How to learn* through Haichuan Forum, understand standards, solve engineering problems, and pass the design examiner exam

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As a designer with no manufacturing experience, I previously had very little understanding of and application of standards. Before the design examiner exam the year before last, I learned through the Haichuan forum, understood the standards, and gained a lot of insight into their application in engineering practice. Here, I would like to give an example that I collected at the time, in the hope of providing inspiration and assistance to those in need. 205. Some practical problems in non-destructive testing engineering? Answer: Applying standards to solve practical engineering problems is the most important task for engineering designers. Examples are as follows: (1) For all Class A and Class B welded joints of low-alloy steel containers with a minimum standard tensile strength of Rm ≥ 540 MPa, if the thickness of those welded joints exceeds 20 mm, an additional local inspection using a different testing method from the original non-destructive testing method shall be carried out; this inspection must cover all weld intersections. Frequently asked questions regarding this clause: The corresponding qualification level for the additional local non-destructive testing is reduced to Level III for radiography or Level II for ultrasonic testing. In fact, the acceptance level for additional non-destructive testing should not be reduced. The reason for the error is that only the “local” requirements were considered, while the “additional” nature was overlooked. Considering the “additional” nature, its detection rate is not 20%, but 120% ； (2) Regarding local non-destructive testing, it should be emphasized that: “The testing length shall not be less than 20% of the length of each weld joint, and shall not be less than 250 mm.” All weld intersections shall be inspected. ”At this point, the total is greater than the sum of the parts ； (3) The design drawings specified the requirement to \"conduct ultrasonic testing on steel plates in accordance with JB4730\", ignoring the requirements regarding the acceptable quality level. For the qualified level, it must be specified by the designer in the technical documents ； (4) Requirements specified in the design drawings: 100% \"radiographic or ultrasonic testing\" as well as \"magnetic particle or penetrant testing\" are required for the welds, while the requirements \"as specified in the technical drawings\" are ignored or neglected. The design documents should specify the exact methods for non-destructive testing to ensure consistency in the technical requirements for pressure vessels. Standards such as GB150 cover more than just newly constructed pressure vessels. From a standard perspective, radiography can be equivalent to ultrasound, and magnetic particle testing can be equivalent to penetrant testing. Standards can be general in nature, while design documents require specificity; it is the designer who must choose between X-rays, ultrasound, magnetic particle testing, or penetrant testing, rather than the manufacturer ； (5) As required by the design specifications: the shell steel plates shall be inspected by UT testing one by one, and must meet the inspection standard of grade ≥Ⅲ. The designer’s intention was that \"the shell steel plates should be subject to UT inspection one by one, with the acceptance level being no lower than Grade III.\" But in terms of numerical values, the grade that is higher than Grade III should be Grade IV, whereas the quality grade of Grade IV is lower than that of Grade III. This style of writing is not standard and can lead to ambiguity ； (6) For small-diameter pipes with a diameter of DN≤65mm in engineering applications, penetrant testing should be chosen instead of magnetic particle testing ； (7) Are non-destructive testing required for the welds on the pressure vessel supports, as well as for the welds connecting the supports to the cylinder? What is the proportion? Answer: The aforementioned welds belong to Class E welding joints. For such welds on certain types of equipment, GB150 10.4 specifies that magnetic materials shall be inspected using MT, while non-ferrous metals and stainless steel shall be inspected using PT; 100% flaw detection is required, with a qualification standard of Grade I ； (8) Radiographic testing is required for the butt welds of pressure vessels, and the hydrostatic test can only be carried out after this testing is completed. Radiographic testing is used to check whether there are any defects in the quality of the butt welds, but the hydrostatic test can also assess the quality of those butt welds as well as the pressure resistance of the entire vessel. If there is a leak at the weld during the hydraulic test, it indicates that the weld has defects; so why is non-destructive testing still necessary? Answer: The purpose of non-destructive testing is to detect defects inside the weld, as well as on its surface or near its surface. Defects that exceed the specified limits require repair, while those that do not exceed these limits can exist inside the weld. However, this does not prove the strength of the weld or whether it will leak, so a hydrostatic test is also necessary. The two cannot be substituted for each other. As special equipment, pressure vessels are subject to inspection and monitoring; if any problems arise during the design validity period, the manufacturer shall be held accountable, with materials and non-destructive testing serving as the primary evidence. Furthermore, even if 100% compliance with non-destructive testing is achieved, it does not guarantee that the equipment is leak-free. A hydraulic test is not merely used to check for leaks in equipment; it is primarily intended to assess