Discussion on degreasing method of oxygen pipeline
The oxygen pipeline has high requirements in the pipeline installation project, which requires not only argon arc welding bottoming and weld shooting, but also degreasing treatment of the pipeline. Based on years of construction experience, the author analyzes and summarizes the degreasing methods of oxygen pipelines, hoping that colleagues can refer to them in the future.
What is degreasing?
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Degreasing, as the name implies, is to remove the oil stain on the inner and outer surfaces of the pipe. Degreasing is the key process of oxygen pipeline construction. Once there is oil in the oxygen pipe, it will react with pure oxygen and easily explode. The oxygen pipe is generally 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 also pickling, neutralization, and passivation treatment are required. During the construction, the pipe orifice after degreasing shall be blocked in time to prevent the pipe from being polluted again. Before delivery, it must be purged with oil-free compressed air.
1. Degreasing, Pickling, and Passivation of Carbon Steel Pipe
The carbon steel pipes shall be degreased before pickling, because the grease is insoluble in acid solution, and the first pickling can neither remove the oil stain nor remove the rust covered by the oil stain. At present, the degreasing pickling method of seamless steel pipes is often used in the construction of the trough immersion method, and the liquid level is more than 50 mm above the external surface of the pipes during immersion. As the oxygen pipeline is subject to degreasing, pickling, and passivation process, four tanks are generally made according to the actual pipe diameter. Namely, the degreasing tank, pickling tank, neutralization tank, and passivation tank.
The degreasing and pickling process flow of the oxygen pipeline is as follows:
Degreasing → water flushing → pickling → water flushing → neutralization → passivation → water flushing → drying.
The formula for degreasing solution is 20g-30g sodium hydroxide, 35g-50g sodium nitrate, and 3g – 5g sodium silicate per liter of solution. The operation process requirements are: the liquid temperature is 70 ℃ – 80 ℃, and the immersion time is determined by the test method according to the amount of oil contamination on the pipe surface, generally about 10min-40min. The heating method can be steam. Degreasing parts shall be inspected by any of the following methods.
- (1) Wipe the surface of degreased parts with clean and dry white filter paper, and it is qualified if there is no grease trace on the paper.
- (2) Use 100% camphor less than 1mm to check the steam purging condensate, and it is qualified if it rotates continuously.
- (3) Check the surface of degreased parts with an ultraviolet ray with a wavelength of 3200-3800 A, and it is qualified if there is no grease fluorescence. After degreasing, the oil stain on the pipe surface can be removed.
1.2 Water flushing
Clean the water with a pressure of 0.8MPa.
Pickling is to remove oxide skin and rust on the surface of seamless steel pipe with an acid solution. Iron oxides (Fe3O4, Fe2O3, FeO, etc.) such as scale and rust react with an acid solution to form salts that are dissolved in the acid solution and removed. The most commonly used pickling methods are sulfuric acid and hydrochloric acid. The formula of the sulfuric acid solution is: the concentration of the sulfuric acid solution is 5% – 10%. The operation 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 operation process requirements are temperature 20 ℃ – 50 ℃, soaking for 5min – 20min. After the pipeline is pickled, the rust on the surface shall be checked and removed, and the pipeline shall be qualified if the metal luster is exposed.
1.4 Water flushing
Rinse with clean water with a pressure of 0.8MPa.
The neutralizer formula is NH4OH solution diluted to pH 10-11. Operation process requirements: soak for 3 minutes at room temperature.
After acid pickling, water washing, and rinsing, the metal surface is very clean, very active, and easy to corroded. Therefore, passivation treatment must be carried out immediately to make the cleaned metal surface generate a protective film to slow down corrosion. The passivation solution formula is: the concentration of NaNO2 is 8% – 10%; The concentration of NH4OH is 2%. The operation process requirement is: to soak for 10min at room temperature.
1.7 Water flushing
Rinse with purified water with a pressure of 0.8MPa.
For drying, clean oil-free dry air or nitrogen must be used.
