Thread Content
1 Grade designation: Given the special characteristics of GC1 pipelines, during design, pipelines carrying media of the GC1 grade with identical properties of the medium, design conditions, and material selection should be assigned a separate grade. Avoid using expensive pipeline materials for ordinary medium pipelines, thereby preventing unnecessary waste. 2 Selection of pipe material standards Table 1 in TSG D0001 specifies certain pipe material standards for media of GC1 grade; the requirements outlined in this table must be followed during design. Regarding the standards for stainless steel welded pipes listed in the table, it is recommended not to choose HG/T20537. Firstly, this standard is quite old; in an era where metallurgy, welding, and testing technologies are constantly evolving, some of the requirements specified in this standard are technically outdated ； Another reason is that, compared to the GB/T12771-2008 standard, this standard has a narrower scope of application, and fewer types of steel pipes can be produced using it. Therefore, GB/T12771 is often chosen in design. Table 1 specifies that GB/T8163, GB3087, and GB/T9711.1 cannot be used for pipelines carrying GC1-type media; however, note 7 adds that those that have passed individual ultrasonic testing and met the required standards may be used for pipelines specified in A1.1(1) of these regulations when the design pressure is ≤4.0 Mpa. This annotation differs from the requirements in GB/T2001.2, and this is related to the years when the two standards came into effect. The reason why the standards impose restrictions on the use of pipes meeting these three criteria is mainly that the requirements for the production, inspection, and testing of pipes under these standards are relatively low; using such pipes in GC1-class pipelines poses certain risks. During design, it is necessary to take into account both the cost-effectiveness and safety of the piping materials in order to make the best choice. The GB/T9711 series of standards has been upgraded to GB/T9711-2017. In the new version, the steel pipe grade PSL1 corresponds to the pipes specified in the original GB/T9711.1, while PSL2 corresponds to the pipes specified in the original GB/T9711.2. In the third edition of the \"Training Materials for National Pressure Pipeline Design Approval Personnel\", the usage restrictions on GB/T9711 are the same as those specified for GB/T9711.1 in Appendix 1. When using this standard, attention should be paid to the areas where its use is not permitted. GB/T9948-2013, GB/T6479-2013, and GB/T5310-2017 are optional for GC1 grade seamless pipelines. From the names of these three criteria, it can be seen the focus of their areas of application. Additionally, attention should be paid to some specific requirements in the standards; for example, only GB6479 specifies requirements for low-temperature impact tests on high-quality carbon steel ； GB9948-2013 specifies requirements for intergranular corrosion testing, as well as additional technical requirements for steel pipes used in H2S-containing environments. GB5310 is a standard specifically established for steel pipes used in boilers, and therefore is commonly used in steam and its condensate pipelines. For GC1-class medium pipelines with a design pressure of 10 MPa or higher, SH/T3059 stipulates that when seamless steel pipes are used under such conditions, their manufacture and inspection must comply with the provisions of GB5310, GB9948, or GB6479. The inspection of stainless steel pipes must meet or exceed the requirements specified in GB/T14976. This requirement differs from that in GB50316. The reason for the difference between these two standards is that, at the time GB50316-2000 (2008 version) was issued, the quality grades of pipes among these three standards, from lowest to highest, were: GB9948 < GB5310 < GB6479. However, at present, these three standards have basically the same requirements in terms of processing and manufacturing as well as inspection and testing. In terms of the application of each of these three standards, the requirements of SH/T3059 are more reasonable. For example, for steam media with a pressure of over 10 Mpa, it is more appropriate to use GB5310 rather than GB6479. Regarding the grade to be adopted based on foreign pipe material standards, TSG D0001 stipulates that when international standards or foreign standards are used directly, they must first be converted into enterprise standards or project specifications. For GC1-grade pipelines, they must also be registered with the General Administration of Quality Supervision, Inspection and Quarantine. If necessary, the **General Administration of Quality Supervision, Inspection and Quarantine entrusts relevant technical organizations or institutions to conduct the evaluation. Among the foreign standards currently used in China, American standards are the most widely adopted, and the technologies related to their use are also well-developed. When using such pipes, it is necessary to comply with the relevant requirements for pipes as specified in ASME B31.3. 3 Fittings (1) Standard fittings. The selection of fitting standards can be specified by the project or chosen independently. For seamless fittings, since they can be manufactured from steel pipes that comply with different standards, once the fitting standard is selected, it is necessary to specify the standard number of the raw material pipe, so as to ensure that the fittings meet the same performance requirements as the pipes. For forged fittings, some designs also specify the grade of the forging. This is very necessary. (2) Non-standard pipe fittings. The most common non-standard pipe fittings include elbows, offset elbows, reducers, and offset tees. The scope of application for elbow fittings is not specified in relevant standards. Given the difficulty in ensuring the quality of on-site fabrication, it is advisable to purchase finished elbow fittings manufactured by the factory for GC1 grade pipelines. Specify the processing and manufacturing requirements in the purchase request to ensure the safe use of the bent pipes. Unless constrained by piping layout and engineering requirements, it is not recommended to design special fittings for material grades. 