the strength of pressure vessels, serving as a strength testing procedure. Moreover, even if there is some slight leakage, it might not be detected during a hydraulic test; only a leak test can determine whether the container is leaking. For ordinary containers, the hydrostatic test can help identify leak points; when pressure is applied and water leaks out, it indicates poor welding. For pressure vessels with slightly higher pressures, hydraulic testing cannot be used to detect leaks; in other words, even if there are leaks, they may not be detected if the inspection is not thorough. Moreover, it is possible that leaks will not be visible during hydraulic testing, nor will any drop in pressure be observed, and this does not guarantee the integrity of the vessel. Both the integrity of the vessel and the tightness of its sealing surfaces need to be verified through air-tightness testing ； (9) **Do media but not Class III containers not require 100% non-destructive testing?** Answer: It is not required according to the fixed-volume regulations, but it is required per GB150 10.3.1; this is an example where technical standards have higher requirements than those set by regulations ； (10) Selection of methods for non-destructive testing. Answer: When it comes to inspecting butt welds, there are specific considerations regarding whether to use radiographic testing or ultrasonic testing. From a detection perspective, each of the two has its strengths. Radiographic inspection is sensitive to volumetric defects, but it is difficult to detect linear defects, especially small gaps or micro-cracks in thick welds ； Ultrasonic testing is sensitive to linear defects, but its quantification of point defects is inaccurate. Radiographic testing has few requirements regarding the workpiece surface. It evaluates weld quality through radiographic films. Its advantages include being intuitive, easy to qualitatively assess, and suitable for documentation. However, it is difficult to determine dimensions in the depth direction ； Ultrasonic testing places strict requirements on the surface to be inspected; it evaluates defects based on the waveforms displayed on a fluorescent screen. Its advantage is that it makes it easy to determine the depth of defects, but it is not intuitive and difficult to archive data (of course, with advances in technology, more applications of ultrasonic testing techniques that allow for data recording are emerging). Qualitative assessment requires comprehensive judgment, and inspectors must possess good technical skills and a strong sense of responsibility. Therefore, for important equipment, two testing methods should be employed; they can complement each other to provide a comprehensive assessment and ensure the internal quality grade of the welds. Of course, surface defects in welds also need to be detected using MT and PT. Additionally, UT testing is generally not used for welds in austenitic stainless steel (the coarse grains of austenitic stainless steel significantly affect the attenuation and propagation of ultrasonic waves). Furthermore, it should be noted that the standards for non-destructive testing depend on the design standards of the equipment; for example, those designed in accordance with GB150 follow JB4730, while those designed per ASME standards follow ASME Volume V, and so on. There are many methods for non-destructive testing, but for special equipment, testing in accordance with the JB/T4730 method is the only one recommended; it also specifies the inspection ratio, quality requirements, and acceptable standards. These are all basic requirements; in other words, they are the necessary conditions to ensure the safe operation of special equipment. However, each type of test has its own advantages and disadvantages; the optimal choice of individual tests or combinations should be made based on the usage conditions of the equipment, with appropriate combinations used. Ensure equipment safety. For general Category I and II equipment, it is sufficient to use a single method for testing. The designer must pay attention to ensuring that the advantages of various detection methods are combined to overcome their disadvantages; the areas that need to be inspected must be checked thoroughly, with no blind spots ； (11) Why is it necessary to perform penetrant or magnetic particle testing on welds that have already been tested by radiography or ultrasonic testing? Answer: X-rays or ultrasound are used to detect internal defects, magnetic particle testing is used to detect defects on the shallow surface, while penetrant testing is used to detect surface defects. Ultrasonic testing has blind spots, so penetrant or magnetic particle testing should be used as a supplement. Welds that are subject to radiographic testing are generally important, and surface testing is also employed as an additional measure (at a lower cost). Due to limitations in their working principles, both radiography and ultrasound have certain drawbacks. For very important welds, a combination of radiography + ultrasound + penetrant testing or magnetic particle testing is used, such as in hydrogenation reactors in the oil refining industry, as well as in cold and hot high-pressure separation tanks. The standards specified generally represent the minimum requirements; if higher standards are required for the welds, they should be increased as appropriate ； (12) For certain equipment, welds of category A and B are required to be inspected by radiographic testing, and at the same time, the weld surfaces are required to be inspected by magnetic particle testing. What should be the order of these inspections, and why? Answer: A pressure test is used to evaluate the strength and integrity of equipment. During this test, the welds in the equipment are subjected to stresses greater than those encountered