The necessity of industrial boiler cleaning is briefly introduced（1）
Waste a lot of fuel: because the thermal conductivity of scale is only one tenth of steel material, when the scale is heated to the surface, it will hinder heat transfer. In order to maintain a certain output of the boiler, the temperature of the fire side must be increased, which leads to the heat loss caused by external radiation and smoke exhaust. According to the different working pressure of boiler, the type and thickness of scale are different, and the fuel consumption is different. Through the test and calculation, the ratio between the scale thickness and fuel consumption is: when the scale thickness (s) ≥ 1 mm, the fuel consumption is 5-13%; when 2mm, the fuel consumption is 13-18%; when 3mm, the fuel consumption is 18-26%.
It is easy to burn steel plate and pipe due to overheating: after the boiler scale, it is necessary to maintain a certain working pressure and evaporation capacity, which can only improve the temperature at the fire side. However, the thicker the scale, the lower the thermal conductivity, the higher the temperature at the fire side. Generally, the temperature of boiler fire side is about 900 ℃ and water side temperature is about 190 ℃. The temperature of the scale free steel plate is about 230 ℃, and once the scale is about 1mm, the temperature of the steel plate is increased by 140 ℃ compared with the non scaling steel plate. At 315 ℃, the plasticity index of 20 ℃ steel plate began to decrease, and when the temperature reached 450 ℃, the steel plate was deformed by overheating and creep. Therefore, the fouling of boiler is easy to cause metal burning.
Increase maintenance cost and shorten service life: boiler accidents caused by scale account for about one third of all boiler accidents, and it still shows an upward trend, which not only causes equipment damage, but also endangers personal safety. Therefore, when the water supply is qualified, the boiler should strictly control the water in the boiler to meet the national standards; in operation, it is necessary to prevent the scale from producing, and timely treatment of the scale, and to thoroughly prevent and remove the scale in the boiler furnace and control the water quality. In order to solve the above problems, the most scientific method is to maintain the boiler with good comprehensive performance and comprehensive efficacy and clean and remove scale regularly.
According to the different working pressure of boiler, the type and thickness of scale are different, and the fuel consumption is different. Through the test and calculation, the ratio between the scale thickness and fuel consumption is: when the scale thickness (s) ≥ 1 mm, the fuel consumption is 5-13%; when 2mm, the fuel consumption is 13-18%; when 3mm, the fuel consumption is 18-26%. It is easy to burn steel plate and pipeline due to overheating: after boiler scaling, it is necessary to maintain certain working pressure and evaporation capacity, which can only improve the temperature at the fire side. However, the thicker the scale, the lower the heat conduction coefficient, the higher the temperature at the fire side. Scale is the "first hundred harm" of boiler, which is the main cause of boiler accidents. The main harm is: fuel waste is large: because the heat conductivity of scale is only several times of steel material, heat transfer will be blocked when scale is heated. In order to keep the normal work of boiler, the furnace body temperature must be increased, so as to radiate and exhaust flue gas outward, resulting in heat loss.
Generally, the temperature of boiler fire side is about 900 ℃ and water side temperature is about 190 ℃. The temperature of the scale free steel plate is about 230 ℃, and once the scale is about 1mm, the temperature of the steel plate is increased by 140 ℃ compared with the non scaling steel plate. At 315 ℃, the plasticity index of 20 ℃ steel plate began to decrease, and when the temperature reached 450 ℃, the steel plate was deformed by overheating and creep. Therefore, the fouling of boiler is easy to cause metal burning. Increase maintenance cost and reduce service life: boiler scale accident accounts for about one third of the total boiler accidents, and it still shows an upward trend, which not only causes equipment damage, but also endangers personal safety. Therefore, when the water supply is qualified, the boiler should strictly control the water in the boiler to meet the national standards; in operation, it is necessary to prevent the scale from producing, and timely treatment of the scale, and to thoroughly prevent and remove the scale in the boiler furnace and control the water quality. In order to solve the above problems, the most scientific method is to maintain the boiler with good comprehensive performance and comprehensive efficacy and clean and remove scale regularly.
