CSB Classic Case Study—The Free Industrial Company Chemical Tank Leak Incident That Polluted Public Water Supplies

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CSB Classic Case Analysis – The Incident of Chemical Tank Leak at Freedom Industries Causing Contamination of Public Water Sources Tang Bin1 Tianjin JustSafety Enterprise Management Consulting Co., Ltd. Website: www.justsafety.com.cn Phone: 13802084672 Email: [9Atangbin@justsafety.com.cn] Email: tangbin@justsafety.com.cn He Chen2 Shanghai Yurui Business Consulting Co., Ltd. Keywords: tank leak, chemicals, integrity management, U.S. Chemical Safety and Hazard Investigation Board (CSB) Abstract: This article analyzes the incident of chemical tank leak at Freedom Industries that led to contamination of public water sources, based on the investigation conducted by the U.S. Chemical Safety and Hazard Investigation Board. It examines the reasons for the contamination of public water sources from the perspectives of tank failure and the release of chemicals into the Elk River, and explores ways to improve tank safety management through the concept of tank integrity management. 1. It is noted that on January 9, 2014, an inspector from the West Virginia Department of Environmental Protection visited the chemical storage and transfer facility of Free Industries located in Charleston, the state’s capital, to investigate reports from the public regarding unusual chemical odors in that area. The ombudsman discovered that chemicals were leaking from the above-ground storage tanks in Tank Area 396; the leaking chemicals were a mixture of methylcyclohexane-methanol precursor and polyethylene glycol ether, with laboratory analysis confirming that the main component was the methylcyclohexane-methanol precursor. The chemicals leaked out through 2 small holes located on the bottom plate of tank 396, seeping into the gravel and soil surrounding the tank. Due to cracks in the fire dike originally designed to contain tank leaks, as well as holes caused by years of neglect, the chemicals flowed through the fire dike into the river. The investigation also found that some of the leaked chemicals entered the river through an underground culvert located at the bottom of a nearby tank. Under the urging of the Environmental Protection Agency, Free Industry Company immediately took measures to control the leak, recover the leaked chemicals, and prevent further pollution. However, approximately 37.9 m3 of methylcyclohexane-methanol material has seeped into the surrounding soil and the Elk River, flowing downstream along the river to the inlet of a public water treatment plant in West Virginia, USA, located about 2.4 km downstream from the Free Industry Company. Since the equipment at public water treatment plants cannot remove all of the methylcyclohexane-methanol compounds from the water, the drinking water in the distribution system becomes contaminated. That night, the public water company issued a \"no use\" order to 93,000 customers (covering approximately 300,000 residents), resulting in widespread closures of businesses, schools, and public institutions. The emergency departments of local hospitals have received a large number of patients exhibiting symptoms such as nausea, rash, vomiting, abdominal pain, and diarrhea. Public safety authorities advise residents to limit their consumption of tap water and bathing over the next 4 to 9 days. Some residents said that even after the pipes were flushed as required, there was still a noticeable odd smell in the drinking water several weeks after the leak. Based on the investigation by the U.S. Chemical Safety Board (CSB) into the accident in which a chemical storage tank belonging to Free Industries Company caused pollution of public water supplies, this article focuses on analyzing the problems related to the failure of the storage tank itself and the shortcomings in controlling the chemical leakage after it occurred. 2. Incident of public water contamination caused by a leak from chemical storage tanks at Free Industrial Company
2.1 Background of Free Industrial Company
