e-book materials ~ Lecture on the Basics of Pressure Vessels (Chapter 10)

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Chapter 10: Case Analysis of Typical Pressure Vessel Incidents
Incident Case 1:
1. Overview of the incident
On the morning of March 27, 2000, at the XX Phosphate Fertilizer Plant in Kunming City, a 400 m³ nitrogen spherical tank needed to be depressurized and vented for maintenance purposes. At that time, the pressure inside the tank was 1.9 MPa. Suddenly, the connection between the vent pipe at the top of the tank and the manhole cover broke. The broken vent pipe flew past two operators and fell to the ground. Fortunately, no one was injured. However, this incident resulted in a prolonged interruption of nitrogen supply, severely affecting the normal production of fertilizers at the plant. 2. Accident cause analysis 1) Original design data and on-site inspection: (1) The process parameters for this spherical tank are as follows: design pressure: 3.06 MPa; design temperature: normal temperature; medium used: nitrogen; container category: Class II; volume: 400 m3. (2) The top of this spherical tank is equipped with a manhole with a diameter of 500 mm. The manhole cover has an elliptical head structure; there is an opening at the top of the cover which is welded to a steel pipe with a length of 10,835 mm. The other end of this pipe is connected to the flange of a Z41H type DN100 globe valve, and the other end of the globe valve is connected to a 90° elbow. The total height of the vent pipe is approximately 3 m. (3) The fracture site of the pipe fitting is in the heat-affected zone of the fillet weld between the manhole and the pipe. At the time of the accident, the opening degree of the DN100 stop valve was around 60 mm, which exceeded half of the valve’s nominal diameter. The direction in which the pipe fitting breaks off and flies out is exactly opposite to the exhaust direction of the 90° elbow. 2) Technical appraisal: (1) Review of completion documents – Upon inspection of the relevant technical documents, it was found that both the manhole cover head and the welded components of the vent pipe had passed the inspection before leaving the factory; the welding of the manhole cover head to the connecting pipes complied with the provisions of the relevant safety standards for pressure vessels. (2) Inspection of the welded joint: Upon inspection, it was found that the joint between the nozzle and the head is of the plug-type structure. According to the design specifications, bevels are provided on both the inside and outside of the head, and it is a fully penetrated welded structure. The weld width on the inner surface of the head is 15 mm, with a weld height of 5 mm; the outer surface features fillet welds, with a weld height of 6 mm. The length of the nozzle is approximately 100 mm, and its other end is welded to a high-neck flange. (3) Inspection of the fracture surface at the joint: The distance from the edge of the fracture on the head side to the top of the fillet weld is 2–20 mm. The fracture surfaces are mostly at a 45° angle; there is obvious plastic deformation on the fracture surface on the tube side. The maximum inner diameter is 103 mm and the minimum is 92 mm, with a radial deformation amount of 11 mm. The thickness of the cross-section at the top of the fracture is 2.5–5.1 mm; it is wavy in shape, with two irregular notches. (4) Non-destructive testing of the nozzle welds: Magnetic particle testing and dye penetrant testing were conducted on the surface of the internal and external fillet welds of the head and nozzle, and no surface cracks or other defects were detected. The wall thickness of the nozzle was measured; except for the outer wall thickness in the deformed area near the fracture, which ranged from 4.9 to 5.2 mm, it can be concluded that the wall thickness of the nozzle is 5 mm. (5) After retesting the samples for chemical composition analysis and mechanical property testing, it was found that both the chemical composition and mechanical properties of the pipes meet the requirements of the GB3087-82 standard for \"Seamless Steel Tubes for Low and Medium Pressure Boilers\". (6) Metallographic analysis of the pipe fracture surface: Through microscopic metallographic examination, its microstructure was found to be ferrite + pearlite. The level of non-metallic inclusions was Grade 1, and the grain size grade ranged from 6 to 8. These results generally meet the requirements of the material standards. Analysis showed that the tissue at the edge of the fracture zone underwent significant deformation, giving rise to secondary cracks in the same direction as the deformation. The hardness of the deformed area of the fracture zone was HV240–248, with an average value of HV245, while the hardness of the undamaged