Maintenance and modification of in-service pressure vessels

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1 Overview Pressure vessels are pressure-bearing devices widely used in production and daily life (such as those used in chemical and petroleum refining processes, as well as the liquefied petroleum gas cylinders commonly found in households). These pressure-bearing devices pose a risk of explosion; over time, due to factors such as temperature, the type of medium involved, the operating environment, and handling methods, any defects present during the manufacturing process of these devices can further develop. On the other hand, wear, corrosion, cracks, deformation and other new defects may occur during use. If these defects are not addressed in a timely manner, they can pose a threat to the safe operation of pressure vessels; in severe cases, it may lead to equipment explosions, resulting in casualties and causing great losses and harm to people’s lives, property and safety. If these defects are not handled properly, they can create new potential hazards for pressure vessels. Due to the wide variety of pressure vessels and the significant differences in their operating conditions, it is necessary to strictly control the factors that affect the quality of maintenance and modification of such vessels. This ensures the quality of maintenance and modification of pressure vessels in service, as well as their safe operation after such work is completed. To this end, relevant regulations have been established for the maintenance and modification of pressure vessels in service, imposing strict requirements on such activities from various aspects. This chapter focuses on the regulations governing the maintenance and modification of pressure vessels in service, the requirements for the organizations responsible for such maintenance and modification, the key points that need attention during the process, as well as some typical examples of maintenance and modification projects for pressure vessels in use. 2 Maintenance: Maintenance of operating pressure vessels is divided into routine maintenance and major maintenance. The replacement, correction, or patching of the main stressed components, as well as the repair of welds such as those at the longitudinal joints of the cylinder shell in welded pressure vessels, the circumferential joints connecting cylinder sections to end caps, the end cap joint seams, the welds between spherical shell plates, and those that require full-thickness penetration welding, all constitute major repairs; all other repairs are considered minor repairs. General maintenance of operating pressure vessels can be carried out by the entity that uses such vessels or by a unit with appropriate technical capabilities hired for this purpose. For major maintenance of operating pressure vessels, whether it is carried out by the using entity itself or by a hired unit, that unit must possess the necessary qualifications. After maintenance, in-service pressure vessels must ensure that their structure and strength meet the requirements for safe use. 2.1 Applicable Regulations The maintenance of in-service pressure vessels shall comply with the \"Regulations on Safety Technical Supervision of Fixed Pressure Vessels\" issued in 2009 by the **General Administration of Quality Supervision, Inspection and Quarantine (hereinafter referred to as the \"Fixed Pressure Vessel Regulations\"). In accordance with the requirements of the Fixed Pressure Vessels Regulations, major repairs to operating pressure vessels shall also comply with the requirements of relevant technical specifications. 2.2 Qualifications The repair organizations and personnel (welders and non-destructive testing personnel performing repairs) of pressure vessels in service must possess the appropriate qualifications. (1) Qualifications of maintenance units: Units engaged in the maintenance of pressure vessels in service must be entities that have obtained the corresponding manufacturing licenses. Such units shall establish and effectively operate a quality assurance system for pressure vessels in accordance with relevant safety technical specifications. The legal representative of the unit must be responsible for the quality of the maintenance work on pressure vessels in service. Maintenance units must strictly comply with laws, regulations, safety technical specifications, and relevant standards. They are also required to provide the user units with technical documents such as maintenance drawings and certificates attesting to the construction quality. (2) Welder qualifications: Welders who are responsible for the maintenance of operational pressure vessels must possess qualifications obtained through examinations conducted by quality and technical supervision departments at or above the provincial level, in accordance with the \"Rules for Assessing Welders of Special Equipment\". (3) Qualifications for non-destructive testing personnel: Those who perform non-destructive testing on operational pressure vessels must possess qualifications obtained through examinations conducted by quality supervision departments at or above the provincial level, in accordance with the \"Rules for Assessing Non-Destructive Testing Personnel for Special Equipment\". 