Aluminum alloy materials are widely used in modern industrial production, especially in air separation devices. It is a commonly used process pipe material. Due to the unique characteristics of aluminum alloy materials and the production process of air separation units, the cleaning, degreasing, processing, assembly, welding and other processes and requirements of aluminum alloy pipelines are different from ordinary carbon steel materials, and some aspects are much more difficult than carbon steel. This article takes the commonly used aluminum alloy pipe LF2 in air separation units as an example to briefly explain the characteristics and relevant technical requirements of aluminum alloy pipeline construction.
1、 Material characteristics
The main chemical composition of aluminum 6082 alloy tube LF2 is: Mg content is 2.0%~2.8%, Mn content is 0.4~1.5%, and the rest is Al; Mechanical properties: In the hot rolled state, its tensile strength shall not be less than 226MPa; The melting point is about 650 ℃, and there is no color change during melting.
2、 Cleaning and degreasing
The air separation unit has the characteristics of low temperature and deep cooling, oxygen combustion support, and explosive when encountering oil. Therefore, it is necessary to clean and degrease the pipes in the cold box of the air separation unit. The cleaning and degreasing process for aluminum alloy pipes is as follows: alkali washing → water washing → nitric acid photochemical treatment → water washing → degreasing → water washing.
Before degreasing aluminum 6063 alloy pipes, dirt, garbage, etc. inside the pipes must be removed. The use of solvents is strictly prohibited for degreasing agents, and polyvinyl chloride or solvents must be used. Due to the fact that degreasing agents are flammable and hazardous substances, measures such as environmental ventilation, avoiding direct sunlight exposure, and strengthening the physical protection of workers must be taken during use and storage.
When degreasing small diameter pipes and fittings, direct immersion in degreasing agent can be used; Large diameter pipelines can be degreased by using degreasing agent infusion and immersion degreasing method, which repeatedly tilts the pipeline during degreasing, thereby fully degreasing the pipeline; According to the actual situation, the scrubbing and degreasing method can also be used. After the degreased pipeline is air dried, it should be sealed in a timely manner to avoid contamination by dirt or oil.
The degreased pipeline should have no odor of degreaser. The inspection method can be done by wiping the degreased surface with clean and dry white filter paper. If there are no traces of oil or dirt on the paper, it is considered qualified; UV lamps can also be used for irradiation, and it is qualified if there is no violet fluorescence.
3、 Pipeline production and installation
The pipe space inside the cold box of the air separation unit is compact, with complex directions and varying pipe diameters, which increases the difficulty of pipeline installation. To ensure construction quality, pipeline construction should be made as much as possible outside the cold box and then assembled inside the cold box. Therefore, before construction, it is necessary to draw a single line diagram of the pipeline based on the pipeline plan and elevation, and sometimes even use the method of making a model to improve the depth, quality, and speed of pipeline prefabrication.
Aluminum alloy pipeline prefabrication needs to be carried out indoors. Due to the softness of 3d printing aluminum alloy, its standard potential value is -1.67 volts. Therefore, a 4mm thick rubber pad should be laid on the prefabrication site to prevent potential corrosion. The production of aluminum alloy pipelines must be separated from carbon steel and stainless steel.
The installation of aluminum alloy pipelines must follow the following principles: first large pipes, then small pipes; First lower, then upper; First in charge, then in charge; Prefabricate first, then configure; The distance between the heating tube and the wall of the low-temperature tube and liquid container shall not be less than 300mm; The pipeline installation starts from the upper part of the cold box of the air separation unit and proceeds downwards. Manufacturing and installation of aluminum alloy pipelines
Link to this article：Processing and installation of aluminum alloy pipe
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