All parameters of the boiler feed water are normal, but what is the reason for the low total alkalinity and relative alkalinity of the boiler water?
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The 20t/h low-pressure boiler, the ammonia synthesis waste boiler, and the methanol waste boiler all use the same water supply for cooling; operation has been normal up to now. However, recent abnormalities have occurred in the boiler water. The parameters of the feed water and the analysis results of the boiler water are as follows: Feed water parameters: Total alkalinity (mmol/l), Total hardness (mmol/l), Suspended solids (mg/l), Iron (mg/l), pH – 0.80, 0.001, 1.5, 0.04–0.21, 7.98–9.08. Boiler water parameters: Total alkalinity, Dissolved solids (mg/l), Phosphates (mg/l), Iron, pH, Relative alkalinity – 0.2–1.2, 760–1310, 7.67–19.46, 0.8–18.22, 3.92–9.21, 0. The wastewater from the 20t/h low-pressure boiler is now black in color, indicating corrosion has occurred. Given that the feed water analysis results appear normal, what could be the reason for the abnormalities in the boiler water of each unit? Please provide analysis and guidance.
Based on the data you provided, the pH level of the water in your boiler’s drum fluctuates, and acidic corrosion has already occurred. If this situation is not brought under control and the corrosion continues, it could lead to a boiler explosion. I think there are two reasons for this situation: 1. The pH value of the water supply is abnormal. 2. The boiler water contains certain substances that decompose in the drum, causing the water to become acidic and thus leading to corrosion. I estimate that the feed water may contain carbon dioxide; when it enters the drum, it decomposes upon heating to form carbonic acid, thereby making the boiler water acidic.
Thank you all for your help. For the treatment of feed water, we used to add trisodium phosphate to adjust pH and alkalinity, and anhydrous sodium sulfite for deoxidation. After fluctuations in the pH of the boiler water occurred during the night shift on February 28th, we adjusted the dosage of chemicals added; 2 KG of NaOH (obtained by diluting 30% alkaline solution) was added per shift. On the night shift of the 29th, the pH of the boiler water rose to 9.0, while the alkalinity remained around 2.0. On March 1, the addition of NaOH was stopped; the pH value of the boiler water was 10–11, with a relative alkalinity of around 0.1. However, from March 2nd to the present, the pH value of the boiler water has remained between 11 and 12, which is outside the specified range. A high OH level may cause alkaline corrosion or caustic embrittlement (when the relative alkalinity is below 0.2).
We have inspected the water supply source, and we encountered the same problem; the reason was that the demineralized water became contaminated in the storage tank.
Such a high iron content in the boiler water indicates that corrosion is quite severe; it is necessary to check each stage of the process, from the feedwater to the boiler, in order to identify the cause. It is possible that there are sand holes or micro-pores inside the waste heat boiler, allowing other substances to enter the boiler water and causing corrosion. In our previous hydroformic acid drum, the boiler water became acidic; it was found that some of the tubes in the synthesis tower had micro-pores or sand holes, which allowed syngas to enter the drum and thus led to the acidity. The above views are for reference only
Try to find the cause from the following aspects: 1. Is there acid leakage in the demineralized water used for utility purposes, resulting in fluctuations in pH levels? 2. Has a pipe in the boiler burst, causing a leak? 3. Is the water inside the furnace not being treated properly, such as due to abnormal dosing of chemicals or the use of the wrong chemicals?
Based on the information you provided, the acidity of your water is too high; you need to increase its alkalinity appropriately
The boiler water is indeed too acidic, which has caused corrosion.
I’m not sure if the original poster has solved this issue; I think it’s best to start by looking for the cause in the water treatment process. It would be good to describe your water treatment methods and procedures.
The pH of the feedwater is 7.98–9.08, while the pH of the boiler water is 3.92–9.21. Based on these two values alone, it can be seen that corrosion in the boiler is quite severe. If the pH of the boiler water is below 3.92 for most of the time, it is necessary to adjust the dosage of chemicals added to the feedwater and to increase the frequency of waste discharge
The reason is that the demineralized water is contaminated inside the water storage tank:$
.1 The pH value of the feed water is abnormal. 2. The boiler water contains certain substances that decompose in the drum, causing the water to become acidic and thus leading to corrosion. I estimate that the feed water may contain carbon dioxide; when it enters the drum, it decomposes upon heating to form carbonic acid, thereby making the boiler water acidic.
Obviously, this problem is caused by the presence in water of substances that hydrolyze to form acids at high temperatures (resulting in an abnormal pH level of the boiler water), while they do not hydrolyze at normal temperatures (with a normal pH level of the feed water). So you need to know your water treatment process. Some netizens have said that carbon dioxide is the cause of this. When carbon dioxide is at saturation, the pH value of water is around 5.5; however, the pH value of the water in your boiler is 3, which indicates a moderately strong acidic level. The water must therefore contain strong acids as well as weak bases and salts. I have encountered this issue many times; since I am not aware of your water treatment process, I speculate that there are three possible reasons: 1) High levels of chloride ions in the water (chlorine leakage during the regeneration of mixed-bed resins). Ammonia is added to the feed water, which is then sent to the boiler. At the feed water temperature, it exists as ammonium chloride, with an alkaline pH value. Upon reaching the boiler, hydrolysis occurs at high temperatures, resulting in the formation of hydrochloric acid, giving the solution a strongly acidic nature. 2) Resin leakage from the mixed-bed. Resin hydrolysis, acidic. 3) The water contains TOC, and it becomes acidic upon hydrolysis. Solution: 1) Improve the water treatment process to identify the cause. Especially the second reason. 2) Increase wastewater discharge and change the water. 3) Add sodium hydroxide to adjust the pH value to above 9.0. In power plants, a pH level below 8 constitutes a serious quality issue; if it isn’t resolved within a few hours, the plant has to be shut down. Petroleum and chemical companies generally do not pay enough attention to this issue; sometimes even when the cause of the problem is identified, no one believes it or takes it seriously, as they are too busy with oil refining and chemical production. This is also the main reason for the low level of boiler operation in such companies. This post was last edited by Qin Yu on 2008-4-8 21:53.]