The necessity of industrial boiler cleaning is briefly introduced（2）
(5) Pickling control requirements are as follows:
(1) Pickling solution concentration: HCI or nitric acid (4-8)%, wash with hydrochloric acid, add fluoride additive: HFL%; or sodium fluoride (5-8) ‰. The curing time is 6-8 hours.
② In the post pickling stage, the pickling end point should be determined according to the following points, and the pickling should be stopped in time.
A. The acid concentration tended to be stable: after 30 minutes, the absolute difference between the two analysis results was less than 0.2 * 10-2.
B. The iron concentration tends to equilibrium.
(6) After pickling, the pickling waste liquid in the pot should be quickly sprayed with clean water. If the flushing water is insufficient, it can also supply water while discharging acid (the acid discharge rate is not greater than the water supply rate), and add an appropriate amount of alkali liquor to neutralize acid in the later stage of flushing. Flush with top acid or deacidification neutralizing water until the pH of effluent is greater than or equal to 4.5.
(7) It must be neutralized and passivated after pickling to prevent metal corrosion. Passivation generally adopts the following methods: after washing, add na3po4.10h2o to make its concentration reach (1-2)%, add NaOH to adjust the pH value, and the pH value should be controlled at 11-12. Generally, passivation with pressure lower than the working pressure can be used. When the concentration of passivation solution in the boiler is uniform, close the circulating system, then light the boiler and slowly increase the pressure to about half of the rated working pressure. The boiler with rated working pressure ≥ 1.6mp and rated evaporation capacity ≥ 10t / h shall be washed and passivated according to the requirements of power station boiler after acid washing.
5. Residual scale treatment: after neutralization, all inspection holes must be opened, and the loose residual scale and sediment in the pot must be cleaned manually to prevent the pipeline from being blocked after scale falling off.
6. Chemical monitoring during cleaning:
(1) Alkali boiling process: measure pH value, total alkalinity and PO43 - concentration of alkali washing solution every 4 hours (every 1 hour when approaching the end point).
(2) Pickling process: measure the acid concentration in the pickling solution every 30 minutes (every 1 hour in the intermediate pickling stage) at the beginning. 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 pH value at the outlet is measured every 15 minutes at the later stage.
(4) Passivation process: measure the pH and PO43 concentration of passivation solution every 3-4 hours.
The determination methods of the above items are shown in Annex 7 chemical monitoring and analysis methods for cleaning process
7. Cleaning quality acceptance requirements:
(1) Descaling rate:
① To clean the carbonate scale, the scale removal area should reach 80% of the original scale coverage area.
② When cleaning silicate or sulfate scale, the scale removal area should reach 609% of the original scale coverage area. If the descaling rate is lower than the above provisions, or the main heating surface of the boiler is still covered with scale which is difficult to clean, the boiler shall be transported for about one month before shutdown, and the residue and residual scale after falling off shall be cleaned manually.
(2) A good passivation film shall be formed on the surface of the boiler after cleaning, and there shall be no secondary floating rust and pitting on the metal surface.
(3) The average value of metal corrosion rate measured by 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 boiler shall be kept unblocked. If the pipe is blocked before cleaning and the cleaning can not be carried out smoothly, it should be repaired by a qualified unit.
3、 For different scale and metal materials, appropriate pickling agent and cosolvent should be selected. Industrial boiler cleaning agent is generally selected as follows:
(1) Hydrochloric acid is generally used to clean carbonate scale.
(2) Silicate scale can be cleaned by adding hydrofluoric acid or fluoride to hydrochloric acid.
(3) Sulfate scale or scale mixed with sulfate and silicate should be boiled with alkali in advance, and then cleaned with hydrochloric acid or fluorine-containing hydrochloric acid.
(4) Iron oxide scale can be cleaned by adding fluoride to hydrochloric acid or nitric acid.
(5) For utility boilers, when the copper content in primary oxide scale is high (CuO > 5 × 10-2), measures should be taken to prevent copper plating on metal surface. Generally, hydrochloric acid, fluoride, thiourea and other cleaning aids can be selected.
(6) Hydrochloric acid should not be used as cleaning agent when cleaning austenitic steel. Generally, hydrofluoric acid, EDTA, citric acid, formic acid, acetic acid and other organic mixed acids can be used as cleaning agents to clean the boiler parts containing chromium materials.
