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Question: The design date for the industrial pipelines at a natural gas station is July 2019; the design pressure is 4.0 MPa, the material used is L245 of the PSL2 series, and the applicable standard is GB/T 9711-2017. Construction began in May this year. During the supervision and inspection process, it was found that according to the provisions of the Pipeline Code, carbon steel pipes specified in GB/T 9711.1 shall not be used for GC1 natural gas pipelines. Although GB/T 9711-2017 no longer divides itself into GB/T 9711.1 and GB/T 9711.2, since the 2011 version also includes PSL2 classifications, it is considered that the material selected for the design does not comply with the requirements of the Pipeline Code. Is this understanding correct? From a technical perspective, the usage restrictions associated with GB/T 9711.PSL2 have been removed. Based on the principle that anything not prohibited by law is permissible, current regulations only specify the usage restrictions for GB/T 9711.1 (PSL 1); there are no such restrictions specified for GB/T 9711.PSL2. Therefore, this cannot be used as evidence that the design does not comply with current regulations. The article I wrote, \"On the Application of GB/T 9711 in the Field of Industrial Pressure Pipelines,\" presents the technical concepts of GB/T 20801 for reference. Q: Inquiry about the design technology of the expanded series of Class2500, DN450 manhole flanges? Image: The design techniques for this case should be based on the fundamental principles outlined in ASME B16.5, so that the design can be integrated into ASME B16.5’s framework, utilizing the pressure and temperature rating system specified for Class 2500. Therefore, the flange design should employ high-neck flanges, high-strength bolts, and octagonal gaskets. The flange strength is calculated using the Whittaker method; the design pressure is 42.0 MPa, the allowable stress for the flange is 150 MPa, and the allowable stress for the bolts is 120 MPa. The bolt areas are arranged in accordance with ASME B16.5 requirements, but once the actual areas are determined, the allowable stress of 175 MPa for the bolts is used in the calculations of bending moments via the Whittaker method. After calculating the flange strength, recheck the flange stiffness verification. When selecting the size of an octagonal gasket, first look in ASME B16.20 for R-series octagonal gaskets with similar pressure and diameter ratings. If none are found, then search within the B or BX series for octagonal gaskets rated for a high pressure of 10,000 Psi. Pay attention to the compatibility between the dimensions and tolerances of the gasket and its corresponding groove. For the outer diameter and thickness of the small end of the flanged tapered neck, and for determining the inner diameter of the flange, an allowable stress of 150 is used, with a corrosion allowance of 1.5 mm added. The outer diameter follows that of standard pipes; therefore, the actual internal diameter may be only 400. Please consider whether this is suitable for use as a manhole, and decide whether to increase it to DN500. It is also reported that the SSEC equipment room designed supplementary series of flanges rated Class 1500 and 2500 using the Thors method and pressure vessel specifications many years ago; these can serve as a model for specific design conditions, but should not be applied within the ASME B16.5 framework. Because there is still a gap compared to the design philosophy of ASME B16.5. Question: For PN250 in the GB flange standards, what standard governs the outer diameter of steel pipes with a DN of 80 and above? Starting from PN250, the outer diameter of pipes above DN80 has increased; it’s no longer the outer diameter specified in ASME B36.10? Image 1. Under high pressure and for larger pipe diameters, the difference between the outer diameter and the nominal bore size becomes too large. Therefore, it is necessary to increase the outer diameter. The European standard follows this approach, while the American standard ASME B16.5 adheres to the principle of keeping the outer diameter series unchanged. 2. The world is diverse; so is technology. 3. The HG/T 20592 standard adopts a technical policy that avoids contradictions, taking into account the level and acceptance capacity of the domestic industry. 