2.3 Formulation and approval of plans: The general maintenance of in-service pressure vessels should be carried out in accordance with the relevant manufacturing technical specifications and approved by the technical supervisor of the maintenance unit; whereas the plans for major maintenance of such vessels must also be approved by the original design unit or a design unit with the appropriate qualifications. The repair plan should include: the standard basis for the repair plan; an analysis of the causes of the defects; the dimensions and location of the defects; the construction procedure; methods for checking the quality of the construction process; and the performance criteria and acceptance standards that the container must meet after repair. 2.4 Construction Requirements The quality of repairs depends largely on the quality of construction, and there are many factors involved in construction quality; any issue at any stage can affect it, and in severe cases it can lead to failed repairs. Therefore, careful and thorough consideration must be given to the construction process when repairing existing pressure vessels, in order to ensure the quality of the repairs. 2.4.1 Safety requirements for maintenance personnel and equipment: Most repairs on operating pressure vessels are carried out outdoors in complex and hazardous environments (such as oil tank areas and spherical storage tank areas in refineries, as well as various industrial installations). Cross-functional work is often required as well; therefore, maintenance personnel must wear appropriate personal protective equipment when working on-site, keep the safety requirements in mind, and be vigilant to any changes in their surroundings. For repairs carried out inside pressure vessels, it is necessary to first check whether the internal medium has been replaced to meet the required standards, whether the safety measures for entering the vessel are adequate, and whether there are safety supervisors present. These safety requirements should be given even greater attention when repairing existing pressure vessel equipment in small and private enterprises, in order to prevent new accidents involving people and equipment. For example, recently a small private gas station experienced an explosion while repairing a fuel tank; this occurred because the gases and oil inside the tank had not been properly removed, and welding triggered the explosion. The welder who was working inside the tank was killed on the spot, while two workers assisting outside the tank suffered severe burns. As for various sources and related reports, there are frequent cases of new personal injuries and equipment accidents occurring during the maintenance of pressure vessels in service, due to non-compliance with maintenance procedures and disregard for safety requirements. Therefore, when repairing existing pressure vessel equipment, the repair plan must include safety considerations. For example, this includes providing safety training for maintenance personnel prior to the work, requiring that the entity undergoing maintenance have the necessary conditions for safe construction, and stipulating that either the maintenance company or the entity being maintained must have at least one safety officer on site, in order to prevent accidents involving people or equipment during maintenance operations. 2.4.2 Materials for maintenance (1) Replacement of pressure components: For pressure component materials that only need to be replaced without welding, in principle, they should be the same as the materials of the original equipment parts. If it is truly necessary to use alternative materials due to special circumstances, approval must be obtained from the original design unit or a design unit with the appropriate qualifications. The replacement materials shall be smelted in open-hearth furnaces, electric furnaces, or oxygen converters. The technical requirements for these materials must comply with the relevant **standards, industry standards, or technical specifications, and they must be accompanied by a quality certificate issued by the steel manufacturer. The maintenance unit shall inspect the steel based on this quality certificate, and re-inspection may be carried out if necessary. (2) Orthodontic treatment of compressed elements: For compressed elements that only require orthodontic treatment, simulation tests should be conducted prior to construction, along with necessary assessments of their mechanical properties (if corrosion resistance is required for such elements, a corrosion resistance assessment is also necessary). Construction can proceed only after the assessment results meet the technical requirements of the equipment. (3) Welding materials for welded pressure components: When repairing in-service pressure vessels, due to the complex circumstances and harsh welding conditions, the correct and appropriate selection of welding materials along with proper management on-site are among the key factors in ensuring the quality of the repairs. When performing welded repairs on in-service pressure vessels, the following principles should be followed regarding welding materials. ① The welding material to be used is chosen to a. ensure the performance of the weld metal. The performance characteristics of weld metal include mechanical properties (strength, toughness), chemical composition (primarily for stainless steel and heat-resistant steel), and corrosion resistance (where the equipment requires such properties). The strength and toughness of weld metal should be considered in conjunction