Free Industrial Company was established in 1992 and provides specialized chemicals for the mining, steel, and cement industries. The Charleston plant site features two computer-controlled loading and unloading areas, where chemical materials such as methylcyclohexane-methanol, calcium chloride, and glycerin, transported to the site by tank trucks, are unloaded into above-ground storage tanks for temporary storage and sale. On December 31, 2013, Liberty Industries merged with the Etowah Terminal and acquired ownership of the 396 accident storage tank; a chemical spill occurred 9 days later. On the day of the accident, there were 19 employees on the company’s roster, 18 of whom were at the scene. To the east of the plant area are a railway and Barrow Avenue; residential areas are located right next to the southern side of the plant area. The Elk River runs along the western boundary of the plant area, as shown in Figure 1. The plant complex includes a two-story office building, warehouses, a cemetery, a fire pump room, a flare stack, fuel filling stations, oil-water separators, a dock, and 19 above-ground storage tanks along with connected product pipelines. Among them, the storage tank involved in the 396 accident was built in 1938 (as shown in Figure 2), with a designed capacity of 174.9 m3; after Free Industry Company acquired ownership of it, it was used to store a mixture of methylcyclohexane-methanol and polyethylene glycol ethers. On the day of the accident, tank 396 contained 88.5% (by mass) of methylcyclohexane-methanol as raw material, 7.3% (by mass) of polyethylene glycol ether, and 4.2% (by mass) of water. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeqcPic0yAxryvTlLbTeJV3iaiazbTFHCbQbWMOick8JRUWUY4MsuEIQJzXg/0?wx_fmt=png Figure 1: Layout plan of the chemical tank farm https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeC5t4x00NznsaVrZx5vVSy3K4nY8UiaTbYm5cgxvibEpgMvYILWRrkCew/0?wx_fmt=png Figure 2: Photos of the storage tanks involved in the Incident 396 (after the incident) 2.2 Chemicals involved in the leakage incident 2.2.1 Crude MCHM Methylcyclohexane-Methanol feedstock The storage tanks in the Incident 396 contained a mixture of methylcyclohexane-methanol feedstock (Crude Methylcyclohexane Methanol, abbreviated as Crude MCHM) and polyglycol ethers (Polyglycol Ethers, abbreviated as PPH, stripped). The methylcyclohexane-methanol feedstock is produced by Eastman Chemical Company and consists of 6 different compounds, as detailed in Table 1. Among them, 4-MCHM (i.e., pure methylcyclohexane-methanol) has the highest content and is the main component that enters drinking water systems. Table 1: Composition of the methylcyclohexane-methanol raw material produced by Eastman Chemical Company. Chemical name | Concentration range: 4-methylcyclohexanemethanol – 68–89%; 4-(methoxymethyl)cyclohexanemethanol – 4–22%; Water – 4–10%; Methyl 4-methylcyclohexanecarboxylate – 4–10%; Dimethyl 1,4-cyclohexanedicarboxylate – 5%; Methanol – 1%; 1,4-cyclohexanedimethanol – 1–2%. The methylcyclohexane-methanol raw material is primarily used in foam flotation processes for coal washing and impurity removal; its combustion causes pollution. According to the safety data sheet provided by Eastman Chemical Company, humans should avoid contact with undiluted methylcyclohexane-methanol, as it can cause eye and skin irritation. At higher temperatures, methylcyclohexane-methanol vapors can also cause discomfort in the eyes and respiratory tract. Currently, there are no established methods or numerical ranges for detecting occupational exposure to methylcyclohexane-methanol. 2.2.2 Polyglycol Ethers (PPH, stripped) Twelve days after the leak incident involving polyglycol ethers occurred, Free Industry Company disclosed another substance present in tank 396, namely polyglycol ethers, which are used to improve flotation efficiency and account for 7.3% of the contents in tank 396. According to the safety data sheet provided by Free Industry Company, this substance causes severe skin and eye irritation and is a flammable liquid. 2.2.3 ShurFlot 944: ShurFlot 944 is a product obtained by appropriately mixing a methylcyclohexane-methanol-based material as the main component with a small amount of polyethylene glycol ether. According to the safety data sheet provided by Free Industrial Company, ShurFlot 944 is composed of alcohols, ethylene glycol ethers, and carboxylates; it is used for flotation, is a deep yellow or brown liquid with a strong odor. Similar to the exposure reaction with the methylcyclohexane-methanol mixture, ShurFlot 944 can cause skin, eye, and respiratory irritation, as well as nausea and vomiting. 