4. A few days ago, someone asked me about the design of a Class 2500 DN450 manhole flange. In reality, its inner diameter is less than 350 mm; therefore, it should be designed as a DN600 flange in order to be used as a manhole flange. Q: According to the “Rules for Periodic Inspection of Pressure Piping – Industrial Piping”, when the overall thinning of piping components exceeds 20% of the nominal thickness, a strength check must be performed in accordance with the design standards. However, neither GB/T 20801.3-2020 nor GB 50316 provides any formulas for the strength check of tees. Is it acceptable for us to use the strength check formula designed for straight pipes instead? The provisions of the inspection regulations listed in the illustration can only be understood as applicable to straight pipes; they probably cannot be mechanically applied to pipe fittings such as tees and elbows. The following two documents are provided for reference: one is ASME B31G, and the other is Chapter 5 of API 579-1. But these two documents can only address the pressure assessment for straight pipes and curved pipes. Additionally, for API 579-1, please consult relevant experts. Using the 0.8 value for straight pipes as specified in the inspection standards, since bent pipes have a larger bending radius, their strength under static pressure is essentially equivalent to that of straight pipes; therefore, 31G can also be applied to bent pipes. The stress distribution and classification caused by pressure in various parts of a tee are much more complex; therefore, it is difficult to apply the evaluation criteria for local thinning in straight pipes. If stress analysis really needs to be done, it is theoretically possible, but it is likely difficult to carry out in practice. GB/T 20801-2020 actually specifies three methods for evaluating the strength of tees: the first is the opening reinforcement method for the extruded outlet, the second is the pressure area method listed in the appendix, and the third is Method B specified in GB/T 12459, namely the 1.5 times thickness method. Therefore, within the reinforcement zone, 0.8 is roughly applied to the thickness of the reinforcement zone to assess feasibility. Specifically, it is subject to in-service inspections; this is for reference only. Question: The bent pipe can be verified directly based on the simplified dimension measurements. The situation with tees is relatively complex; generally, the strength of the tee’s design drawings is evaluated using methods such as hole reinforcement or pressure area calculation. During evaluation, prototypes are measured to determine whether the processes employed meet the design requirements. But in reality, the shoulders inside many extruded tees fail to meet the reinforcement requirements, especially when used at high temperatures. For in-service tees, it is not possible to verify them since the specific internal shape and dimensions are unknown; only it is possible to check whether the wall thicknesses of the main pipe and the straight pipes meet the design requirements. We once removed a defective tee fitting and used laser scanning to determine its dimensions. It turned out that the actual shape and size of the manufactured part deviated significantly from those specified in the drawings, failing to meet the strength requirements. Images: There are several basic concepts that need to be clarified in the assessment of the static pressure resistance of operating equipment and pipelines. One is that it involves only the evaluation of static pressure strength, without considering other loads; this is particularly important for pipelines. The second thing to be used as a reference for evaluation is whether it is the physical object, the design conditions, or the subsequent operating conditions. The evaluation results vary depending on the reference conditions. This issue relates to the physical value, namely the nominal wall thickness; this means it is generally more conservative compared to the other two reference conditions. If it exceeds the specified limits, then the reliability is re-evaluated using the design conditions or the actual operating conditions. This evaluation requirement stipulates that the physical design and construction must meet the standard requirements, and that the reliability after 20% corrosion has been verified as a prerequisite. A 20% local corrosion rate can be considered a moderate realization of the excess safety margin of the pipeline as well as the safety margin specified by the design codes; for example, there is usually a 12.5% margin between the nominal wall thickness of a pipeline and its minimum thickness. The rule that a negative wall thickness deviation of no more than 0.25 mm is not considered during the design of pressure vessels translates to a deviation of 4% for a plate thickness of 6 mm. Question: Based on the definition of GC3-grade pipes in GB/T 20801.1-2020, please help confirm whether the pipe classification for the following medium conditions is correct: 1. The medium is circulating water, with a design pressure of 0.7 Mpa, a design temperature of 65°C, and a pipe diameter of DN80 – should the pipe be classified as having no grade or as GC3? 2. The medium is circulating water, with a design pressure of 0.7 Mpa, a design temperature of 65°C, and a pipe diameter of DN40 – should the pipe be classified as having no grade or as GC3? 