with those of the components being repaired. Based on years of practical experience, the failure of pressure vessels in service is often not due to insufficient strength, but rather to inadequate toughness, which leads to the formation of cracks or brittle fracture. Therefore, when selecting welding materials, it is advisable to choose ones whose deposited metal grade is slightly lower than that of the base metal, while still having toughness that is equal to, similar to, or slightly higher than that of the base metal. This principle is particularly important when repairing high-strength steel. When repairing stainless steel and heat-resistant steel pressure vessel equipment, factors such as the corrosion resistance and temper brittleness of the equipment must be taken into account; therefore, the weld material chosen should have a chemical composition for its deposited metal that is similar to that of the base metal. b. Ensure the crack resistance of the weld. There is a significant difference between maintenance and manufacturing. For pressure vessels that are in use and require maintenance, the material of their casing undergoes certain changes after a period of operation. When carrying out maintenance, it is essential to take into account the tendency for cracks to form; therefore, welding materials with good crack resistance should be preferred, provided that the mechanical properties of the weld metal remain satisfactory. c. Good weldability. The weldability of welding materials includes: arc stability, slag removal properties (except when using TIG welding), porosity sensitivity, fluidity of molten slag, weld formation, spatter, etc. d. The complexity and rigidity of the repair components. When repairing components with complex shapes and large thicknesses, the cooling rate during welding is fast, resulting in high welding stresses that can lead to cold cracks. Therefore, electrodes with good crack resistance, toughness, plasticity, and low hydrogen content should be used (such as low-hydrogen, ultra-low-hydrogen, or high-toughness electrodes). When repair is required in all positions and the component cannot be turned over, electrodes suitable for all-position welding are preferable. e. Welding conditions. When certain welding areas are indeed difficult to clean thoroughly, it is advisable to use acidic electrodes with strong oxidizing properties that are not sensitive to water, rust, oil, and similar substances, in order to avoid defects such as pores. When welding in a closed container, acidic electrodes (such as iron-type or titanium-calcium type) or alkaline electrodes with low dust and low toxicity should be preferred as much as possible. When both acidic and basic electrodes can meet the requirements, acidic electrodes should be preferred whenever possible. f. Cost-effectiveness. Taking the aforementioned a~e factors into consideration, welding materials with low cost and high efficiency should be selected (such as iron powder electrodes, gravity electrodes, root pass electrodes, vertical-down welding electrodes, and high-efficiency stainless steel electrodes). Of course, whether the welding electrodes selected according to the above principles can be used must also be assessed in accordance with the NB/T 47014—2011 standard \"Welding Procedure Qualification for Pressure Equipment\" and the technical requirements of the equipment to be repaired; they can only be used if they pass such assessment. ② Welding material management a. Acceptance of welding materials. The units responsible for the maintenance of operational pressure vessels shall inspect the welding materials purchased in accordance with the requirements of relevant welding material standards, as well as the product instructions, factory certificates of conformity, and quality assurance documents provided by the manufacturers of those welding materials; re-inspection may be carried out if necessary. b. Drying of welding materials. Drying of welding materials is one of the important factors in ensuring the quality and success of repairs. The purpose of drying welding materials is to remove the adsorbed water in them, reduce the content of diffused hydrogen in the weld metal, and decrease the likelihood of pores forming in the weld metal. Welding electrode arc welding is commonly used for on-site repair of operating pressure vessels; therefore, a detailed explanation is provided here regarding the drying of welding electrodes. The purpose of drying welding electrodes is to remove the moisture from the wet coating as well as the crystalline water in the flux; this helps to reduce the hydrogen content in the weld metal, thereby preventing the formation of cold cracks and pores to a certain extent. The drying of welding electrodes must strictly adhere to the specified drying process parameters. When the drying temperature is too low or the holding time is insufficient, the water in the flux is not completely removed, which can lead to the formation of pores and cold cracks in the weld. If the drying temperature is too high, certain components in the flux decompose, causing the flux to crack; as a result, the flux tends to fall off during welding, reducing its protective effect on the molten pool. c. Welding material issuance. After being dried according to the drying procedure, the welding electrodes are placed in a constant-temperature oven set at 100°C ± 20°C (this step must be strictly observed, especially on-site in southern regions during maintenance). Welders carry insulated containers and retrieve the electrodes as needed; the amount taken each time should not exceed 5 kg. Welding rods that are not used up on the same day must be stored separately, and they must be dried again using a drying process before being used once more. The number of re-drying sessions should not exceed two. 2.4.3 Maintenance personnel: All personnel entering the site to perform maintenance on pressure vessels must receive safety training in accordance with the established relevant safety operating procedures. Those who serve as repair welders on in-service pressure vessels and as non-destructive testing personnel after repairs must possess the qualifications specified in 8.2.2.2 and 8.2.2.3. 