4 Valves: At present, only GB/T20801.3 provides relatively detailed requirements for valves used in GC1-grade pipelines. The selection criteria for such valves will be analyzed one by one below: Specialized petrochemical valves should be used ； (1) Valves under such standards have undergone years of practical use and testing, and the requirements regarding quality and acceptance are also high. Most of the valve standards listed in GB/T20801.3 have been updated; therefore, when designing, it is advisable to select those standards specified in the tables of SH/T3059 that better meet the requirements of these regulations. (2) Leakage of the fluid medium at the valve stem packing should be prevented ； This requirement is primarily based on the fact that GC1-type media have higher toxicity and flammability compared to other types of fluid media. External leakage from valves can cause environmental pollution and safety hazards. Diaphragm-sealed valves offer the best performance in preventing leaks at the valve stem, but their widespread use in the petrochemical industry is limited by the high difficulty of manufacturing them, the stringent technical requirements, the fact that diaphragm materials cannot yet be fully produced domestically, and the resulting high costs. For extremely hazardous and highly hazardous media, from the perspective of personal safety, it is advisable to consider using such valves. In the standards for valves, the sealing test is a requirement aimed at preventing leakage from the packing at the valve stem ； Furthermore, in ISO15848, the leakage at the valve stem is calculated based on the diameter of the stem, and leakage is classified into different levels. In actual design, it is possible to select an appropriate valve structure and suitable leakage level by taking into account the properties of the medium, thereby preventing external leakage at the valve stem. (3) The sealing structure of the valve cap or bonnet shall adopt one of the following forms: 1) Flange connection, with a minimum of 4 bolts, and gaskets that comply with the requirements specified in 5.1.9 of GB/T20801.3 ； GB50316 and ASME B31.3 also require that valves with valve covers fixed to the valve body using fewer than 4 bolts or U-bolts can be used only in applications involving Class D fluids. The valve standards listed in the table all specify that flanged valve covers must have no fewer than 4 bolts. This requirement, along with those in the following paragraphs, is intended to prevent leakage of the medium at the valve cover. The type and material of gaskets are also part of the valve specification sheet. API 600 lists the available seal surface types for the connection between the valve body and the valve cover. Once the seal surface type has been determined, the appropriate type of gasket can be selected based on the actual conditions of the medium, in accordance with this provision as well as the requirements outlined in HG/T20592–20635. 2) Self-tightening structure ； This structure can effectively prevent leakage of the medium at the valve cover. This structural form is mentioned in API 6D and NB/T47044, but no detailed specifications are provided in either standard. From the perspective of sealing principles, the sealing at the valve cover can be divided into forced sealing and self-sealing. The typical structure for forced sealing is gasket sealing, which is commonly used in valves with low to medium pressures as well as those with small to medium diameters ； Compared to forced sealing, in a self-sealing structure, as the valve cover moves upward under the action of the medium pressure, the sealing specific pressure between the valve cover and the wedge-shaped gasket, as well as between the valve body and the wedge-shaped gasket, gradually increases as the pressure rises. In self-sealing media, the pressure always tends to increase the pre-sealing specific pressure, thereby enhancing the sealing performance. The higher the medium pressure, the greater the specific pressure on the working seal, and the better its sealing performance. Based on this characteristic, self-sealing is used as a high-pressure sealing technology for high-temperature or high-pressure large-diameter valves. 3) Fully penetrative welded structure ； This structure is the most effective way to prevent leaks at the valve cover, and it is not limited by the valve size, operating pressure, or temperature. However, for valves that can be repaired online, the welding areas need to be removed during repair; therefore, they are commonly used in valves that can operate for a long time without requiring repair ； Or in situations where it is more important for this type of valve to achieve reliable sealing at the valve cover connection than to overcome the difficulties associated with maintaining the interior of the valve. 4) Cylindrical threaded connection: strength check passed; metal seal welding is used ； Given the usage limitations of cylindrical threads and the need for metal seal welding, with the same disadvantages listed in c) above, valve covers of this type are rarely chosen in engineering designs. 