A.14.1 Equipment Overview
The gas tank that exploded was manufactured in 2004 by a regular pressure vessel factory, with a design pressure of 0.88MPa, a design temperature of 100℃, a volume of 0.6m³, a maximum working pressure of 0.8MPa, an inner diameter of 650mm, a total height of 2138mm, and a main pressure component made of Q235B with a nominal wall thickness of 5mm. The normal working pressure of the equipment is 0.6MPa, the working temperature is room temperature, and the medium is compressed air delivered by the air compressor.
A.14.2 Brief description of the accident
In October of a certain year, an air compressor gas tank in a factory exploded, causing damage to the equipment in the air compressor room and the collapse of the wall, but fortunately no casualties. According to the investigation, the accident occurred at about 4 a.m., when an unattended gas storage tank in the air compressor room suddenly exploded on its own, and the entire device exploded longitudinally from the upper head to the lower head. The huge impact force caused the equipment to fly into the air, and after landing, it hit the ground hard, causing the support legs and the lower head to be severely deformed and torn. At the explosion site, it was found that the safety valve was rusted and there was no sign of opening. The pressure gauge dial was damaged, the pointer was bent and deformed, and the pressure displayed exceeded 1.6MPa.
A.14.3 Cause Analysis
(1) Since it was installed and put into use for one year, it has not been registered and has never been regularly inspected, and the safety accessories have not been regularly calibrated.
(2) After visual inspection, it was found that the weld structure and layout of the main pressure-bearing components of the container (cylinder and head) were reasonable. Before leaving the factory, it was tested by 20% X-ray. The weld quality met the requirements of Level III, so the influence of the weld quality can be basically ruled out. No abnormalities were found in the other appearance and geometric dimension inspections. During the thickness measurement process, the thickness of most of the inspected parts of the head and the cylinder was consistent with the nominal wall thickness in the factory data, and no corrosion or erosion thinning was found. However, the edge of the explosion tear on the cylinder was obviously in a tensile thinning state. Based on this, it can be inferred that the container explosion did not occur instantaneously, but was due to the gradual increase in internal pressure. The cylinder material was stretched by the circumferential tensile stress, the weak parts yielded, and plastic deformation occurred, and finally failed and tore and exploded.
(3) The chemical composition of the main pressure-bearing components of the gas storage tank was detected using a SpectroSortCCD spectrometer. The test found that the composition of the head material was basically consistent with the data, while the Mn content of the cylinder material was only 0.09%, which was not only lower than the 0.30%~0.70% range specified in GB 700-1988 "Carbon Structural Steel", but also lower than the 0.28% indicated in the quality certificate. It is well known that Mn, as a beneficial element in steel materials, can play a greater role in solid solution strengthening of ferrite and austenite, increase the amount of pearlite, and make the steel have high strength and hardness. If the Mn content does not meet the standard, the strength of the steel may decrease and the performance may be reduced. In this gas tank, the Mn content of Q235B, the main pressure component material, is unqualified, and its mechanical properties (especially tensile strength) do not meet the design requirements, which may also be one of the important factors affecting the explosion of the container.
A.14.4 Preventive measures
(1) The user unit shall make good arrangements for the ordering of pressure vessels, equipment acceptance, installation acceptance and commissioning.
(2) In accordance with the "Inspection Procedures for In-use Pressure Vessels", the regular inspection plan for pressure vessels shall be implemented, and detailed records of the main problems and their treatment shall be kept and reported to the superior department regularly.