under normal operating conditions, which may lead to damage to the welds; therefore, surface inspection after a pressure test is necessary. Radiographic testing should be carried out first, followed by magnetic particle testing; since radiographic testing is used to detect defects inside the welds, it is more reasonable to conduct surface testing after the radiographic testing yields satisfactory results ； If magnetic particle testing is carried out first, and radiographic testing is done after the surface inspection is successful, any internal defects detected will require rework, followed by further radiographic testing and surface inspection ； On the other hand, performing magnetic particle testing first and then radiographic testing has an impact on the photographic imaging; therefore, in non-destructive testing, radiographic testing should be carried out first followed by magnetic particle testing ； (13) 100% RT requires gap-free RT testing of Class A and Class B welds on the equipment, with the test pieces overlapping each other ； For the 20% of areas subject to local non-destructive testing, in addition to the T-joints, the radiographs do not need to be placed sequentially; they can be placed at intervals. Of course, aside from the junction areas which must be inspected, the rest can be chosen freely; for simplicity, it is possible to proceed sequentially. But it’s best not to do this; it is recommended that, in addition to T-joints, X-rays be taken of the first weld made by each welder as well ； (14) Generally, to simplify the process, our company welds the lower head to the shell first, and then conducts RT inspections on the longitudinal and circumferential seams of the shell. Is this sequence of manufacturing steps reasonable? Answer: No standards or regulations specify the order in which RT inspections should be carried out for longitudinal and circumferential seams; therefore it is allowed, but such an inspection order carries risks (such as the need for rework due to defects in the longitudinal seams of the cylinder) ； (15) According to HG/T 21574-1994 \"Equipment Lugs\", \"the remaining welds shall also be subject to magnetic particle or penetrant testing in accordance with JB/T 4730\" – does this requirement apply to the welds connecting the head or cylinder as well? Sometimes, the equipment has a relatively small diameter and thin head wall thickness, which does not meet the standard minimum wall thickness requirements; in such cases, custom-designed lugs are used, followed by verification calculations. If the standard lifting lugs mentioned above are required, is magnetic particle or penetrant testing also necessary for the welds between the self-designed lifting lugs and the head? If necessary, should it be specified in the technical requirements of the overall layout diagram or component diagram? Answer: Lugs connected to the head or shell sections must be 100% MT or PT, whether the lugs are selected from standard options or designed as custom ones; this is a matter of safety and should be specified in the technical requirements. Technical requirements generally apply only to the non-destructive testing of the lugs used in the weld joints between the housings; as for the lugs themselves, the priority is to ensure the quality of their material, and this is guaranteed by the manufacturer ； (16) After non-destructive testing has been performed on the weld, is it permissible to grind the weld surface? Answer: If radiographic testing is conducted and the result is satisfactory, grinding is possible, but it is necessary to control the weld bead height and ensure the weld surface is smooth. If there are requirements regarding surface roughness, then attention must be paid to it ； If it is an MT or PT test, it is best not to polish; if polishing is done, it is advisable to conduct the MT or PT test again. Because MT or PT examines the near-surface or surface of the workpiece, and grinding destroys the original testing surface, meaning the results obtained using MT or PT become invalid ； (17) I saw some drawings where the surfacing layer required PT in some cases, and UT in other cases. The standards state that a PT test is required for the surfacing layer, but they do not specify that a UT test is necessary. Under what circumstances is a UT test needed? Are there any relevant standards or specifications? Answer: UT is used to detect defects within the surfacing layer, defects resulting from poor bonding between the surfacing layer and the base material, as well as reheat cracks in the base material beneath the surfacing layer ； PT for the weld overlay is used to detect defects on the surface of the weld overlay; these two processes are different from each other. Both processes are required for reactors. For the inner wall of the shell, important components such as tube sheets, or in cases with severe operating conditions, UT+PT ； For ordinary components in general situations, PT is sufficient ； (18) Austenitic stainless steels have bimodal grain boundaries, which interfere with the ultrasonic waves used in UT, making it difficult to make accurate judgments about defects. Ultrasonic testing cannot be used on welds of austenitic stainless steel, as factors such as twin grain boundaries in austenitic stainless steel significantly affect the attenuation and propagation of ultrasonic waves; however, austenitic stainless steel plates, forgings, etc. can still be inspected using ultrasonic testing. The twin grain boundaries refer to the planar network-like grain boundaries formed by chromium and nickel; they are planar and network-like due to their high toughness and plasticity, and are similar in structure to graphite ； (19) Is non-destructive testing related to weld strength? Answer: Non-destructive testing can only detect defects within the weld, on its surface, and