How to carry out spectral analysis correctly?

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Spectral analysis is essential in pipeline construction and the inspection and supervision of pressure pipelines. Relevant standards only specify the inspection requirements and sampling ratios, but do not define the exact number of testing points for spectral analysis. We now invite our colleagues to discuss together how the number of points in spectral analysis is determined and what the relevant standards are. The specifications are as follows: SH/T3501-2021 Code for Construction and Acceptance of Steel Piping Systems for Toxic and Flammable Media in Petrochemical Industries. 5.1.6 Components made of chromium-molybdenum alloy steel, nickel-containing low-temperature steel, and stainless steel shall be subject to verification tests using spectral analysis or other methods to determine the content of major alloying elements, in accordance with the provisions of this standard; records and markings must be kept. 5.1.7 The quality certificate for pipeline components requiring resistance to hydrogen sulfide shall include the results of hardness tests, and the hardness values shall not exceed the requirements of the standards or the values specified in the design. If the design documents require tests for hydrogen-induced cracking (HIC) and sulfide stress cracking/stress corrosion cracking (SSC/SCC), the relevant test results shall be provided. 5.1.8 Pipes, fittings, and flanges with hardness requirements shall be subjected to hardness testing; 1% of each batch shall be sampled, with a minimum of 1 sample per batch. 5.1.9 For materials specified in the design documents to undergo low-temperature impact tests, the product quality certificate shall include the results of such tests; otherwise, supplementary tests shall be conducted in accordance with the provisions of GB/T 229. 5.1.10 For stainless steel materials specified in the design documents to undergo intergranular corrosion tendency tests, the product quality certificate shall indicate the results of such tests; otherwise, supplementary tests shall be conducted in accordance with the relevant provisions of GB/T 4334. 5.1.11 For duplex stainless steel materials specified in the design documents to have ferrite content checks, the product quality certificate shall include the results of ferrite content determination; otherwise, supplementary tests shall be carried out according to the methods specified in GB/T 1954. Regarding the testing of highly toxic pipes and fittings (for liquid chlorine media): 5.2.5 For the pipes and fittings that are part of the piping components as specified in clause 5.1.6 of this standard, verification tests shall be conducted on the content of the main alloying elements; 10% of each batch shall be sampled, with a minimum of 1 sample per batch. 5.2.6 For extremely hazardous media in SHA1(1) and SHA1(2), as well as for pipes with a design pressure of 10 MPa or higher, the pipe quality certificate must include ultrasonic testing results; otherwise, supplementary tests shall be carried out on each pipe in accordance with the provisions of GB/T 5777. 5.2.7 For pipes and fittings with a design pressure equal to or greater than 10 MPa, the external surfaces shall be subject to individual non-destructive testing, and no linear defects are allowed. 5.2.8 For pipes and fittings used for transporting SHA1(1) media or highly hazardous media in SHA1(2) with a design pressure of less than 10 MPa, 5% of each batch shall be sampled, with a minimum of 1 piece per batch, to undergo surface non-destructive testing; no linear defects are allowed. When excessive defects are detected in the sampling inspection, they shall be handled in accordance with the provisions of Clause 5.1.12 of this standard. 5.3.9 For the valves among the piping components specified in Clause 5.1.6 of this standard, verification tests shall be conducted on the contents of the main alloying elements in their valve bodies, valve covers, and connecting bolts; 10% of each batch shall be sampled, with a minimum of 1 piece per sample. 5.3.15 Surface non-destructive testing shall be conducted on the weld joints of the following welded valves, and the test results must show no linear defects: a) 100% testing shall be performed on the grooves of steels and chromium-molybdenum alloy steels whose minimum standard tensile strength is equal to or greater than 540 MPa ； b) For non-austenitic stainless steel grooves designed for temperatures below -29°C, 5% of each batch shall be sampled, with a minimum of 1 sample. 5.4.3 For the flanges, flange covers, and flanged splices among the pipeline components specified in Clause 5.1.6 of this standard, verification tests shall be conducted on the content of their major alloying elements; 10% of each batch shall be sampled, with a minimum of 1 piece per sample. 5.5.4 For stainless steel and chromomolybdenum alloy steel studs and nuts used in the following pipelines, verification tests on the content of their main alloying elements shall be carried out by spectral analysis; 5% of each batch shall be sampled, with a minimum of 10 pieces. a) The design pressure is equal to or greater than 10 MPa ； b) Design temperature below -29℃ ； c) The design temperature is equal to or greater than 400°C. 5.5.5 Stainless steel and chromium-molybdenum alloy steel studs and nuts used for pipes with a design pressure equal to or greater than 10 MPa shall be subjected to hardness testing; no less than 2 specimens per batch shall be inspected, and the hardness values shall fall within the range specified in the design documents or product standards. If it fails to meet the requirements, it shall be handled in accordance with the provisions of Article 5.1.12 of this standard. 5.6.3 Metal gasket rings and lens gaskets shall be tested for hardness individually. The inspection location should be kept away from the sealing surface, and the inspection results should comply with the provisions of the design documents or product standards. 