Regarding design changes to pressure vessels

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There are always people who don’t understand this: what exactly are design changes? Design changes refer to alterations made to pressure vessels before they leave the factory for various reasons. In accordance with the contents of design documents specified in Clause 3.1.4.1 of TSG 21-2016 “Safety Technical Supervision Regulations for Fixed Pressure Vessels”: (1) The design documents for pressure vessels shall include a risk assessment report (if required), strength calculation sheets or stress analysis reports, design drawings, and manufacturing technical specifications; if necessary, they shall also include instructions for installation, operation, and maintenance ； (2) For pressure vessels equipped with overpressure relief devices such as safety valves and rupture discs, the design documents shall also include calculations regarding the safe relief volume of the pressure vessel, the discharge capacity of the safety valve, and the discharge area of the rupture disc ； When software simulation calculations are unavailable or impossible to conduct, the design entity shall, in consultation with the client that commissioned the design or the user entity, decide on the use of overpressure relief devices. 3.1.4.2 Approval of design documents: The risk assessment reports, strength calculation sheets or stress analysis reports, as well as the overall design drawings included in the design documents, must be signed at least at three levels – design, verification, and review ； For Category III pressure vessels and pressure vessels designed through analytical methods, approval must also be given by the technical responsible person of the pressure vessel design firm or their authorized representative (Level 4 signature). 3.1.4.3 Retention period The retention period for design documents shall be no less than the design service life of pressure vessels. 3.1.4.4 General design diagram 3.1.4.4.1 Main contents of the general diagram The general design diagram of a pressure vessel must indicate at least the following information: (1) Name and classification of the pressure vessel, as well as the main regulations and product standards applicable to its design and manufacture ； (2) Working conditions, including working pressure, working temperature, and medium properties (toxicity and explosion hazard level, etc.) ； (3) Design conditions, including design temperature, design loads (all loads to be considered, including pressure), medium (composition), corrosion allowance, welding joint factor, natural conditions, etc. For storage tanks used for liquefied gases, the filling coefficient must also be specified; for storage vessels prone to stress corrosion, the permissible concentration of corrosive media must also be indicated ； (4) Material grade and material standard of the main compression members ； (5) Main characteristic parameters (such as the volume of the pressure vessel, the heat exchange area and number of stages of the heat exchanger, etc.) ； (6) Design service life of pressure vessels (number of cycles indicated for fatigue-resistant vessels) ； (7) Special manufacturing requirements ； (8) Heat treatment requirements ； (9) Non-destructive testing requirements ； (10) Requirements for pressure testing and leak testing ； (11) Requirements for corrosion prevention (corrosion rate of the medium, susceptibility to stress corrosion, etc.) ； (12) Specifications of safety accessories and instruments and special ordering requirements (excluding those already considered in the process system) ； (13) Location of the pressure vessel nameplate ； (14) Packaging, transportation, on-site assembly/welding and installation requirements. 4.1.5.1 (1) As-built drawings: These drawings must bear the design unit’s official design seal (copies of such seals are not valid, except for pressure vessels produced in bulk); in addition, they must be stamped with an as-built drawing seal (on which the name of the manufacturing unit, the manufacturing license number, the signature of the reviewer, and the word “As-Built Drawing” are indicated) ； In cases where material substitutions, changes in non-destructive testing methods, or alterations to processing dimensions occur during manufacturing, the manufacturing unit must make clear markings on the as-built drawings in accordance with the requirements specified in the written approval documents from the design unit. The marked areas must include the signature of the person making the modifications and the date of modification ； Review of design documents by inspection personnel: 6.2.1.1 Review of technical documents. Before commencing manufacturing, the unit under inspection shall submit the design documents of the pressure vessel, quality plan, welding procedure specifications (or bonding procedure specifications; the same applies hereinafter), heat treatment procedures, and other relevant process documents to the inspection personnel for review. The inspector reviews the design documents, quality plans, and relevant process documents for each pressure vessel one by one, and signs on the overall design diagram. If the pressure vessel under inspection is a standard product (Note 6-2), the inspector can review the design documents based on the model. 6.2.1.1.1 The review of design documents shall include at least the following: (1) The qualifications of the design unit, the overall design plan, and the approval procedures for design changes (including material substitutions) ； (2) Witnessing documents for the review of external drawings and processes by the unit under inspection ； (3) Completeness of design documents as required in Clause 3.1.4.1 of these procedures ； (4) The regulations and product standards adopted in the design, as well as the validity of the material standards for the main pressure-bearing components; when designing using international or foreign standards, it is also necessary to examine the statements in the design documents regarding their compliance with China’s basic safety requirements ； (5) Review the review of new technologies and corresponding approval procedures required by Clause 3.1.5 of these regulations ； When designing the test method, verify the monitoring documents for the test process ； (6) Type test report (certificate) for the product subject to type testing ； (7) Check whether the requirements for non-destructive testing, heat treatment, pressure testing, and leak testing specified on the general design plan comply with the provisions of these regulations. TSG 21-2016 \"Regulations on Safety Technical Inspection of Fixed Pressure Vessels\" 5.2.1 (1) The modification of a pressure vessel refers to changing the structure of its main pressure-bearing components, or altering the operating parameters of the vessel, the medium it contains, or its intended use ； Major repairs to pressure vessels refer to the replacement, reshaping, or patching of the main pressure-bearing components, as well as the rewelding of butt joints that meet the requirements of clause 3.2.2.1 of these regulations, or the repair of adhesive joints in non-metallic pressure vessels ； (2) The renovation or major repair plan for pressure vessels shall obtain written consent from the original design unit or a design unit with corresponding capabilities ； (3) For the modification or major repair of pressure vessels, their original product standards may be applied; after such modification or major repair, it shall be ensured that their structure and strength meet the requirements for safe use ； (4) The construction processes for the modification and major repair of pressure vessels that meet the requirements of 6.1.2.2 of these regulations shall be subject to supervision and inspection by a special equipment inspection agency with corresponding qualifications; pressure vessels that have not undergone such supervision and inspection or that fail such inspections shall not be put into use ； (5) Fixed pressure vessels shall not be converted into mobile pressure vessels. ......................................................................................................................................................................................................................... The above is the original text of the technical specifications. It can be concluded that: 1. The design documents for pressure vessels require signatures at level 3 or 4 as well as the use of a special seal for design approval; thereafter, they must be reviewed by inspection personnel on a case-by-case basis and signed on the overall design diagram before they can be used for manufacturing. Design changes also occur only during the manufacturing process of pressure vessels, and such changes likewise need to be approved by inspection personnel. Therefore, what information is required in the design change order: it must be submitted by the manufacturing unit, approved in writing by the original design unit, after which the manufacturing unit shall make clear markings on the as-built drawings; the person making the changes must sign it along with the date of the change, which must be before the date of shipment. 2. Design changes after leaving the factory: Re-inspection is required as part of renovation or major repairs, and approval from inspection personnel is still necessary. Traces that should be retained in normal programming documents: signature and date at level 3 or 4 → special seal for design approval → signature of the inspection officer or inspection seal → completion seal ； Design change during manufacturing process: In the standard procedure, insert the date of approval at level 3 or 4 for the design change document → official seal for design approval → signature by the inspection officer or stamp by the inspection officer → completion seal ； Design changes after the container leaves the factory: handled in accordance with the sealing and major repair procedures.