Construction Requirements for Oxygen Pipelines (Part One)
The construction of oxygen pipelines should not only comply with relevant national standards for construction of the pressure pipeline but also special safety regulations, such as strict rust removal, degreasing, welding, flaw detection, pressure and air tightness tests, purging, etc. After construction, there should be a strict acceptance check. It should be strictly checked and accepted after completion. If these processes are not strictly controlled, they will lead to an explosion for the oxygen pipeline, resulting in great losses and painful lessons.
1. The requirements for cleanliness
Before installation, oxygen pipelines, valves, and pipe fittings should be inspected in accordance with the requirements of Construction and Acceptance Specification of Industrial Metal Pipelines (GB50235). Oxygen can be treated as a combustible fluid. Their cleanliness should also meet the following requirements:
(1) The rust of carbon steel oxygen pipes and pipe fittings should be strictly removed. Derusting can be done by sandblasting, pickling and other methods. Burrs, weld beads, sand, rust and other combustibles should be thoroughly removed for surfaces exposed to oxygen, and the inner wall should be kept smooth and clean. When removing rust from the pipes, we should stop until the natural color of the metal appears.
(2) All parts in contact with oxygen such as oxygen pipes and valves should be strictly derusted and degreased before installation and after maintenance.
(3) The degreasing treatment of all parts in contact with oxygen should be carried out in accordance with the requirements of the Degreasing Engineering Construction and Acceptance Specification (HG20202), including the surfaces of all components in contact with the fluid. If there are different requirements for engineering design documents, they shall be implemented in accordance with the provisions of the engineering design documents. Degreasing can be performed with inorganic non-flammable cleaning agents, dichloroethane, trichloroethylene and other solvents, and inspected by ultraviolet inspection, camphor inspection or solvent analysis until it is qualified.
The degreased carbon steel oxygen pipeline should be passivated immediately or filled with dry nitrogen to close the entrance of the pipeline. For pipes undergoing hydraulic tests, the inner wall of the pipe should be passivated after degreasing to prevent corrosion. The degreased piping components should be blown with nitrogen or air and then sealed to prevent re-contamination, and the remaining degreasing medium from forming a dangerous mixture with oxygen.
Effective measures should be taken during and after installation to prevent the oxygen pipeline from being contaminated by grease, and avoid combustibles, rust, welding slags, sand and other debris from entering or staying in the pipe. Strict inspections should be carried out.
2. Requirements of installation, welding, construction, and acceptance of pipelines
The installation, welding, construction, and acceptance of oxygen pipelines should not only meet the requirements of the GB50235 standard and the Specifications for Construction and Acceptance of Field Equipment and Welding Engineering of Industrial Pipelines (GB 50236) but also meet the following requirements:
(1) When welding carbon steel and stainless steel oxygen pipes, argon arc welding should be used for bottoming to facilitate good welding and prevent welding slags from forming on the inner wall of the pipe.
(2) The cutting and beveling of the pipe should adopt mechanical methods, and gas welding cutting and beveling are not allowed to ensure the processing accuracy and facilitate welding.
(3) The prefabricated length of the pipeline should not be too long, and it should be easy to check the quality of installation, welding, and cleanliness of the inner and outer surfaces of the pipeline.
(4) Radiographic testing should be used for welding of pipelines. When hydraulic testing is used, the number and standards of testing should be implemented in accordance with relevant requirements (see Table 6). Table 6 is quoted from the GB 16912 standard.
Table 6 Inspection requirements for the welding of oxygen pipelines
1. The requirements for cleanliness
Before installation, oxygen pipelines, valves, and pipe fittings should be inspected in accordance with the requirements of Construction and Acceptance Specification of Industrial Metal Pipelines (GB50235). Oxygen can be treated as a combustible fluid. Their cleanliness should also meet the following requirements:
(1) The rust of carbon steel oxygen pipes and pipe fittings should be strictly removed. Derusting can be done by sandblasting, pickling and other methods. Burrs, weld beads, sand, rust and other combustibles should be thoroughly removed for surfaces exposed to oxygen, and the inner wall should be kept smooth and clean. When removing rust from the pipes, we should stop until the natural color of the metal appears.
