Installation of oxygen pipeline
A large amount of oxygen is used in ironmaking, steelmaking, on-line cutting, semi-finished product renovation and other processes in metallurgical plants, which need to be sent to users from oxygen generation station through storage tank and pressure reducing device by oxygen pipeline. The material of oxygen pipe is mostly carbon steel pipe; Stainless steel pipe should be used for the parts with serious corrosion; Small diameter can also be used copper tube or aluminum tube. The diameter of oxygen pipe varies, and the oxygen pipe used in large converter steelmaking can reach 350mm.
Oxygen is a kind of strong combustion supporting agent, which is easy to explode when mixed with combustible gas such as gas. It may also cause spontaneous combustion or explosion when contacting with grease. In order to ensure safety, the installation of oxygen pipeline must ensure no leakage, and the pipeline system must be degreased and purged strictly.
Before construction preparation and installation, the following preparations shall be made according to the design drawings and relevant technical regulations of oxygen pipe network:
- (1) Establish the pipeline installation plan, determine the degreasing method and pipeline purging process of pipes, valves, gaskets, etc. Make specific technical regulations for the hoisting, assembly, welding and test of the pipeline.
- (2) Quality inspection of pipes, pipe fittings and valves.
- (3) Measure the position and elevation of pipeline center line, support foundation, embedded parts and trench.
There are two construction procedures: degreasing before installation and degreasing after installation:
- (1) Degreasing before installation: degreasing of pipes and accessories – Installation – pressure test – purging – oxygen test.
- (2) Degreasing after installation: the pipeline is installed in one section (or whole section circulation), degreasing, pressure test, purging and oxygen test.
The oxygen pipeline in oxygen station, buried oxygen pipeline and oxygen pipeline with more valves on the pipeline usually adopt the method of degreasing before installation; Most of the pipes in the plant and workshop are installed before degreasing. When there are many valves on the pipeline, the method of flushing before pressure test can also be adopted.
Derusting is one of the key links to ensure the installation quality of oxygen pipeline. Before the installation of oxygen pipes, valves and joints, the surface contacting oxygen shall be inspected to remove dirt, rust and burr. Derusting methods include pickling, sandblasting, mechanical derusting, manual brush derusting, etc. When the number of pipelines is large, pickling is better. The process is pickling water washing neutralization water washing and passivation drying. After pickling, apply antirust primer and finish coat on the outer surface of the pipe (not on the welded joint) to prevent rust return.
Degreasing is one of the key links to ensure the installation quality of oxygen pipeline. The degreasing solvents are carbon tetrachloride, dichloroethane, rectification ethanol, trichloroethylene and so on. Carbon tetrachloride and dichloroethane are toxic oxygen and corrosive. Dichloroethane and distilled ethanol are flammable and explosive. Protective measures should be taken when using them. The degreasing place should be well ventilated and no fireworks are allowed.
The degreasing methods include trough degreasing method, lavage method, pull washing method and online degreasing method:
- (1) Trough degreasing method. Make degreasing tank according to the length and quantity of pipes, put degreasing agent into the tank, clean the pipes and pipe fittings to be degreased in the tank for 10 ~ 15min, take them out for drying and sealing.
- (2) Lavage method. It is suitable for pipes and containers. One end of the pipe should be plugged tightly with a wooden plug and filled with degreasing agent. Then block the other end, turn it over or rotate it to make the contact time between the inner surface and degreaser not less than 15min.
- (3) Lavage. The plug is made of old cloth and iron wire, dipped in degreasing agent, dragged back and forth in the lumen, and finally wiped clean with dry cloth, and sealed after air drying.
- (4) Online degreasing method. There are two kinds of degreasing: non circulation degreasing and circulation degreasing. Non circulating degreasing is to install the pipeline in place first, and degreasing is generally carried out in sections of 150-200m. The method is to install temporary pump, water distribution tank and degreaser barrel at one end of the pipe, connect them in series with temporary pipes, and install temporary discharge pipe and vent pipe at the other end of the pipe. In the degreasing process, it is necessary to master the filling time, pay attention to reduce the temperature of the pipe, and connect the degreased pipe sections after degreasing. Circulating degreasing is to connect the pipe into a loop, so that the degreasing agent can be recycled in the pipe, which can save the time and cost of degreasing.
Attention should be paid during degreasing:
- (1) When degreasing with carbon tetrachloride and dichloroethane, the surface of metal pipe fittings shall be kept dry without moisture, so as to prevent degreasing agent from corroding metal.