3. The medium is high-pressure flushing water, with a design pressure of 35 Mpa, a design temperature of 65°C, and a pipe diameter of DN100 – should the pipe be classified as having no grade or as GC1? The issues related to pipe classification mentioned in the question can essentially be summarized into two questions. First, second, and third are how to understand the issue of how regulatory standards are consistent with GB/T 20801, yet they serve different purposes and have different scopes of application. The classification of the several questions posed is based on the requirements of relevant regulatory standards; from a surveillance perspective, it is reasonable. However, from the viewpoint of the design team, especially engineering and technical personnel, and considering which basic safety standards should be followed in design, the classification scheme established in GB/T 20801-2020 represents one of the best options. As for how to express this in the design documents, regulations and standards do not specify any requirements; it is up to each design institute and each project to decide on the details. At the level of standards and regulations, how should the pressure pipeline category be filled in the pipeline characteristic table? It’s all about implementation; it is the result of balancing and coordinating the interests and demands of various parties involved in implementing regulations and standards. It is inconvenient under the standards, and it is not possible to provide clear opinions or solutions. Therefore, in the aforementioned cases, the key point is that, whether within the scope of regulations or not, standard grading serves as a symbol for determining the corresponding technical requirements. Therefore, the pipe grade in GB/T 20801-2020 serves as a synonym for the basic technical requirements of the code coordination standards within the scope of application of the pipe code. However, for industrial pipelines outside the scope of pipe codes, they serve as recommended technical standards for users to choose from. The classification of pipeline regulations represents the different supervision and management requirements imposed by regulatory authorities on pipelines within their statutory oversight scope, using a classification system. Of course, the technical requirements for regulation are summarized using harmonized standards. However, the grading of regulatory standards, in addition to technical requirements, also represents the core requirements of such standards regarding licensing, qualifications, management procedures, format, responsibilities, and so on. The GB/T 20801-2020 standard should not cover management issues such as management, responsibilities, and qualifications. Based on this, the answer to the above question is that, in terms of pipeline regulation classification, if it is considered that pipelines do not cover liquids that lack the three necessary specifications, then it can be said that such liquids cannot be classified. However, from the perspective of GB/T 20801-2020, whether a liquid lacks those three specifications is not a factor taken into account when determining the applicability of the standard; therefore, such liquids still fall under the scope of GC1. For this operating condition, it is simply a matter of choosing between following the conventional design specifications in GB/T 20801-2020 or the pipeline specifications for Class K in ASME B 31.3. Therefore, a few words are added after the high-pressure pipelines above 420 bar, which are not covered by GB/T 20801.1-2020; this establishes a binary transition zone within a certain pressure range between high pressure and what is known as ultra-high pressure, thereby defining Class K pipelines as another set of construction specifications available for design and construction purposes. Question: Must the bolt height exceed that of the nut after installation? Are there any standards that specify this? Are washers required when installing flange bolts on nuclear power pipeline? Neither GB/T 20801 nor the ASME piping codes and ASME PCC-1 require bolts to be exposed; on the contrary, the relevant ASME technical documents seem to stipulate only that after the bolts are pre-tightened, all of the threads within the nuts must be in contact. It seems that ASME PCC-1 also recommends allowing the bolt to be slightly below the nut, with one or two teeth not in contact. Because the exposed bolt threads are prone to corrosion, it will lead to difficulties in disassembly. However, domestic pipeline construction standards, with the aim of ensuring construction quality and an orderly appearance, once required that bolts be exposed, limited to one or two threads. To this end, when formulating the 1997 version of the standard for HG pipe flanges, the bolt length was determined by mechanically copying the bolt length calculation formula from ASME B16.5; limits such as the positive tolerance for flange thickness, the negative tolerance for bolt length, and the chamfering at both ends of the