5) A valve cover sealing structure with threaded connections shall not be used ； This type of structure is not only used for GC1-class medium pipelines; valves for other types of process medium pipelines are also rarely chosen for this purpose, and small valves that experience little vibration during operation and do not require frequent disassembly are generally used instead. 6) Valves for flammable fluids that use non-metallic sealing materials shall meet the fire resistance test requirements, and their pressure-temperature ratings shall be determined based on the pressure-temperature ratings that the non-metallic materials can withstand. This requirement is primarily aimed at ensuring that the valves remain capable of being shut off in case of a fire hazard, thereby preventing the occurrence of secondary disasters. Due to their inherent properties, non-metallic materials have limited resistance to temperature and pressure; therefore, when selecting such materials, it is necessary to strictly follow the requirements specified in the standards and verify the temperature and pressure thresholds. Care should be taken when using materials not listed in the standards, as different manufacturers may provide different temperature and pressure values. When it is not the only option, it is not recommended to use non-metals that exceed the standard as seal surface materials. 5 Flanges: The key considerations when selecting flanges are their type and the sealing surface. According to GB/T20801.3, expansion flanges and threaded flanges shall not be used for GC1 grade pipelines ； In cases of severe cyclic loading or for GC1-class pipelines, where the nominal diameter of the socket weld joint is greater than DN50, socket weld connections shall not be used. SH/T3059 stipulates that plate flat welding flanges shall not be used for pipelines carrying toxic and flammable media. GB50316 specifies for Class A1 fluids: (1) Flat welding (plate-type) flanges shall not be used ； (2) In addition to using weld lip gaskets, when selecting the nominal pressure of the flange, a margin of ≥25% should be reserved, and it should not be lower than 2.0 Mpa in nominal pressure ； (3) When using soft gaskets, flanges with male and female surfaces or tenon and mortise surfaces should be selected. All of the above require strict adherence to the specifications. The selection of gaskets, bolts, nuts, and gaskets should be based on a comprehensive consideration of factors such as the properties of the fluid, operating temperature, pressure, and the flange sealing surfaces. HG/T20592–20635 provides detailed specifications on the operating temperatures for various types of gaskets as well as their compatibility with different types of flanges. When designing, it is possible to select the appropriate material and type of gasket by referring to these standards along with the requirements specified in GB/T20801.3 and SH/T3059. In addition, special attention should be paid to the usage limitations of certain gaskets; for example, gaskets made of flammable materials such as rubber must not be used in pipelines transporting highly oxidizing media. It is worth noting that among the fasteners specified in HG/T20592–20635, the European standard series includes hex head bolts, equal-length double-headed studs, and fully threaded studs ； The American standard series includes only hex head bolts and fully threaded studs; different selection standards result in different types of fasteners. HG/T20634 specifies that the use of hex head bolts and Type I hex nuts must comply with the following requirements: nominal pressure class ≤ Class150 ； Non-toxic, non-flammable media and applications without severe cycling ； Equipped with non-metallic flat gaskets. Except in this case, special-grade fully threaded studs and special nuts should be used. Based on the properties of the media used in GC1-class pipelines, it is clear that for such pipelines, only specialized fully threaded studs and corresponding nuts can be used as fasteners. The types of fasteners specified in SH/T3404 are the same as those in HG/T20634, and the requirements for the use of fasteners in this standard are identical to those in HG/T20634. 7 Branch connections: The branch connection fittings are an important part of pipeline classification. According to GB/T20801.3, for GC1-grade pipelines, branch connection fittings should be of the type with integral reinforcement or tees ； For Fluids Class A1 as defined in GB50316, and Fluid Class A2, Grade II, standard tees should be given priority for branch connections, followed by branch pedestals or embedded branches. In accordance with these specification requirements, for branch connections of GC1 grade pipelines, standard branch fittings should be used when standard connection fittings are available. If branch pipe fittings cannot be used, the provisions regarding reinforcement calculations in GB/T20801.3 and SH/T3059 shall be followed: reinforcement plates should not be used to reinforce pressure pipelines of grade GC1. This requirement is mainly due to the fact that in the reinforcement plate structure, the distribution of the reinforcing metal is too scattered, resulting in low reinforcement efficiency ； The reinforcement ring cannot fit perfectly against the shell metal, resulting in poor heat transfer. When used at medium temperatures or above, there is a significant difference in thermal expansion between the two, which causes large thermal stresses in the reinforced area ； In addition, the reinforcement ring is connected to the housing by lap welding, making it difficult to form a single unit with the housing, resulting in poor fatigue resistance ； Compared to other reinforcement methods, reinforced welded structures also have higher welding stresses. Given these defects in reinforcement by gussets, an overall reinforcement approach can be adopted for branch design.