Everyone knows that oxygen pipes must be oil-free and degreased. Today we will discuss the method of degreasing oxygen pipes. Degreasing, by definition, is the removal of oil stains from the inner and outer surfaces of the pipe.
It is a crucial step in the construction of oxygen pipelines. If there is any oil contamination inside the oxygen tube, an explosion can easily occur when it comes into contact with pure oxygen.
Oxygen pipelines generally use seamless steel pipes made of 20# steel or stainless steel. For stainless steel pipes, only a degreasing treatment is required.
However, for pipes made of 20# steel, not only degreasing but also pickling, neutralization and passivation treatment are required.
During construction, degreased pipe ends must be immediately sealed to prevent secondary contamination. Before being handed over for use, they must be carefully cleaned with oil-free compressed air.
1. Degreasing, Pickling and Passivation of Carbon Steel Pipes
Carbon steel pipes must be degreased before pickling as the grease is not soluble in acid. If pickling is done first, it will not remove the oil stains or the rust beneath the oil stains.
The dipping method is commonly used for degreasing and pickling seamless steel pipes during construction. During immersion, the liquid level must be at least 50 mm above the outer surface of the tube.
Since oxygen pipelines undergo degreasing, pickling and passivation treatment, four tanks are generally prepared based on the actual diameter of the pipeline. These are the degreasing tank, the pickling tank, the neutralization tank and the passivation tank.
The degreasing and pickling process flow for oxygen pipelines is as follows: degreasing → water rinse → pickling → water rinse → neutralization → passivation → water rinse → drying.
1.1 Degreasing
The formula of the degreasing solution is: each liter of solution contains 20g to 30g of sodium hydroxide, 35g to 50g of sodium nitrate and 3g to 5g of sodium silicate.
The operational process requires: a liquid temperature of 70°C to 80°C and an immersion time determined by the amount of oil on the surface of the tube, generally between 10 and 40 minutes.
Steam can be used for heating.
Degreased items should be inspected using any of the following methods:
- Clean the surface of the degreased piece with a clean, dry, white filter paper. If there are no traces of oil on the paper, it is considered qualified.
- Use 100% camphor less than 1 mm to inspect steam drying condensate. It is considered qualified if it continues to spin.
- Inspect the surface of the degreased item with ultraviolet light with a wavelength of 3,200 to 3,800 Angstroms. It is considered qualified if there is no oil shine. After degreasing, the oil on the pipe surface can be removed.
1.2 Rinse with Water
Rinse thoroughly with clean water at a pressure of 0.8 MPa.
1.3 Pickling
Pickling uses an acidic solution to remove oxide film and rust from the surface of seamless steel pipe.
Iron oxides (Fe3O4, Fe2O3, FeO, etc.) react with the acidic solution to form salts that are soluble in the acidic solution and are thus removed. Sulfuric acid and hydrochloric acid are most commonly used for pickling. The formula of sulfuric acid solution is: the concentration of sulfuric acid is 5% to 10%.
The operational requirement is: temperature of 60°C to 80°C and immersion time of 5 to 20 minutes. The formula of hydrochloric acid solution is: the concentration of hydrochloric acid is 5% to 20%.
The operational requirement is: temperature of 20°C to 50°C and immersion time of 5 to 20 minutes.
After stripping the pipe, check whether the surface rust is completely removed and the metal shine is exposed. You are considered qualified if this is the case.
1.4 Rinse with Water
Rinse thoroughly with clean water at a pressure of 0.8 MPa.
1.5 Neutralization
The formula of the neutralization solution is: NH4OH diluted to a pH value of 10 to 11. The operational requirement is: soak at room temperature for 3 minutes.
1.6 Passivation
After pickling, water rinsing and bleaching, the metal surface is very clean and highly activated, being very susceptible to corrosion. Therefore, passivation treatment must be carried out immediately to form a protective film on the clean metal surface to delay corrosion.
The formula of the passivation solution is: the concentration of NaNO2 is 8% to 10%; the concentration of NH4OH is 2%. The operational requirement is: soak at room temperature for 10 minutes.
1.7 Rinse with Water
Rinse with purified water at a pressure of 0.8 MPa until clean.
1.8 Drying
Drying must be done with clean, oil-free and dry air or nitrogen.
1.9 Inspection
Clean and inspect with white filter paper. If there are no traces of oil on the paper, it is considered qualified. After passing inspection, seal both ends of the pipe with plastic cloth and tape.
Carbon steel pipes treated in this way have clean, shiny inner walls and can remain rust-free for about 2 months; if stored well, this period can be extended. This method can also be applied to carbon steel pipe fittings.
II. Degreasing, Pickling and Passivation of Stainless Steel Pipes
For stainless steel pipes that only require degreasing, in addition to the above-mentioned degreasing methods, the cleaning technique can also be used for larger diameter pipes.
When using the cleaning method, cotton thread is not recommended; instead, cloths, silks, or fiberglass fabrics that are less likely to shed fibers should be used.
For longer tubes, clean iron wires can be used to attach fabric strips, dragging and wiping bidirectionally to remove oily contamination from the inner wall of the tube.
After degreasing, a careful inspection of the surface of the degreased component is mandatory to avoid the attachment or falling out of organic textile fibers. Once inspected and found to be satisfactory, the pipe ends must be sealed immediately.
III. Degreasing, pickling and passivation of stainless steel valves and pipe fittings
Depending on the larger volume of the valve, a customized degreasing container can be created from steel plates (or a simple modification of an iron drum used to store carbon tetrachloride).
After removing the rust and dirt from its internal wall, it must be cleaned with a white cloth.
Stainless steel pipe fittings and valves that require degreasing should be wiped with a dry white cloth (Note: It is recommended to use a high-quality white cloth that does not shed fibers), tied with iron wire or thin steel cables, and placed in the created container.
Fill the container with carbon tetrachloride to soak the stainless steel pipe fittings and valves. Avoid overfilling; Ideally, maintain a distance of 200 mm between the liquid level and the top of the container to avoid spillage of carbon tetrachloride, causing pollution or personal injury.
As carbon tetrachloride is toxic and evaporates easily, the operator must stand upwind during filling to avoid inhalation and, consequently, poisoning.
Then cover the container, soak for 1-2 hours, remove the pipe connections and valves, dry the inner and outer surfaces with a silk cloth and let them dry naturally.
After passing inspection, pipe fittings and valves must be individually packaged and sealed in plastic. Before degreasing valves, grind and pressure test them, then disassemble them to remove rust and other debris before soaking them in carbon tetrachloride solvent for 1 to 1.5 hours.
Screws and metal joints must be degreased using the same method. Valve bodies that are not suitable for immersion can be cleaned.
Non-metallic gaskets should be soaked in carbon tetrachloride for 1.5 to 2 hours during degreasing and then hung in a well-ventilated area or inside a ventilation device to be individually dried until no solvent odor remains.
For asbestos fillings inside valves, burn at about 300°C for 2-3 minutes (do not use a smoldering flame) and then dip in a specific layer.
4. Construction safety measures
Before construction, technical personnel must inform construction personnel about safety and techniques. Tools, measuring instruments and other devices used for degreasing, inspection and installation must be pre-degreased in accordance with the requirements of the degreased parts.
Use without degreasing is not permitted. Safety measures for degreasing and pickling must follow the relevant regulations of the “Technical Regulation on Safety in Petrochemical Construction”, and degreasing and pickling work must be carried out in a well-ventilated outdoor area.
Personal protection must be reinforced during work, including the use of work clothes, rubber gloves, masks, protective glasses, rain boots, aprons and, when necessary, a gas mask.
