Industrial boiler chemical cleaning process and acceptance requirements
1. Industrial boilers refer to boilers that provide steam and hot water for industrial production or life, and generally refer to boilers with a rated working pressure of ≤2.5MPa.
2. Determination of chemical cleaning conditions: The chemical cleaning of industrial boilers includes alkaline cleaning or acid cleaning. When the boiler produces scale or rust, it should be cleaned in time, but the pickling method should meet one of the following conditions, and the pickling interval of each boiler should not be less than two years.
(1) The scale of the boiler heating area is more than 80X10-2, and the average scale thickness reaches or exceeds the following values; for boilers without superheater: 1mm. ; Boiler with superheater: 0.5mm; hot water boiler: lmm.
(2) The heating surface of the boiler is corroded seriously.
3. Cleaning system: Industrial boilers only use alkaline cleaning, and generally do not need a circulation system. When pickling is used, a combination of circulation and static soaking should be used.
4. Chemical cleaning process
(1) The chemical cleaning process of the operating boiler includes various stages such as alkali boiling conversion, water washing, water washing after pickling, rinsing, and passivation. Among them, alkali boiling conversion and rinsing can be avoided according to specific conditions.
(2) Before chemical cleaning, the sediment and dirt accumulated in the pot should be removed. If there are clogged pipes, try to remove them in advance.
(3) For scales dominated by sulphate, alkali boiling reform should be carried out, and the requirements are as follows:
(1) According to the thickness and composition of the scale, add the mixed solution of sodium carbonate and trisodium phosphate dissolved in the solution into the boiler to make the reagent concentration in the boiler water evenly reach na2co3:(0.3-0.6)X10-2na3po412h202(0.5-1.0) X10-2 (equivalent to P043-concentration 1250-2500mg/l).
②Slowly increase the boiler pressure, increase the boiler pressure to half of the rated working pressure within 5 hours, and maintain it for 36-48 hours. If the scaling is serious, the conversion time of alkaline boiling should be appropriately extended.
(3) The boiling time should be sampled and analyzed regularly. When the boiler water alkalinity is lower than 45mmol/L and the P043-concentration is lower than 1000mg/L, sodium carbonate and trisodium phosphate should be appropriately added.
(4) After the alkali boiling conversion, the lye should be drained and rinsed with water until the PH value of the effluent is less than 9.
(5) The pickling control requirements are as follows:
(1) Concentration of pickling solution: HCI or nitric acid (4-8)%, wash with hydrochloric acid, add fluoride additives: hfl%; or sodium fluoride (5-8)‰. The marinating time is 6-8 hours.
②After pickling, the end point of pickling should be determined according to the following points, and pickling should be stopped in time.
A. The acid concentration tends to be stable: after 30 minutes, the absolute difference between the two analysis results is less than 0.2*10-2.
B. The iron ion concentration tends to balance.
(6) After pickling, the waste pickling liquid in the pot should be sprayed out with clean water quickly. If the amount of flushing water is insufficient, you can also supply water while discharging the acid (the rate of discharging acid is not greater than the rate of water supply), and add an appropriate amount of lye to neutralize the acid at the end of the flush. Rinse with top-acid or deacidified neutralized water until the pH of the effluent is greater than or equal to 4.5.
(7) After pickling, it must be neutralized and passivated to prevent metal corrosion. Passivation generally adopts the following method: After washing with water, under the system cycle, add Na3PO4.10H2O to make the concentration reach (1-2)%, add NaOH to adjust the pH value, and the pH value should be controlled at 11-12. Generally, passivation at a pressure lower than the working pressure can be used. When the concentration of the passivation liquid in the boiler is uniform, close the circulation system, then light the boiler, and slowly increase the pressure to about half of the rated working pressure. For boilers with rated working pressure ≥1.6MP and rated evaporation ≥10t/h, after pickling, they should be flushed and passivated according to the requirements of power station boilers.
5. Residual scale treatment: After neutralization, all inspection holes must be opened, and the loose residual scale and sediment in the pot must be manually cleaned to prevent the scale from clogging the pipeline after it falls off.
6.Chemical monitoring during the cleaning process:
(1) Alkali cooking process: Measure the pH value, total alkalinity and PO43-concentration of the caustic lotion every 4 hours (every 1 hour when approaching the end point).
(2) Pickling process: The acid concentration in the pickling solution is measured every 30 minutes at the beginning (every 1 hour in the intermediate stage of pickling). When the concentration of Fe3- and Fe2- is close to the end point, the analysis time should be shortened.
(3) During the washing process of neutralization and deacidification water, the outlet PH value shall be measured every 15 minutes in the later period.
(4) Passivation process: Measure the pH and PO43 concentration of the passivation solution every 3-4 hours.
7. Cleaning quality acceptance requirements:
(1) Descaling rate:
① To clean carbonate-based scale, the scale removal area should reach 80% of the original scale coverage area.
②Clean up silicate or sulfate scale, the scale removal area should reach 609% of the area covered by the original scale. If the descaling rate is lower than the above-mentioned regulations, or the main heating surface of the boiler is still covered with scale that is difficult to clean, the boiler should be transported for about one month before the boiler is shut down, and the sludge and residual scale should be manually cleaned up.
(2) A good passivation protective film should be formed on the surface of the boiler after cleaning, and there should be no secondary floating rust and pitting corrosion on the metal surface.
(3) The average metal corrosion rate measured by the corrosion indicator should be less than 6g/m2.h, and the total amount of corrosion should be less than 72g/m2.
(4) All water wall pipes and convection pipes in the pot should be kept unblocked. If the pipe is blocked before cleaning, the cleaning still cannot be carried out smoothly, it should be repaired by a qualified unit.
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