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          What are the rules of chemical cleaning for low pressure boilers?

          Release date:2021-10-19 18:38 Browse times:
          Article 1 In order to prevent improper chemical cleaning from endangering the safe operation of boilers, to ensure the safety and reliability of boiler chemical cleaning and to improve the cleaning effect, these rules are formulated.
          Article 2 these rules apply to the chemical cleaning of fixed steam boilers and confined hot water boilers whose rated outlet steam pressure is less than or equal to 2.45MP(25kgf/cm2).
          Article 3 Chemical cleaning of boiler includes two types: alkali boiling and pickling. Cleaning can be divided into soaking, cycling cleaning and soaking and cycling cleaning combined with the way.
          Article 4 Boiler water treatment is an important measure to ensure the safe and economic operation of the boiler. Chemical cleaning shall not replace or relax the regular and effective water treatment work. Each boiler pickling interval should not be less than two years.
          Article 5 professional boiler chemical cleaning units must obtain the qualification approval of boiler and pressure vessel safety supervision institutions of provincial or above (including provincial) labor departments. Professional boiler chemical cleaning units shall have the following conditions:
          1. Have a sound organizational structure and obtain the legal person status.
          2. Professional and technical personnel suitable for the chemical cleaning work undertaken.
          The cleaning unit responsible for cleaning class C boilers shall have at least one intermediate technician and two junior technicians. Units with qualification for cleaning class C boilers can undertake chemical cleaning of boilers below class C.
          The cleaning unit responsible for cleaning class D and below boilers shall have at least one intermediate technician or two junior technicians.
          3. Cleaning operation and testing personnel should be approved by boiler and pressure vessel safety supervision institutions at or above prefectural level (including prefectural level).
          4. Complete and safe and reliable cleaning equipment and analysis means.
          5. Have perfect management system, and formulate safe operation rules.
          Units in line with the above conditions can pickling boilers by themselves after approval by local labor departments.
          Only alkali boiling units do not need to be accredited.
          Article 6 Before chemical cleaning of boilers, chemical cleaning units must work out cleaning plans and submit them to boiler pressure vessel safety supervision institutions of local or municipal labor departments for record before implementation.
          Article 7 The boiler and pressure vessel safety supervision organization shall inspect and supervise the qualification and quality of cleaning units according to these rules.
           
          Chemical cleaning conditions and preparation
          Article 8 Chemical cleaning can be carried out only when the boiler meets any of the following conditions:
          1. The boiler heating surface is covered by scale more than 80% and the average scale thickness reaches or exceeds the following values.
          For boiler without superheater: 1mm,
          For boiler with superheater: 0.5mm,
          For hot water boiler: 1mm.
          2. There are obvious oil stains or rust on the boiler heating surface.
          Article 9 Before cleaning, the boiler should be inspected in detail inside and outside, in order to determine the cleaning method and formulate safety measures. If there is leakage and blockage of boiler, effective measures should be taken to deal with it in advance.
          Article 10 The scale type must be determined before cleaning. Representative scale samples should be taken from different parts of the boiler for analysis. See Appendix 1 for classification identification of scale.
          Article 11 The cleaning system must be designed and the amount of chemical cleaning agent calculated before cleaning.
          Article 12 Equipment, materials, safety supplies and other items should be prepared before cleaning.
           