2.4.4 Equipment for maintenance All types of equipment used for maintaining pressure vessels in service must be calibrated and certified; a certification label must be affixed to a prominent location on the equipment. 2.4.5 Pretreatment: The maintenance of in-service pressure vessels is largely related to welding. Since most of the equipment under maintenance has been in operation for some time, its structural materials have suffered damage to varying degrees; the equipment itself and its welds exhibit corrosion, new cracks, as well as welding defects (cracks that existed during manufacturing but were not detected, lack of full penetration, slag inclusions, porosity, undercutting, etc.). For repairing these defects, pre-treatment of the area to be repaired and its surrounding areas is crucial before the repair is carried out; the quality of this pre-treatment directly determines whether the repair will be successful. Based on the common defects of in-service pressure vessels, preprocessing mainly takes into account the following aspects: dehydrogenation treatment; removal of corrosion layers; removal of grease; cleaning of scale and absorption layers; drilling of crack-stopping holes. 2.4.6 Construction: The construction and maintenance unit develops a scientific and reasonable maintenance construction procedure based on the construction plan and on-site construction conditions. The construction technique is the core element of repairing operating pressure vessels; it serves as the technical guarantee for the quality of such repairs and determines the technical standards that must be met after the repair of operating pressure vessels. The construction processes include: pre-treatment process for the area to be repaired, welding process, heat treatment process, and quality inspection processes (visual inspection, non-destructive testing, pressure testing). 2.4.7 Welding environment: Effective protective measures must be taken when any of the following conditions exist in the welding environment; otherwise, welding is prohibited: ① When using shielded metal arc welding, if the wind speed is greater than 10 m/s; ② When using gas shielded welding, if the wind speed is greater than 2 m/s; ③ If the relative humidity is greater than 90%; ④ In rainy or snowy conditions; ⑤ When the temperature in the welding environment is below 0°C. 2.5 Inspection The repair of pressure vessels in service is a complex engineering task that involves many different departments. It requires analysis of the causes of defects, formulation of repair plans and approval processes, as well as various procedures such as material procurement, welding, heat treatment, non-destructive testing, and pressure testing. To ensure the safe use of these pressure vessels after repair, it is essential to strengthen quality control throughout the entire process, so as to guarantee that the repaired vessels meet the required quality standards. 2.6 Safety assessment There are two types of standards for the safety assessment of defects in operating pressure vessels: one type is the standards for safety assessment aimed at quality control, and the other type is the standards for safety assessment aimed at meeting operational requirements. (1) Quality control standards: Quality control standards are established with the aim of ensuring the quality of manufacturing or repair processes. They consider all welding defects as factors that weaken the strength of welds and pose risks to structural safety. Without taking into account differences in specific usage conditions, these standards require that defects be reduced to the lowest possible level. Such standards are welding quality acceptance criteria designed for the purpose of controlling quality in the manufacturing or repair of welded products. Including GB/T12467, GB/T12468, GB/T12469, NB/T47013, etc. (2) Compliance with usage standards: For certain \"defects that exceed acceptable limits\" present in operational pressure vessels, which are deemed unacceptable according to \"quality control standards,\" if all such defects were to be corrected or the vessels declared defective, it would result in unnecessary and excessive repair work, as well as the dismissal of vessels that are actually still usable – which is highly uneconomical. In fact, fixing defects that do not affect functionality often results in more harmful or harder-to-detect defects. Of course, pressure vessels that pass the quality control inspections can undoubtedly be put into use. However, many pressure vessels that fail quality control inspections are still in use. In other words, substandard quality does not mean