bolts were all taken into account. In reality, for domestically supplied flanges, the positive tolerance of the flange thickness after machining is usually zero. This results in three or four threads, or even more, protruding after the bolts are installed. Subsequently, it was found that foreign engineering companies often made adjustments when establishing bolt length databases; for this reason, adjustments were also made during the formulation of the 2009 version of the HG pipe flange standard. Regarding the question of whether washers are necessary and how to adjust the bolt length after using washers, ASME PCC-1 as well as various foreign technical documents on determining bolt tightening torque generally consider that the function of flat washers is to reduce the friction coefficient between the lower surface of the nut and the surface of the flat washer when the bolt is tightened – whereas in the absence of washers, it is the lower surface of the nut that comes into contact with the upper surface of the flange. The harder material of the washer, along with its smoother surface, contributes to this reduction in friction, allowing the tightening torque to be converted more effectively and steadily into compressive force on the bolt. Furthermore, spherical washers help to adjust and reduce the bending stress on bolts caused by various factors during bolt installation. Therefore, using washers when installing flange bolts helps improve the quality of installation, but whether it is necessary depends on the actual circumstances as well as the level of concern shown by the owner or the designers. Therefore, no provisions are made in the standards; it is usually limited to additional design requirements when necessary. Indeed, if washers are added, they should be taken into account when calculating the bolt length. Additionally, please note that for metric-threaded fasteners, the length includes the chamfers at both ends; whereas for imperial-threaded fasteners, the length *traditionally* does not include the end chamfers, but refers only to the length of the threads. The two marking methods are different. Question: According to clause 9.1.7 of GB/T20801.5, the three conditions under which pressure testing can be waived are: 100% radiography, sensitivity testing, and flexibility analysis. If this pipeline is not exempt from pressure testing, the radiographic inspection ratio is 10%, and grade III compliance is sufficient. If the pressure test is waived and the radiation ratio is 100%, will it still be considered grade III compliant? Rather than being Grade II qualified? Image: 100% radiation, grade 2 pass. The logic for non-destructive testing is 100% + grade 2 compliance, reflecting high requirements for the reliability of weld quality. Local proportionality testing with a grade 3 pass indicates that the reliability of the weld quality is relatively low. Precisely because the pressure test is omitted, it is necessary to enhance the reliability of weld quality in order to compensate for and reflect the emphasis on the construction quality of the project. As for the rework issues caused by the contractor reducing welding quality in order to carry out partial flaw detection as originally planned, from the perspective of project quality assurance or clarification of standards, there is no room for compromise. This issue can only be resolved through negotiation among the property owner, the designers, and the contractors. Question: Explanation of gases in GBT 20801: Gases other than those specified in GB/T20801. Apart from the definition in 1-2020 3.10, the \"gases\" referred to in GB/T 20801-2020 also include steam, liquefied gases, liquids with a maximum operating temperature equal to or higher than their standard boiling point, as well as two-phase or multi-phase fluid media that include a gaseous phase. Does the multiphase flow medium in this sentence include a mixture of gas and dust? The toxicity hazard category of a mixture is determined based on its acute toxicity LC50 (for inhaled dust and fumes). Question: Section 4.2.4 of GB/T 20801.2-2020 states that \"recycled materials shall not be used in pipeline components.\" Does this mean that it is not allowed to use components from old pipes that have been removed? For example, in a new pipe installation project, is it prohibited to use used valves, elbows, or tees? In other words, recycled materials (such as Q235, 20#, 30408, etc.) cannot be used to manufacture pipeline components (valves, elbows, etc.). The correct interpretation of clause 4.2.4 of GB/T 20801.2-2020 is that \"recycled materials shall not be used as raw materials for the remanufacturing of pipeline components.