Everyone knows that oxygen pipelines should avoid oil and be degreased. Today we will introduce the degreasing method for oxygen pipelines. Degreasing, as the name suggests, means removing oil stains from the inner and outer surfaces of pipes. Degreasing is a key process in the construction of oxygen pipelines. Once oil contamination is inside the oxygen pipeline, it reacts with pure oxygen and is prone to explosion. Oxygen pipelines generally use seamless steel pipes made of 20# steel or stainless steel. For stainless steel pipes, only degreasing treatment is required, while for 20# steel pipes, not only degreasing but pickling, neutralization, and passivation treatment are required. During the construction process, the pipe mouth that has been degreased should be promptly sealed to prevent secondary pollution of the pipeline. Before delivery, it must be blown clean with oil-free compressed air.
Degreasing and pickling passivation of carbon steel pipes
For carbon steel pipes, degreasing should occur before acid washing, as grease is insoluble in acid. If acid washing is carried out first, it will not remove oil stains or rust covered by oil stains. The degreasing and acid pickling method for seamless steel pipes is commonly used in construction using the trough immersion method, where the liquid level exceeds the outer surface of the pipe by more than 50mm during immersion. Due to the need for degreasing, acid pickling, and passivation processes on oxygen pipelines, four grooves are generally made based on the pipe diameter used – namely, degreasing tank, pickling tank, neutralization tank, and passivation tank. The process flow of oxygen pipeline degreasing and acid washing is: degreasing → water flushing → acid washing → water flushing → neutralization → passivation → water flushing → drying.
The formula of degreasing solution is 20g-30g sodium hydroxide, 35g-50g Sodium nitrate, and 3g -5g Sodium metasilicate per liter of solution. The operating process requirements are: the liquid temperature is 70 ℃ – 80 ℃, and the soaking time is determined by testing methods based on the number of oil stains on the surface of the pipe, usually around 10min to 40min. The heating method can be steam. Degreasing parts should be inspected using any of the following methods.
- (1) Wipe the surface of the degreased part with clean and dry white filter paper. If there are no traces of grease on the paper, it is considered qualified.
- (2) Use 100% camphor with a thickness of 1mm or less to check the steam-blowing condensate; it is qualified if it rotates continuously.
- (3) Check the surface of the degreased part with ultraviolet light with a wavelength of 3200-3800 angstroms, and it is qualified if there is no grease or fluorescence. After degreasing treatment, the oil stains on the surface of the pipeline can be removed.
2. Water flushing
Flush clean water with a pressure of 0.8MPa.
3. Acid washing
Acid pickling uses an acid solution to remove oxide scale and rust on the surface of seamless steel pipes. Iron oxides such as oxide scale and rust (Fe3O4, Fe2O3, FeO, etc.) undergo chemical reactions with acid solutions, forming salts that dissolve in the acid solution and are removed. The most commonly used pickling methods are sulfuric acid and hydrochloric acid. The formula for sulfuric acid solution is: the concentration of sulfuric acid solution is 5% -10%. The operating process requirements are temperature 60 ℃ – 80 ℃, soaking for 5min -20min. The formula of the hydrochloric acid solution is: the concentration of the hydrochloric acid solution is 5% -20%. The operating process requirements are temperature 20 ℃ – 50 ℃, soaking for 5-20 minutes. After the pipeline is pickled, it is deemed qualified if the surface rust is removed thoroughly and the metal luster is exposed.
4. Water flushing
Rinse thoroughly with clean water at a pressure of 0.8MPa.
The neutralization solution formula is: dilute NH4OH to a solution with a pH value of 10-11. The operating process requirement is: to soak at room temperature for 3 minutes.
After acid washing, water flushing, and rinsing, the metal surface is very clean, highly activated, and prone to corrosion. Therefore, passivation treatment must be carried out immediately to form a protective film on the cleaned metal surface and slow down corrosion. The passivation solution formula is: NaNO2 concentration is 8% -10%; The concentration of NH4OH is 2%. The operating process requirements are: soak at room temperature for 10 minutes.
