Accident Analysis of Dangyang Madian Gangue Power Plant (Official Version)

Analysis of the accident cause: According to the information currently available, the material of the main steam pipe is 12Cr1MOV with a diameter of 426 mm; the material of the flow meter is also 12Cr1MOV. The operating temperature is 540°C and the pressure is 9.8 Mp. The aperture is 273*25 mm; the working temperature for 12Cr1MOV does not exceed 570–585 degrees. It is a steel grade widely used in high-pressure, ultra-high-pressure, subcritical power plant boilers for superheaters, header tanks, and main steam pipes. It retains high thermal strength and oxidation resistance at 580°C, as well as good permanent plasticity. The production process is relatively simple, and it has good weldability; however, it is sensitive to the quenching rate during normalizing. Preheating to 300°C is required before welding, and stress relief treatment is necessary after welding. Long-term use at 580°C can lead to type II temper embrittlement caused by the segregation of impurity elements such as phosphorus at the grain boundaries. Prolonged use can also lead to the spheroidization of pearlite. The weld of the flow meter valve in the main pipeline at the steam outlet of Boiler No. 2 cracked, resulting in the failure of that valve weld. The reasons for this failure are as follows: 1. There are issues with the 12Cr1MOV material; it is most likely that the quality of the main steam pipe is substandard or that an old pipe section was used instead. 2. The welding quality was substandard; the weld processing techniques were inadequate, and 100% flaw detection of the welds was not carried out after welding. Two days before the accident, leaks began to occur at those welds. On-site technical personnel were working on solutions when the accident happened. 3. The pipeline layout is unreasonable; the faulty pipelines are located on the 8th floor. This area is where the main steam mains of the entire plant meet the outlet main of Boiler No. 2, and it also serves as the boiler feedwater platform. With a large number of pipelines intersecting here and limited space, pipeline expansion is hindered, which makes it easy for thermal stresses to cause the welds to fail. 4. Over-temperature and over-pressure conditions occurred during operation. The company registered a boiler with a capacity of 220 tons, but the actual capacity was approximately 300 tons. Before the accident, the unit was operating normally, and the tuning carried out involved thermal energy adjustments; this process could have led to an excess flow rate in the main pipeline at the steam outlet of Boiler No. 2, with the highest stress levels occurring at the welds of the flow meter valves.

Lessons learned from the accident: 1. The layout of the control room was unreasonable; it was located in the passage between the turbine hall and the boiler island, with its main door facing Column B and its back wall facing the passage and equipped with double-glazed windows. There were numerous high-temperature and high-pressure pipes in this passage, some of which even passed through the ceiling of the control room, which was the main cause of the heavy casualties. 2. 26 people were injured or killed, of whom 19 were on duty while the other 7 were technical staff from the company who were working to fix leaks through debugging efforts. The company failed to increase the automation level of the units as required by the State Administration of Work Safety, which calls for using automation to replace human workers in order to improve safety standards. 3. The design pressure for this high-temperature and high-pressure boiler is 9.8 Mpa, with a temperature of around 540°C. It uses a mother tube system; the material used for the main steam pipes is 12Cr1MOV. It is clear from the data that the company in question followed all relevant standards to the fullest extent, but the safety margin of the equipment was insufficient. Especially when the plant also provided heating using its own generators, the main steam pipes were operated beyond their allowable limits for extended periods of time. 4. The company involved in the accident has had similar pressure pipeline explosion incidents, yet it has failed to learn from them. 5. The level of emphasis placed on technical supervision, particularly regarding the metal inspection of high-pressure pipelines, supports and hangers, as well as inspections related to expansion. 6. From the perspective of the power plant’s approval process, it is clear that the construction and design took place simultaneously, with continuous changes being made. The design firm responsible for the power plant was a non-specialized electrical design agency from the chemical industry sector, while the boilers were manufactured by Wuhan Boiler Factory.

Production can’t be done carelessly; if you’re not responsible toward him, he won’t be responsible toward you

By the 14th, 22 people had already died!

Any minor hazard can turn into a major disaster; safety must not be taken lightly at all...

It’s a matter of playing it by the rules; there is a lack of oversight in the process.

It’s been some time since this accident analysis was completed