Regular inspection of atmospheric pressure storage tanks and requirements for inspection qualifications

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Storage tanks are storage facilities widely used in petrochemical production. Atmospheric storage tanks usually have connections that lead directly to the atmosphere; in other words, the operating pressure of such tanks is atmospheric. In contrast, there are pressure storage tanks (pressure vessels). With the increasing demands for the safe management of atmospheric-pressure storage tanks for hazardous chemicals, how can such companies effectively mitigate the safety risks associated with these tanks? How can they ensure that the tanks remain in good condition, while also standardizing and regulating the inspection and testing processes for them? The author has analyzed and compiled information on the qualifications and inspection (testing) requirements that many enterprises are concerned about, and would like to share it with you all. There are numerous standard specifications related to the integrity management of atmospheric pressure storage tanks. The most common ones, which are closely related to the maintenance and inspection (testing) of such tanks, are as follows: 1. “Integrity Management of Atmospheric Pressure Storage Tanks” (GB/T 37327-2019); 2. “Risk-Based Inspection and Evaluation of Atmospheric Pressure Storage Tanks” (GB 30578-2014); 3. “Safety Technical Regulations for Vertical Cylindrical Welded Steel Storage Tanks” (AQ 3053-2015); 4. “Design Code for Vertical Cylindrical Welded Steel Oil Tanks” (GB 50341-2014); 5. “Welded Steel Atmospheric Pressure Vessels” (NB/T 47003.1-2022); 6. “Technical Specifications for Inspection and Suitability Assessment of Operational Atmospheric Pressure Storage Tanks” (T/CCSAS 042-2023); 7. “Maintenance and Repair Procedures for Vertical Cylindrical Welded Steel Storage Tanks” (SHS 01012-2019). Currently, there is a fragmentation of standards related to the integrity management of atmospheric pressure storage tanks, and the revision of some of these standards is lagging behind ； Even regarding the specific provisions, there are cases where the requirements among the standards are not consistent. In terms of legal effect and applicability, among the aforementioned standards and specifications regarding the inspection (testing) of atmospheric pressure storage tanks during their use, particular attention should be paid to “Integrity Management of Atmospheric Pressure Storage Tanks” (GB/T 37327-2019). When it comes to the concept of atmospheric pressure storage tanks, those working in hazardous chemicals industries are familiar with the term. However, a considerable number of them may not be able to accurately define what an atmospheric pressure storage tank is, or determine its scope of regulation. One important reason is that different standards define different ranges for atmospheric storage tanks. For example, among the standards mentioned above, some define the scope of \"vertical cylindrical welded steel storage tanks at normal pressure\" as \"vertical cylindrical welded steel storage tanks with an internal pressure not exceeding 18 kPa.\" Furthermore, different standards specify varying ranges for the media stored in atmospheric-pressure tanks and their volume capacities. Considering the characteristics and risks of atmospheric storage tanks, the author supports the following concepts, which also serve as the foundation for the discussions in this paper. 1. Category, volume, and range of media: The scope of application as specified in GB/T 37327-2019 applies here; that is, “This standard is applicable to vertical cylindrical welded steel storage tanks at normal pressure. The integrity management of other storage tanks at normal or low pressure (operating pressure less than 0.1 MPa) can be carried out with reference to this standard.” ”2. Pressure range: GB/T 37327-2019 does not specify the pressure range for storage tanks at normal pressure. Currently, the definition that is widely used is the one outlined in GB 50160, namely: “Storage tanks whose design pressure is less than or equal to 6.9 kPa (gauge pressure at the tank top).” ”Within the above range, those responsible for the management of special equipment will notice that there is a gap between the pressure range of the aforementioned atmospheric storage tanks and the pressure range of pressure vessels. In this regard, GB 50160 provides a definition for low-pressure storage tanks, which are defined as: “Storage tanks with a design pressure greater than 6.9 kPa and less than 0.1 MPa (gauge pressure at the tank top).” ”This is exactly what is referred to in the scope of application of GB/T 37327-2019 – the \"low-pressure storage tanks\" that can be used as a reference for implementation. It should be further noted that some standards classify storage tanks with a design pressure greater than 6.9 kPa and less than 0.1 MPa (gauge pressure at the tank top) into two categories: \"micro-pressure storage tanks\" and \"low-pressure storage tanks\", with the boundary set at 18 kPa. Risk analysis of atmospheric pressure storage tanks: This article focuses on how to