in the near-surface area. It serves to prevent stress concentration at those defects in the welded joint when the joint is under stress; if the stress at such concentration points exceeds the material’s yield strength, it can lead to the failure of the weld. Defects in the weld will inevitably affect its strength, but weld strength has no direct relationship with non-destructive testing. Weld strength refers to properties such as the material’s yield strength and tensile strength. It is determined by factors like the welding process and heat treatment conditions, as well as the composition of the weld metal, the type of welding electrode, the welding method, the amount of weld metal deposited, and the operating conditions during welding; these factors influence the hardness and other properties of the weld ； (20) A flash tank has been modified with an internal coil at its conical bottom. What is the best way to carry out non-destructive testing on the joints? Answer: Before bending, weld the joints first; ensure that the joints pass RT testing. If RT testing is not possible, UT testing can be used instead. After bending to give the part its final shape, perform penetrant testing on the joints to confirm they meet the requirements – this should be a reliable approach. Finally, conduct a hydrostatic test, and once it passes, drain the water and install the part in the equipment ； (21) In non-destructive testing, RT and UT are mainly used to detect internal defects in the object under inspection, while MT and PT are used to detect surface and near-surface defects. Why does GB150 require RT or UT only for Class A and B welds (i.e., butt welds), while requiring only MT and PT for Class C and D welds? Are there no internal defects in Class C and D welds? How is this ensured? Answer: After all, Class C and D welds are not that important; as long as there are certified welders to carry out the welding while ensuring no surface defects, and pressure tests are conducted to verify quality, the quality can be guaranteed. Of course, its reliability still differs from that of RT and UT. Furthermore, welds of types C and D are generally located in areas such as fillet welds that are difficult to inspect; RT testing is not feasible there, nor is UT testing, so the only way to ensure quality is by relying on qualified results from welding procedure qualification ； (22) GB150 specifies ultrasonic or radiographic testing requirements only for weld joints of types A and B, while for type D joints, only surface testing is required. Under what circumstances are Class D welds required to undergo radiographic or ultrasonic testing? Today I came into contact with a set of equipment designed for operation in the presence of hydrogen and wet H2S; its design pressure is 7 MPa and its design temperature is 280°C. One of the technical requirements specifies that \"Welded joints of category D with an inner diameter greater than 200 mm must undergo 100% ultrasonic testing in accordance with JB/T4730-2005, and they must meet grade I standards.\" May I ask what the basis and source for this is? Answer: The main considerations are: a) Importance: The joints connecting the take over to the shell are subject to high restraint, resulting in significant stress concentration; moreover, the weld metal usually has lower plasticity than the base material, and cracks generally originate from these areas. In conventional design calculations such as GB150 (primary film stress), the effect of stress concentration is not taken into account; instead, it is controlled through special manufacturing requirements ； b) The conventional approach without any special requirements: During welding, gas cutting and gas gouging are commonly used to create the saddle-shaped openings manually; it is difficult to ensure the correct bevel angle and edge thickness, the oxide scale on the surface of the bevels is hard to remove, and the working position is not conducive to proper shaping. Therefore, defects such as cracks, lack of penetration, lack of fusion, and slag inclusions are very likely to occur. With no requirement for non-destructive testing, welders’ sense of responsibility also decreases. Quality is hard to guarantee ； c) Make requests: These are generally specified only under special operating conditions (fatigue, low temperatures, large temperature gradients, toxic or hazardous media, and sometimes flammability and explosivity). It is best to use a saddle-type cutting machine to create the groove; even in the absence of such a machine, the quality of the groove must still be ensured. A fully penetrative welding process should be employed, either through double-sided welding with root cleaning on the reverse side, or through single-sided welding with TIG welding for the root portion. In summary, it is reasonable and necessary to propose non-destructive testing for the special operating conditions of Class D connectors ； (23) For the surfacing of a certain tube sheet, there are two manufacturing sequences – which one is more reasonable? a) Cladding transition layer - Heat treatment - PT - UT ； b) Cladding transition layer-PT-UT-heat treatment, which one is more reasonable? Answer: The surfacing process is related to the material of the substrate being surfaced. The requirements for Cr-Mo steel are different from those for ordinary carbon steel. Cr-Mo steel is prone to delayed cracking and reheat cracking; therefore, heat treatment is carried out before non-destructive testing, and test plates are used to evaluate and ensure welding quality. Theoretically, for materials prone to reheat cracking, two non-destructive inspections should be conducted, one before and one after heat treatment. Therefore, Cr-Mo steel equipment is suitable for the first method ； Cladding with ordinary materials is suitable for the second method ； (24) When