portion of the matrix was HV183–186. Technical inspections have shown that the vent pipes and end caps come with complete factory documentation and meet the requirements of relevant technical standards; the material selection and dimensions meet the specifications outlined in the design drawings, and no defects exceeding acceptable limits were found in the structural fillet welds as determined by surface flaw detection. However, macroscopic examination of the fracture surface showed that it was grayish in color with severe plastic deformation; microscopic metallographic analysis also indicated significant slip in the tissue at the fracture edges. Therefore, this can be identified as a typical plastic fracture accident. 3) Force analysis: According to the theories of engineering mechanics of materials, the vent pipe at the top of this spherical tank constitutes a typical cantilever beam structure. When nitrogen is discharged, the fluid changes direction by 90° at the outlet, resulting in a lateral force on the fluid that, together with the total length of the vent pipe (the lever arm), creates a torque. The maximum bending moment occurs at the junction where the vent pipe meets the manhole cover seal. The maximum bending moment exerted on the pipe fittings when the fluid is discharged is related to the degree of valve opening and the angle of the outlet elbow. This requires that during the emptying process, operators must strictly follow the operating procedures and control the degree of valve opening carefully. Additionally, it is necessary to avoid using 90° elbows in the design to ensure operational safety. 3. Accident conclusion: The cause of the fracture in the vent pipe at the top of this spherical tank was that, during maintenance, the vent pipe valve was opened fully in a short period of time, which resulted in excessive stress at the junction between the vent pipe and the manhole cover. This led to the average stress on the pipe wall exceeding the material’s yield limit and strength limit, thereby causing plastic fracture damage at that junction (on the pipe wall). Therefore, the pipe rupture is related to the valve being opened too wide in a short period of time and to an unreasonable structural design. 4. Several suggestions: The vent pipe at the top of a pressure vessel is an exhaust device installed in accordance with the equipment’s process requirements, as well as for manufacturing, installation, maintenance, and testing. While strengthening the safety management of the main pressure-bearing components of pressure vessels, the safety requirements for venting devices cannot be ignored. This sudden breakage of the vent pipe should draw our serious attention. The author puts forward the following suggestions for colleagues’ reference: (1) Operators of pressure vessels should conscientiously follow safety operating procedures and enhance their safety awareness. During the venting process, it is absolutely forbidden to open the valve fully in a short period of time; the degree of opening should preferably not exceed 1/3 of the valve’s nominal diameter, and pressure release must be carried out carefully and slowly. (2) In the design of the vent structure, 90° elbows at the airflow outlet should be avoided as much as possible; instead, 120°–135° elbows can be used to reduce the lateral force exerted by the fluid. Given the large bending moment load on the bottom during exhaust, it is recommended to use thick-walled steel pipes. Additionally, to enhance the stability of this structure, overall reinforcement measures should be considered in the design to prevent excessive exhaust vibrations. (3) During manufacturing and installation, it is necessary to strictly comply with the regulations and technical standards applicable to pressure vessels, adhere to strict construction protocols, prevent errors in the use of materials for vent pipes, and eliminate defects caused by welding. (4) During routine visual inspections and periodic internal and external inspections of pressure vessels, enhanced safety inspections of this component should be carried out. The focus should be on whether there are any surface fatigue cracks, deformations, or leaks at the corresponding welds and base materials. Any such issues found must be repaired promptly. Accident Case 2: Serious explosion of a pressure vessel at XX Feed Factory in Gushu Town, Yutian County, Hebei Province. Source: Safety Management Network. (I) Overview of the accident: At 14:56 on October 26, 2001, an explosion occurred in a pressure vessel (steaming tank) in the steaming tank workshop of XX Feed Factory in Dazhuang Village, Gushu Town, Yutian County, resulting in 3 deaths and direct economic losses of around 150,000 yuan. At 13:30 on October 26, 2001, the XX Feed Factory in Dazhuang Village, Gushu