8.2.4 When hot-working methods are used for the grooves of the following pipes, surface non-destructive testing shall be carried out on the groove surfaces. The testing ratio shall meet the following requirements, and the test results must show no linear defects: a) For pipes with chromium-molybdenum alloy steel, or steel whose minimum specified tensile strength is 540 MPa or higher, the testing ratio for the pipe grooves shall be 100% ； b) For non-austenitic stainless steel pipes designed for temperatures below -29°C, the groove inspection rate shall be 5%; in the event of any non-conformities, the inspection frequency shall be doubled in accordance with Clause 5.1.12 of this standard. 8.2.15 The alignment fixtures welded to the pipeline shall not be removed by hammering or twisting. When using flame cutting, the cutting should be performed 2 mm to 3 mm away from the pipe surface, followed by grinding. The following steel grades shall also undergo surface non-destructive testing after grinding, and the pass grade shall meet the requirements corresponding to the pipeline grade: a) Chromium-molybdenum alloy steel ； b) Steel with a minimum standard tensile strength of 540 MPa or greater. GB50184—2011 Code for Acceptance of Construction Quality of Industrial Metal Piping Projects 4.0.2 For piping components made of chromium-molybdenum alloy steel, nickel-containing low-temperature steel, stainless steel, nickel and nickel alloys, titanium and titanium alloys, sampling inspections of the material shall be carried out, and proper labeling shall be applied. The test results shall comply with the **current relevant standards and the provisions of the design documents. Inspection quantity: For each inspection lot (with the same furnace batch number, same model and specifications, and delivered at the same time), 5% shall be sampled, with a minimum of one item. Testing method: Spectral analysis or other material retesting methods are used. 4.0.5 For pipes and pipe fittings used in GC1-class pipelines and Class C fluid pipelines, which transport media with extremely hazardous toxicity levels or have a design pressure of 10 MPa or higher, magnetic particle testing or penetrant testing shall be conducted on their outer surfaces. The test results must meet at least Grade I as specified in the current standards “Non-destructive Testing of Pressure Equipment – Part 4: Magnetic Particle Testing” JB/T 4730.4 and “Non-destructive Testing of Pressure Equipment – Part 5: Penetrant Testing” JB/T 4730.5. The actual wall thickness after grinding away the surface defects detected shall be not less than 90% of the tube’s nominal wall thickness, and shall also be not less than the designed wall thickness. Inspection quantity: 5% of each inspection lot shall be sampled for inspection, with a minimum of 1 sample. Inspection method: Magnetic particle or penetrant inspection. GB50235—2010 Code for Construction of Industrial Metal Piping Projects 4.1.4 For piping components made of chromium-molybdenum alloy steel, nickel-containing low-temperature steel, stainless steel, nickel and nickel alloys, as well as titanium and titanium alloys, the material composition shall be rechecked using spectral analysis or other methods, and proper labeling shall be applied. 4.3.2 Alloy steel bolts and nuts shall have their material retested using spectral analysis or other methods, and shall be properly labeled. Bolts and nuts used for GC1-class pipes and Class C fluid pipes with a design pressure of 10 MPa or higher shall undergo hardness testing. 8.3.1 Unless otherwise specified in the design documents, the socket welds of pipes and pipe components welded on-site, the branch connection welds (except butt-type branch connection welds), the welds of reinforcement rings, the sealing welds, the welds where supports and hangers are directly welded to the pipes, as well as other fillet welds on the pipes, shall be subject to magnetic particle testing or penetrant testing on their surfaces. GB/T20801.4-2020 Industrial piping fabrication and installation 5. Inspection and acceptance of piping components and materials 5.1 Acceptance of material markings and quality certification documents In addition to being inspected in accordance with the design documents as well as the provisions of 9.1 and 9.2 in GB/T20801.2—2020, the markings and quality certification documents of piping components and materials must also meet the following requirements: a) The supplier (manufacturer) shall provide various performance data or test results in line with the design documents and the terms of the supply contract, and such data and results must comply with the provisions of the design documents and product standards ； b) If the performance data or test results provided in the quality certification documents do not meet the requirements of the product standards and design documents, or if the recipient has objections to such performance data or test results, necessary verification tests or additional tests shall be conducted ； c) The markings on pipeline components and materials shall be clear and complete, and shall allow traceability to the product quality certification documents. 5.2 Visual Inspection: Pipeline components and materials shall have their material, specifications, models, and quantities verified in accordance with the design documents and product standards; furthermore, an inspection and acceptance of their visual quality and geometric dimensions shall be carried out on a per-item basis, with the results meeting the requirements specified in the design documents and relevant product standards. 