of regenerants. (4) Anion exchange resins have a weaker ability to resist contamination by organic substances and other factors compared to cation exchange resins; therefore, they should not be used directly with raw water. What is the impact of water pH on silicon removal by anion exchangers? Answer: The pH level of the water has a direct effect on the efficiency of silicon removal. A low pH value of water facilitates silicon removal, as silicon in such water exists in the form of silicic acid. The ion exchange reaction is as follows: R-OH + H2SiO3 → R-HSiO3 + H2O. A high pH value makes it difficult to remove silicon, because at high pH levels silicon exists in the form of silicates, and the counterion OH- is generated. The higher the concentration of this OH- counterion, the greater its inhibitory effect on silicon removal. The reaction is as follows: R-OH + NaHSiO3 → R-HSiO3 + NaOH. The reverse reaction of this process occurs much more rapidly than the forward reaction, so the concentration of HSiO3- in the water remains high. What are the characteristics of a fixed-bed counter-current regeneration ion exchanger? Answer: The main characteristic of a fixed-bed counter-current regeneration ion exchanger is that the direction of water flow during operation is opposite to the direction of the regeneration fluid during regeneration; it generally operates in a forward-flow manner while being regenerated in a counter-current fashion. During countercurrent regeneration, the fresh regenerant fluid first contacts the resin that has less degradation, flowing from the bottom to the top of the exchanger, while the lower-quality regenerant fluid contacts the resin in the upper layers that has more degradation. Based on the ion balance in the solution, the regeneration liquid can be utilized effectively either at the lower or upper part, thereby **significantly improving the resin’s regeneration efficiency and the economic viability of regeneration. When operating in the forward flow direction, the water to be treated enters from the top of the exchanger, first coming into contact with the resin whose regeneration degree is lower. As the water flows downward, the amount of ions that need to be exchanged in the water gradually decreases, while the regeneration degree of the resin the water comes into contact with increases. According to the equilibrium relationship of ion exchange, the higher the regeneration degree of the resin in the protective layer, the greater the purity of the effluent. Therefore, the ion exchange resin in a fixed-bed counter-current regeneration ion exchanger achieves a high degree of regeneration, consumes less regenerant, and produces water of good quality. What are the requirements for the concentration and flow rate of the regeneration fluid during counter-current regeneration of ion exchangers? Answer: The selection of concentration is based on the effectiveness of regeneration; the optimal concentration is determined through adjustment experiments taking into account factors such as water quality. Generally, when hydrochloric acid is used as the regenerant in cation exchangers, the regeneration concentration is mostly in the range of 2% to 5%, but lower concentrations are also used in some cases ； When regenerating anions, a NaOH solution at a concentration of 0.5% to 2.5% is generally used, and it yields good results. The flow rate of the regeneration liquid during regeneration is generally in the range of 4 to 6 m/h. Excessively high flow rates can cause turbulent layers, disrupting the regeneration process ； If the flow rate is too low, the regeneration time will be too long, and the results may not be good. What are the requirements for the water used for backwashing in counter-current regeneration ion exchangers? Why? Answer: The exchange resin at the bottom of counter-current regeneration ion exchangers is generally fully regenerated, with a resin regeneration degree of nearly 100%. If water with a high salt content is used for backwashing, the cationic (or anionic) ions in the backwash water are exchanged and adsorbed by the resin at the bottom layer; during operation, these ions are released again, affecting the quality of the effluent water. Therefore, desalinated water (or softened water) is the best choice for backwash water. What issues should be considered during the storage and preservation of ion exchange resins? Answer: The following aspects need to be taken into account when storing and preserving ion exchange resins: (1) During long-term storage, the resins should be converted into their neutral salt form, washed with pure water, and then stored properly. (2) To prevent the resin from cracking during drying, it