Town, Yutian County, Hebei Province, began operations, using a self-made pressure vessel (steaming tank) to steam chicken feathers for use as feed. After the first tank was finished steaming, steam was supplied to the second tank, and the pressure rose to 0.24 MPa. At 14:56, the steaming tank exploded; its lid was flung out. The lid and the wave of steam knocked down one worker who was operating in front of the tank, scratched the corner of the wall at the entrance, and then, after changing direction, knocked down another two workers who were grinding materials outside the workshop. The lid rolled to a distance of 31 meters from the steaming tank. Meanwhile, the explosion caused the tank to shift position; part of the roof (which was made of asbestos tiles) was destroyed by the blast wave. In total, three people died, and the direct economic losses amounted to approximately 150,000 yuan. (II) Analysis of the accident causes 1. The pressure vessels (steam boilers) used in this factory were homemade, substandard products with no relevant documentation; the welds at the bolt mounting sites were superficial welds that did not penetrate fully, resulting in insufficient strength. Moreover, these vessels were used illegally despite a notice issued on December 31, 1998, ordering their cessation of use, and this was the main cause of the accident. 2. In terms of management, first, the factory has no rules and regulations regarding work safety management nor any safe operating procedures ； Second, workers entering the factory do not receive proper safety education and training, nor do they have work permits issued by the relevant county authorities ； Third, the implementation of notices issued by safety supervision departments is ineffective, and supervision is not timely. 3. The work safety management system is not standardized, and the town-level authorities fail to provide sufficient education on work safety awareness to the factory. (III) Measures to prevent similar accidents 1. Strictly enforce the safety regulations for pressure vessels, and design, produce, install, and use them in strict accordance with laws, technical specifications, and rigorous standards. Relevant authorities must resolutely ban pressure vessels that are illegally designed, produced, installed, or used. 2. Pressure vessels must be equipped with devices such as safety valves, and regular inspections must be carried out to ensure their sensitivity and reliability. 3. Cylinder inspection units conduct inspections on cylinders in accordance with specified items and schedules, paying special attention to the manufacturing stamp and the inspection stamp. Cylinders that do not meet safety requirements, such as those that exceed standards or have unclear stamps, are disposed of through destructive methods. 4. Before use, units utilizing gas cylinders must inspect them; cylinders that have exceeded the standard specified service life shall not be used ； It is necessary to verify the cylinder manufacturing process, the inspection stamp markings, as well as the type of gas and its pressure; cylinders that do not meet the safety technical requirements must not be used. When using gas cylinders, it is necessary to strictly adhere to the requirements and specifications outlined in the instructions or warning labels. 5. The entity using the gas cylinder shall not weld the cylinder body or alter its steel stamp or color mark ； Do not use scrapped cylinders, cylinders whose inspection period has expired, or cylinders with obvious surface defects. Do not dispose of any residual liquid inside the cylinders on your own. Case 3: The Serious Accident Involving the Explosion of a Liquid Chlorine Cylinder at XX Paper Industry Co., Ltd. Source: Safety Management Network (I) Overview of the accident: At around 13:30 on September 6, 2008, a liquid chlorine cylinder at XX Paper Industry Co., Ltd. in Yongning County, Ningxia, exploded, resulting in 119 people experiencing irritation and poisoning symptoms; 33 of them were taken to hospitals for medical observation and treatment. The accident-prone cylinder was an 8001-chlorine liquid cylinder, manufactured in 1981 and filled with chlorine liquid at the filling station of Yinchuan Sodium Production Factory. At the time of the incident, the bottle had exceeded its inspection period, was left outdoors without being in use. The blast opening is an annular crack in the base metal, located about 25 mm inside the fixed weld of the end cap shield on the side of the gas cylinder where there is a corner valve; it is approximately 750 mm long, with a minimum wall thickness of less than 4 mm. The inner surface of the base metal is smooth, while the outer surface shows obvious strip-like signs of corrosion. (II) Analysis of the accident cause: 1. The accident-related gas cylinder was a scrapped cylinder that had exceeded its safe service life and was severely corroded. After being filled with liquid chlorine, it was used and stored exposed to the sun, and the explosion that occurred was the direct cause of the leakage of liquid chlorine. 