prevent and control the risks associated with operational atmospheric pressure storage tanks from the perspective of equipment integrity. Risks associated with operations on atmospheric pressure storage tanks (such as hot work) and safety risks related to process operations (such as liquid overflow inside the tank) are not covered in this document. The main equipment-related risks that need to be guarded against in atmospheric pressure storage tanks are, first of all, corrosion-induced thinning of the tank body or local settlement of the tank foundation, as well as cracks in the welds at the bottom of the tank, all of which can lead to leaks of the substances stored inside the tank and consequently cause accidents such as fires, explosions, poisoning, and burns. Maintaining the integrity of tank-related accessories and instruments is also a crucial step in preventing and controlling safety risks associated with tanks. From the perspective of the normal operation of atmospheric pressure storage tanks and risk prevention and control, GB/T 37327-2019 stipulates that the integrity assessment of various components of atmospheric pressure storage tanks includes the tank body and foundation, sealing systems, cathodic protection, anti-corrosion coatings, vent valves, instrument and electrical systems, lightning and static electricity protection facilities, as well as other related accessories. Given that the scope of atmospheric-pressure storage tanks discussed in this paper includes not only tanks for flammable liquids but also tanks for acids and alkalis, users must pay close attention to the risks associated with the tank roofs. In particular, since many enterprises lack platforms for personnel to walk on atop their tank roofs, special care must be taken to prevent accidents where workers performing duties or inspections might fall into the tanks due to corrosion or thinning of the roof structures. While providing consulting services at a company in Shandong, the company’s equipment management staff mentioned that one of their alkali tanks had a serious problem of thinning at the top of the tank; fortunately, it was detected in time. Regarding the routine inspections (monthly inspections), annual inspections, maintenance, and inspection/testing of atmospheric pressure storage tanks, there are certain differences between the requirements stipulated in this document’s Part II and those for atmospheric pressure storage tanks. Given that routine inspections (monthly inspections) and annual inspections can be carried out without the need to open the tank, making them easy to implement, they will not be discussed in detail in this document. Below, taking three representative standards as examples, we will focus on discussing the requirements for maintenance and inspection (testing). 1. \"Safety Technical Regulations for Vertical Cylindrical Welded Steel Storage Tanks\" (AQ 3053-2015) 11.3 Determination of Inspection Periods The determination of the inspection period for storage tanks shall meet the following requirements: c) The periodic inspection interval should be determined based on the measured corrosion rate and the minimum allowable thickness of the tank body; the actual inspection period should be set such that, by the time of the next inspection, the thickness of the tank body is not less than the minimum thickness required by the standards. d) When the corrosion rate is unknown, it can be determined based on the predicted corrosion rate derived from operating experience of storage tanks under similar operating conditions ； When there is no operational experience or data on similar storage tanks, the periodic inspection interval shall not exceed 6 years; for large storage tanks, this interval shall not exceed 4 years. e) For storage tanks with severe corrosion, the user should reasonably shorten the interval between regular inspections based on the actual conditions. 11.4 Users of tanks subject to Risk-Based Inspection (RBI) may also employ RBI procedures to determine the interval between periodic inspections. The RBI conducts a comprehensive assessment of the likelihood and consequences of leaks or accidents in storage tanks, in order to determine the acceptable risk level for the minimum thickness of these tanks prior to the next regular inspection. Based on this assessment, the interval between regular inspections specified in 11.3 can be increased or decreased accordingly. 2. \"Code for Maintenance and Overhaul of Atmospheric Vertical Cylindrical Welded Steel Storage Tanks\" (SHS 01012-2019) 3.1 Overhaul Cycle 3.1.1 The overhaul cycle for storage tanks shall be determined based on the requirements of comprehensive inspection and maintenance (regular inspections); it must be ensured that by the time of the next overhaul, the thickness of the tank shell is not less than the minimum thickness specified in the standards. It is generally 6 years. 3.1.2 For storage media with a corrosion rate of ≤0.1 mm/a and reliable anti-corrosion measures, and where the technical condition of the storage tank was confirmed to be good during the last comprehensive inspection and maintenance, ensuring safe operation, the period between inspections (comprehensive inspection and maintenance) may be extended upon approval by the enterprise’s technical supervisor; however, this extension shall not exceed 9 years in principle. 