is best to soak it in boiled water. The water used for soaking the resin should be changed regularly to avoid bacterial growth that could contaminate the resin. (3) Once the resin has been dehydrated, it must not be soaked in plain water; instead, it can be soaked in saturated saltwater, after which the salt solution should be gradually diluted to allow the resin to expand slowly. Once restored, the resin can then be soaked in boiled water. (4) The storage temperature of the resin should not be too high; it is generally between 5 and 20°C, with a maximum of 40°C. (5) During storage, the resin should be kept away from substances that can easily contaminate it, such as rust, strong oxidizing agents, organic compounds, and oils. How to identify different ion exchange resins? Answer: Take 2 mL of the resin and place it in a 30 mL test tube. Add 5 mL of 1 mol/L HCl solution, shake for 1–2 minutes, and use a pipette to remove the clear liquid from the top. Repeat this process 2–3 times, then wash the resin with distilled water 2–3 times. After that, add 4–5 mL of 10% CuSO₄ solution, shake for 1 minute, discard the liquid remaining on top, and rinse again with distilled water 2–3 times. If the resin turns light green, add another 2 mL of 5 mol/L NH3·H2O and shake for 1 minute. If the resin turns dark blue, it is a strongly acidic resin ； If it remains light green, it is a weakly acidic resin. If the resin does not change color after the above treatment, add 5 mL of 1 mol/L NaOH solution, shake for 1 minute, wash it 2–3 times with distilled water, then add 5 drops of phenolphthalein solution and shake for 1 minute. If the resin is red, it is a strongly basic resin. If the resin does not change color, add 5 mL of 1 mol/L HCl solution, shake for 1 minute, wash it 2–3 times with distilled water, then add 5 drops of methyl red solution and shake for 1 minute. If the resin turns peach-colored, it is a weakly basic resin. After the above treatment, if the resin does not change color, it indicates that the resin has no ion-exchange capacity. What are the factors that affect the ion exchange rate? Answer: The main factors that influence the ion exchange rate are: ① The exchange groups of the resin ； ②Degree of crosslinking of the resin ； ③Size of resin particles ； ④Concentration of the solution ； ⑤Water temperature ； ⑥Water flow velocity ； ⑦The inherent properties of the ions being exchanged, etc. What is the specific consumption of a regenerant? What is the ratio consumption of a regenerant? Answer: The amount of regenerant required to restore the exchange capacity of 1 mole of the exchanger is called the specific consumption of the regenerant. When regenerated with table salt, it is called salt consumption. When regenerated with acid, it is called acid consumption. When regenerated with alkali, it is called alkali consumption. The formula for calculating the specific consumption is as follows: The specific consumption is the ratio of the actual amount of regenerant used to the theoretical amount required, that is, Specific Consumption = During the operation of the ion exchanger, the higher the flow rate of the incoming water, the greater the working exchange capacity of the exchange medium, and consequently the greater the amount of water that can be processed per cycle. ( ) Answer: ×. What is the working exchange capacity of an ion exchanger? What factors influence the magnitude of this working exchange capacity? Answer: During operation, the effective exchange capacity of the ion exchanger is referred to as its working exchange capacity. Factors that affect the working exchange capacity include: ① the ion concentration of the incoming water ； ②Control indicators for exchange endpoints ； ③Height of the exchange resin layer ； ④Water flow rate ； ⑤The pH value of water ； ⑥Particle size of the exchanger ； ⑦Form of the substituent group ； ⑧Whether regeneration is sufficient, etc. What are the differences in the regeneration conditions between strongly basic anion exchangers and strongly acidic cation exchangers? Why? Answer: The main differences lie in the following aspects: (1) Amount of regenerant used. Cation exchangers have a low regeneration specific consumption, while anion exchangers have a high regeneration specific consumption. (2) Regeneration solution concentration. The concentration of the regeneration solution for cation exchangers is generally 3% to 5%, while that for anion exchangers is generally 1.5% to 4%. (3) Temperature of the regeneration liquid and regeneration time. There are no requirements regarding the temperature of the regeneration fluid for cation exchangers, and the regeneration time is short, generally completing within 30–45 minutes ； The temperature of the regeneration fluid for anion exchangers is generally controlled at 40 ± 5°C, and the regeneration process takes 45 to 60 minutes to complete. The main reason for the different regeneration conditions between the two is that the exchangeable groups in strongly basic anion exchange resins have low activity, resulting in a easily compressible double layer; furthermore, HSiO3- is difficult to displace, and the replacement process occurs at a slow rate.
