Measures for Leakages of Hydrogen Pipelines in Refining & Chemical Hydrogen Production Units

3. The research on the failure
3.1 Design

All stainless steel pipes and pipe fittings of this device are ordered according to the "Comprehensive Material Lists" of the detailed engineering design of the design institute, and the material is 0Cr18Ni10Ti. Comply with design specifications "General Principles for Layout and Design of Petrochemical Pipelines" SH3012-2000, "Specifications for Design of Thermal Insulation of Petrochemical Equipment and Pipelines" SH3010-2000, "Design Specifications for Industrial Metal Pipelines" GB50316-2000). According to the design requirements, the pipe fittings are manufactured in accordance with the SH3408 "Steel Butt Welded Seamless Pipe Fittings" standard and the GB14976 "Stainless Steel Seamless Steel Pipe for Transporting Fluids" standard. After careful analysis and discussion by the relevant professional and technical personnel of the design institute on the problem of the reducer of the hydrogen production unit, the following conclusion came: The pipeline made from SS321 will not cause hydrogen corrosion based on the working temperature and working pressure, and complies with the "Petrochemical Pipeline Design" According to the material selection regulations of "General Rules for Equipment Selection"; this pipeline is based on the maximum working temperature of the medium of 420°C and the maximum working pressure of 3.5MPa. According to the calculation formula in the "General Rules for Equipment Selection of Petrochemical Pipeline Design", the maximum wall thickness of the elbow is only 7.85mm after taking into account the negative deviation. According to SH 3405, the design wall thickness should be SCH 40s, that is, 9.5mm. After considering the negative deviation, the inner wall thickness of the elbow only needs a maximum of 9.6mm. The design wall thickness should be 13mm. After considering the negative deviation, the maximum inner wall thickness of the elbow only needs to be 14.2mm, and the design wall thickness is 15mm. Therefore, there is no problem with wall thickness design; the water injection position is based on the equipment drawings and technical requirements provided by the desuperheater and pressurizer manufacturer. There is nothing wrong with the piping; in the operating state of this device, the corrosion of chloride ions should be mainly considered for the stress corrosion of stainless steel, while the conversion catalyst has more stringent requirements for chloride ions; the catalyst has not yet been poisoned, so it is considered that exceeding chloride ions can be eliminated.
 
3.2 Manufacturing pipe fittings
During the investigation, the manufacturers were required to provide quality certificates of raw materials, manufacturing process data and quality certification documents for these three batches of pipe fittings (original records of all production and manufacturing processes, intergranular corrosion test reports of raw materials and finished products and quality certification documents), original heat treatment records (all original heat treatment records and temperature curves. According to the information provided by the manufacturer, no problems were found when the raw materials for pipe fitting manufacturing were inspected at the factory and the quality control of the pipe fitting manufacturing process met the design standards.
 
3.3 Construction
Check the construction design instructions of the hydrogen production unit and the welding process evaluation of the construction unit to confirm that the design does not require post-weld heat treatment. Considering that stainless steel is prone to chloride ion corrosion, the re-inspection report of the insulation material entering the factory was inspected, and the manufacturer was required to provide an analysis report on the quality of the cooling water used for rinsing after pickling. No excessive chloride ions were found. At the same time, the project management department and supervision company also re-examined all on-site inspection and verification data for the construction of the hydrogen production unit and no problems were found.
 
3.4 On-site inspection of cracked pipe fittings
At the installation site, inspection and testing personnel were arranged to conduct a hardness test on a reducer of DN500*400 where cracks were found. The average Brinell hardness (HB) was 146 and the highest value was 161. According to the standard provisions of SH3408-96 "Steel Butt Welded Seamless Pipe Fittings", the hardness value (HB) of austenitic stainless steel pipe fittings should be less than and equal to 190, meeting the standard requirements. At the same time, the hardness of the remaining stainless steel reducers of DN200*150 was tested. The average Brinell hardness value was 140 and the highest value was 180, which met the standard requirements. For the failed pipe fittings, a technical center was entrusted to conduct macroscopic, low-magnification analysis, chemical analysis, scanning electron microscope analysis and metallographic analysis, and the following conclusions were drawn: the material of the reducing pipe is 0Cr18Ni10Ti austenitic stainless steel, and the metallographic structure is solid solution Austenitic structure. The cracking at the reducer is caused by corrosion fatigue.
 
3.5 Process media and process operation inspections
Through the analysis of chloride ions in the deoxygenated water, no chlorine ions were found, and there were no excessive media; the investigation of process operation records showed that the temperature and pressure during operation were in compliance with the process indicators and design specifications. The influence of process operations is therefore excluded.
 
4. Conclusion
In response to the leak, we organized a comprehensive and systematic investigation and related technical research from aspects such as pipeline design, pipe fitting manufacturing, pipeline installation, relevant inspection and testing items of in-service pipe fittings, and production operations. It was concluded that there was an issue with manufacturing for 6 pipe fittings and fatigue for 1 pipe fitting.
 
5. Measures
5.1 After research and inspection of other similar devices and technical consultation with its design institute, it is believed that there is no problem in pipeline design and material selection. However, through investigation of similar devices and comparison of the pipeline design in the same process area with similar devices, it was found that the pipe support layout is different because there is no support at one end. This time it has been redesigned by another design institute. The original design institute has confirmed and approved that new pipe support can be provided to the failed reducer.
5.2 Conduct a comprehensive inspection of the supports of high-temperature hydrogen-facing pipe fittings, and rectify any problems found.
5.3 The process enhances smooth operation, reduces the impact of the medium on the pipeline, and prevents pipeline vibration.
5.4 Analyze the sulfur, chlorine and oxygen ion content in the process medium once a week, focusing on monitoring changes in these parameters.
5.5 Establish a pipe fitting ledger for the manufacturer's pipe fittings; mark the model, part, process parameters and serial number, and list them as key parts for daily inspection, weekly inspection and post inspection. Achieve timely discovery, timely treatment, and timely elimination of hidden dangers.
5.6 During each maintenance of the hydrogen production unit, all pipe fittings inspections shall be listed as required and subjected to non-destructive testing.
5.7 Make spare parts for the pipe fittings of the feed line and other key parts of the furnace area to prepare for the next maintenance and replacement.