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Selection of GC1 steel pipe materials

Attention:     Issuing time:2020-05-21 15:16
Selection of GC1 steel pipe materials
 
After reading this article, you will fully understand the selection of GC1 steel pipe materials!


 
1 Grade(materials) setting
In view of the particularity of GC1 pipelines, the same GC1 grade media pipelines with the same media properties, design conditions, and material selection should be set separately in the design. Avoid using expensive pipeline materials on ordinary media pipelines to cause unnecessary waste
 
 
2 Selection of pipe standards
In Table 1 of TSG D0001, the requirements for some pipe standards for GC1 grade media are specified, and the requirements in this table should be implemented during design.
 
It is recommended not to choose HG / T20537 for the listed stainless steel welded steel pipe standards. One is because the standard is very old. Today, metallurgy, welding, and inspection technology are changing rapidly. Some of the requirements in the standard have fallen behind from a technical perspective; another reason Yes, compared with the GB / T12771-2008 standard, this standard has a small range of application and can produce fewer steel pipe materials. So choose GB / T12771 when designing.
 
Table 1 stipulates that GB / T8163, GB3087, GB / T9711.1 cannot be used for GC1 type media pipelines, but there is a supplementary note in its Note 7: After ultrasonic testing one by one and meeting the qualification requirements, it is allowed to be used for design pressure The pipeline specified in A1.1 (1) of this regulation ≤ 4.0Mpa. This note is different from the requirements in GB / T20 01.2, which is related to the effective year of the two standards. The reason why the specification restricts the use of these three standards is mainly because the requirements of the three standards in terms of manufacturing, inspection, and testing are relatively low. For GC1 pipelines, there will be certain risks. When designing, it is necessary to comprehensively consider the economics and safety of the pipes and make the best choice.
 
At present, the GB / T9711 series standard has been upgraded to GB / T9711-2017. In the new version, the steel pipe grade PSL1 corresponds to the pipe in the original GB / T9711.1, and PSL2 corresponds to the pipe in the original GB / T9711.2. In the third edition of the "Training Textbook for National Pressure Pipeline Design Approvers", the restrictions on the use of GB / T9711 are the same as those specified in Schedule 1 for GB / T9711.1. When choosing this standard, it is necessary to pay attention to the range that it is not allowed to use.
 
GB / T9948-2013, GB / T6479-2013 and GB / T5310-2017 are optional for GC1 seamless pipeline. From the names of these three standards, we can see the emphasis of their scope of use. In addition, some specific requirements in the standard should also be noted, for example: only GB6479 has provisions for low-temperature impact test of high-quality carbon steel; while in GB9948-2013 there are requirements for intergranular corrosion test and the supplement of steel pipes for H2S-containing environments skills requirement. GB5310 is a standard set specifically for boiler steel pipes, so it is mostly used in steam and condensate lines.
 
For the GC1 media pipeline with a design pressure greater than or equal to 10Mpa, it is specified in SH / T3059: when the seamless steel pipe is used for design pressure ≥10Mpa, the manufacturing and inspection should meet the requirements of GB5310, GB9948 or GB6479, and the inspection of stainless steel pipes should not be low In accordance with the provisions of GB / T14976 [4]. This requirement is different from the requirements in GB50316. The reason why the two specifications are different is mainly because when the GB50316-2000 (2008 version) was issued, the order of the quality of the pipes in the three standards from low to high was: GB9948 <GB5310 <GB6479. However, the requirements of these three standards are basically the same no matter in terms of manufacturing, inspection or testing. In terms of the use of these three standards, the requirements of SH / T3059 are more reasonable. For example, for steam media over 10Mpa, GB5310 is more suitable than GB6479.
 
For the selection of foreign pipe standards, TSG D0001 [1] stipulates: When directly adopting international standards or foreign standards, they should be converted into enterprise standards or engineering regulations. The GC1 pipeline should also be reported to the AQSIQ for record. When necessary, the AQSIQ commissions relevant technical organizations or technical institutions to conduct the review. At present, among the foreign standards used in China, the American standard is the most used, and the technology for applying American standard pipes is also very mature. When using it, it should be implemented according to the corresponding requirements for pipes in ASMEB31.3.
 
 
 
3 Pipe fittings
(1) Standard pipe fittings. The selection of pipe fitting standards can be specified by the project or selected by oneself. For seamless pipe fittings, because they can be manufactured from steel pipes of different standards, after selecting the pipe fitting standard, it is necessary to indicate the pipe standard number of the raw material, so that the pipe fitting can meet the same requirements as the pipe. For forged pipe fittings, some designs also limit the grade of forgings. This is necessary.
 