How to prevent corrosion of the feed water pump? Answer: Preventing corrosion of the feed water pump mainly involves deoxygenating the feed water, adjusting its pH value, and improving the material used for the pump. The specific measures are as follows: (l) Ensure the proper operation of the thermal deaerator to improve deoxygenation efficiency, and combine this with the addition of hydrazine to the feedwater in order to completely eliminate residual dissolved oxygen in it. (2) Select the feed pump material appropriately. The guide vanes and impellers of the feed water pump are made of corrosion-resistant materials such as chromium steel (2Cr13) and stainless steel (lCr18Ni9Ti). (3) Stabilize the quality of the supply water and carry out ammonification treatment to raise the feedwater pH value to the range of 8.5–9.2. (4) Prevent air from entering the pump or avoid phenomena such as water vaporization, so as to prevent cavitation. How to prevent corrosion in the water supply system? Answer: The main factors causing corrosion in the water supply system are oxygen and carbon dioxide in the water. Therefore, preventing corrosion in the feedwater system should start with removing oxygen and carbon dioxide from the water. Currently, power plants mainly adopt the following measures: (1) Feedwater deoxygenation. Thermal deoxidation is primarily used, that is, the method of heating with steam to raise the water temperature to its boiling point at the corresponding pressure, thereby releasing the dissolved oxygen from the water. This is supplemented by chemical deoxygenation, namely the addition of hydrazine to the water, in order to completely eliminate any residual oxygen in it. (2) Ammonia treatment of feed water. By utilizing the alkalinity generated by ammonia dissolving in water, the pH value of the feedwater is increased and adjusted to remain between 8.5 and 9.2, thereby forming a stable protective film on the metal surface and preventing corrosive agents from corroding the metals in the feedwater system. Furthermore, by taking advantage of the volatility of ammonia, the pH value of the condensate water can be kept above 8, thereby preventing carbon dioxide corrosion in the condensate water system. (3) Reduce the carbonate alkalinity of the make-up water. Generally, H-Na softening of water can be used, along with the addition of acid and chemical desalination, to reduce the carbonate alkalinity in water to below 0.01 mmol/L. Why is it necessary to carry out anti-corrosion measures on boilers that are not in use? Answer: The metal surface of boilers that are not in use contains salts, scale, and deposits; when exposed to O2 and CO2 in the air, corrosion occurs. This type of corrosion is much more severe than corrosion that occurs during operation. When the economizer is in operation, it is generally prone to corrosion at its inlet section; if no anti-corrosion measures are taken for boilers that are not in use, the entire piping system will suffer from corrosion. Superheaters generally do not suffer from corrosion during operation, but corrosion may occur when they are out of use, especially in the elbow sections. The boiler water wall tubes and drum are rarely subjected to oxygen corrosion during operation, but are highly susceptible to it when the boiler is out of service. Corrosion occurs during standby periods; on one hand, this increases the amount of corrosion products in the water, and these products such as Fe2O3 and CuO act as catalysts that accelerate corrosion. This is an important cause of corrosion and scaling during operation. Therefore, special attention must be paid to corrosion prevention for boilers that are out of service. What are the basic principles for preventing corrosion in boilers that are out of service? Answer: There are many methods for preventing corrosion in such boilers, but the basic principles mainly include the following: (1) Preventing air from entering the water and steam systems of boilers that are not in use. Such as maintaining a certain steam pressure or feedwater pressure inside the boiler. (2) Keep the metal surface of the boiler equipment that is on standby fully dry. If hot, pressurized water drainage is used, drying can be