Influence of low maximum temperature on chemical c

2022-08-11
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The influence of too low temperature on boiler chemical cleaning

Abstract too low temperature not only can not achieve the purpose of descaling and derusting, but also affects the passivation effect, unable to form a dense passive film, resulting in significant economic losses

1 preface

boiler chemical cleaning includes the boiling of the new furnace and the descaling of the old furnace. The boiling process includes: water washing, alkali washing and degreasing, water washing after alkali washing, acid washing, water washing after acid washing, rinsing, neutralization and passivation. Temperature control is extremely important in the cleaning process. Too high a temperature is not conducive to on-site control, and it will also cause new corrosion of the boiler. Too low a temperature will not achieve the purpose of acid pickling and descaling. At the same time, too low a temperature will also affect the passivation effect and cannot form a dense protective film. The 5# furnace in the thermal power plant of Qilu Petrochemical Company had an accident of low temperature during cleaning and poor cleaning quality

2 overview

hg410/type natural circulation single drum single stage evaporation centralized downcomer boiler with two-stage arrangement at the tail of 5 boilers in thermal power plant. The boiler is surrounded by water-cooled walls, a platen superheater is arranged at the furnace outlet, and a two-stage convection superheater is arranged at the horizontal flue. At the tail, the company launched a new crowdfunding activity on indiegogo, with staggered shafts, arranged with two-stage economizers and two-stage air preheaters, and the horizontal flue steering chamber at the top of the furnace and the tail cladding wall are wrapped with fins. The drum diameter is 1600mm, the barrel thickness is 90mm, the barrel length is 1300mm, the head wall thickness is 100mm, the total length of the drum is 14520mm, 56 (315) cyclone separators are set in the drum, phosphate dosing devices and continuous discharge devices are also set in the drum, and 60 × 5 finned tubes with a pitch of 80mm are welded with fully sealed membrane water-cooled walls, and the four walls of the furnace are 4 in total × 124 risers, 7 (377 × 25 centralized downcomers are led from the lowest point of the drum to the lower part of the operation layer, and then 48 (133 × 10 connecting pipes are introduced into the lower header of the water wall, and the front water wall, the rear water wall, and the upper header of the side water wall share 42 (133 × 10 steam and water outlet pipes, 12 (133) for inclined wall header × 10. The steam water outlet pipe is connected with the steam drum

this chemical cleaning is the first cleaning of this boiler, which is the responsibility of the steel plate reprocessing unit reduced by the construction customer. Its main purpose is to degrease and remove rust, clean welding slag and passivate, so as to ensure the safety and steam quality of the boiler during its initial operation. The cleaning scope includes economizer, water wall, downcomer and steam drum

3 chemical cleaning process control

3.1 chemical cleaning process design

3.1.1 alkaline cleaning: use 0.05% detergent +0.05%na3po4+0.2%na2hpo4 solution to cycle for 12 hours, and the cleaning temperature is ℃

3.1.2 water washing after alkaline washing: wash with demineralized water until the outlet water ph=8.4, the water quality is transparent, drain the washing water, check whether there is leakage of welding joints or other defects in the system, and eliminate them immediately if any. To reduce possible problems during chemical cleaning, and prepare enough demineralized water

3.1.3 pickling: cycle cleaning in 5%hci+0.3%tp-1 corrosion inhibitor +0.5% acid urea +0.5% NaF solution for 8~10h, The temperature of pickling solution is 50 (5 ℃), the analysis of acid solution concentration and iron ion concentration should be strengthened in the acid washing process. Check whether the balance is reached and remove the monitoring tube to check the acid washing effect. When the iron ion concentration in the acid washing process becomes stable, monitor the basic cleaning in the tube, and the acid washing can be stopped after recirculation for about 1H.

3.1.4 water washing after acid washing: if there is more undissolved sediment accumulated in the dead corner of the cleaning system after acid washing, it can be washed with water after the acid washing solution is discharged Discharge when the pH of the effluent is 4~5, and manually clean the sediment in the hand holes of the steam drum, header and lower header

3.1.5 passivation: maintain N2H4 concentration of the passivation solution at 200~300mg/l, ph=9.5~10, At 90 (after circulating in the passivation solution at 5 ℃ for 24h, the hot furnace discharges water. At the same time, NaCl solution is put into the waste liquid treatment tank for decomposition and treated to N2H2 10ug, which is mini/L.

3.2.1 I loop:

3.3 supervision of chemical cleaning process

because the pipe diameter of hot mixing pipeline is not calculated during on-site construction, the pipe diameter is too small and the boiler inlet pipeline is not provided with insulation layer, the actual cleaning temperature is low, and the maximum temperature during alkaline cleaning is only 43 ℃, acid When washing, the liquid temperature in the cleaning tank can not meet the requirements of pickling process, only about 35 ℃, so the whole pickling process is basically carried out at room temperature. The whole pickling process starts at 13:00 p.m. and ends at 23:00 p.m. See pickling process monitoring table 3~1:

table pickling process monitoring table for the analysis results

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