Analysis of Typical Accident Cases Involving Special Equipment

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The major pressure pipeline accident on August 11 in Dangyang, Hubei: A brief description of the incident: At 14:45 on August 11, 2016, during the commissioning phase of a cogeneration project operated by a power generation company in Dangyang, Yichang City, Hubei Province, a high-pressure steam pipeline suddenly ruptured, resulting in the release of large amounts of high-temperature, high-pressure steam and causing injuries to the workers on duty. To date, the accident has resulted in 22 deaths and 4 injuries. Upon receiving the report of the accident that day, the General Administration attached great importance to it; officials from the relevant department rushed to the accident site the following day. Accompanied by leaders from the Hubei Provincial Quality Supervision Bureau and experts from the Provincial Special Equipment Inspection Institute, they went to the scene to assess the situation and conduct an on-site investigation. This accident was placed under supervision by the State Council’s Work Safety Committee. Hubei Province established an investigation team for the \"8·11\" accident, inviting experts in areas such as power plant design, boilers, welding, and materials to serve on the expert group to provide technical support; the deputy director of Hubei Province’s Quality Supervision Bureau led this expert group in carrying out its tasks. The area in question is the Phase II thermoelectric project under construction; during the commissioning and operation of the project, the high-pressure steam pipeline at the boiler outlet, located on the third-floor framework, suddenly ruptured, resulting in a large release of high-temperature, high-pressure steam. The two-story frame below the pipeline houses two control rooms; the partitions between these rooms and the pipeline corridors are made of glass windows and doors. It is understood that at the time of the incident, there were 8 to 10 members of the pipeline maintenance team in the control room, about 8 members of the boiler maintenance team, as well as several dispatchers on duty. The pipe diameter is DN426, the temperature is around 530–540°C, and the steam pressure is approximately 8.8 MPa. The steam erupted from the broken section of the pipe; the two severed sections of the pipe, driven by the force of the steam, struck the wall and caused the glass windows and doors to shatter. The high-temperature, high-pressure steam destroyed the glass windows and doors and entered the control room, resulting in severe injuries and deaths among the people inside those two rooms in an instant. Inspection findings at the scene: Upon on-site inspection, it was initially determined that the break in the high-pressure steam pipeline at the steam outlet of Boiler No. 2 occurred at the circumferential weld connecting the long-necked nozzle flow meter to the two main pipelines. The welds connecting the long-necked nozzle body to those main pipelines appeared intact when inspected visually, whereas the circumferential weld on the long-necked nozzle body itself suffered a brittle fracture; the fracture surface was clean, suggesting the presence of a lack of proper welding penetration. The long-neck nozzle flow meter is a measuring device in which changes in pipe diameter cause variations in flow pressure; it is manufactured by Chongqing Angoli Instrument Co., Ltd. Comprehensive accident analysis: According to the available data, the flow meter was subjected to a pressure resistance test prior to leaving the manufacturer’s facility, with the pressure maintained at 14 MPa for 6 minutes. After installation, no pressure test was conducted before the commissioning of the entire piping system. The quality inspection system and the power system impose the following specifications and standard requirements for industrial pipelines respectively: 1) According to Article 92 of TSG D001—2009, for pipelines in which it is not possible to conduct pressure tests using liquids or gases due to on-site conditions, alternative testing methods may be employed upon obtaining approval from the design unit. Alternative testing must meet the following requirements: ① For all welded joints of pressure components in pipes that have not undergone hydraulic or pneumatic testing, including those in pipes and fittings, longitudinal welds, and spiral welds, 100% penetrant testing or 100% ultrasonic testing must be conducted, with successful results; for other welded joints not covered by this requirement, 100% penetrant testing or magnetic particle testing must be carried out, with successful results as well ； ②Conduct the flexibility analysis of the piping system in accordance with the provisions of 7.3.3 in GB/T 20801.3—2006 ； ③Leakage testing of piping systems is carried out using sensitive gases or by immersion in liquid, and the requirements for such testing must be specified clearly in the design documents. 2) Clause 6.2.11 of the electrical code DL/T 5190.5—2012 stipulates that the inspection of welds in the main steam, reheat steam, and high-pressure feedwater pipeline systems shall comply with the requirements of DL/T 869, \"Welding Techniques for Thermal Power Plants\"; if the welds pass 100% inspection, a hydrostatic test is not required. All welds of this pipeline were subjected to 100% non-destructive testing by Yichang Special Inspection Institute. However, since this long-neck nozzle flowmeter is a measuring instrument and is purchased as an industrial finished product, it has only obtained a certification for measuring purposes, and no approval is required for pipeline components; therefore, the range of non-destructive testing does not include the circumferential welds of this long-neck nozzle flowmeter. It is suspected that the ring weld has weld defects such as lack of penetration. This defect may be the direct cause of the cracking and explosion incident involving the long-neck orifice flowmeter in the high-temperature steam piping system. Since the long-neck nozzle flowmeter is not a pressure pipeline or pipeline component and therefore does not fall under the category of special equipment, Hubei Province classified this incident as the \"8·11\" major high-pressure steam pipeline rupture safety accident – it is not considered an accident involving special equipment. However, given that boilers and pressure pipeline systems were involved in this incident, the quality supervision authorities promptly sent experts to provide technical support. At the same time, on-site inspections revealed several other issues: 1) Quality control problems regarding flowmeter products: The flowmeters installed in the main pipelines are pressure-bearing components, and there are gaps in the oversight of their product quality. 2) Supervision inspection issue: The Yichang branch has issued the supervision inspection report for this pipeline, yet among the alternative solutions for not conducting a pressure test, the most important element – namely the flexibility analysis of the piping system provided by the design team – is missing. It remains to be verified whether the stress condition of the piping system is reasonable. 3) Design issue: Why are the high-temperature and high-pressure steam pipes located so close to the control room? Moreover, why is glass used as a barrier to separate this area from the high-temperature and high-pressure steam pipes, which represent a major hazard? If solid walls were used, the consequences of an accident might be **mitigated. 4) Approval issue: Unit 2, where the accident occurred, may have faced problems regarding project approval, with construction proceeding beyond the approved limits. 5) Maintenance issues: According to information on site, the plant’s equipment department detected a pipeline leak the day before the accident; it is possible that repairs were carried out while the pipeline was still under pressure. 6) Operational issues: Whether there is overpressure operation needs to be investigated.