achieved by utilizing the residual heat in the furnace or by using hot air from an adjacent operating boiler. Practice has shown that corrosion can be prevented when the relative humidity inside standby boiler equipment is below 20%. (3) Form a protective or adsorptive film on the metal surface that provides corrosion resistance. Corrosion protection can be achieved by using vapor-phase corrosion inhibitors such as cyclohexylamine carbonate after shutting down the furnace and draining water. (4) Immerse the metal surface in an aqueous solution containing a deoxidizer or other protective agent. Such as immersion in hydrazine or ammonia solutions. (5) Fill the standby boiler equipment with inert gas. Such as filling with high-purity nitrogen or ammonia. In fact, the aforementioned principles can be summarized as preventing electrochemical corrosion through three approaches: causing cathodic polarization by removing the cathodic depolarizer, causing anodic polarization by forming a stable protective film or adsorption film, or ensuring that there is no electrolyte solution on the metal surface. How to choose the protection method for a boiler that is out of service? Answer: The selection of the protection method for a boiler out of service should be based on specific conditions, taking into account the following key factors: ① The structural design of the boiler itself ； ②Length of downtime ； ③Temperature of the surrounding environment ； ④On-site equipment conditions ； ⑤The source and quality of water, etc. What are the causes of iron oxide scale formation? What are its characteristics? Answer: Iron oxide scale is the most common type of scale found in the water wall tubes of boilers in thermal power plants. The main reason for its formation is: excessive local heat load on the boiler’s heating surfaces ； The boiler water contains a high level of iron ； Poor boiler water circulation ； There are many corrosion products on the metal surface, etc. Iron oxide scale typically appears in a shell-like shape, with some areas showing scale-like protrusions; the surface of the scale layer is brown, while its interior and bottom are black or gray. After the scale is removed, there is a small amount of white substance on the metal surface; this white substance consists mainly of compounds of silicon, calcium, magnesium, and phosphates, with some of the scales also containing a small amount of sodium hydroxide. The most significant characteristic of iron oxide scale is that the metal surface beneath the scale layer suffers varying degrees of corrosion damage, ranging from pitting and ulcers to perforation. How to prevent iron oxide scale from forming in boilers? Answer: Preventing the formation of iron oxide scale in boilers should be approached from the following aspects: (1) Newly installed boilers must undergo chemical cleaning. Remove impurities such as scale, slag, and corrosion products from the boiler equipment. (2) Minimize the oxygen and iron content in the feed water. (3) Improve chemical dosing in the boiler and enhance boiler blowdown. (4) During unit startup, strictly monitor the water quality in the boiler water circulation system, such as by intensifying drainage and water replacement tasks. (5) Carry out anti-corrosion measures during the period when the equipment is out of use or under maintenance. Furthermore, in terms of boiler structure and operation, it is necessary to avoid excessive local thermal loads on the heating surface metals, in order to maintain normal combustion and a good water circulation condition within the boiler. How is copper scale formed on the heating surfaces of boilers? How can it be prevented? Answer: Copper scale on the heating surfaces of boilers is primarily caused by an electrochemical process in which copper oxide, which enters the boiler along with the feedwater, is reduced to metallic copper. This process is unrelated to the pressure in the boiler; it occurs mainly in areas where the heat load on the heated surfaces is too high. In such areas, the oxide layer on the metal surface is damaged, resulting in local potential differences. This causes the metal in the boiler to convert into ferrous ions in the boiler water, while the released electrons are absorbed by copper ions, leading to the formation of copper deposits on the tube walls. The amount of copper precipitated increases as the thermal load of the boiler rises. The electrochemical process is as follows: Fe → Fe2+ + 2e-, Cu2+ + 2e- → Cu. To prevent the formation of copper scale, two approaches should be taken: first, to prevent corrosion of copper components in thermal equipment as much as possible, and second, to reduce the copper content in the feed water ； Secondly, in terms of boiler operation, efforts should be made to avoid situations where the local heat load becomes too high. What is the phenomenon of \"temporary disappearance of salts\" in boiler water? What are its hazards? Answer: When the load on a drum boiler increases, certain soluble sodium salts in the boiler water precipitate out and deposit on the walls of the boiler tubes, resulting in a significant decrease in their concentration in the boiler water. When the boiler load decreases or the boiler is shut down, the sodium salts deposited on the tube walls dissolve again, causing their concentration in the boiler water to rise once more. This phenomenon is known as the \"temporary disappearance of salts,\" or also as the \"hidden salts\" phenomenon. "The hazards of the salt hiding \"phenomenon\" are similar to those of scale, and they include the following: (l) It can react with other deposits on the furnace tubes, such as metal corrosion products and silicon compounds, to form insoluble scale. (2) Poor heat transfer performance can lead to overheating, deformation, and even explosion of the furnace tube metal. (3) It can cause corrosion of metals beneath the sediment. How to prevent the phenomenon of \"temporary disappearance of salts\" in boiler water? Answer: To prevent this phenomenon, the following measures should generally be taken: (1) Improve the combustion conditions in the boiler so that the heat load on each section of the boiler tubes is uniform ； Prevent coking and slagging inside the furnace, and avoid excessive local heat load on the furnace tubes. (2) Improve the flow conditions of the boiler water inside the boiler tubes to ensure the proper operation of the water circulation. For example, remove the horizontal evaporation tube and increase the inclination of the furnace tube to 15°C to over 30°C. (3) Improve the chemical dosing process in the boiler to limit the phosphate content in the boiler water. Such as low-phosphate treatment or pure-phosphate treatment, etc. (4) Reduce deposits inside the boiler tubes and improve their cleanliness, etc. What is a corrosion inhibitor? What are its characteristics? Answer: During the acid cleaning of boilers, adding a small amount of a certain chemical substance to the acid cleaning solution can prevent or reduce the corrosion of metals by that acid solution; such a substance is called a corrosion inhibitor. The characteristics of corrosion inhibitors are as follows: (l) An extremely small amount is required (a few parts per thousand or parts per ten thousand on a dry basis) to **significantly reduce the corrosion rate of acids on metals ； (2) It will not reduce the ability of the pickling solution to remove deposits ； (3) Its ability to inhibit corrosion does not decrease over time as cleaning proceeds ； (4) It has no effect on the mechanical properties or microstructure of the metal ； (5) Non-toxic, safe and convenient to use ； (6) The waste liquid discharged after cleaning will not cause environmental pollution or public hazards. Why do corrosion inhibitors work to slow down corrosion? How should corrosion inhibitors be chosen during pickling? Answer: There are two main reasons why corrosion inhibitors can reduce corrosion: (1) The molecules of the corrosion inhibitor adsorb onto the metal surface, forming a thin protective film that prevents corrosion. (2) The corrosion inhibitor reacts with other ions on the metal surface or in the solution; the products of this reaction cover the metal surface, thereby suppressing corrosion. Determining the type of corrosion inhibitor and its dosage during pickling is related to the type and concentration of the cleaning agent, as well as the cleaning temperature and flow rate, since each corrosion inhibitor has its own suitable range of temperature and flow rate. The effect of corrosion inhibitors in reducing the corrosion rate generally decreases as the temperature of the cleaning solution rises and the flow rate increases. Due to various factors, the selection of a corrosion inhibitor should be determined through small-scale tests. Why does the \"copper plating\" phenomenon occur during the acid cleaning of operating boilers? What are its hazards? How can it be eliminated? Answer: During the acid cleaning of operating boilers, if the sediment inside the boiler contains a high amount of copper, the acid solution reacts with this copper-rich sediment according to the following reactions: Fe – 2e → Fe2+, Cu2+ + 2e → Cu. As a result of these reactions, the steel is corroded, and copper deposits on the surface of the steel, causing an uneven coating of metallic copper on that surface. Due to the different electrode potentials of copper