          Chemical cleaning system
          Article 13 Chemical cleaning units shall design chemical cleaning systems according to boiler structure (especially water circulation system and water volume), boiler room conditions, environment, cleaning tasks, cleaning media, cleaning methods and other specific conditions.
          Article 14 The basic principles of cleaning system design are as follows:
          1. "alkali cooking" and soaking do not need circulation system;
          2. Forced circulation hot water boiler must use circulation cleaning;
          3. Circulating cleaning systems of various furnace types are shown in Figure 1, 2 and 3. The cleaning fluid can be entered from the upper or lower part of the boiler.
          Article 15 The cleaning circulation system consists of cleaning liquid tank, cleaning pump, temporary cleaning pipe and cleaning loop of boiler, and shall meet the following requirements:
          1. The cleaning liquid tank should have enough volume to ensure that the cleaning liquid is smooth and the sediment can be discharged smoothly;
          2. The cleaning pump should be corrosion-resistant, and the output of the pump should ensure the flow rate and head of the cleaning liquid required for cleaning;
          3. The inlet of the cleaning pump or the outlet of the cleaning liquid tank should be equipped with a filter screen, the mesh aperture should be less than 5mm, and there should be enough circulation area;
          4. The inlet pipe and return pipe of cleaning liquid should have sufficient cross-sectional area to ensure the flow of cleaning liquid;
          5. The top of the boiler and the top of the closed cleaning liquid tank should be equipped with a exhaust pipe, the exhaust pipe should be led to a safe place, and there should be enough circulation area;
          6. Boiler down pipe shall be equipped with throttling device;
          7. The valves in the cleaning system should be flexible, strict and corrosion resistant. The valves and measuring instrument pipes containing copper parts should be removed, blocked or replaced with pipe accessories coated with anticorrosive paint before pickling, and the pipes irrelevant to boiler cleaning should be blocked;
          8. There should be sampling points in the cleaning system;
          9. Spray liquid injection device and steam heating device can be installed if necessary;
          10 economizer cleaning should be carried out separately from the boiler.
          Article 16 The cleaning system shall be strict and leak-free. After the system is installed, the sand, welding slag and other sundries in the system should be cleaned.
           
          Choice of cleaning agent and corrosion inhibitor
          Article 17 When boiler is "alkali cooking", cleaning agent composed of sodium hydroxide, sodium carbonate and trisodium phosphate is generally used. Surfactant can be added to the formula if necessary.
          Article 18 In boiler pickling, pickling agent is selected according to scale type as follows:
          1. For carbonate scale, hydrochloric acid is generally used for cleaning.
          2. For silicate scale, hydrofluoric acid or adding fluoride in hydrochloric acid can be used to clean.
          3. For sulfate scale or sulfate and silicate mixed scale, can be pre-" alkali boiling "and then using hydrochloric acid or adding fluoride in hydrochloric acid cleaning.
          Article 19 during boiler pickling, if the amount of Fe3+ in the cleaning solution reaches 500mg/l, an appropriate amount of strong reducing agent such as sodium sulfite, stannous chloride or phosphite should be added to the pickling agent. The amount of addition may be calculated by chemical reaction formula or determined by pre-conducted small-scale tests.
          Article 20 Corrosion inhibitors shall be selected based on the results of a prior determination of corrosion inhibitor efficiency, as described in Appendix II. The inhibition efficiency of corrosion inhibitor should be above 98% without pitting.
          When choosing corrosion inhibitor, the toxicity, odor and water solubility of corrosion inhibitor should also be considered.
          Article 21 Corrosion inhibitors shall be carefully inspected and measured according to package or batch number before use. Other agents related to boiler cleaning should also be inspected.
           