\" Used old pipes, valves, elbows, and tees can be used in pipelines with different requirements, provided they come with complete quality certification documents, have undergone the necessary in-service inspection and evaluation, and meet the principles required for their use. Regarding the evaluation requirements in this regard, since GB/T 20801-2020 is a code for the construction of industrial pipelines, it does not cover these aspects; see clause 1.4 a) of GB/T 20801.1-2020 for details. For relevant requirements, please follow the provisions of TSG Industrial Pipe Code, or refer to the relevant regulations on ‘fitness for service’ in API 579-1/ASME FFS-1 2016. Question: A pipeline transports a flammable liquid; its diameter is DN80 and the maximum operating pressure is 0.09 MPa.G. The safety valve of the equipment connected to this pipeline is set at 0.6 MPa.G. Is this pipeline considered a pressure pipeline? How should the “maximum operating pressure” in the definition of pressure pipelines be understood? “Should “maximum operating pressure” be understood as the “highest pressure during normal operation,” rather than under abnormal conditions, during start-up or shutdown, or in case of accidents? According to the definition in the catalog of special equipment, shouldn’t it not be considered a pressure pipeline? Illustration: Understanding of “maximum working pressure”: The definition of “maximum working pressure” as a measure of pressure appears in relevant regulations and documents pertaining to the safety supervision of pressure pipelines. However, GB/T 20801, the standard for the construction of industrial pipelines, does not define “maximum working pressure”; instead, it defines and describes pipelines based on their “design pressure”. Although the \"maximum operating pressure\" is mentioned in safety supervision regulations and documents, the specific maximum operating pressure for a pipeline is not determined by the safety supervision authorities; rather, it is specified by the pipeline design team in the relevant design documents. Safety inspection departments usually determine this based on the maximum operating pressure specified in the design documents; if the maximum operating pressure is not indicated in those documents, the design pressure given therein has to be considered as the maximum operating pressure. It is the responsibility of the design firm to determine in the design documents the maximum operating pressure, design pressure, normal operating pressure, and other values for each pipeline. Although GBT20801 does not define pressures other than the design pressure, other design standards related to pipelines – whether they are ** or industry, professional, corporate, project, or process package standards – may specify the maximum operating pressure and normal operating pressure of pipelines. If there is no applicable standard to follow, it is up to the design department to determine these values, and it assumes corresponding responsibility, as this is part of its duties. Question: From the designer’s perspective, it is actually very difficult to define the actual maximum operating pressure; could it be changed to the maximum operating pressure permitted by the design? The image shows the maximum allowable working pressure, commonly referred to as MAWP, which is the pressure definition used in Part 1 of Volume 8 of ASME BPVC for determining the load-bearing capacity of pressure vessels. However, in the domestic pressure equipment and piping industry, this definition of MAWP is not referenced in the relevant standards and specifications. It seems therefore that it is not possible to adopt the above suggestions. Question: To address the issue of misalignment when welding pipes together or pipes to fittings in power plant piping systems, it is required that the welding reference values (welding C-value) for pipes and fittings be calculated in accordance with the standard \"Piping for Power Stations\" DL/T 850-2004, in order to ensure that the welding requirements are met. Are there no specific requirements for industrial pipelines? In chemical plants, there are also steam pipelines operating at pressures of 8.8–9.81 MPa and temperatures of 460–540°C. If the design specifications do not explicitly state the welding C-value, would it be unsafe to construct these pipelines in accordance with the standards for industrial pipeline construction and acceptance? The concept of the inner diameter C of the groove, as mentioned in DL/T 850 for power plants, is as described in the first paragraph of Appendix C of this standard. The calculation formulas provided in this appendix can be applied to the four main types of pipelines in power plants as well as certain high-temperature and high-pressure pipelines; however, they are not applicable to all pressure pipelines in power plants. The objective is to minimize the internal misalignment at the groove during assembly by adjusting the inner diameter of the joint through processing of the circumferential welds, thereby ensuring that the designed thickness is maintained and improving the quality of the circumferential welds. As a general construction standard for industrial