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Key points for the purging and cleaning of chemical pipelines. The purging and cleaning of chemical pipelines are crucial processes for ensuring safe production, preventing contamination by various substances, and maintaining the stable operation of equipment. Given the flammable, explosive, toxic, or corrosive nature of chemical media, strict adherence to specifications is required during operation to ensure that pipeline cleanliness meets the required standards. The following are the key details for the operation: I. Purpose and necessity of the operation 1. Safety risk control: 1) Remove construction residues such as slag, grease, and dust to prevent reactions with substances that could lead to fires or explosions. 2) Displace the air or residual gases in the pipeline to prevent the formation of explosive mixtures (for example, hydrogen pipelines need to be purged with nitrogen to reduce the oxygen content to <0.5%). 2. Process assurance: 1) Ensure medium purity to prevent impurities from contaminating the product (e.g., catalyst deactivation). 2) Avoid cross-contamination from shared pipelines for multiple products. 3. Equipment protection: Prevent particulate matter from wearing out valve and pump sealing surfaces, as well as from causing scaling that reduces heat exchange efficiency. 4. Regulatory compliance: Meet the requirements of standards such as GB 50160 Code for Fire Protection Design of Petrochemical Enterprises and TSG D0001 Regulations on Safety Technical Supervision of Pressure Pipelines. II. Purging and Cleaning Methods (I) Physical Purging Method 1. Compressed air/nitrogen purging: Applicable scenarios: pipelines for non-flammable media, inert gas displacement. Key points: 1) Flow rate ≥ 20 m/s (per ASME B31.3) to ensure impurities are removed. 2) Segmental purging (each segment ≤ 500m), with target plate inspection (no rust or dust within 5 minutes). 3) Due to its stable chemical properties, nitrogen is often used in pipelines carrying flammable and explosive substances (such as ethylene and propylene) to prevent reactions with residual materials inside the pipes during purging, thereby ensuring safety. 2. Steam purging: Applicable scenarios: high-temperature pipelines (such as the outlet of cracking furnaces), oil pipelines (for dissolving oils). Key points: 1) The steam flow rate should be greater than or equal to 30 m/s, and the temperature must be above the freezing point of the medium; this prevents the accumulation of condensate and thus avoids damage to the pipes caused by water hammer effects. 2) Before operation, the pipes should be warmed up and drained first, and thermal displacement should be checked; the heating-cooling-reheating cycle must be repeated to ensure that the pipes fully adapt to temperature changes. 3) Steam purging should be carried out after the insulation layer has been installed, as the integrity of the insulation layer prevents steam leakage, thereby ensuring the safety and effectiveness of the purging process and avoiding heat loss or injury to personnel due to steam leakage. (II) Chemical cleaning methods 1. Acid washing/alkaline washing: Applicable scenarios: oxide scale, scaling (such as CaCO3), polymer residues. Reagents and key points: 1) Acid cleaning: Hydrochloric acid is used (for carbon steel) or citric acid (for stainless steel); a corrosion inhibitor such as urucol must be added to protect the pipes. After cleaning, neutralization is required followed by passivation using sodium nitrite. During operation, it is necessary to control the acid concentration and temperature to prevent damage to the pipes. 2) Alkali washing: Sodium hydroxide solution is used, primarily for degreasing. It is also necessary to pay attention to controlling the concentration of the alkaline solution and the operating temperature in order to ensure effective cleaning and pipeline safety. 2. Solvent cleaning: Applicable scenarios: Grease or organic residues (such as benzene, **pipes). Common solvents: carbon tetrachloride (be cautious due to its high toxicity; use it with care), alcohol (suitable for electronic-grade pipelines). When using solvents for cleaning, ensure good ventilation to avoid the risk of poisoning. (III) Mechanical cleaning method: PIG pigging technology: For long-distance pipelines (such as crude oil pipelines), pigs are sent to scrape off the deposits on the inner wall, along with liquid flushing. III. Operation Procedures and Key Points 1. Preliminary preparations: System isolation: Install blind flanges to cut off the equipment and set up warning signs. Plan formulation: Define the medium, pressure, flow rate, and acceptance criteria. Equipment inspection: Verify that the pressure test is successful, then remove sensitive instruments (such as orifice plates and control valves). 2. Phased implementation: Sequence: main pipe → branch pipe → drainage pipe, to prevent the backflow of dirt. Direction: From high to low, to ensure smooth drainage. 3. Effect verification: Target plate method: A white cloth or aluminum target plate is placed at the outlet, and visual inspection confirms the absence of impurities. Detection and analysis: 1) Gas pipelines: Flammable gas concentration < 10% of LEL, oxygen content < 1% (inerted pipelines). 2) Liquid pipelines: pH and turbidity must meet the specified standards (e.g., resistivity of pure water ≥ 1 MΩ·cm). 4. Waste liquid treatment: The acid washing waste liquids are neutralized to a pH of 6–9, and oily wastewater is treated in an oil separation tank to meet environmental standards. IV. Special pipeline treatment Pipeline type Key requirements Oxygen pipelines: Degreasing treatment using oil-free solvents; contact with grease is prohibited, and after purging, they should be wiped with a clean cloth to ensure no oil residues remain. Chlorine pipelines are dried and purged (dew point ≤ -40°C) to prevent corrosion caused by hydrochloric acid. For hydrofluoric acid pipelines, glass and ceramic components are prohibited; after purging, a protective film is formed through fluorination treatment. Polymer pipes: High-temperature steam to dissolve residues, or specialized solvents (such as DMF to remove PTFE residues). V. Safety Measures 1. Personal protection: All personnel involved in purging and cleaning operations must wear gas masks, acid- and alkali-resistant gloves, and protective clothing to ensure safety when coming into contact with harmful chemicals. 2. Emission control: When releasing chemical substances, the emission outlets should be kept away from open flames and densely populated areas. For flammable gases, they should be directed to a flare system for safe combustion to prevent fires or explosions. 3. Emergency plan: An emergency plan should be prepared in advance; in the event of emergencies such as acid leaks, lime should be used immediately for neutralization, and emergency flushing systems should be activated to minimize harm to personnel and the environment. 4. Environmental isolation: Clearly mark the purge area as a no-entry zone and assign dedicated personnel to monitor it, to ensure that unauthorized persons do not enter the dangerous area. At the same time, it is necessary to ensure that emergency evacuation routes are clear, so that people can evacuate the site quickly and safely in the event of an accident. 5. Emergency evacuation: Clearly establish emergency evacuation routes and post them in prominent locations within the work area to ensure that all staff are familiar with the escape paths. Conduct regular emergency drills to improve the emergency response capabilities of all personnel. VI. Common Problems and Solutions 1. Incomplete purging: Increase the time or perform purging in reverse stages. 2. Residual debris in hard-to-reach areas: Remove the flange and clean it locally or use high-pressure water jets. 3. Corrosion risk: Avoid using hydrochloric acid to clean stainless steel (use citric acid instead). Summary: The purging and cleaning of chemical process pipelines requires the precise selection of methods based on the medium, material, and process. It is essential to strictly follow the sequence of \"isolation → purging/cleaning → testing → handling\" to ensure \"zero residues and zero risks\", thereby guaranteeing safe production and compliant operation. Reference standards: GB 50160 \"Code for Fire Protection Design of Petrochemical Enterprises\", TSG D0001 \"Regulations on Safety Supervision of Pressure Pipelines\".