and iron, when they come into contact, a corrosion cell is formed, which causes severe corrosion of the metal being cleaned. To eliminate the \"copper plating\" phenomenon during the pickling process, the following measures can be taken: (1) When the CuO content in the deposits inside the boiler is below 5%, a masking agent can be added to the cleaning solution to remove copper ； (2) When the content of CuO in the deposits inside the boiler is greater than 5%, an ammonia washing step must be considered during the pickling process; copper ions form stable copper-ammonia complex ions in ammonia water, thereby preventing the occurrence of \"copper plating\". Why is it necessary to rinse with a dilute citric acid solution after the boiler has been pickled? The answer is that rinsing with citric acid serves to take advantage of its ability to form complexes with iron ions, thereby removing any residual iron ions in the pickling system as well as any secondary rust that may form during the rinsing process after pickling, thus creating more favorable conditions for the passivation treatment. Additionally, it can also shorten the rinsing time after pickling, reducing water consumption. What is corrosion beneath boiler deposits? How can it be prevented? Answer: When scale, sludge, or metal corrosion products accumulate on the metal surface inside the boiler, severe corrosion occurs beneath them; this type of corrosion is known as corrosion beneath boiler deposits. This type of corrosion is related to the local concentration of boiler water, and therefore is also known as media concentration corrosion. To prevent such corrosion, the following measures are generally taken: (1) Necessary chemical cleaning should be carried out on both newly installed boilers and those that have been in operation. (2) Carry out anti-corrosion measures for the water supply system to reduce the copper and iron content in the water. (3) Carry out anti-corrosion measures for the boilers that are out of service to prevent corrosion within the boilers during that period. (4) Improve the quality of the feedwater to minimize the corrosive components carried into the boiler. (5) Choose a suitable method for treating water inside the boiler in order to adjust the quality of the boiler water and eliminate or reduce the erosive impurities present in it. What is coordinated pH-phosphate treatment? Answer: Coordinated pH-phosphate treatment is a strict yet rational method for regulating water quality inside the boiler. It not only prevents the formation of calcium and magnesium scale but also prevents corrosion of the boiler tubes. This treatment essentially involves adding different proportions of phosphates—namely trisodium phosphate and disodium hydrogen phosphate (or sodium dihydrogen phosphate)—to the boiler, depending on the hardness and alkalinity of the feed water. Sodium dihydrogen phosphate or sodium hydrogen phosphate is added mainly to neutralize the free sodium hydroxide that enters the boiler water along with the feedwater. The reaction is as follows: Na2HPO4 + NaOH → Na3PO4 + H2O. Sodium triphosphate, on the other hand, can establish a hydrolysis equilibrium in water according to the following equation: Na3PO4 + H2O ⇌ Na2HPO4 + NaOH. Therefore, the addition of sodium triphosphate not only helps to maintain a certain level of excess phosphates in the boiler water, but also enables the generation of a certain amount of sodium hydroxide through its hydrolysis, thus helping to maintain the pH value of the boiler water. When local evaporation and concentration occur in the boiler water, the hydrolysis equilibrium shifts toward the formation of trisodium phosphate, which prevents sodium hydroxide from concentrating to levels that could be harmful to metals. Even under high heat loads, this mechanism helps to avoid corrosion of the metals by concentrated alkalis. What is the blowdown rate of a boiler? How is it calculated? Answer: The percentage of the boiler’s blowdown volume per unit of time relative to the boiler’s evaporation volume is called the boiler’s blowdown rate. That is, P = formula, where DP represents the amount of wastewater discharged by the boiler, in t/h ； D--Boiler evaporation rate, t/h ； P--Boiler blowdown rate, %. The calculation of the boiler’s blowdown rate is generally not done using the formula above; instead, it is calculated based on the principle that the amount of salt brought into the boiler by the feedwater equals the sum of the amount of salt removed by the boiler’s blowdown and the amount of salt carried away by the saturated steam (i.e., the principle of salt balance). The derived calculation formula is as follows: p = – × 100%, where P represents