7. Water flushing
Flush with purified water at a pressure of 0.8MPa until clean.
Drying must be done with clean and oil-free dry air or nitrogen gas.
Wipe and inspect with white filter paper. If there are no traces of grease on the paper, it is considered qualified. After passing the inspection, seal the two ends of the pipe with plastic cloth and tape.
The inner wall of the carbon steel pipe treated according to the above method is clean and bright and can be maintained for about 2 months without rusting; If stored properly, the time can also be extended. The above method can also be used for carbon steel pipe components.
Degreasing and pickling passivation of stainless steel pipes
Because stainless steel pipes only need to be degreased, in addition to the above-mentioned degreasing methods, large diameter pipes can also be degreased by wiping. When using the wiping method for degreasing, the cotton yarn should not be used, and fabrics such as cloth, silk, and glass fiber that are not easy to fall off should be used. Long pipes can be pulled and wiped in both directions using clean iron wire and cloth strips to remove oil stains on the inner walls of the pipes. After degreasing, the surface of the degreased part must be carefully inspected. It is strictly prohibited for organic fabric fibers to fall into or adhere to the surface of the degreased part. After passing the inspection, the pipe mouth should be immediately sealed.
Degreasing and pickling passivation of stainless steel pipe fittings and valves
According to the maximum valve volume, a degreasing container made of steel plate (or modified using an iron bucket containing carbon tetrachloride) is used to remove rust and dirt from its inner wall and then wiped clean with a white cloth. Wipe the stainless steel pipe fittings and valves that need to be degreased clean with a dry white cloth (note: buy high-quality white cloth without shedding yarn), and string them with iron wire or fine steel wire rope into the container made. Inject carbon tetrachloride into the container and soak the stainless steel fittings and valves. When injecting, fill in the right amount. It is best to keep the liquid level 200mm away from the top of the container to prevent carbon tetrachloride from overflowing, causing pollution or personal injury. Because carbon tetrachloride is toxic and easily volatile, operators should stand upwind when injecting liquid to prevent poisoning caused by inhaling carbon tetrachloride. Then cover the container and soak for 1 to 2 hours before removing the pipe fittings and valves. Dry the inner and outer surfaces of the pipe fittings and valves with a silk cloth; then air dries them naturally. After the inspection, wrap and seal the pipe fittings and valves in plastic cloth. Before degreasing the valve, it should be ground and pressure tested to be qualified. Then, it should be disassembled into parts to remove rust and other debris and soaked in carbon tetrachloride solvent for 1 hour to 1.5 hours. Use the same method to degrease bolts and metal gaskets. Valve shells that are inconvenient to soak can be wiped. When degreasing non-metallic gaskets, immerse them in carbon tetrachloride solvent for 1.5h-2 hours, and then take them out and hang them in an air circulation or ventilation device to blow dry one by one until there is no solvent odor. The asbestos packing inside the valve can be ignited at around 300 ℃ for 2 to 3 minutes (no smoky flames are allowed) and then impregnated with the specified coating.
Construction safety protection measures
Before construction, technicians must provide safety and technical briefing to the construction personnel. The tools, measuring tools, instruments, etc., used for degreasing, inspection, and installation must be pre-degreased according to the requirements of the degreased parts. Do not use without degreasing. The safety measures for degreasing and acid washing should comply with the relevant provisions of the “Technical Regulations for Safety in Petrochemical Construction,” the degreasing and acid washing work should be carried out outdoors in a well-ventilated area. Personal protection shall be strengthened during work, and work clothes, rubber gloves, masks, protective glasses, rain boots, aprons, and Gas masks shall be worn if necessary.
Remember not to deceive the degreasing of oxygen pipelines. If the degreasing is thorough, it will avoid safety hazards for future production and operation, and it is more likely to lead to safety accidents. So, from the beginning of construction, it is necessary to do a good job of safety technical disclosure and make every construction personnel attach great importance to it from the ideology perspective. After degreasing, please ask the quality inspector to check and confirm that it is qualified before proceeding to the next process.