a pressure vessel is undergoing non-destructive testing, it is difficult or impossible to perform radiographic inspection on certain areas; in such cases, ultrasonic inspection is considered as an alternative. If the design drawings specify only radiographic inspection, but standards such as TSG R0004-2009 and GB150 do not provide clear requirements, the manufacturing unit may use ultrasonic inspection instead of radiographic inspection. Do we have to submit design changes every time there is an inspection? Answer: Changing the testing method generally requires a design change by the design institute, as the choice of testing method is also part of the design process. Ultrasonic testing and radiographic testing each have their advantages, and generally cannot replace one another ； (25) For a certain weld that requires 100% inspection, when pulse-reflectance ultrasonic testing, which involves recording, cannot be used, radiographic testing must be supplemented by local non-destructive testing – what is the required proportion for this supplementation? Also 20% and not less than 250 millimeters? Answer: There is no specified additional testing ratio; generally, it is set at a rate of not less than 20% ； (26) What non-destructive testing requirements apply to the butt joints of stainless steel coiled tubes in pressure vessels – radiography or penetrant testing? Answer: In general, penetration is sufficient, but it is also necessary to consider the toxicity of the medium, whether it is flammable or explosive, and the pressure level. If the outer diameter is large, that is also important; radiation treatment can also be considered ； (27) The equipment is designed for a pressure of 7.2 MPa and has a volume of 13 m3 ； Medium: Air, engine oil ； The weld coefficient is taken as 1 ； Wall thickness 50mm. Requirements: 1. 100% RT testing shall be conducted on Class A and Class B welds, with an additional 20% UT testing ； 2. 100% magnetic particle testing shall be conducted on the surfaces of all weld joints after the final heat treatment of the container, with the acceptable grade being Grade I ； 3. After the hydraulic testing of the container, its Class A and Class B weld joints shall also undergo 100% ultrasonic testing, with the acceptance standard being Grade II ； Are the above requirements appropriate? Answer: The welding factor is 1; 100% RT or 100% UT inspection is required for Class A and Class B welds, with the RT acceptance level to be no lower than Grade II and the UT acceptance level to be no lower than Grade I ； However, since the plate thickness exceeds 38 mm, it is recommended to conduct 100% RT + 100% UT inspections; both inspection methods must yield satisfactory results, and in case of doubts regarding the test results, the UT results shall take precedence. For butt joints that are required to undergo 100% UT testing, the acceptable grade should be Grade I ； (28) I would like to ask about the sequence of non-destructive testing for Cr-Mo containers? Answer: The most commonly used Cr-Mo steels in our country are 15CrMoR, 14Cr1MoR, and 12Cr2Mo1R; their strength increases in sequence, and their tendency to develop delayed crack formation in welds also increases accordingly. Whether combined non-destructive testing is required for Class A and Class B welds should generally be determined by the designer, as there are no mandatory requirements in the standards. For particularly important Cr-Mo vessels, it is necessary to consider a combination of non-destructive testing methods: UT for Class D joints, additional UT and MT after heat treatment, as well as additional UT and MT after hydrostatic testing ； (29) For a certain vessel, Class A and Class B weld joints in the cylinder section are required to undergo 20% RT with a acceptance level of Grade III, while butt joints are required to undergo 100% RT. How should the acceptance level for radiographic inspection of the head be determined? Answer: The head should also be determined based on the overall inspection qualification level of the container; although the inspection rate here is 100%, the qualification level is still Grade III ； (30) Recently, a heat exchanger was manufactured, with the fluid on the tube side being hydrocyanic acid + acetonitrile; this fluid is classified as highly toxic. The material used for the shell on the tube side is S30403, with a wall thickness of 14 mm. I included the requirement for ultrasonic testing of the tube-side steel plates in the technical specifications, but it was removed after review, on the grounds that the specifications for ultrasonic testing of shell steel plates used to contain highly toxic substances apply only to carbon steel and low-alloy steel. I would like to ask: 1. Why does the fixed-volume standard specify ultrasonic testing only for carbon steel and low-alloy steel, without imposing restrictions on stainless steel? 2. Are there any properties of stainless steel that allow it to be exempt from ultrasonic testing of steel plates? 3. Referring to Appendix E of GB150-2011 regarding the basic design requirements for cryogenic pressure vessels, Clause E1.1 specifies that \"this appendix applies to cryogenic vessels made of carbon steel and low-alloy steel.\" In other words, cryogenic vessels made of stainless steel and used at temperatures above -196°C can also be exempt from certain provisions of Appendix E. Why is that? Answer: When using ultrasound on stainless steel, there is a great deal of noise, making it almost impossible to determine what constitutes defects; therefore, ultrasound testing is generally not used. As for low temperatures, austenitic stainless steel possesses excellent toughness; brittle fracture caused by low temperatures hardly occurs at temperatures above -196 degrees Celsius, so it is not necessary to treat it as a low-temperature pressure vessel.