Wipe and check with white filter paper. If there is no grease trace on the paper, it is qualified. After the inspection is qualified, seal the pipe orifices at both ends with plastic cloth and tape.
The inner wall of carbon steel pipe treated according to the above method is clean and bright, and can be kept free of rust for about 2 months; If well preserved, it can be extended. The above methods can also be used for carbon steel pipe fittings.
2. Degreasing of stainless steel pipes
As stainless steel pipes only need to be degreased, in addition to the above degreasing methods, the large diameter pipes can also be degreased by wiping. Cotton yarn shall not be used for degreasing by wiping, and cloth, silk, glass fiber fabric, etc. with fibers that are not easy to fall off shall be used. Long pipes can be dragged and scrubbed in both directions with clean iron wire binding cloth strips to remove oil stains on the inner wall of pipes. After degreasing, the surface of degreased parts must be carefully checked. It is strictly forbidden for organic fabric fibers to fall into and adhere to the surface of degreased parts. After the inspection is qualified, the pipe orifice shall be blocked immediately.
3. Degreasing of stainless steel pipe fittings and valves
According to the volume of the largest valve, a degreasing container made of steel plate (or simply refitted from an iron bucket containing carbon tetrachloride) is used to remove the rust and dirt on its inner wall, and then it is wiped clean with a white cloth. Wipe the stainless steel pipe fittings and valves to be degreased with dry white cloth (note: white cloth shall be of better quality without shedding wool), string them with iron wire or thin wire rope, and put them into the container. Inject carbon tetrachloride into the container and soak the stainless steel pipe fittings and valves. Do not fill too full when filling. It is better to keep the liquid level 200mm away from the upper mouth of the container to prevent carbon tetrachloride from overflowing, causing pollution or personal injury. Since carbon tetrachloride is toxic and easy to volatilize, operators should stand upwind when injecting liquid to prevent poisoning due to inhalation of carbon tetrachloride. Then cover the container, soak for 1h-2h, take out the pipe fittings and valves, wipe the inner and outer surfaces of the pipe fittings and valves with silk cloth, and then air dry them naturally. After passing the inspection, the pipe fittings and valves shall be wrapped and sealed by category with plastic cloth. Before degreasing, the valve shall be grinded and pressure tested to be qualified, then disassembled into parts to remove rust and other impurities, and put into carbon tetrachloride solvent to soak for 1h-l 5h. Bolts and metal gaskets shall be degreased in the same way. The valve shell that is inconvenient to soak can be wiped. During degreasing, non-metallic gaskets shall be immersed in carbon tetrachloride solvent for 1.5h-2h, and then taken out and hung in the air circulation place or ventilation device to dry one by one until there is no solvent smell. The asbestos packing in the valve can be burned for 2min-3min at a temperature of about 300 ℃ (no smoky flame is allowed), and then impregnated with the specified paint.
4. Construction safety protection measures
Before construction, technicians must make safety and technical disclosure to the construction personnel. Tools, measuring tools, and instruments used for degreasing, inspection, and installation must be degreased in advance according to the requirements of degreased parts. Do not use without degreasing. The safety measures for degreasing and pickling shall comply with the relevant provisions of Technical Specification for Petrochemical Construction Safety, and the degreasing and pickling work shall be carried out in an outdoor well-ventilated area. Personal protection shall be strengthened during work. Work clothes, rubber gloves, masks, protective glasses, rain boots, aprons, and gas masks shall be worn when necessary.
The oxygen pipeline degreasing must not be fooled. If the degreasing is not complete, it will leave a potential safety hazard for future production and operation, which is more likely to lead to safety accidents. Therefore, from the beginning of construction, safety technical disclosure should be done well, and every construction personnel should pay attention to it from the perspective of ideology, and carefully degrease. After degreasing, the next process can be carried out only after the quality inspector checks and confirms that it is qualified.
Author: Hua Dejun
Source: China Oxygen Pipeline Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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