substandard performance. Therefore, from a practical standpoint, \"defects exceeding standards\" should be treated differently: only those defects that pose a threat to the safe operation of the pressure vessel should be repaired, while those defects that do not pose such a threat can be left as they are. Standards established for the purpose of ensuring suitability for use are referred to as \"suitability for use\" standards. These include: DW-X-749 \"Recommended Methods for Defect Evaluation from the Perspective of Fracture Failure\" (proposed by the International Welding Society), BSI-PD6493 \"Guidelines on Various Methods for Acceptance Criteria of Welding Defects\" (proposed by the British Standards Institution), WES-2805K \"Acceptance Criteria for Welding Defects Based on Fracture Evaluation\" (proposed by the Japanese Welding Engineering Society), the U.S. \"Code for Boilers and Pressure Vessels\" (ASME Section III, Appendix G; Section XI, Part I, Appendix A), and China’s CVDA-84 \"Code for Evaluation of Pressure Vessels\", among others. Of course, the above specifications are all guiding documents and not mandatory standards; they generally require the consent of both the container user and the evaluation party before they can be applied. (3) Principles for selecting safety assessment criteria ① In the following situations, quality control criteria can be used for assessment: a. The pressure vessel has only a small number of \"defects exceeding standards\"; b. A longer maintenance interval is desired; c. There are no data or capabilities available for conducting reliable fracture mechanics calculations; d. There is a lack of experience in using such pressure vessels. ② In the following situations, evaluation based on applicable standards can be used: a. It is difficult to repair existing pressure vessels in accordance with quality control standards, and there is a tendency for them to be scrapped; b. There is experience and capability to make comprehensive judgments regarding welding defects on-site; c. There are qualified fracture safety analysts available; d. There is extensive experience in the use of existing pressure vessels. (4) Qualifications, rights, and responsibilities of safety assessment agencies ① For the safety assessment of defects in operational pressure vessels, **approval is granted item by item by the safety supervision agency. The entity using pressure vessels shall enter into a safety assessment contract for defects in operational pressure vessels with an assessment agency that possesses the appropriate inspection qualifications and has been approved by the **safety supervision authority. ② The entity responsible for conducting safety assessments of defects in operational pressure vessels must provide clear assessment conclusions based on the nature of the defects, the causes behind them, and predictions regarding their development, thereby indicating the impact on safe operation. These include: usage conditions, monitoring measures for use, and the usage period, which should not exceed one inspection cycle. ③ Units undertaking the safety assessment of defects in operating pressure vessels must be responsible for the inspection results of the defects, the conclusions of the defect assessment, and the safety performance of the pressure vessels for continued use, and shall bear corresponding responsibilities. The evaluation report and conclusions must be reviewed by the technical supervisor of the evaluating entity and approved by its legal representative. They shall be sent to the users of the pressure vessels in service; simultaneously, copies must be submitted to the competent authorities of the user entities as well as the ** and provincial/municipal safety supervision agencies. 2.7 Acceptance Upon completion of all items stipulated in the contract and maintenance plan, and when the equipment meets the required operating standards, allowing the repaired pressure vessel equipment to be put into service, the maintenance acceptance is considered complete upon passing the inspection and acceptance. (1) Main contents of the acceptance for the repair of in-service pressure vessels: ① Conduct a systematic examination and summary of the repair plan and process. ② Analyze the quality and cost of repairs. ③ Transfer of the maintenance report. ④ Handle the final settlement for repairs. (2) Standards for the acceptance of repaired in-service pressure vessels: ① The standards specified in 2.6 Section (1) and (2). ② GB150 \"Steel Pressure Vessels\". ③ \"Regulations on Safety Technical Inspection of Fixed Pressure Vessels.\" ④ GB713 \"Steel Plates for Boilers and Pressure Vessels\". ⑤ NB/T47015 \"Welding Code for Pressure Vessels\". ⑥ Other relevant ** and industry standards and specifications. In addition, maintenance contracts and technical requirements must also be complied with. (3) Technical documents (reports) to be submitted for the acceptance of repaired operating pressure vessels: After the repair of operating pressure vessels is completed, the handover documents (reports) serve as the basis for final acceptance. These documents must be prepared simultaneously with the construction work, be complete in scope, thorough in content, accurately recorded, and in a uniform format; they must also bear the official seals of the relevant departments as well as the signatures of the responsible personnel. The specific contents of the report are as follows: ① Repair plan; ② Welder’s qualification certificate; ③ Qualification certificate for non-destructive testing personnel; ④ Quality certificate for welding materials; ⑤ When pressure-bearing components need to be replaced, their material quality certificates must also be provided; ⑥ Welding procedure qualification report; ⑦ Welding records; ⑧ Heat treatment records; ⑨ Visual inspection records; ⑩ Non-destructive testing report; ⑪ Pressure test report. 