5.3 Material inspection: For pipeline components made of chromium-molybdenum alloy steel, nickel-containing low-temperature steel, stainless steel, as well as nickel and nickel alloys, titanium and titanium alloys, spectral analysis (PMI) or other methods should be used before use to check the content of the main alloying elements; such contents must meet the following requirements: a) For GC1-grade pipelines, the number of components to be inspected shall be 10% of each batch, with a minimum of 1 pipeline component to be inspected ； b) For other pipes, 5% shall be sampled from each inspection lot, with no less than 1 pipe component. Note: Each inspection lot refers to a batch of pipe components or materials with the same furnace batch number, the same model and specifications, and delivered at the same time. GB/T20801.5-2020 Inspection and Testing of Industrial Piping – 6 Scope of Inspection and Acceptance Criteria 6.1 Inspection Grades 6.1.1 Pressure pipelines are classified into five inspection grades: I, II, III, IV, and V, based on factors such as pipeline class, material type, and nominal pressure. When pressure pipelines are assigned to different inspection grades according to 6.1.2–6.1.6, the higher of these grades shall be applied as the inspection grade for that pipeline. When the design documents have specific requirements, they must also comply with the requirements of the relevant design documents. 6.1.2 Pressure pipelines that meet one of the following conditions are classified as Grade I: a) Pipelines specified in the design documents as being under severe cyclic service conditions ； b) GC1 class pipelines ； c) Pipelines used under high-temperature creep conditions (as specified in 3.2 of GB/T20801.3—2020) ； d) Titanium and titanium alloys, nickel and nickel-based alloys, high-chromium-nickel-molybdenum austenitic stainless steels, zirconium and zirconium-based alloy pipes ； e) Pipes with a nominal pressure greater than PN160 ； f) Inner tube of the jacketing tube ； g) Pipelines for which the pressure test can be waived in accordance with the provisions of 9.1.7 ； h) Piping for which design requirements specify 100% non-destructive testing of weld joints. 6.1.3 Pressure pipelines that meet one of the following conditions are classified as Grade II: a) Carbon steel pipelines with a nominal pressure greater than PN50 (impact testing is required per this section) ； b) Austenitic stainless steel pipes with a nominal pressure greater than PN110 ； c) Low-temperature nickel-containing steel, chromium-molybdenum alloy steel, duplex stainless steel, aluminum and aluminum alloy pipes ； d) Pipelines requiring 20% non-destructive testing of weld joints. 6.1.4 Pressure pipelines that meet one of the following conditions are classified as Grade III: a) GC2-grade pipelines with toxic media of toxic severity ； b) Carbon steel pipes with a nominal pressure not exceeding PN50, except for GC3-class pipes (impact testing is required per this section) ； c) Austenitic stainless steel pipes with a nominal pressure not exceeding PN110, except for GC3-class pipes ； d) Piping for which design requirements call for 10% non-destructive testing of weld joints. 6.1.5 Pressure pipelines that meet one of the following conditions are classified as Grade IV: a) GC2 grade pipelines other than those specified in 6.1.4a) ； b) Carbon steel pipes with a nominal pressure not exceeding PN50, except for GC3-class pipes (no impact test requirement in this section) ； c) Pipelines for which design requirements specify 5% non-destructive testing of weld joints. GB50517-2023 Code for Acceptance of Construction Quality of Metal Piping in Petrochemical Projects 5.2 Inspection and Acceptance of Piping 5.2.7 For chromium-molybdenum alloy steel, nickel-containing low-temperature steel, molybdenum-containing austenitic stainless steel, nickel and nickel-based alloys, titanium and titanium alloys, and zirconium and zirconium alloys, verification tests for the main alloying metal elements shall be carried out using spectral analysis or other methods; the sampling rate shall be 5% of each batch (with the same furnace number and batch number), with a minimum of one sample per batch. 5.3 Valve inspection and acceptance 5.3.9 For valves made of chromium-molybdenum alloy steel, nickel-containing low-temperature steel, molybdenum-containing austenitic stainless steel, nickel and nickel-based alloys, titanium and titanium alloys, as well as zirconium and zirconium alloys, spectral analysis or other methods shall be used; moreover, a verification test shall be conducted on the main alloying metal elements of each valve body individually. 5.4 Inspection and acceptance of other pipeline components 5.4.4 For pipe fittings made of chromium-molybdenum alloy steel, nickel-containing low-temperature steel, molybdenum-containing austenitic stainless steel, nickel and nickel-based alloys, titanium and titanium alloys, and zirconium and zirconium alloys, verification tests for the major alloying metal elements shall be carried out using spectral analysis or other methods. The sample size should be 5% of each batch, with a minimum of one unit. 5.4.8 For bolts and nuts made of chromium-molybdenum alloy steel used in pipes with a design pressure equal to or greater than 10 MPa, 5% of each batch, with a minimum of one piece, shall be subject to verification testing for the major alloying elements using spectral analysis; two sets of bolts and nuts from each batch shall be taken for hardness verification testing. 5.4.9 For chromium-molybdenum alloy steel bolts and nuts used in low-temperature pipelines designed for temperatures below –29°C, 5% of each batch, with a minimum of one piece per batch, shall undergo verification testing of the major alloying elements by spectral analysis; two bolts from each batch shall be tested for low-temperature impact resistance. 5.4.10 For chromium-molybdenum alloy steel bolts and nuts used in pipes whose design temperature is 400°C or higher, spectral analysis shall be employed for verification testing of the major alloying elements; the sampling rate shall be 5% of each batch, with a minimum of one specimen per batch. The above content is merely an excerpt from relevant standards for ease of understanding; for specific implementation, please refer to the official standards.