2.3 Course of the leakage incident At 8:16 a.m. on January 9, 2014, the West Virginia Department of Environmental Protection received reports of an unusual odor in the air, with claims that the odor originated from the manufacturing facilities of Freedom Industries located in Charleston. At around 10 a.m. that day, the 911 emergency center in Kanawa County also received reports of a chemical odor, located approximately 0.8 km away from Freedom Industries. The EPA ombudsman arrived at Freedom Industries around 11:05 a.m. to meet with the company’s president to discuss the issue of air odors. Almost at the same time, an employee of Freedom Industries reported to the president that there was a leak from a storage tank containing methylcyclohexane-methanol feedstock. The company’s president accompanied the inspector to the vicinity of Tank 396, where they saw that the ongoing leak had formed a liquid pool covering an area of 37.2 m², with a depth of about 7–10 cm, resembling a gushing spring. Water is continuously flowing into an underground tunnel with a diameter of about 30.5 cm at the northwest corner of the liquid pool; at the same time, it seeps from beneath the tank’s fire dike and through internal holes into the nearby Elk River. After the accident, the supervisor of the public water treatment plant in West Virginia rushed to the scene to gather more information; he then reported to CSB investigators that there was a noticeable shine on the surface of the Elk River in the area near the leak site. At 11:56 a.m., he was told that the leaked MCHM was a coagulant. At around 1:30 p.m., a site worker possibly from Free Industrial Company or an automobile transportation company told the public water treatment station supervisor that MCHM is a foaming agent, not a coagulant. Subsequently, the Environmental Protection Agency estimated the leakage volume to be approximately 3.8–18.9 m3. At around 2:00 p.m., the operator at the public water treatment plant noticed a slight unpleasant odor in the river water flowing into the treatment equipment. Based on the estimated amount of leakage and other available information, the public water treatment plant decided to rely on filtration processes and purification systems to eliminate the odor problem caused by the leaked chemicals. Just after 4 p.m., the public water treatment plant determined that the filters had not completely removed the chemicals from the water, and an unusual odor was detected downstream of the filters; the public water distribution system may have been contaminated. At 6:00 p.m., following consultations with the ** Office, the Environmental Protection Agency, and the Public Health Authority, the Public Water Company issued a “prohibition of use” notice. At 9:30 p.m. that day, West Virginia Governor Tomlin announced a state of emergency to the public. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSetJksUCufraoUYt8yAIlNfw64DCCcFPh9AyHybfIXkL8Z0cVmkUPpaQ/0?wx_fmt=png Figure 3: Emergency response timeline for leakage incidents. 3. Analysis of the incident involving contamination of public water sources due to a chemical leak from Free Industrial Company’s storage tanks. To determine the cause of the chemical leak from the tanks and its entry into the Elk River, the CSB investigation team conducted a thorough inspection of the tanks, the fire dikes, and the surrounding soil. They also reviewed relevant technical documents and records related to tank inspections. 3.1 Tank failure analysis 3.1.1 Pitting API-certified tank inspectors conducted internal and external inspections of the tank involved in the 396 accident, and determined that the source of leakage was two holes located on the bottom plate of tank 396, with diameters of approximately 1.9 cm and 1.0 cm respectively. Through a structural inspection of the tank material samples, it was determined that the leakage holes were caused by pitting, which started on the surface of the inner bottom plate of the tank and spread toward the soil side, gradually weakening the wall thickness until holes formed. Additionally, there are relatively deep, isolated corrosion spots in the area near the tank shell, as shown in Figures 4 and 5. Similar pitting issues exist on the bottom plates of the adjacent MCHM tanks (395, 397). https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeovgRWT8oTV7Tc9iaASDuz20oa5L7wOhvQNVul9iaecxnAysMcexGYYsQ/0?wx_fmt=png Figure 4: Corrosion holes and pits on the bottom plate of the storage tank involved in the Incident No. 396.
https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSecDozfJAEfOETgt2nMqUREpqakC2jVnE0XibxxNDHLwLUiahcuDUdFZaA/0?wx_fmt=png Figure 5: Dimensions of corrosion on the bottom plate of the storage tank involved in the Incident No. 396.