- (2) Trichloroethylene solvent shall be used as degreasing agent for aluminum pipe fittings.
- (3) The nonmetallic liner can only be degreased with carbon tetrachloride and then dried in a cool and ventilated place. Asbestos liner degreasing is to burn in smokeless flame at about 300 ℃ for 2-3 min, and then apply graphite powder.
- (4) When the degreased pipes and valves are dried with compressed air, there shall be no oil or water in the air.
- (5) For degreased valves, air tightness test shall be conducted after degreasing.
- (6) Check the degreasing quality. Generally, wipe the inner wall of the pipe with dry and white filter paper, and there should be no oil stains on the paper; The degreased surface should have no violet fluorescence.
Most of the oxygen pipelines in metallurgical plants are overhead, and some of them are underground; The oxygen pipe network in the plant area is generally laid together with the gas pipeline, and the pipeline in the workshop is usually laid along the wall and column. When the overhead oxygen pipeline and the gas pipeline are supported together, the supports are welded on the gas pipeline, and they need to be installed after the gas pipeline is fixed. Before hoisting, the large parts shall be assembled on the ground as far as possible. Generally, 2-3 steel pipes shall be assembled into a group. The self-propelled crane shall be used to lift and place them on the bracket whose center line and elevation have been found, and then the alignment and connection shall be carried out group by group. The curvature radius of elbow and branch pipe of oxygen pipeline is more than 5 times of pipe diameter, and the angle of branch pipe should be 45 ~ 45 with the direction of main pipe air flow 60。 The laying of buried oxygen pipeline generally adopts the method of section by section construction. Dewatering measures shall be taken for construction in the section with high groundwater level, and degreased pipe fittings shall be protected to prevent any dirt from entering. The pipe should keep a certain slope along the direction of the drainer. The oxygen pipeline laid in trench shall be designed as a separate trench, which shall not be connected with other trenches. After being qualified by purging and pressure test, it shall be filled with dry sand and sealed with cover plate.
When welding is used for pipe connection, the first layer of carbon steel pipe is welded by argon arc welding, and the other layers are welded by electric arc welding. Argon arc welding is commonly used for welding of aluminum and copper pipes. Non destructive testing is used for spot check of welds. When the flange is used for connection, the welding joint of the flange shall be checked for coloring. In order to export static electricity, the pipe flange connection should be welded with conductive sheet, generally copper sheet or flat steel. The welding length between the two ends and the pipe should be greater than 50 mm, and the static grounding device should be made according to the design requirements. Generally, the gasket at the flange joint shall be annealed and softened aluminum sheet or copper sheet, and the tools for installing flange and valve parts shall be degreased.
After the pressure test pipeline is installed, the system strength test and tightness test shall be conducted according to the design requirements. The test medium can be oil-free dry air, nitrogen or oil-free water, and the gas medium is more. When the test pressure is not specified in the design, 1.25 times of the working pressure can be taken as the strength test pressure. The pressure should rise slowly step by step and last for 10min after rising to the strength test pressure value. It is qualified if there is no deformation and leakage. After that, the working pressure is reduced to the working pressure for tightness test, and the inspection is carried out by means of soapy water. It is qualified if there is no leakage and the pressure does not drop for half an hour at constant pressure.
Generally, oil-free dry compressed air or nitrogen is used for purging pipeline system. Remove the valve and orifice plate before blowing and connect them with short pipe temporarily. Pay attention to:
- (1) All temporary short pipes and accessories shall be degreased with carbon tetrachloride.
- (2) The pressure of gas source should meet the design requirements, and the flow rate should not be less than 20m/s.
- (3) Check the outlet with white cloth or target plate coated with white paint, and it is qualified if there is no rust, dust, moisture and other stains within 5 minutes.
- (4) Both sides of the welded joint shall be hammered to blow out the welding slag in the pipe.
- (5) When purging with nitrogen, the staff should stand in the upwind or take other protective measures to prevent suffocation.
- (6) All pipes shall be blown to the bottom without dead end. If the pipe network is branched, each branch pipe shall be blown separately. If the blockage is found, it is necessary to conduct it with the reverse purge tool in time.
After pressure test and purging, the pipeline shall be coated. When applying oil, tape the flange to prevent the paint from flowing into the gasket. After the installation and acceptance, if the factory can not send oxygen in time, the pipe shall be sealed with oil-free nitrogen, and the pressure value of 0.1MPa shall be maintained to prevent rust.
Source: China Oxygen Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)