Thread Content
SHL20 I. Requirements for acid-washed boilers: 1. Conduct simulation experiments to ensure that hydrochloric acid can effectively remove scale from such boilers. 2. The average thickness of the scale buildup in the boiler is over 0.5 mm, and the scale covers more than 80% of the heating surfaces. 3. The riveting, expansion joining, welding, and other areas of the boiler have been inspected for any leaks, and there is no severe corrosion on the pressure-bearing components. II. Preliminary work for pickling: 1. Determine the scope of pickling based on the inspection results: the upper and lower boiler drums, various header tanks, water wall tubes, and convection tube banks. 2. Material for cleaning: the upper and lower boiler drums are 20g; the convection tube bundles, as well as the various header tubes and water wall tubes, are 20 (GB3087-86). 3. Cleaning medium: 4–5% hydrochloric acid solution and 0.3% “02–corrosion inhibitor”. 4. Scale condition: (1) Nature of the scale: carbonate scale ； (2) Thickness of scale: 3 mm. 5. Boiler water capacity: 12.5 m3. III. Relevant calculations for acid cleaning and descaling of the SHL20---13/A boiler: (1) 31% industrial hydrochloric acid is used, and a 5% dilute hydrochloric acid solution is to be prepared. Amount of pure hydrochloric acid: 12500 * 5% = 625 kg. (2) Amount required for “02 – Corrosion inhibitor”: 12500 * 0.3 = 37.5 kg. Aniline and formaldehyde are used in equal amounts; thus, aniline = 18.75 kg and formaldehyde = 18.75 kg. According to the mixing ratio specified for “02 – Corrosion inhibitor”, it is 20 : 0.5 : 1 : 1. For 70100, since 18.75 kg each of formaldehyde and aniline is required, the amount of hot water needed is 18.75 * 20 = 375 kg. The amount of pure HEL required is 18.75 * 0.5 = 9.375 kg. Converting this to industrial nicotinic acid with a concentration of 31%, we get 9.375 / 31 = 30.24 kg. This amount of hydrochloric acid must be taken from the total amount of 2016 kg of hydrochloric acid available. Therefore, the total amount of acid required for pickling this boiler is 2016 – 30.24 = 1985.76 kg of 31% industrial hydrochloric acid. The amount of the “02 – Corrosion Inhibitor” solution needed is: 375 + 30.24 + 18.75 + 18.75 = 442.74 kg. The amount of cold water required to prepare the acid solution is: 12500 – 1985.76 – 442.74 = 10071.5 kg. After the pickling process, it is necessary to neutralize the remaining acid using an alkaline solution with a concentration of 0.2–0.3; 2–3 kg of NaOH per ton of water is required. Thus, the mass of NaOH to be added is 3 * 12.5 = 37.5 kg. II. Process steps: 1. Before preparing the solutions, it is necessary to retest the concentration of the purchased industrial hydrochloric acid in order to determine its exact concentration and compare it with the concentration indicated on the label. 2. Prepare a 5% dilute hydrochloric acid solution: It can be prepared in batches depending on the capacity of the corrosion-resistant container. 1985.76 kg of 31% industrial hydrochloric acid is required, along with 10071.5 kg of cold water, which are then mixed together thoroughly. 3. Prepare the “02-Corrosion Inhibitor” solution: (1) Aniline: 18.75 kg, formaldehyde: 18.75 kg ； 31% industrial hydrochloric acid, 30.24 kg ； 70 hot water, 375 kg. (2) Preparation sequence: hot water ---- 31% industrial hydrochloric acid ---- aniline ---- formaldehyde. (3) When pouring each type of chemical into hot water, it must be stirred vigorously with a stick. (4) The prepared corrosion inhibitor should be added to the prepared dilute hydrochloric acid in batches, while stirring vigorously with a stick to ensure uniform mixing. It is necessary to ensure the correct ratio between hydrochloric acid and the corrosion inhibitor in each batch. (5) The prepared solution is pumped into the boiler using an acid-resistant pump, from the drain valve (or safety valve seat or vent valve). Once the boiler is filled, the solution is returned to a corrosion-resistant container through the drain valve at the bottom, thus enabling circulation. It cycles every 0.5 to 1 hour, with each cycle lasting 1 hour. 6. Perform cyclic pickling for about 5 to 7 hours (usually not exceeding 6 hours). During the pickling process, test the concentration of the acid every 15 minutes to determine when the pickling is complete. (1) A continuous decrease in the acid concentration indicates that the descaling process is in progress. (2) If the pickling concentration drops rapidly below 0.5%, acid should be added to maintain it at 2–3%. (3) If the decrease in pickling concentration slows down, and then the concentration remains constant for a period of time, it indicates that the scale has been essentially removed, at which point pickling should be stopped. (When the difference in acid concentration between two side measurements is less than 0.2%, it indicates that the process should be essentially complete.) 7. After the pickling is complete, the acid in the tank should be immediately drained, and it should be prevented from entering the slag flushing ditch through water transfer hoses, in order to neutralize the alkalinity in the slag flushing tank. 8. Rinse the acid residue remaining in the pot with clean water for 15 to 20 minutes. 9. Fill with water to the highest level, heat to 60°C, then add 0.2~0.3% of an alkaline solution (i.e., 2~3 kg of NaOH per ton of water), and circulate for 1~2 hours without applying pressure. 10. After natural cooling, the alkaline solution is discharged into the dust removal ditch to neutralize the acidity in the dust removal water. Rinse with clean water 1–2 times. III. Precautions: (1) It is necessary to determine the nature, thickness, and extent of scale accumulation. (2) Acid cleaning for descaling must be carried out under the supervision of experienced technicians, and the acid cleaning plan should be thoroughly reviewed by the team to develop a practical and feasible approach. (3) No matter what type of corrosion inhibitor is used, it is inevitable that there will be some slight corrosion on the boiler. (The corrosion rate of 20-grade steel is 0.85 g/m2·h); therefore, pickling for descaling should not be used repeatedly. (Not more than 10 times). (4) For boilers with cracks or leakage defects in areas such as rivet seams and flared joints, these defects must be removed; otherwise, acid cleaning for descaling is not permitted. (5) During acid washing, operators must wear appropriate protective equipment (acid-resistant gloves, plastic aprons, masks, and goggles), and they should open doors and windows to prevent injury. Welding, smoking, or using open flames is strictly prohibited in the work area, to avoid casualties and other damages caused by hydrogen explosions. (6) Before acid cleaning, the boiler’s water supply lines, steam outlet, and drain pipes should be securely isolated using blind flanges. The drain valve should be opened to allow the gases generated during acid cleaning to escape properly. (7) The pickling temperature should be controlled at 5 degrees Celsius. (8) The problems encountered during the pickling process and the test data must be carefully recorded. Units lacking experience in pickling technology should entrust a professional pickling team to carry out the pickling work.