pipelines, GB/T 20801 does not include such additional technical requirements, just like the ASME B31.3 standard and the DL standards; however, similar limits on edge misalignment are specified in the construction requirements for Class K pipelines under ASME B31.3. In my second article on carbon steel engineering issues in pressure pipelines, I also provided, based on the principles of DIN 2559, a general formula for determining the inner diameter of circumferential weld grooves, for reference. This is a matter of the quality and level of design and construction; it is not an issue of conformity assessment regarding whether or not standards and specifications are met. It’s also a concept that cannot be evaluated in terms of safety or security. I remember that over twenty years ago, Director Chen of the Special Inspection Institute established the basic concepts for formulating the regular inspection regulations for industrial pipelines in service. These concepts were based on the axial stress level in the circumferential welds; if the flexibility-related stresses of the pipelines were not taken into account, this value represented half of the tangential stress. Accordingly, the assessment of defects that exceeded acceptable limits in such circumferential welds was carried out using the principles of fracture mechanics for classification purposes. This may also be the basic concept underlying the current inspection regulations for the quality assessment of circumferential weld inspections. Therefore, when evaluating the design and construction quality of high-temperature and high-pressure pipelines, it is inappropriate to mechanically apply concepts of safety or insecurity; rather, using concepts of reliability or its level is more suitable. It may be more appropriate to adopt the concept of usability assessment from API579-1. At pipeline construction sites, welding of the circumferential joints can only be carried out on the outer side; moreover, issues such as uneven pipe ends and wall thickness variations lead to misalignment, making it extremely difficult to meet the requirements for double-sided welding. To reduce the quality issues in circumferential welding that arise as a result of this, German DIN 2559 provides a good example. On the inner sides of the two ends to be butt-welded of the steel pipe, during the processing of the pipe end bevels, the inner diameters of the pipe ends are pre-machined to the value given by the following formula and shown in Figure 4: dp = dO min – 2 tmin. Where: dp is the machined inner diameter; dO min is the minimum outer diameter of the steel pipe, i.e., the nominal outer diameter of the steel pipe minus the maximum allowable negative deviation specified in the standards ； When the negative deviations of the outer diameter of the steel pipes on both sides are different, the larger value is taken; for welded pipe ends of cast or forged pipe components such as valves and flanges that are machined, the negative deviation of the outer diameter is set to 0; tmin represents the minimum wall thickness of the steel pipe, which is the nominal thickness of the pipe minus the maximum allowable negative deviation specified by the standards ； When the negative thickness deviations on both sides of the steel pipe are different, the larger value is taken; for butt-welded pipe ends of cast or forged pipe components such as valves and flanges that are machined, the negative thickness deviation is taken as 0. Q: 1. Several years ago, ASME B31.3 eliminated the requirement for heat treatment in a sufficiently preheated state for welding of carbon steel, regardless of thickness. Will we consider this in the future? ； 2. The W temperature coefficient used in calculating wall thicknesses is specified as \"to be determined by the designer based on experience,\" which facilitates the calculation of high-temperature wall thicknesses for applications such as styrene and POSM at present; however, domestic standards have not yet adopted this approach. However, all explanations consider only the attenuation at high temperatures. This inconsistent approach—where some standards are strict while others are not—is quite confusing. I believe that domestic standards can be appropriately innovated in this regard by specifying different temperature-dependent attenuation coefficients for various high-temperature materials. 