3.1.3 For stainless steel storage tanks with a nominal volume of not more than 5000 m³ and containing clean storage media, where leak prevention and operational monitoring measures are in place, the timing of maintenance (comprehensive inspection and upkeep) can be determined as needed upon approval by the enterprise’s technical supervisor. 3.1.4 For storage tanks with severe corrosion, the maintenance (comprehensive inspection and repair) cycle should be appropriately shortened based on the actual conditions. 3.1.5 The cycle for tank inspection and maintenance (comprehensive inspection and upkeep) should be relatively fixed, and this should be specified in the company’s regulations ； When making changes, the corresponding procedures must be followed. 3.2 The maintenance strategy can utilize risk-based inspection and evaluation results to determine the inspection strategy for storage tanks. 3. \"Integrity Management of Atmospheric Pressure Storage Tanks\" (GB/T 37327-2019) 8.1.1 Atmospheric pressure storage tanks are generally subject to their first integrity assessment 3 to 6 years after being put into use; the timing of subsequent assessments is determined by the inspection agency based on the results of those assessments. 8.1.2 The integrity assessment of the various components of an atmospheric pressure storage tank includes the inspection and evaluation of the tank body and foundation, the sealing system, cathodic protection, anti-corrosion coatings, vent valves, instrumentation and electrical systems, lightning and static electricity protection facilities, as well as other related accessories; a usability assessment is conducted when necessary. Chapter 8 of this standard also clearly stipulates the specific contents regarding the inspection and evaluation of various components of atmospheric storage tanks, including related accessories. Fifth, the issue of inconsistent requirements regarding the qualification standards for maintenance and inspection (testing) agencies is a concern shared by hazardous chemicals enterprises, as are the qualification issues related to such agencies. And once the latter is resolved, the former will also be easily solved. To implement the requirements set out in the \"Notice of the General Office of the State Council on Matters Related to Comprehensive Administrative Law Enforcement in Emergency Management\" (Guo Ban Han [2023] No. 51), on July 17, 2023, the Ministry of Emergency Management issued the \"Guidance Catalogue for Comprehensive Administrative Law Enforcement Matters in Emergency Management (2023 Edition)\". The interpretation of this document emphasizes that, in principle, the name of an enforcement matter is derived by summarizing and extracting the contents of the laws, administrative regulations, and departmental rules that establish such a matter. ”Based on this, it can be considered that, from the perspective of the qualifications of inspection (testing) agencies, the \"regular inspections\" and maintenance of atmospheric pressure storage tanks differ from the \"mandatory\" regular inspection requirements imposed by laws and regulations on pressure vessels. Everything has two sides. From the perspective of risk management, the reality is that some companies do not pay enough attention to the maintenance and inspection of atmospheric pressure storage tanks. The author recently witnessed a petrochemical company that had not carried out any tank opening inspections on its gasoline storage tanks for over 10 years, nor had it conducted any assessments of the integrity of those tanks ； Although a local refining company provided a test report on the atmospheric pressure storage tanks issued by a third party, the contents of the test were incomplete, and there was no date scheduled for the next inspection (completeness assessment), leaving the risks uncontrolled. In summary, it is necessary to establish regulations regarding the maintenance and inspection (testing) tasks for atmospheric pressure storage tanks, as well as the capabilities of the organizations responsible for such maintenance and inspection (testing), in order to guide users in preventing safety risks and maintaining the integrity of the equipment. The entity that operates the atmospheric pressure storage tank in Section 6 is the primary responsible party for its safety management. It shall, in accordance with relevant standards and regulations, establish a management system for such tanks (maintenance procedures), specifying the cycles and scope of maintenance and inspection (integrity assessment) in order to effectively control the risks associated with these tanks. When selecting agencies for the maintenance and inspection (testing) of atmospheric pressure storage tanks, emphasis is placed on the capabilities of these agencies; that is, the agencies responsible for such maintenance, inspection (testing) must possess the appropriate skills for the tasks at hand, including those related to non-destructive testing. -End-