Corrosion, as commonly understood, refers to an electrochemical reaction that occurs between a metal alloy and its environment, resulting in the gradual degradation of structural materials along the surface at a certain rate. Pitting is a type of corrosion that develops as small holes that penetrate deeper into a metal surface at a single point or in a very small area; it takes the form of cavities. It usually starts from the upper surface of the horizontally placed metal and progresses in the direction of gravity, capable of penetrating through the entire thickness of the wall. Since pitting generally occurs in localized areas, and the longitudinal corrosion rate is usually higher than the transverse corrosion rate, it is difficult to detect pitting issues. The rust formed by pitting covers the corrosion site, making it more difficult to detect during inspection. Although the soil side at the bottom of the storage tank is also affected by corrosion, it is not as severe as the pitting that directly caused the leakage accident, as shown in Figure 6. One characteristic of pitting is that it usually starts from the metal surface and progresses in the direction of gravity. As can be seen in Figure 6, there are distinct individual corrosion spots on the inner surface of the bottom plate of the 396 tank, progressing toward the wall thickness. In contrast, the corrosion characteristics on the soil side are relatively consistent, resulting in very shallow pits and a layer of iron oxide. Typically, the pitting corrosion rate in a localized area is many times higher than the general corrosion rate on the entire metal surface. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSek11YrbwESXgs5v6K2JibsaOQkQNqaG6eOT851I6RIlofWRTQA49f6yA/0?wx_fmt=png Figure 6: Changes in wall thickness caused by corrosion of the storage tank bottom plate. 3.1.2 Corrosion rate Based on the morphological analysis of the corrosion spots on the bottom plate of the tank involved in Accident No. 396, it was found that the penetrating corrosion holes originated from the upper surface of the bottom plate, rather than the lower surface. The CSB investigation team assigned an expert in storage tanks to conduct a corrosion rate analysis. Although we recognize that the corrosion rate varies and there are many uncertain factors, given the state of research at the time, the best reasonable assumption was to consider the corrosion rate of Tank 396 to be constant throughout its lifespan—specifically, 12.3 mpy (mils per year), with a range of 10–15 mpy. Due to the lack of original tank drawings and documentation, since its construction in 1938, tank experts have assumed that the original bottom plate of Tank 396 was used for 25 years, and was replaced in 1963 with the 6.4-mm-thick welded steel bottom plate that was in use at the time of the leak incident. Based on the condition of the tank bottom plate after the accident, the tank inspector estimated that the replaced bottom plate had been in use for at least 25 years. At a certain point after the replacement of the tank bottom plate, polyvinyl acetate material was added to it to prevent corrosion, as shown in Figure 7. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeogd5kdcYDPXLfFcRBKPWEdg6ekic9hwJTiau54VVIj3VvT7GRSXxRm6Q/0?wx_fmt=png Image 7: Timeline of the service life of the tank involved in the 396 accident 3.1.3 Tank integrity inspections The CSB investigation team found that Free Industry Company did not establish proper procedures for regular inspections and tests to ensure that above-ground tanks and related equipment were properly maintained. Part of the reason is that there are few regulatory requirements forcing companies to have the aforementioned tank maintenance procedures, and businesses also do not voluntarily comply with these requirements. Free Industrial Company stated that the MCHM tanks had not been inspected for at least 10 years prior to the leakage incident in January 2014. The CSB investigation team obtained an informal assessment report on the storage tanks at the Free Industry Company’s facility, consisting of 2 pages and prepared by a third-party consulting firm in December 2013. This brief report summarizes the visual inspection of the storage tanks, mentioning that riveting work was carried out on tanks 395/396/397, but it provides no information regarding the interior condition of the tanks. The report states that “the aforementioned tanks possess a certain degree of structural integrity, but may not fully comply with API 653 and EPA standards.” Without undergoing upgrades or renovations, these tanks are unsuitable for storing oil and controlled products. The report recommends that companies develop plans to have qualified personnel conduct a thorough inspection of each storage tank over the next 5 years. The CSB investigation team found that Free Industry Company did not have any leakage prevention or detection systems, nor any effective measures to contain leaks. Additionally, the storage tanks were not equipped with level indication devices or measurement systems, making it impossible to determine the actual amount of leakage from MCHM, which led to inaccurate estimates of the leakage volume. At that time, West Virginia and federal regulations did not require leak detection systems to be installed on above-ground storage tanks. 