1、 Preparation before cleaning of industrial boiler
(1) Before cleaning, the structure and materials of boiler shall be understood in detail, the boiler interior shall be inspected, cleaning method shall be determined and safety measures shall be formulated. If the boiler has leakage, blockage and other defects, effective measures shall be taken in advance to deal with it.
(2) The scale category must be determined before cleaning. Representative scale samples should be taken from different parts of boiler for analysis. See Appendix 1 "identification method of scale category" for the identification method of scale category. For boilers with rated pressure greater than 1.5MPa, qualitative analysis of scale of samples is required
(3) Before cleaning, according to the actual situation of boiler, the cleaning plan must be formulated by professional personnel and approved by the technical director. The cleaning plan shall include the following:
① Boiler user name, boiler model, registration number, service life and last cleaning time, etc.
② Whether the boiler has defects:
③ Boiler structure and corrosion conditions, including scale distribution, thickness (or deposition), scale analysis results and equipment condition.
(4) Cleaning scope and cleaning method.
⑤ According to the "small test of cleaning process", boiler structure, boiler material, scale amount and cleaning system, the concentration and dosage of cleaning agent, corrosion inhibitor, passivator and other auxiliary agents, as well as cleaning temperature and time, etc. are determined;
⑥ Chemical cleaning system diagram:
⑦ Throttling, isolation and protection measures required for cleaning;
⑧ Items to be monitored and recorded during cleaning;
⑨ Cleaning waste liquid removal and treatment;
⑩ After cleaning or residual scale cleaning, cleaning quality acceptance conditions; boiler chemical cleaning should strictly implement cleaning plan. In case of special circumstances, the original approved technical director shall sign and agree to change the cleaning scheme.
(5) Before cleaning, chemical cleaning drugs, such as purity of original solution and corrosion inhibition efficiency of selected inhibitor, shall be rechecked, and equipment and materials shall be prepared according to technical and safety measures.
2、 Chemical cleaning process and acceptance requirements of industrial boiler
1. industrial boiler refers to the boiler providing steam and hot water for industrial production or life, generally refers to the boiler with rated working pressure ≤ 2.5MPa.
2. determination of chemical cleaning conditions: chemical cleaning of industrial boiler includes alkali washing or pickling. When the boiler is dirty or rusty, it shall be cleaned in time, but the pickling method shall meet one of the following conditions, and the acid cleaning interval of each boiler shall not be less than two years.
(1) The boiler has a heating area of more than 80x10-2, and the average fouling thickness reaches or exceeds the following values; for boilers without superheater: LMM. Boiler with superheater: 0.5mm; hot water boiler: LMM.
(2) The boiler heating surface is seriously corroded.
3. cleaning system: industrial boiler only uses alkali washing, and generally does not need circulation system. When pickling is used, the method of combining cycle and static soaking shall be adopted.
4. chemical cleaning process
(1) The chemical cleaning process of the boiler includes alkali boiling conversion, water washing, washing after pickling, rinsing, passivation and other stages, in which the alkali boiling transformation and rinsing can be avoided according to the specific conditions.
(2) Before chemical cleaning, the accumulated sediment and dirt in the pot shall be removed. If there is any blocked pipe, try to clear it in advance.
(3) For the scale mainly sulfate, the alkali boiling transformation shall be carried out, and the requirements are as follows:
(1) According to the thickness and composition of scale, the mixed solution of sodium carbonate and trisodium phosphate dissolved in the solution is added to the boiler, so that the reagent concentration in the boiler water reaches na2co3: (0.3-0.6) x10-2na3po412h202 (0.5-1.0) x10-2 (equivalent to p043 concentration 1250-2500mg/l).
② Slowly 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 scale is serious, the conversion time of alkali boiling should be extended appropriately.
(3) The boiling time shall be sampled and analyzed regularly. When the alkalinity of boiler water is lower than 45mmol/l and the concentration of p043 is lower than 1000mg/l, sodium carbonate and trisodium phosphate should be added appropriately.
(4) After the alkali boiling transformation, the alkali solution shall be drained and washed with water to the pH value of the effluent less than 9.
