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The \"1.11\" pressure vessel accident in Tongren, Guizhou: At 16:20 on January 11, 2016, an explosion occurred in the drying drum of a ironing machine at Yuping Xingfa Laundry Co., Ltd. in Tongren City, Guizhou Province, resulting in 4 deaths and 2 injuries. The members of the investigation team visited the accident site on several occasions to conduct inspections, review relevant documents and surveillance footage, and interview those responsible, thereby gaining an initial understanding of the situation. Course of the accident and investigation at the accident scene: Through questioning the boiler operator, the details of the accident were as follows: Around 4 p.m. on January 11, 2016, the steam supply valve of the ironing machine was closed; there was still an hour left until the normal end of work time (5 p.m.), and four female workers were still carrying out ironing tasks. At around 16:10, it was noticed that the temperature was a bit low; the pressure gauge indicated a pressure of 0.4 MPa. A staff member in the office who was also near the ironing machine reported that the pressure was roughly 0.4 MPa as well. She then went back to the office to answer a phone call, and a few minutes later the explosion occurred. After the explosion, the worker, dragging his injured leg, turned off the water and electricity valves and switches. According to the on-site inspection, the workshop where the accident occurred was severely damaged; the walls were cracked, the partition wall in the laundry area was blown down, and the dryer next to the ironing machine was displaced entirely and rotated 180 degrees. The three-roller ironing machine was destroyed, and the No. 3 drying cylinder among the three stainless steel cylinders of the ironing machine broke into three parts: the two end caps of the cylinder separated completely from the cylinder body, both end caps remained relatively intact, and the fracture lines were located in the heat-affected zone on the cylinder side of the circumferential joint between the end caps and the cylinder body. The fracture lines were straight with no obvious plastic deformation ； The cylinder section has three longitudinal welds in total. Unfolding along one of these longitudinal welds yields a flat plate; aside from slight plastic deformation at the fracture site at one corner of the flat plate (the plastic deformation length is approximately 60 mm, and this area is preliminarily suspected to be the initiation point, pending further verification), the other fracture surfaces are level with no significant plastic deformation. Measurements show that the length of the plate is approximately 2.99 m and its width is about 2.48 m, which roughly matches the dimensions of the drying drum of φ800×3000 mm ； The thickness measurement of 3.4 mm is consistent with the quality certificate ； Further PT testing was conducted on the inner surfaces of the three longitudinal welds in the cylinder; no defects were detected in the two undamaged longitudinal welds, while more than 10 cracks were found along the fusion line of the cracked longitudinal weld. The maximum length of these cracks was 60 mm, but it is not possible at present to determine whether these cracks are related to this explosion ； Analysis of the main material was carried out using a quantitative spectrometer, and the results showed that the cylinder, internal support rings, and weld deposit met the standards for 304 stainless steel, while the head met the standards for Q235 material, which is consistent with the quality certification documents. The other two stainless steel drying drums, which were originally arranged horizontally side by side, have been stacked at an angle one on top of the other; there is no significant deformation of the drum bodies. The bracket of the ironing machine has broken, and the drive shaft, driven shaft, coupling gears, etc. are still connected to the head. The ironing machine involved in the pressure vessel accident on January 11 in Tongren, Guizhou, has been dismantled. The condition of the other two drying drums following the explosion in that accident. Accident investigation: Information on the equipment involved in the accident. The ironing machine was manufactured by Shanghai Chuan Island Washing Equipment Company and features a three-drying-drum design. The cylinder dimensions are (diameter × length) φ800×3000 mm, made of stainless steel, with an operating pressure of 0.4~0.6 MPa. The ironing machine came online in June 2015, just 6 months before the explosion. This drying drum falls under Category I pressure vessels; the entity using it has not applied to the authorities responsible for supervising special equipment for registration, and the operators do not possess certificates required for working with such equipment. Review of the supervision and inspection certificates for stainless steel drying drums, product quality certificates, and as-built drawings: 1) Review of the supervision and inspection certificates. The copies of the supervision and inspection certificates issued by the Jiangsu Provincial Special Equipment Safety Supervision and Inspection Institute (certificate numbers: YC-RC-2015-C-0449~0498) indicate that the stainless steel drying drum with product number YW150338 was designed and manufactured by Zhehe Machinery Manufacturing Co., Ltd. in Sheyang County, Jiangsu Province. The design license number is TS1232013-2019, while the manufacturing license number is TS2232034-2015; the manufacturing license level is D1. The product numbers applicable to this certificate are YW150291~YW150340; it is a supervision and inspection certificate for pressure vessels manufactured in batches. The product numbers selected for inspection include YW150291, YW150292, YW150293, YW150294, and YW150296. Therefore, the drying drum with product number YW150338, which was involved in the explosion, is not within the scope of this supervision and inspection. The inspection certificate clearly states that \"in accordance with the Safety Law of the People’s Republic of China on Special Equipment, such equipment shall go through the registration process with the local quality and technical supervision department where it is used.\" 2) Review of the product quality certificate: According to the on-site inspection, the cylinder has 3 longitudinal welds, with a total weld length of 3 × 3000 mm = 9000 mm. Based on a 20% RT sampling rate, the weld length to be inspected is 9000 × 20% = 1800 mm. With an effective length of 300 mm per film, 6 films shall be taken for radiographic inspection. A copy of the product quality certificate shows that the cylinder has only 2 longitudinal welds, and 4 radiographic films were taken in total; this indicates that the quality certification document does not match the actual situation, as it fails to meet the technical requirement of an inspection rate of ≥20%. 3) Review of the as-built drawings: On the copy of the as-built drawings, there is a handwritten alteration stating “operating pressure 0.75 MPa”, and no signature of the person who made the alteration is present; all other details are consistent with those indicated on the product label. Analysis of the accident cause: Preliminary summary of issues from on-site investigation: The RT testing rate for dryer drums does not meet the standard technical requirements ； The drying drum has not undergone the registration procedures required for the use of pressure vessels ； Does manual drainage ensure that the condensate level in the drying drum is controlled to meet the design requirements? ； It is recommended that future investigations focus on the following aspects: ① Conduct a detailed failure analysis, paying particular attention to the properties of the cylinder material and whether the quality of the longitudinal welds meets the standard requirements, in order to determine the causes of cracks in the fusion lines of the longitudinal welds on the inner surface of the cylinder. ②Thermodynamic calculations were performed for the operating conditions of the drying drum to analyze whether the pressure inside the drum would increase after the air supply was shut off, in order to determine the source of the explosion energy.