2. Improper safety use and storage management of gas cylinders by the entities that use them are the main causes of leaks. 3. The gas cylinder filling units failed to scrap the cylinders that had exceeded their service life and instead continued to fill them, which was a major cause of the accident. (III) Measures to prevent similar accidents 1. Relevant departments should urge filling units and user units to intensify the safety inspections of gas cylinders in use, so as to avoid using cylinders that have passed their inspection deadline or have been scrapped. 2. Cylinder filling units shall strictly comply with the \"Regulations on Safety Supervision of Cylinders\" and relevant standard requirements, assign dedicated personnel to conduct pre-filling inspections on each cylinder, and measure the wall thickness of the cylinders when necessary. Regulatory standards prohibit filling cylinders with blurred markings; such cylinders must not be filled under any circumstances. Case 4: Major explosion of a pressure vessel at Minfu Zinc Industry Co., Ltd. in Xihe County, Gansu Province Source: Safety Management Network (I) Overview of the accident On May 12, 2004, at 12:31 p.m., a major explosion occurred in an leaching tank (a pressure vessel) at XX Zinc Industry Co., Ltd. in Xihe County, Gansu Province. The accident resulted in 1 death and 1 injury, with direct economic losses amounting to 4.555 million yuan and indirect losses of 18.008 million yuan. The equipment involved in the accident was the leaching tank, a key component of the complex catalytic oxidation direct leaching process for zinc ore; its design pressure is 0.25 MPa and its design temperature is 125°C℃ ； The medium used in the design is zinc sulfate, which is non-toxic, non-flammable, and highly acidic; it consists of 9% sulfuric acid and 1% nitric acid ； Operating pressure: 0.2 MPa, operating temperature: 90–120℃ ； The cylinder has a diameter of 3600/312 mm, the head has a diameter of DN3600/312 mm; the container’s height is 5390 mm. The material used for both the cylinder and the head is Q235B. The volume of the container is 47 m³, and its own weight is 1.08 tons. The inner wall of the container is lined with polytetrafluoroethylene, while the outer wall insulation layer is rock wool/50 ； The container is equipped with a steam-heating (or tap water-cooling) heat exchange coil with a heat exchange area of 11.6 m², as well as a YCK three-blade inclined impeller-type, detachable turbine mixer ； The waist of the container cylinder is connected using flanges (referred to as equipment flanges, made of 16Mn forged steel). The bolts used to connect the upper and lower flanges are M303380-T double-headed bolts, in total 72 of them, made of 35 steel. A trial was conducted on April 30, 2004, but it was not successful. On May 12, 2004, the second test was conducted. At 12:30, the liquid phase temperature was 77°C and the pressure was 0.04 MPa; oxygen was then introduced. The oxygen flow rate was 50 m³/h, later adjusted to 170 m³/h, and then adjusted again to between 150 and 130 m³/h. At 12:31, an explosion occurred in Leaching Tank No. 1. The accident caused all 72 double-headed bolts at the flange connections of the equipment in Leach Tank No. 1 to break. The upper part of the leaching tank was lifted into the air, hitting and destroying the roof beams of the factory building, penetrating the roof, breaking the casing of the mixer motor; the container casing tilted and fell, while the motor rotor was flung out and landed on the roof more than 10 meters away ； 1 crack in the container shell (located between the equipment flange and the ear mounts in the lower half of the equipment) ； Severe inward deformation occurred around the flange of the upper shell head due to impact forces; there were 6 cracks in the equipment flanges (2 on the upper flange and 4 on the lower flange) ； Damage to mixers, heat exchange (cooling) coils, pipes, and valves; local damage to linings and insulation layers ； Cracking in the 100 mm section at the top of the reinforced concrete pier and abutment supporting the container ； All the windows and their glass on the first and second floors of the workshop were completely destroyed ； The circular single-girder crane track in the workshop is scrapped. The duty officer and Zhang, the deputy workshop director, fell into the leaching tank and died ； Lu, the chief commander of the test drive, was blown away by the shock wave, resulting in minor abrasions to his skin. (II) Analysis of the accident causes 