the salt content or silicon content in the feed water, in mg/L ； P-furnace -- Salt content or silicon content in the furnace water, mg/L ； Pv--Salt content or silicon content in saturated steam, mg/L ； P--Boiler blowdown rate, %. What is the impact of the salt content in boiler water on steam quality? Answer: As long as the salt content in boiler water does not exceed a certain level, it has little effect on steam quality; however, once the salt content exceeds that level, its impact on steam quality increases significantly. (1) As the salt content in the boiler water increases, its viscosity rises, making it difficult for the water bubbles in the water layer to combine into larger bubbles. As a result, the drum water chamber is filled with small bubbles, and these small bubbles rise more slowly through the water. This leads to increased water level expansion and a reduced height of the steam space, which hinders the separation of steam from water. (2) When the impurity content in the boiler water increases to a certain level, a foam layer forms at the interface between steam and water. This foam layer reduces the height of the steam space, thereby affecting steam-water separation. If the foam layer is too thick, steam can carry away the foam, resulting in a large amount of water being carried along with the steam. When the salt content in the boiler water increases to a certain level, both of these factors degrade the efficiency of steam-water separation, resulting in a large amount of water being carried in the steam and a sharp increase in its salt content. What is the impact of the boiler’s operating conditions on steam quality? Answer: Operating conditions such as the boiler’s load, the rate of load change, and the water level in the drum have a significant influence on steam quality. (l) Drum water level. When the drum water level is too high, the height of the vapor space at the top of the drum inevitably decreases. This reduces the distance over which water droplets can splash to the steam outlet, hindering natural separation and resulting in an increased amount of water carried in the steam. (2) Boiler load. When the boiler load increases, the kinetic energy of the steam-water mixture rises, which in turn increases the amount and kinetic energy of the water droplets formed by mechanical impact and splashing. Additionally, as the flow rate of steam exiting the drum increases, the capacity of steam to carry moisture also increases, resulting in a higher amount of water carried by the steam. (3) Changes in the boiler’s load, water level, pressure, etc. Excessive fluctuations in the boiler’s load, water level, and pressure can also cause a large amount of water to be carried in the steam. For example, when the boiler load increases suddenly and the pressure drops abruptly, the boiling point of the water decreases, causing the water in the boiler to boil violently and generating a large number of steam bubbles. This causes the bubbles to burst, producing a large number of tiny water droplets, and it also **intensifies** the expansion of the water level. Reduce the vapor space. All of these will lead to an increase in the amount of water carried by the steam, thereby increasing its salt content. What are the rules governing the dissolution and carrying of impurities by saturated steam? Answer: The rules regarding the dissolution and carrying of impurities by saturated steam are as follows: (1) The ability of saturated steam to dissolve and carry impurities is related to the boiler pressure. The greater the pressure, the stronger the dissolving and carrying capacity. (2) Saturated steam has selectivity in dissolving and carrying impurities. Saturation steam has different dissolving capacities for various substances. For the substances commonly found in boiler water, they can be divided into three categories based on their degree of solubility in saturation steam: The first category includes silicic acids (such as H2SiO2, H2Si2O3, H4SiO4, etc.), which have the highest solubility ； The second category includes substances such as NaCl and NaOH, which have a much lower solubility than silicic acid ； The third category includes substances such as Na2SO4, Na3PO4, and Na2SiO3, which are difficult to dissolve in saturated steam. (3) The dissolution carryover increases with increasing pressure. Because as the saturated vapor pressure increases, the vapor density also increases, and the amount of various substances that can dissolve in it also increases. (4) The dissolution and transport of saturated steam in silicon compounds have specific characteristics. The forms of silicon compounds in boiler water include dissolved silicates and silicic acid in