Degreasing of oxygen pipelines

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As we all know, oxygen pipelines must be kept free of oil and degreased. Today, we’ll introduce the degreasing methods for oxygen pipelines. As the name implies, degreasing involves removing oil stains from the inner and outer surfaces of the pipe. Degreasing is a critical step in the installation of oxygen pipelines; if oil is present inside these pipelines, it will react with pure oxygen, leading to an explosion very easily. Oxygen pipelines are generally made of seamless steel tubes, with materials being 20# steel or stainless steel. For pipes made of stainless steel, only degreasing is required, while for pipes made of 20# steel, degreasing is necessary in addition to pickling, neutralization, and passivation. During construction, the openings of degreased pipes must be sealed promptly to prevent any secondary contamination of the pipes. Before putting it into use, it must be thoroughly purged with oil-free compressed air. Degreasing, pickling, and passivation of carbon steel pipes: For carbon steel pipes, it is necessary to first degrease them before pickling, as grease is insoluble in acid solutions. If pickling is carried out first, neither the grease nor the rust covered by it can be removed. The degreasing and pickling method for seamless steel pipes commonly uses a tank immersion method in practice, with the liquid level being more than 50 mm above the outer surface of the pipes during immersion. Since oxygen pipelines require degreasing, acid washing, and passivation treatments, generally 4 tanks are fabricated based on the actual diameter of the pipes in use. That is, the degreasing tank, pickling tank, neutralization tank, and passivation tank. The degreasing and pickling process flow for oxygen pipelines is: degreasing → water rinsing → pickling → water rinsing → neutralization → passivation → water rinsing → drying. 1. Degreasing: The formula for the degreasing solution is as follows: each liter of the solution contains 20g–30g of sodium hydroxide, 35g–50g of sodium nitrate, and 3g–5g of sodium silicate. The operational requirements are as follows: the liquid temperature should be between 70°C and 80°C, and the immersion time is determined through testing based on the amount of oil on the pipe surface; it is generally around 10 minutes to 40 minutes. Steam can be used as the heating method. Defatted parts shall be inspected using any one of the following methods. (1) Wipe the surface of the degreased parts with clean, dry white filter paper; it is considered qualified if no oil traces remain on the paper. (2) Use 100% camphor particles smaller than 1 mm to inspect the steam-flushed condensate; continuous rotation is considered acceptable. (3) Use ultraviolet light with a wavelength of 3200–3800 Å to inspect the surface of degreased parts; the absence of grease fluorescence indicates compliance. Through degreasing, the oil on the pipe surface can be removed. 2. Rinse with water: Clean thoroughly with clean water at a pressure of 0.8 MPa. 3. Pickling: Pickling involves using an acid solution to remove scale and rust from the surface of seamless steel pipes. Oxides of iron such as scale and rust (Fe3O4, Fe2O3, FeO, etc.) undergo a chemical reaction with acid solutions, forming salts that dissolve in the acid and are thus removed. The most commonly used acids for pickling are sulfuric acid and hydrochloric acid. The formula for the sulfuric acid solution is: the concentration of the sulfuric acid solution is 5%–10%. The operational requirements are: temperature of 60°C to 80°C, and soaking time of 5 minutes to 20 minutes. The formula for the hydrochloric acid solution is: the concentration of the hydrochloric acid solution is 5% to 20%. The operational requirements are: temperature of 20°C to 50°C, and soaking time of 5 minutes to 20 minutes. After acid cleaning, the pipeline is inspected; it is considered qualified if the surface rust has been completely removed and a metallic luster is visible. 4. Rinse with water: Rinse thoroughly using clean water at a pressure of 0.8 MPa. 5. Neutralization: The formula for the neutralizing solution is a solution of NH4OH diluted to a pH value of 10–11. The required processing procedure is: soak at room temperature for 3 minutes. 6. Passivation: After acid washing, water rinsing, and bleaching, the metal surface becomes very clean and highly activated, making it susceptible to corrosion. Therefore, passivation must be carried out immediately to form a protective layer on the cleaned metal surface, thereby reducing corrosion. The passivation solution formula is: NaNO2 concentration of 8%–10% ； The NH4OH concentration is 2%. The required processing procedure is: soak at room temperature for 10 minutes. 7. Rinse with water: Rinse until clean using purified water at a pressure of 0.8 MPa. 8. Drying: Drying must be carried out by blowing with clean, oil-free dry air or nitrogen. 