1. Direct cause During the commissioning of Leaching Tank No. 1, zinc sulfide and iron sulfide present in the concentrate powder reacted chemically with dilute sulfuric acid under the heating conditions in the leaching tank, resulting in the generation of hydrogen sulfide gas. Due to the rudimentary control and shut-off devices for supplying oxygen, the amount of oxygen delivered to the leaching tank reached a level sufficient to initiate chemical reactions. Due to the electrostatic properties of the container lining—polytetrafluoroethylene—which cause non-conductors to generate static electricity, and since the container is not equipped with any conductive or non-conductive anti-static devices, static electricity accumulates until it discharges in the form of sparks, thus providing the ignition condition required for chemical reactions. After the chemical reaction H2S+3/2O2=SO2↑+H2O+519 kg/mol occurs, sulfur dioxide gas is produced along with a large amount of heat. The heat released by these chemical reactions can only cause the sulfur dioxide gas to expand in volume in order to release that energy; as a result, the pressure inside the container rises sharply in an instant. A safety valve with a nominal diameter of DN150 is simply not fast enough to relieve this pressure, leading to damage at the weakest point in the container’s structure. 2. Main reason: The contract stipulated that the equipment manufacturer did not possess the qualifications for manufacturing pressure vessels. Without the consent of the technology provider and the original designer, it entrusted another party to make changes to the location of the equipment flanges, as well as the location, quantity, and speed of the mixers. This was done without a clear understanding of the possible reaction processes and hazardous substances that could arise during the manufacturing process, resulting in stress concentration on the bolts ； The actual as-built drawings of the containers provided do not meet the requirements specified for the medium in the original design, and are not in line with the actual conditions ； The container category does not match the actual requirements, and the defect of not having designed anti-static devices for conductors and non-conductors has not been corrected ； The discharge capacity of the safety valve was not calculated ； Meanwhile, the container had bolts improperly spot-welded before leaving the factory; during installation, the required reporting procedures were not followed, and someone without the necessary qualifications for welding pressure vessels was allowed to carry out the welding work on site. The ear plate ribs were connected incorrectly, which resulted in cracks appearing in the container’s shell. The design parameter provider failed to clearly specify the hazardous substances that should be present in the main chemical reaction processes, and blindly approved the equipment drawings despite their design flaws, resulting in the pressure vessel being designed for a medium that does not match its actual usage conditions. 3. Indirect cause: Inadequate pre-job training for operators. The oxygen control device cannot ensure proper control. (III) Measures to prevent similar accidents 1. It is strictly prohibited to apply unproven or non-expert-evaluated processes in large-scale industrial production. 2. Strengthen supervision over units responsible for the design and manufacture of pressure vessels, ensuring that such design and manufacture meet the required design conditions. 3. Further strengthen the promotion and enforcement of the Regulations on Safety Supervision of Special Equipment, in order to prevent the illegal installation and use of such equipment. 4. It is necessary to fully demonstrate the feasibility of the direct leaching process using zinc concentrate composite catalytic oxidation. If this process is indeed feasible, formal tests or demonstrations should be carried out to improve and refine the key equipment of this process—the pressure vessel (leaching tank): (1) The pressure vessel shall be designed and manufactured in accordance with the requirements for Class III pressure vessels. (2) Determine appropriate parameters such as volume, structural dimensions, and design pressure. (3) For safety accessories, a combination of safety valves and burst discs should be preferred. (4) Install anti-static devices on the conductive or non-conductive parts of containers and pipelines. (5) Through automated control, the total amount of oxygen supplied (in moles) is regulated, with automatic shutdown in case of excess. Control the appropriate temperature and flow rate to prevent overheating, overpressure, and the mixture gas from reaching its explosive limit. (6) For the cooling of gas-phase media, the spray cooling method should be preferred.