solution. Saturated steam mainly carries silicic acid in solution, having little capacity to dissolve silicates. How are the salt deposits distributed within the superheater? Answer: The various impurities carried by saturated steam deposit in the superheater as follows: (l) Na2SO4 and Na3PO4. The higher the temperature, the lower the solubility of these impurities; therefore, they deposit in the superheater (or are carried to the turbine by steam as solid particles). (2) NaOH. The higher the temperature, the greater the solubility; therefore, it moves as a concentrated liquid droplet stream toward the turbine. However, concentrated NaOH droplets also adhere to the walls of the superheater tubes, where they react with CO2 to form Na2CO3, which then deposits within the superheater. (3) NaCl. When the pressure is greater than 9.8 MPa, its solubility is very high, and it often dissolves in superheated steam and is carried to the turbine. (4) H2SiO3 or H4SiO4. Both lose water to become SiO2, and SiO2 has a high solubility in superheated steam; it is generally carried to the turbine. Therefore, the deposition of salts in the superheaters can be summarized as follows: (l) The deposits in the superheaters of medium and low-pressure boilers are mainly sodium compounds (such as Na2SO4, Na3PO4, Na2CO3, and NaCl). (2) The deposits in the superheater of high-pressure boilers are mainly Na2SO4 and Na3PO4, with very low contents of other sodium salts. (3) The amount of salt deposits in the superheater of ultra-high pressure boilers is very small. What are the reasons for the formation of deposits in turbines? What are their characteristics? Answer: The reasons for the formation of deposits in turbines are as follows: (l) As superheated steam does work within the turbine, its pressure and temperature gradually decrease; consequently, the solubility of sodium compounds and silicic acids in the steam also decreases, leading to their deposition within the turbine. (2) Tiny concentrated NaOH droplets in the steam, along with some solid particles, adhere to the steam flow passages of the turbine, forming deposits. The deposition characteristics of various impurities within the turbine are as follows: (l) Sodium compounds deposit in the high-pressure section of the turbine. (2) Silicic acid dehydrates to form quartz crystals, which deposit in the medium and low pressure sections of the steam turbine. (3) Iron oxides can deposit on the blades of all stages of the turbine. How is the distribution of salt deposits inside the turbine? Answer: The distribution of salt deposits within the turbine is as follows: ① The amount of deposits varies across different stages ； ②The chemical composition of sediments varies among different grades ； ③The distribution is uneven on the baffles and impellers at all levels ； ④The amount of sediment in heating units and units that are started and stopped frequently is very low. What specific measures should be taken to obtain clean steam? Answer: To obtain clean steam, the following measures must be taken: (l) Minimize impurities entering the boiler water. Specific measures include: ① Improving the quality of feedwater ； ②Reduce the feedwater rate ； ③Prevent corrosion in the water supply system ； ④Perform chemical cleaning of the boiler in a timely manner. (2) Strengthen the boiler’s blowdown. Ensure proper continuous and periodic discharge of waste. (3) Improve the internal equipment of the steam drum. This includes improvements to the soda separation device and the steam cleaning device. (4) Adjust the operating conditions of the boiler. This includes adjusting the boiler load, drum water level, as well as the pressure and temperature of saturated steam, avoiding too rapid changes in operating parameters, and reducing the salt content in the boiler water. What are thermochemical tests for drum boilers? What is the purpose of thermochemical tests? Answer: Thermochemical tests involve adjusting the salt content in the boiler water and varying the operating parameters of the boiler, in order to determine appropriate standards for the quality of the boiler water and ensure a boiler operation mode that guarantees good steam quality. In other words, they are specialized tests conducted on the characteristics of the boiler and the quality of the water. The purpose of thermochimical tests is to determine, through experiments, the relationships between water quality, steam quality, the thermal processes of the boiler, the operating conditions of the boiler, and the characteristics of the equipment ； Under conditions of good steam quality, determine the water quality standards for boiler water and the optimal operating conditions for the boiler.