3.2 MCHM leakage into the Elk River: After leaking from the corrosion holes in the bottom plate of the tank involved in the 396 accident, MCHM entered the Elk River through two pathways: (a) through the damaged and neglected fire dike ； (b) Pass through the damaged underground culvert. 3.2.1 Geological analysis of the tank farm: The CSB investigation team assigned a geological analyst to examine the soil characteristics and permeability beneath the MCHM storage tanks. The analysis indicated that the 10–15 cm thick gravel base at the bottom of the tanks exhibited high permeability, allowing MCHM to penetrate it rapidly. The soil beneath the gravel foundation is clayey; the minimum permeability coefficient of the surface clay is less than 10-7 cm/sec, indicating moderate permeability. Gravel has high permeability and offers little resistance to fluid flow, as shown in Figure 8. Based on the size of the leak hole in tank 396, the CSB investigation team estimated that the rate at which MCHM leaked from the bottom of the tank into the soil was 11.5 GPM (gallons per minute, approximately 0.0038 m3/minute). Therefore, the leak from the 396 tank should be observable on the surface surrounding it, or it should seep into the soil around the tank. However, none of the Free Industry Company employees interviewed by the CSB investigation team stated that they had noticed any signs of MCHM leakage prior to the day of the accident. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeU14py4guRmVkP4GgkVHBqraXB6dLyeic0wzRRQLybpCr0Feibr733fMA/0?wx_fmt=png Figure 8: Schematic diagram of the permeation resistance of a crushed stone foundation. 3.2.2 Fire dikes: All storage tanks on site are surrounded by brick walls constructed from masonry; these can serve as overflow control measures in the event of a leak. The first fire dike contains tanks 398–405, while the second fire dike contains tanks 393–397. They are constructed from bricks, concrete blocks, and cast concrete, and are designed to be able to hold all the materials in case all of the tanks rupture completely. A concrete wall between Tank 398 and the pump house separates the two fire dikes. MCHM and PPH continued to leak from the bottom of Tank 396 and flowed to the low point in the northwest corner of the fire dike. The dike became structurally damaged due to years of neglect, allowing the MCHM that leaked on the day of the incident to escape through those breaks in the dike. As shown in Figure 9, many parts of the fire dike have large holes and cracks, making it impossible to contain the leaking material. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeoLrC6HazRSAVWPvqibtYstSm7AgtqXcQXrSf7D2IxwGDDicHpKKPEzug/0?wx_fmt=png Figure 9: Location where the fire dike is damaged. 3.2.3 The rainwater drainage system for the underground culvert tank farm consists of an underground corrugated steel pipe culvert with a diameter of 30.5 cm. It starts from the northeast side of the tank farm, traverses the area of the fire dike, and reaches the Elk River on the northwest side of the tank farm. The culvert passes through between Tank 394 and Tank 395, approximately 9 m north of Tank 396. Figure 10 shows a schematic cross-section of the underground structure of the culvert; the exact location of the starting point of the culvert entrance is uncertain. The leaked MCHM seeps into the culvert and flows along it until it reaches the discharge point or outlet located outside the fire dike area, from where it is discharged into the Elk River. https://mmbiz.qlogo.cn/mmbiz_png/fnBs2C3PMfDZhibKNyE9dOSXRBFY4TPSeSEf5au6XtCsl5SqMBfQJicHCym0MKqica9JkZIc22ibf6UXQPibAdsMcGg/0?wx_fmt=png Image 10: Schematic diagram of underground culverts 3.3 Recommendations from the CSB investigation team (1) Owners and operators of above-ground storage tanks located near water sources should establish regular inspection procedures and implement routine monitoring measures for the tanks and fire dikes, in order to assess the integrity of the tanks and their ability to control leaks. It is also necessary to coordinate with nearby public water utilities and emergency response agencies, and to thoroughly communicate information regarding the characteristics, quantities, and toxicity of the stored chemicals, so that this critical information can be obtained immediately in the event of a leak ； (2) Owners and operators of above-ground storage tanks should update and strictly implement corresponding leakage prevention and protection plans in accordance with existing management requirements, in order to reduce the potential risk of leaks from the tanks and fire dikes ； (3) Since toxicological information on many chemicals is unknown, **measures should be taken immediately to protect water sources and the public from these hazardous chemicals.** Increase the frequency of inspections of chemical storage facilities in areas near water sources, and improve the coordination mechanisms between emergency response agencies and public health agencies ； (4) **Standards for water source assessment should be established, requiring public water companies to develop a \"Water Source Protection Plan\" that includes information on the characteristics of the water sources, potential major pollution sources, management strategies, water source monitoring, and alternative water sources. The Water Source Protection Plan should be updated at least every 3 years, or whenever there are significant changes in potential major pollution sources within the areas of focus. 