(2) Non-standard pipe fittings. The most common non-standard pipe fittings include elbows, miter elbows, reducers and miter tees. The scope of use of elbows is not limited in the relevant standards. In view of the difficulty in ensuring the quality of on-site processing and manufacturing, it is best to buy elbows for GC1 grade pipes. At the time of purchase, please indicate the processing and manufacturing requirements in the requisition document to
Ensure the safe use of elbows. Unless restricted by pipeline layout and engineering requirements, it is not recommended to design special pipe fittings in the material grade.
 
 
 
4 Valve
At present, only GB / T20801.3 has more detailed requirements for valves for GC1 pipelines. The requirements for its selection will be analyzed one by one: the special petrochemical valves should be selected;
 
(1) Valves under such standards have undergone many years of practice and inspection, and have high requirements for quality and acceptance. Most of the valve standards listed in GB / T20801.3 have been upgraded. At the time of design, the selection according to the listed standards in SH / T3059 is more in line with the requirements of this article.
 
(2) The leakage of the fluid medium at the stem packing should be prevented; this requirement mainly considers that the GC1 type medium has higher toxicity, flammability and higher than other types of fluid medium. The leakage of the valve will cause environmental pollution and safety risks. The best anti-leakage effect is the bellows sealed valve, but due to its difficulty in processing, high technical requirements, and the bellows material has not been fully localized, the cost is too high, which limits its large number of Promotion. For extremely hazardous and highly hazardous media, this type of valve can be considered from the perspective of personal safety.
 
In the standard of the valve, the upper seal test is a requirement for the leakage of the packing at the valve stem; in addition, the leakage at the valve stem in ISO15848 is calculated according to the diameter of the valve stem, and the leakage is graded. Combined with the characteristics of the medium, select the appropriate valve structure and appropriate leakage level to fundamentally prevent external leakage at the valve stem.
 
(3) The sealing structure of the bonnet or bonnet should adopt one of the following forms:
①Flange connection, the number of bolts is ≥4, and the gasket meets the requirements of 5.1.9 in GB / T20801.3; it is also required in GB50316 and ASME B31.3: the valve cover is fixed with less than 4 bolts or U-shaped bolts Valves with bolts on the valve body connected to the bonnet can only be used in Class D fluid conditions. The valve standards listed in the table all stipulate that there should be no less than 4 bolts connecting the flange to the bonnet. This and the following requirements are to prevent the leakage of medium at the valve cover. The type and material of the gasket are also a part of the valve specification. API 600 lists the sealing surface types that can be used at the connection between the valve body and the bonnet. / T20592 ~ 20635 requirements, according to the actual situation of the medium, select the appropriate gasket type.
 
②Self-tightening structure; this structure can effectively prevent the media leakage at the valve cover. This type of structure is mentioned in API 6D and NB / T47044, but there are no more detailed regulations. From the principle of sealing, the seal at the bonnet is divided into a forced seal and a self-tightening seal. The typical structure of a forced seal is a gasket seal, which is usually used for low-pressure, medium-pressure, and small-medium diameter valves; Self-tightening sealing structure When the bonnet moves upward under the action of medium pressure, the sealing specific pressure between the bonnet and the wedge seal and between the valve body and the wedge seal gradually increases with the increase of pressure. The medium pressure in self-sealing always tends to increase the specific pressure of the pre-tightened seal and increase the sealing performance. The higher the medium pressure, the greater the specific pressure of the working seal, and the better the sealing performance. According to this feature, self-tightening seals are commonly used with high-temperature or high-pressure large-diameter valves as high-pressure sealing technology.
 
③ Fully-welded welded structure; this structure is the most effective way to prevent leakage at the bonnet and is not limited by the valve size, operating pressure and temperature. However, for valves that can be repaired online, the welding parts need to be removed during maintenance, so they are often used on valves that can be used for a long time without maintenance; Difficulties in maintenance are more important occasions.
 
④Cylinder thread connection, strength check is qualified and metal seal welding is used; in view of the limitation of the use of cylindrical thread and the need for metal seal welding, the disadvantages are the same as listed in c) above, the valve cover connection of this structure is rarely used in engineering design Optional.
 
⑤ The sealing structure of the bonnet with screw connection is not allowed; this structure is not only the GC1 type media line, but also the valves on other types of process media lines are rarely used. It is generally used on small valves that have little vibration during use and do not require frequent disassembly.
 