9. Inspection: Wipe with white filter paper; if no oil traces remain on the paper, it is considered acceptable. After passing the inspection, seal both ends of the tube with plastic sheeting and tape. The inner wall of the carbon steel pipe treated in this way remains clean and shiny, and it can stay free from corrosion for about 2 months ； If it is well preserved, the duration can be extended. The above method can also be applied to carbon steel pipe fittings. Degreasing, pickling, and passivation of stainless steel pipes: Since stainless steel pipes only require degreasing, in addition to the degreasing methods mentioned above, the wiping method can also be used for large-diameter pipes. When using the wiping method for degreasing, cotton yarn should not be used; instead, fabrics with fibers that do not easily fall off, such as silk or glass fiber fabrics, should be employed. Longer pipes can be tied with clean wire and cloth strips to be pulled and scrubbed in both directions in order to remove oil stains from the inner walls of the pipes. After degreasing, the surface of the degreased parts must be carefully inspected; it is strictly prohibited for fibers from organic materials to fall into or adhere to the surface of these parts. Once the inspection is satisfactory, the pipe openings should be sealed immediately. Degreasing, pickling, and passivation of stainless steel pipe fittings and valves: Depending on the volume of the largest valve, a degreasing container is made from steel plates (or can be simply modified from an iron barrel used for storing carbon tetrachloride); after removing rust and dirt from its inner walls, it is cleaned with a white cloth. The stainless steel fittings and valves that need to be degreased should be wiped clean with a dry white cloth (note: choose a high-quality cloth that does not shed fibers), and then tied together with wire or thin steel cable and placed in the container that has been prepared. Carbon tetrachloride is poured into the container to soak the stainless steel fittings and valves. Do not fill it to the brim when injecting; it is best to keep the liquid level about 200 mm away from the top of the container, to prevent carbon tetrachloride from spilling and causing contamination or injury. Since carbon tetrachloride is toxic and volatile, operators should stand upwind when pouring it to prevent poisoning from inhaling carbon tetrachloride. Then cover the container, soak it for 1 to 2 hours, remove the fittings and valves, wipe the inner and outer surfaces of these components with linen cloth, and let them dry naturally. After passing the inspection, wrap and seal the pipe fittings and valves in plastic sheeting in a categorized manner. Before degreasing the valve, it must first be ground and pressure-tested to ensure it meets the requirements; thereafter, it is disassembled into its components to remove rust and other impurities, and then immersed in carbon tetrachloride solvent for 1 to 1.5 hours. Bolts and metal gaskets are degreased in the same way. For valve housings that cannot be soaked, the wiping method can be used. During degreasing, the non-metallic gaskets are immersed in carbon tetrachloride solvent for 1.5 to 2 hours, after which they are taken out and hung in a well-ventilated area or ventilation system to be dried individually until no odor of the solvent remains. The asbestos packing inside the valve can be burned at a temperature of around 300°C for 2 to 3 minutes (a flaming flame that produces smoke must not be used), and then immersed in the specified coating. Construction safety protection measures: Before commencement of work, the technician must provide safety and technical instructions to the workers. Tools, measuring instruments, gauges, etc., used for degreasing, inspection, and installation must be pre-degreased in accordance with the requirements of the parts to be degreased. It must not be used without degreasing. The safety measures for degreasing and acid washing shall comply with the relevant provisions of the \"Safety Technical Regulations for Petrochemical Construction,\" and such work should be carried out in well-ventilated outdoor areas. Personal protection should be strengthened at work; one should wear work clothes, rubber gloves, a mask, protective goggles, rain boots, and an apron, and a gas mask should be worn if necessary. Conclusion: When degreasing oxygen pipelines, care must be taken; inadequate degreasing can create safety hazards in future operations and may even lead to accidents. Therefore, from the very beginning of construction, it is necessary to provide proper safety and technical instructions, ensuring that every worker attaches great importance to these matters. They must carry out thorough degreasing, and only after the quality inspector checks and confirms that the degreasing has been done properly can they proceed to the next stage of work.