          Chemical cleaning process
          Article 22 The alkali cooking process of new furnace shall be implemented in accordance with the provisions concerning the installation of industrial boilers in volume 6 of TJ231(vi) Code for Construction and Acceptance of Mechanical Equipment Installation Engineering. See Appendix 3 for cooking methods.
          Article 23 Pickling in boiler includes alkali boiling, water flushing, pickling, water top acid or rinsing and passivation. Alkali boiling and rinsing can be avoided according to specific circumstances.
          Article 24 Before chemical cleaning, the inside of the boiler shall be flushed with a high speed stream of water, followed by the removal of sediment and dirt.
          Article 25 The requirements for alkali cooking in boiler are as follows:
          1. Add 0.3-0.5% sodium hydroxide and 0.5-1% trisodium phosphate mixture to the boiler.
          2. Boiler slowly boost pressure, generally in 24 hours to make the boiler pressure up to 50% of the rated pressure, and maintain 24-48 hours. When scaling is serious, alkali cooking time can be appropriately extended.
          3. After alkali cooking, the lye should be drained and washed with water until the PH value of the outlet water is less than 9.
          Article 26 Requirements for boiler pickling are as follows:
          1. Concentration of acid washing solution:
          Hydrochloric acid 4-8%, no more than 10%;
          Hydrofluoric acid 1-2%, if combined with hydrochloric acid 1%;
          Fluoride 0.5%(with hydrochloric acid).
          Corrosion inhibitor must be added to acid washing solution, and its concentration is determined according to test results.
          2. The temperature of pickling liquid shall not exceed 60℃. The heating method of pickling liquid can be external heating method or internal heating method, as shown in Appendix IV. Acid shall not be heated directly with an open fire in a furnace.
          3. The total pickling time from acid injection to acid discharge should generally not exceed 12 hours. In the process of pickling, the difference between the two analysis results of hydrochloric acid concentration is less than 0.2%, then the cycle pickling can be ended.
          4. When soaking pickling, the prepared acid is added to the boiler, and soaked for a period of time according to the scale dissolution, the acid is discharged. In order to improve the cleaning effect, the acid can be pre-heated to 50℃-60℃ outside the boiler before adding to the boiler. Concentrated acid shall not be injected directly into the boiler.
          5. When circulating cleaning, the flow rate of acid liquid in the boiler tube is generally controlled within the range of 0.05-0.5m/s, and the highest is not more than 1m/s.
          Article 27 Requirements for boiler rinsing are as follows:
          Generally, citric acid solution with a concentration of 0.1-0.3% is used, and ammonia water is added to adjust the PH value to 3.5-4 before rinsing. The solution temperature was maintained at 75℃-90℃, and the cycle lasted about 2 hours.
          Article 28 Passivation must be carried out after pickling. In order to ensure the quality of passivation film, after the end of pickling, the waste liquid should be quickly jacked out with clean water or rinsed. When water apex acid is used, the PH of the effluent should be in the range of 4-4.5.
          Article 29 Passivation requirements are as follows:
          The passivation solution is 1-2% trisodium phosphate solution, and the PH value is adjusted to 10-11 with sodium hydroxide, and passivation is performed at 80℃-90℃ for more than 8 hours.
          After passivation, sediment and dirt in the pot should be eliminated.
          5. Passivation process: Determination of trisodium phosphate concentration and PH value every 2 hours.
          Article 32 At the beginning of pickling, corrosion test sheets should be put into the acid box and the pot respectively (try to hang two pieces in the pot). After pickling, take out and process the corrosion test sheet, and calculate the metal corrosion rate. Please refer to Appendix 6 for the preparation and processing method of test film.
          Metal corrosion rate (A) is calculated as follows:
          W-W1
          A = ─ ─ ─ ─ h. g/m2
          T. F
          Where: W-- weight of corrosion test sheet before pickling, g;
          W1-- Weight of corrosion test sheet after pickling, g;
          T-- total pickling time, h;
          F-- Total area of test piece, m2.
           
          Chapter vii evaluation of chemical cleaning results
          Article 33 The scale removal rate shall meet the following requirements:
          1. When cleaning carbonate scale, the scale removal area should reach more than 80% of the original scale coverage area.
          2. When cleaning silicate or sulfate scale, the scale removal area should reach more than 60% of the original scale coverage area.
          Article 34 the average corrosion rate of metals measured by corrosion specimens shall be less than 10g/ m2·h.
          Article 35 The cleaning surface of the boiler shall be cleaned and a good passivation film formed.
          Article 36 Chemical cleaning units shall complete materials, record clear and complete, accurate data, and issue completion acceptance report. The completion acceptance report is accepted by both parties.
           
          Liquid waste processing
          Article 37 The discharge of waste liquid of boiler chemical cleaning shall conform to the relevant provisions of GB8978-88 Comprehensive Sewage discharge standard. Waste liquid that does not meet discharge standards should be treated in advance before discharge.
           
          Security measures
          Article 38 Before boiler chemical cleaning, workers must be familiar with cleaning safety operation procedures, the characteristics of various agents used and first-aid methods, and do a good job of their own protection.
          Article 39 The cleaning site shall be equipped with fire fighting equipment, complete lamps, adequate lighting, flushing water, first-aid medicine and labor protection articles. No fireworks during cleaning.
          Article 40 Special tools should be used to carry concentrated acids and bases.
          Article 30 If the boiler cannot be put into operation immediately after pickling, anti-corrosion measures shall be taken.
           
          Chemical supervision of cleaning processes
          Article 31 The cleaning liquid shall be analyzed regularly during chemical cleaning. The analytical method is shown in Appendix V.
          The analysis items are generally as follows:
          1. Alkali cooking process: determination of sodium phosphate, sodium hydroxide concentration and PH value of alkali cooking solution every 2 hours.
          2. Pickling process: Acid concentration and Fe3+ concentration are measured every 30 minutes.
          3. Water top acid process: test outlet water PH value every 10 minutes.
          4. Rinse process: Measure acid concentration, PH value and iron content of rinse solution every 30 minutes.
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