⑥ Valves that use non-metallic sealing materials for flammable fluids should meet the fire test requirements, and the pressure-temperature rating of the valve should be determined according to the pressure-temperature rating that the non-metallic material can withstand. This requirement is mainly to ensure that the valve still has the ability to shut off in the event of a fire hazard to avoid the occurrence of secondary disasters. Non-metallic materials have limited temperature-pressure capacity due to their own properties. When selecting, the temperature-pressure value must be checked in strict accordance with the requirements of the standard. If materials not listed in the standard need to be selected carefully, different manufacturers may give different temperature-pressure values. When it is not the only option, it is not recommended to use non-metals that exceed the standard as the sealing surface material.
 
 
 
5 Flange
The focus of flange selection is on the flange type and sealing surface. GB / T20801.3 stipulates: expansion flanges and threaded flanges shall not be used in GC1 grade pipelines; in severe cycling conditions or GC1 grade pipelines. When the nominal diameter of the socket welding joint is greater than DN50, socket welding connection shall not be used. It is stipulated in SH / T3059: toxic and flammable medium pipelines shall not use flat weld flanges. GB50316 stipulates for A1 fluid: (1) Flat welded (flat plate) flanges should not be used; (2) In addition to welded lip gaskets, the flange nominal pressure should be selected with a margin of ≥25%, and It should not be lower than the nominal pressure of 2.0Mpa; (3) When soft gaskets are used, flanges with concave or convex surfaces or tongue and groove surfaces should be used [3]. The above are required to strictly follow the specifications


 
6 Washers, bolts, nuts
The selection of gaskets should be considered based on factors such as fluid properties, operating temperature, pressure, and flange sealing surface. HG / T20592 ~ 20635 has a detailed description of the use temperature of various gaskets and the supporting use of various types of flanges. The design can be combined with the standards and the requirements in GB / T20801.3 and SH / T3059 Material and type of gasket. In addition, special attention should be paid to the restrictions on the use of some gaskets. For example, gaskets made of combustible materials (such as rubber) should not be used for pipelines that transport strong oxidizing media.
 
It is worth noting that HG / T20592 ~ 20635 fasteners, the European standard series has hexagon head bolts, equal-length double-head studs and full thread studs; the American standard series only has hexagon head bolts and full thread studs, the selection standard Different forms of corresponding fasteners are different. HG / T20634 stipulates that the use of hexagon head bolts and type I hexagon nuts should meet the following requirements: nominal pressure class ≤ Class150; non-toxic, non-combustible media and non-violent circulation occasions; equipped with non-metallic flat gaskets. In addition to this situation, special grade full thread studs and special nuts should be used.
 
According to the nature of the GC1 pipeline medium, we can know that the fasteners used for the GC1 pipeline grade can only use special grade full thread studs and special nuts. The types of fasteners in SH / T3404 and HG / T20634 are the same, and the requirements for the use of fasteners in this standard are the same as HG / T20634.

 
 
7 Branch connection
The branch table is an important part of the pipeline grade. It is stipulated in GB / T20801.3: GC1 grade pipe branch pipe connection fittings should use integrally reinforced branch pipe connection fittings or tees [6]; Class A1 fluid defined in GB50316 As with class II fluids in class A2, the branch pipe connection should prefer the standard tee, followed by the branch pipe station or the embedded branch pipe [3]. According to these requirements of the specification, for branch connections with GC1 pipe grades, those with standard connection pipe fittings shall use standard branch pipe fittings. If it is not possible to use branch pipes to connect the pipe fittings, according to the provisions of the reinforcement calculation section in GB / T20801.3 and SH / T3059: It is not appropriate to use reinforcement plates for reinforcement for GC1 pressure pipes. For this requirement, the distribution of the reinforcing metal of the reinforcing plate structure is considered to be too scattered, and the reinforcing efficiency is not high; the reinforcing ring and the metal of the shell cannot be completely bonded, and the heat transfer effect is poor. When used at medium temperature or above There is a large thermal expansion difference between the two, which results in a large thermal stress in the local reinforcement area; in addition, the reinforcement ring and the shell are connected by lap welding, which is difficult to form an integral part of the shell, and the fatigue resistance is poor; Compared with other reinforced forms, the welding stress is also greater [11]. In view of these defects reinforced by the reinforcement circle, the design of the branch can be carried out in a way of overall reinforcement.

 

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