3. ASME, and even the current SH standard’s new inspection specifications, have introduced non-destructive alternatives; PAUT and even TFM (total focusing method) should also be incorporated into GB/T 20801 in the future, I believe. Image 1. In recent years, relevant ASME specifications have undergone multiple revisions concerning preheating prior to welding, inter-pass temperature control, post-weld heat treatment, as well as hardness verification. The main objective of these revisions is to address historical and habitual discrepancies and inconsistencies among various standards. They also reflect the importance of enhanced management practices and higher preheating temperatures in reducing the tendency toward hardening and cold cracking. This intention was taken into account during the revision of GB/T 20801, but given the level of technological management at construction sites in China, it is not appropriate to relax the exemption conditions for PWHT as a result. Additionally, regarding the post-weld hardness testing, although there are numerous practical issues such as inaccuracies and difficulty of operation, there are no other feasible methods to monitor the quality of local heat treatment; therefore, it is still retained. 2. Regarding the decrease in the long-term strength of welds in alloy steels, stainless steels, etc., within their creep temperature ranges: first, this is a practically existing problem; second, ASME has derived the data in the specification tables based on the currently available limited amount of actual measurement data. For details, please refer to the relevant appendices in Section NH of Part III and Section D of Volume II of the ASME Boiler and Pressure Vessel Code. Precisely because data is limited and its underlying patterns and mechanisms are not yet understood, it is acceptable for users or welding departments to adopt other measures based on the data they actually have at hand in order to control risks. The use of cobalt-nickel-based welding materials for INCOLOY 800HT in the styrene plant, as mentioned in the question, addresses the issue of the W factor. To date, there has been no attention in China devoted to testing, accumulating, and analyzing long-term or even short-term strength data for metal materials, let alone strength data for welded joints; hence, there is no possibility for innovation. Therefore, the wording in the ASME codes is practical and realistic; similarly, a similar provision has also been included in clause 4.2.7.4 of GB/T 20801.3-2020. 3. Regarding the criteria for incorporating new NDT methods, I believe several factors should be considered regularly: ① The interests of various parties vary, and due to different positions, their levels of enthusiasm and the necessity for adopting such methods also differ. ② Secondly, stability, operability, human factors, storability, reproducibility, reliability, etc., are important considerations. Therefore, including it as a basic standard as an alternative approach is not yet feasible; instead, it should be addressed through coordination among all parties as a supplementary requirement, without the need to follow trends in basic standards. Question: Bolts and nuts are generally used in combination with ASTM A193 B7 and ASTM A194 2H. However, manufacturers are now using ASTM A193 B7 bolts together with ASTM A194 7 nuts. Is it acceptable to use them this way? The hardness of the two seems to be similar. Is it possible to find out the load capacity guaranteed by the nut? Image: Since the principle for selecting nuts is to ensure that the load they can handle is greater than that of the bolts, 2H is still higher than B7. And since 7 is even higher than 2H, it is acceptable. The guaranteed load is specified in A194; grades 2H and 7 fall into category 1. The guaranteed load represents an important indicator of the strength of screws. 7 is chrome-molybdenum steel, which has a wider operating temperature range at both high and low temperatures than 2H. Even though the A194 standard classifies the guaranteed loads for 2H and 7 as the same, the strength margin is greater. It is even more necessary to prevent relaxation at high temperatures and maintain low impact strength at low temperatures; ASTM A193 B7, L7 in combination with A194 7. Question: For equipment with an external pressure of 5 Bar and diameters of 3600 mm and 2800 mm, where the flanges use double O-ring sealing, how should the calculations be carried out? Image 1. The traditional flange design principle is based on narrow face/float, forced sealing, and internal pressure; whereas the flange in this case is completely opposite to traditional flange designs in terms of these three characteristics. Therefore, the calculation formulas for externally pressurized self-tightening full-flat flanges should be used as shown in Table 16 of Chapter 3.8 of BS PD5500-2015 for design verification. 2. Although Table 16 in Chapter 3.8 of PD5500-2015 is intended for internal pressure, it can also be applied to external pressure. The formula for calculating the bending moment of the flange under external pressure remains unchanged; however, as the force direction changes from internal to external pressure for HD and HT, the reaction force generated by the contact between the two metal surfaces of the flange under the static head of external pressure changes from HR, which is located on the outer periphery of the flange in Table 16, to HB, which is located on the inner periphery of the flange. At the same time, Wm1 also becomes the difference between H and HB. Of course, HB is the quotient of M and (C-B)/2, while for WDN3600, the sum of the inner and outer diameters as given in the original calculation sheet can also be used, that is, 395 plus 388.1 mm2. It can be seen that therefore, it is safe and feasible to conduct design verification on the standard WN flanges of PN6 DN3600 using the revised Table 16. 