(2) All parts in contact with oxygen such as oxygen pipes and valves should be strictly derusted and degreased before installation and after maintenance.
(3) The degreasing treatment of all parts in contact with oxygen should be carried out in accordance with the requirements of the Degreasing Engineering Construction and Acceptance Specification (HG20202), including the surfaces of all components in contact with the fluid. If there are different requirements for engineering design documents, they shall be implemented in accordance with the provisions of the engineering design documents. Degreasing can be performed with inorganic non-flammable cleaning agents, dichloroethane, trichloroethylene and other solvents, and inspected by ultraviolet inspection, camphor inspection or solvent analysis until it is qualified.
The degreased carbon steel oxygen pipeline should be passivated immediately or filled with dry nitrogen to close the entrance of the pipeline. For pipes undergoing hydraulic tests, the inner wall of the pipe should be passivated after degreasing to prevent corrosion. The degreased piping components should be blown with nitrogen or air and then sealed to prevent re-contamination, and the remaining degreasing medium from forming a dangerous mixture with oxygen.
Effective measures should be taken during and after installation to prevent the oxygen pipeline from being contaminated by grease, and avoid combustibles, rust, welding slags, sand and other debris from entering or staying in the pipe. Strict inspections should be carried out.
2. Requirements of installation, welding, construction, and acceptance of pipelines
The installation, welding, construction, and acceptance of oxygen pipelines should not only meet the requirements of the GB50235 standard and the Specifications for Construction and Acceptance of Field Equipment and Welding Engineering of Industrial Pipelines (GB 50236) but also meet the following requirements:
(1) When welding carbon steel and stainless steel oxygen pipes, argon arc welding should be used for bottoming to facilitate good welding and prevent welding slags from forming on the inner wall of the pipe.
(2) The cutting and beveling of the pipe should adopt mechanical methods, and gas welding cutting and beveling are not allowed to ensure the processing accuracy and facilitate welding.
(3) The prefabricated length of the pipeline should not be too long, and it should be easy to check the quality of installation, welding, and cleanliness of the inner and outer surfaces of the pipeline.
(4) Radiographic testing should be used for welding of pipelines. When hydraulic testing is used, the number and standards of testing should be implemented in accordance with relevant requirements (see Table 6). Table 6 is quoted from the GB 16912 standard.
Table 6 Inspection requirements for the welding of oxygen pipelines
| Design pressure/MPa | Radiographic ratios | Quality assessment for welding seams |
| P being greater than 4.0 | 100 | Class II |
| P being between 1.0 and 4.0 | 40 (fixed welded ports) 15 (rotated welded ports) |
Class II |
| P being less than and equal to 1.0 | 10 | Class III |
| Pipelines with cryogenic liquids | 100 | Class II |
Please note that the quality assessment standard for welding seams is The Radiograph of Welding Joints of Metal Fusion (GB/T 3323). Table 6 divides the design pressure into three pressure ranges. The higher the pressure is, the greater the risk becomes, and the higher the quality requirements for welding becomes. The greater the proportion of weld Х-ray inspection is, the higher the quality level requirements for welding becomes.
When gas is used for the pressure test, the danger is great, and the radiographic inspection ratio of welding seams should be increased. The specific requirements are as follows: when the design pressure is not greater than 0.6MPa, the inspection ratio is not less than 15%, and the quality level for welding is not less than grade Ⅲ. When the design pressure is greater than 0.6MPa and less than or equal to 4.0MPa, the detection ratio is 100%, and the quality level of welding seams is not lower than grade Ⅱ.
(5) For welding seams that are not required for non-destructive testing, the quality inspector shall conduct the visual inspection for all visible parts of the welding seam, and the quality shall comply with the relevant provisions of the GB 50236 standard.