4. Lessons learned from the accident caused by a chemical storage tank leak at Free Industry Company, which polluted public water supplies. The safety management of storage tanks was initially based on an accident-driven approach, which involved dealing with accidents after they occurred and carrying out emergency repairs, as well as periodic maintenance based on the time of use, as in this case. At present, most tank users in our country are still at this stage, while in the developed countries in the West, an integrity management approach based on predictable risks and reliability has been widely adopted. The integrity management approach represents a higher level of management for tank systems. From safety and economic perspectives, it involves carrying out integrity management activities centered on risk assessment for tanks. By analyzing historical data related to tank systems, it is possible to predict the development trends of risks. Based on the acceptable level of risk, targeted inspection strategies and emergency response measures are formulated, providing a basis and management platform for managers to carry out dynamic supervision and make decisions. 4.1 The Risk Assessment API Publ 353 provides a quantitative risk assessment method suitable for storage tanks, which determines the risk value by identifying tank leakage scenarios, leakage frequencies, and consequences, and calculating the product of these leakage frequency and consequences. 4.1.1 Tank leakage frequency The tank leakage frequency is the sum of the leakage frequencies of evaluation items such as the tank bottom plate, tank shell, overflow, and central drain pipe of floating roof tanks. The leakage frequency for each evaluation item can be obtained by adjusting the baseline leakage frequency using a correction factor. The baseline leak frequency for each evaluation item is obtained from statistical data, as shown in Table 2. Due to the differences in the actual operating conditions of various storage tanks, the correction factor is related to factors such as the tank’s service life, corrosion rate, original thickness, inspection level, and category. The mathematical expression is: Leakage frequency = Base leakage frequency × Correction factor. Table 2: Baseline leakage frequencies for storage tanks
| Type of component failure | Leakage frequency (times/year) | Remarks |
|---------------------------|--------------------------------|---------|
| Minor leaks at tank bottom | 7.2e-03 | — |
| Rapid failures | 2.0e-05 | — |
| Minor leaks in tank shell | 1.0e-04 | For welded tanks: 1.0e-03; for riveted tanks: — |
| Rapid failures | 4.0e-06 | Tanks not maintained in accordance with API 653 |
| Properly maintained tanks per API 653 | 1.0e-07 | — |
| Fittings failures | 1.0e-05 | — |
| Rupture of drain hose on tank roof | 5.0e-04 | — |
| Leakage from a normally open valve with a 3.175 mm hole | 2.0e-02 | — |
| Rupture of hinged pipe | 3.0e-04 | — |
| Leakage from a normally open valve with a 3.175 mm hole | 3.0e-02 | — |
| Rupture of drain hose on tank roof | 5.0e-06 | — |
| Leakage from a normally closed valve with a 3.175 mm hole | 2.0e-04 | — |
| Rupture of hinged pipe | 3.0e-06 | — |
| Leakage from a normally closed valve with a 3.175 mm hole | 3.0e-04 | — |

4.1.2 Consequences of storage tank leaks
The consequences of leaks include environmental impacts, such as effects on soil, water bodies, and ecosystems ； Public consequences refer to the effects on the safety, health, and facilities of personnel in the tank farm and its surroundings ； Business consequences refer to financial losses and loss of reputation. The calculation formula for the leakage consequence value LCV is: LCV = ECOF·EWF + PCOF·PWF + BCOF·BWF. In the formula, ECOF represents the environmental consequence value ； PCOF is the public consequence value ； BCOF is the Business Consequence Value ； EWF, PWF, and BWE are the weight factors for environmental consequences, public consequences, and business consequences respectively, with values ranging from 0 to 1. These values are determined based on the company’s value orientation and risk tolerance, and it holds that EWF + PWF + BWF = 1. The main factors affecting the consequences of a leak include: the type of material leaking and the quantity ； Scope and targets affected by the leak ； Soil conditions, recyclability of materials ； Damage to the environment, society, equipment, and operations, as well as the duration of such damage ； The specific environment that affects the consequences of a leak includes the surrounding ecosystem, population density, and facility layout, among others. Based on the determination of the leakage frequency and consequences of storage tanks, corresponding risk values are calculated and risks are ranked. Targeted risk management strategies are developed according to the risk levels and their distribution, such as inspection activities for high-risk items, in order to effectively control the overall risk. 4.2 Integrity Assessment The integrity assessment is conducted on the basis of risk evaluation by detecting potential defects in the storage tank, and it involves evaluating the severity of these detected defects to ensure that risks do not increase as a result of such defects, thereby providing a direct basis for taking measures to mitigate those risks. 