Can flange bolts be tightened under pressure?

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Chapter 4, Section 5 of \"TSG D0001-2009 Regulations on Safety Inspection of Pressure Piping – Industrial Piping\": Pressure tests and leakage tests. Article 89: Hydraulic tests must meet the following requirements: (8) The gas within the pipeline must be completely removed during the test; in the event of a leak during the test, it must not be repaired while under pressure. When draining the liquid after the test, care must be taken to prevent the formation of negative pressure. Article 90: The pressure test shall meet the following requirements: (6) During the test, the pressure should be increased gradually and slowly. When the pressure reaches 50% of the test pressure, if no abnormalities or leaks are detected, the pressure should be increased further in steps of 10% of the test pressure until the test pressure is reached. Afterwards, the pressure should be reduced to the design pressure for inspection; a test is considered successful if no leaks are detected using a foam agent. It is strictly prohibited to tighten bolts while the pressure is still applied during the test. Chapter 5, Section 3: Maintenance and Repair Article 115: Generally, no major repairs shall be carried out on pressure-bearing components when there is pressure inside the pipes. In cases where the production process is special and requires bolts to be tightened under temperature and pressure, or where emergency sealing operations are needed while under pressure, the using unit shall establish effective operating procedures and safety measures. These must be implemented with the approval of the technical supervisor and under the supervision of safety management personnel on site. Operators who carry out pressure-sealing leak repair operations must receive professional training and hold a \"Special Equipment Operator Certificate\" for the relevant category*. Editor’s note: For TSGZ6001-2019 pressure-sealing leak plugging, there is no longer a requirement for a special equipment operation permit. \"TSG 21-2016 Regulations on Safety Technical Inspection of Fixed Pressure Vessels\" 4.1.9.2 General requirements for pressure resistance tests: (2) During the pressure retention period, continuous pressurization shall not be used to maintain the test pressure; no tightening under pressure or application of external force to the pressure-bearing components is allowed during the pressure resistance test. 7.1.10 Safety requirements for repairs and sealing under pressure: No repairs shall be carried out when there is pressure inside the pressure vessel. In the event of an emergency leak that requires sealing under pressure, the user entity shall establish valid operational requirements and safety measures in accordance with the design specifications, and these must be approved by the person in charge of safety management at the user entity. Personnel performing pressure-sealing operations must undergo professional training and assessment to obtain a certificate for working with special equipment, and they must work with such a certificate*. In actual operation, the safety management department of the using unit should send personnel to conduct on-site supervision. Editor’s note: For TSGZ6001-2019 pressure-sealing leak plugging, there is no longer a requirement for a special equipment operation permit. Explanations for the revision of the \"Regulations on Safety Technical Inspection of Fixed Pressure Vessels\" (TSG21—2016): The original regulations stipulated that for special production processes where it is necessary to tighten bolts while the vessel is under pressure and at temperature, or in cases of emergency leaks that require sealing while under pressure, the user unit must establish effective operational requirements and safety measures in accordance with the design specifications, and these must be approved by the technical supervisor of the user unit. ”The phrase “In the case of special production processes where tightening under temperature and pressure is required” is now removed; such situations generally fall under normal tightening and do not require special approval. GB 50184-2011 Code for Acceptance of Construction Quality of Industrial Metal Piping Projects 7.3.1 The bolts of flanges on high-temperature or low-temperature pipelines shall be tightened hot or cold during the commissioning phase in accordance with the following provisions*: 1 The temperatures for hot tightening and cold tightening of the pipelines shall comply with those specified in Table 7.3.1. 2 Hot tightening or cold tightening should be carried out 2 hours after reaching the operating temperature. 3 When tightening the bolts, the maximum internal pressure of the pipeline shall be determined based on the design pressure. When the design pressure is less than or equal to 6 MPa, the maximum internal pressure for hot tightening should be 0.3 MPa ； When the design pressure is greater than 6 MPa, the maximum internal pressure for hot tightening should be 0.5 MPa. Cold-state tightening should be performed after depressurization. Editor’s note: The \"GB 50517-2010 Code for Acceptance of Construction Quality of Metal Piping Works in Petrochemical Industries\" (with partial revisions in 2023) specifies that once the bolts are tightened to the specified torque value using a torque wrench during installation, and no leakage is detected, there is no need to carry out further bolt tightening under hot or cold conditions. In summary, bolts shall not be tightened under pressure during the pressure test phase, the leak test phase, and the commissioning phase of pressure pipelines or pressure vessels ； For normal production processes, bolts can be tightened under pressure.