3. The current calculation is more conservative when based on internal pressure, but it still passes the verification; moreover, since the actual operating condition involves external pressure, it is safer from a strength calculation perspective. Please refer to the final calculation sheet in conjunction with the above comments. Question: According to GB/T20801.2-2020, clause C.5.5.1, carbon steel and chromium-molybdenum alloy steels with a chromium content of 3% or less may suffer from high-temperature, high-pressure hydrogen erosion when the operating temperature is above 177°C and the hydrogen partial pressure is at least 0.345 MPa. Why are the limits on hydrogen partial pressure and temperature set so strictly? What is the basis for these limits? Image 1. The hydrogen partial pressures and temperatures listed above represent the safety limits in practical engineering applications for carbon steel equipment to prevent hydrogen erosion. 2. In recent years, especially after the catastrophic accidents that occurred in 2010 and 2012 in the United States involving carbon steel heat exchangers and pipes in refineries, under carbon steel temperature and pressure conditions below the Nelson curve, and which took place in the weld areas that had not undergone post-weld heat treatment, doubts were raised regarding the reliability of the Nelson curve. As a result, there has been a surge in research within the international engineering community into the mechanisms of high-temperature hydrogen erosion, as well as into non-destructive testing methods for detecting hydrogen-induced damage in operating equipment. 3. Research and numerous NDT inspections of existing equipment have shown that for carbon steel heat exchangers and pipes that have not undergone stress relief after welding, and whose actual operating temperature is higher than the intended design temperature, a margin of over 50 degrees should be reserved on the Nelson curve. Q: This time, there was no specific mention regarding the inapplicability of Grade GC1 pipes for ductile iron pipes. The industry has expressed concerns: 1. Among industrial pipes made of ductile iron, those primarily used for transporting gaseous media—do any of them fall under Grade GC1 pipes? Or conditions greater than 4.0 MPa? 2. GB/T20801.2-2020 version 020 specifies in the text that the elongation rate should be greater than 15%; ball-milled cast iron with a high elongation rate is thus selected for use in industrial pipelines. But does this alone suffice to eliminate the safety risks associated with the use of GC1 grade material? 3. According to GB/T20801-2006, ductile iron with an elongation of 15% or more as specified in the table shall not be used for GC1 grade applications. The 2020 version, on the surface, removes materials such as QT400—15. 6.1.1 Ductile iron? Image 1. GB/T20801-2006 refers generally to all ductile irons specified in GB/T 1348, including those with an elongation of less than 15% as well as those with an elongation greater than 15%; therefore, they are classified as brittle materials, and various restrictions are imposed on the pressure, temperature, and hazard level of the media in which they can be used. 2. Clause 6.1.1 of GBT20801.2-2020 specifies the requirements regarding the elongation and impact properties of ductile iron used in industrial pressure pipelines; these requirements limit the use of ductile iron to those with elongation and impact properties at least equivalent to those of QT400-18. Even QT400-15 is excluded as the standard does not require specific impact properties for this grade of ductile iron. Therefore, the quality requirements for ductile iron outlined in GB/T 20801.2-2020 are equivalent to those for ductile iron used in high-temperature pressure components specified in ASME B31.3 under grade A395. To this end, all the requirements of 6.1.1 in GB/T 20801.2-2020 have also been established in accordance with the requirements of ASME B31.3 323.3.2a and M323.3.2. At the same time, referring to the pressure limits for ductile iron valves in European pressure equipment regulations, a PN50 pressure limit was added. 3. Since GB/T13295, which is applicable to gas pipelines, includes ductile iron with an elongation of less than 15%, clause 6.1.1.1 of GB/T 20801.2-2020 specifies that the scope of application for ductile iron gas pipelines defined in GB/T13295 must comply with the pressure and temperature limits specified in this standard; such pipelines can only be used for low-pressure gas pipelines of PN16 or lower.