4.2.1 Tank inspection methods Tank inspection includes the inspection of the tank bottom, walls, roof, and related accessories, with routine inspections, online inspections, and tank-opening inspections being the main methods used. The routine inspection involves a visual check to determine whether there is any structural damage to the storage tank ； Online inspection refers to the use of methods such as visual inspection, ultrasonic thickness measurement, and acoustic emission testing without interrupting production ； Cask testing requires the storage tank to be taken out of service, its contents to be emptied, the tank to be opened, and it to be cleaned through displacement. Inspectors enter the tank to carry out various tests, primarily using methods such as magnetic flux leakage, ultrasound, radiography, magnetic particle inspection, and penetrant testing. Currently, abroad, acoustic emission technology is primarily used for online detection of active defects on the walls of atmospheric storage tanks, as well as corrosion and leakage signals on the tank bottom plates. The magnetic flux leakage method is employed to regularly detect corrosion and leakage points on the tank bottom plates, while ultrasonic testing technology is used to inspect the tank walls and roof. Penetrant testing can detect surface defects such as leaks and cracks with high sensitivity. The vacuum chamber testing method can be used for detecting the bottom plate of tanks after repair and for identifying suspected leak points. The pulsed eddy current method can determine wall thickness without removing the insulation layer. Furthermore, low-pressure air, soapy water, or gas detectors can also be used to detect small corrosion holes and welding defects in storage tanks. 4.2.2 Evaluation Methods The methods for evaluating tank defects mainly include direct evaluation of external corrosion, direct evaluation of internal corrosion, and stress corrosion evaluation. The evaluation steps are as follows: data collection and integration, determination of defect evaluation methods, conduct of defect evaluation, derivation of integrity evaluation conclusions, and provision of recommendations for repair measures and re-inspection intervals. The evaluation focuses on residual strength assessment and residual life prediction. The remaining strength evaluation is based on the collected and integrated data; by using the selected defect evaluation method, the remaining strength and safety factor of the tank are calculated to determine the maximum equivalent stress in the defective material sections. Based on the evaluation results, suggestions for defect repair methods and timelines are provided. Remaining life prediction is based on information such as the time since the tank was put into use and the size of defects; a remaining life prediction model is established, and estimated growth rates are used to determine the future behavior of these metal defect characteristics and their impact on the material’s integrity. Recommendations regarding methods and time intervals for further inspection are provided. 4.3 Risk mitigation measures are taken based on the results of risk assessment and integrity evaluation, in order to reduce the probability of occurrence or the consequences of specific leakage incidents. Risk mitigation measures can generally be divided into 3 categories: preventive control measures, detection control measures, and protective control measures. Preventive control measures are designed to prevent leakage incidents from occurring in the first place, while detection control measures aim to identify leaks of hazardous materials as early as possible after such incidents occur. Protective measures refer to those actions taken to mitigate the impact of leaks and prevent them from causing more severe damage to surrounding areas. 5. Conclusion: Adopting a maintenance strategy based on risk assessment and integrity evaluation for tank integrity management is an inevitable trend. In the future, it will be necessary to establish a tank integrity management system and platform to enable a cycle of data collection, evaluation, and maintenance. On this basis, such approaches can be further extended to the integrity management of equipment and safety instrument systems in tank areas, in combination with process safety assessments for those areas, such as QRA and HAZOP, ultimately achieving overall integrity management for the tank area. 6. Reference documents: CSB INVESTIGATION REPORT, Chemical Spill Contaminates Public Water Supply in CHARLESTON, WEST VIRGINIA. REPORT NO. 2014-01-I-WV; API Publ 353, Managing Systems Integrity of Terminal and Tank Facilities. 2006; API RP 575, Inspection Practices for Atmospheric and Low-Pressure Storage Tanks, 3rd ed; **Notice from the State Administration for Work Safety on Further Strengthening the Safety Management of Chemical Tank Areas, An Jian Zong Guan San [2014] No. 68 ； Chen Jianfeng, Shi Biyuan, Shen Yuxin, et al. Integrity management of storage tanks and process pipelines. Oil & Gas Storage and Transportation, April 2011