3 Modification: The modification of existing pressure vessels refers to changing the structure of the main pressure-bearing elements, or altering the operating parameters of the pressure vessel, as well as the medium it contains or its intended use. (1) Regulations to be followed for renovation: The renovation of in-service pressure vessels shall comply with the \"Regulations on Safety Technical Supervision of Fixed Pressure Vessels\" issued in 2009 by the **General Administration of Quality Supervision, Inspection and Quarantine». These regulations specify the qualifications required of entities responsible for the renovation of in-service pressure vessels, as well as the approval procedures, responsibilities, and relevant aspects of such renovations. In accordance with the requirements of the Fixed Pressure Vessels Regulations, the modification of in-service pressure vessels shall also comply with the requirements of relevant technical specifications. (2) Notification of modification: Before modifying an in-service pressure vessel, the entity carrying out such modification shall provide written notification to the authority responsible for registering the use of pressure vessels. (3) Modification qualifications: The modification plan for existing pressure vessels must be approved by the original design unit or a design unit with the appropriate qualifications. (4) Modification quality: After the modification of in-service pressure vessels, it is necessary to ensure that their structure and strength meet the requirements for safe use. (5) Supervision and inspection during modification: During the modification process of pressure vessels in use, supervision and inspection must be carried out by specialized equipment inspection and testing institutions with the appropriate qualifications; pressure vessels that have not passed such supervision and inspection shall not be put into use. (6) Preparatory work before modification: Before the personnel responsible for modifying existing pressure vessels enter the vessels to carry out work, the using unit shall, in accordance with the requirements of the \"Rules for Periodic Inspection of Pressure Vessels\", carry out necessary preparatory and cleaning tasks. Access is strictly prohibited when the requirements are not met. (7) Modification requirements: When replacing sections of existing pressure vessels, a construction plan shall be formulated with reference to the relevant design and manufacturing standards, and it may only be carried out upon approval by the technical supervisor. In addition to the above, for entities responsible for modifying pressure vessels, other requirements regarding construction, etc., can refer to 2. 4 Maintenance and modification of mobile pressure vessels. Mobile pressure vessels refer to transportation equipment that consists of a tank body (pressure vessels used for holding fluids in railway tank cars, road tank cars, and tank containers) or large-volume seamless steel gas cylinders (pressure vessels used for holding fluids in tube trailers and tubular containers), combined with a traveling mechanism or a frame that is permanently connected to it. The maintenance of mobile pressure vessels is divided into routine maintenance and major maintenance. For the replacement, reshaping, and patching of the main pressure-bearing components of the tank, as well as for the longitudinal joints of the cylinder, the circumferential joints connecting the cylinder sections to the end caps, and the joint seams of the end caps, butt joints with full penetration across the entire cross-section should be used. For the joints between the nozzles (flanges) and the tank (jacket), the joint seams of the jacket, and the joints between the jacket and the cylinder or end caps, repairs should be carried out using butt joint welds that meet the requirements for full penetration. The replacement of gas cylinders constitutes major maintenance, while all other repairs are considered minor maintenance. Modification refers to changing the purpose of mobile pressure vessels, the pipeline structure, and the local structure of the main pressure-bearing components of the tank. The maintenance and modification of mobile pressure vessels shall comply with the \"Regulations on Safety Technical Supervision of Mobile Pressure Vessels\" issued in 2011 by the **General Administration of Quality Supervision, Inspection and Quarantine**. This regulation covers general provisions, tank materials, design, manufacturing, use and management, filling and unloading, modification and repair, periodic inspections, safety accessories and handling accessories, as well as supplementary provisions. Chapter 7 specifically sets out the requirements regarding the qualifications of entities responsible for the repair and modification of mobile pressure vessels, the procedures for approving such entities, their responsibilities, and the details related to modification and repair. For the rest, refer to 2 and 3.