Low temperature stainless steel pipeline for LNG
According to the ultra-low temperature operation conditions of LNG industry, the classification, manufacturing process, executive standard and design selection of stainless steel pipeline are discussed.
In recent years, with the development of the world economy and the improvement of China’s national economy, the demand for clean energy such as natural gas is increasing day by day, and the LNG industry, as a form of new energy, is also booming. For the LNG project, because the pipeline transports – 162 ℃ ultra-low temperature medium, there are higher requirements for the equipment, pipeline, valve and other materials used in the LNG project. Ensuring the quality, safety and reliability of the materials plays an important role in the safe operation of the LNG project. LNG project involves ultra-low temperature working condition, so stainless steel products which are more resistant to low temperature than carbon steel products are widely used in LNG projects. In order to ensure the safe operation of the project, the characteristics, manufacturing process and executive standards of stainless steel pipeline are discussed as follows.
Basic characteristics of stainless steel
Stainless steel can be divided into martensitic stainless steel, ferritic stainless steel, austenitic stainless steel and austenitic ferrite duplex stainless steel. The common type of stainless steel used in LNG project is austenitic stainless steel. Compared with other kinds of stainless steel, austenitic stainless steel is characterized by stable metallography, good plasticity, toughness, weldability and corrosion resistance, especially its low temperature resistance, which is suitable for ultra-low temperature of LNG project Working conditions. The following is a comparison of various types of stainless steel.
Martensitic stainless steel
Martensitic stainless steel has high strength and hardness due to its high carbon content, but poor plasticity and weldability. It is commonly used in mechanical parts with high mechanical properties, such as valve stem in valve and blade of steam turbine.
Ferritic stainless steel
The corrosion resistance, toughness and weldability of ferritic stainless steel are improved due to the increase of tensile strength. High temperature oxidation resistance and thermal expansion performance are also high, but the mechanical properties and process properties are poor, so it is not suitable for ultra-low temperature conditions. It is commonly used in acid resistant equipment and parts under high temperature conditions.
Austenitic ferrite duplex stainless steel
Austenite ferrite duplex stainless steel has both the advantages of these two types of steel, with superplasticity. Compared with ferrite, it has higher plasticity and toughness, better intergranular corrosion resistance and welding performance; compared with austenite, it has higher strength and better chloride corrosion resistance, but its working temperature range is narrow, which is generally used between – 50 and 250 tons, so it is not suitable for LNG project.
Austenitic stainless steel
Austenitic stainless steel is the most important type of stainless steel. Its production and use account for more than half of the total batch of stainless steel. It is characterized by good comprehensive performance and corrosion resistance in various media. Chromium and nickel are the two most important elements in austenitic stainless steel. Chromium promotes the passivation of stainless steel and keeps the stainless steel in a stable and passive state. The increase of chromium content in austenitic stainless steel can also reduce the transformation temperature of martensite, so as to improve the stability of austenite matrix Body of the body. For stress corrosion, it is also beneficial to improve the corrosion resistance of steel such as oxidation resistance and sulfide resistance.
Nickel is an element that strongly stabilizes austenite and expands its phase zone. When the steel contains 18% chromium, the minimum nickel content required is 8%, which is the basic composition of 18-8 chromium nickel austenitic stainless steel 3. The effect of nickel on mechanical properties of austenitic stainless steel is mainly to stabilize its microstructure and toughness, including ultra-low temperature toughness. Therefore, austenitic stainless steel can adapt to ultra-low temperature conditions.
There are two kinds of stainless steel pipes used in LNG projects: seamless steel pipes and welded steel pipes. Due to the price of stainless steel pipe base materials and the diameter of stainless steel pipes used in LNG projects is relatively large, when large quantities of stainless steel pipes are purchased, except for small diameter pipes (generally ≤ 6), most of the other pipelines are welded steel pipes. The seamless and welding two kinds of pipe making process are introduced below.
Manufacturing process of seamless steel pipe
At present, there are two kinds of seamless steel pipe manufacturing process: one is the traditional piercing technology, the other is the hot extrusion technology.
Cold drawing process
It is a traditional process to produce seamless stainless steel pipe by piercing. It is mainly made of steel ingot and then hot rolled, cold rolled or cold drawn. The flow chart of cold drawing process is shown in Figure 1.
Fig.1 flow chart of cold drawing process for seamless stainless steel pipe
round tube → heating → perforation → Heading → annealing → pickling → oiled (copper) → multi-pass cold drawn (cold rolled) → blank tube → heat treatment → straightening →hydrostatic testing (testing) → mark → storage.
The general cold strip mills, volume should go through continuous annealing (CAPL unit) to eliminate cold hardening and rolling stress, or batch annealing reach the mechanical properties of the corresponding standard specifies. Cold rolled steel surface quality, appearance, dimensional accuracy better than hot-rolled plate, and right-rolled thin product thickness is about 0.18mm, so the majority of users favor.
Hot extrusion production process
Hot extrusion production process is a new pipe making process developed in recent years. It mainly adopts three-dimensional compression stress one piercing technology and deformation. The metal density of such pipe is good and the organization is uniform. Compared with the traditional cold drawing production technology, the quality of steel pipe is higher. The flow chart of hot extrusion production process is shown in Fig. 2.
Fig.2 flow chart of hot extrusion process for seamless stainless steel pipe
Most of the stainless steel pipes used in LNG are welded pipes, which are mainly due to the simple process, high production efficiency, many varieties and specifications, and relatively cheap price. First, with the progress of welding and inspection technology and the improvement of automation, the quality of welded pipe has also been greatly improved, which can replace seamless steel pipe in more and more fields. In particular, the JCO welding pipe production process developed in recent years reflects the advanced level of edge treatment and forming of straight seam welding in the world. See Fig. 3 for the sketch of JCO production process.
Fig.3 JCO process of stainless steel welded pipe
Material retesting → vacuum hanging → board probe → flap → planing curl before demi plate (J molding) on the board edge → loose → enter demi steel plate (C molding) demi steel plate → curl → loose out → bend → pre-welded → welding cited the (quenching) arc board → welding → Qing root weld → mechanical repair end to cited hydrostatic test (put out) arc the board → ultrasonic inspection → mechanical expanding → tube ends of ultrasonic testing ring the stratification test to the UT → X-ray inspection → Magnetic particle inspection of the finished product inspection → weighing and measuring the length → outer anticorrosion → internal corrosion → mark → delivery.
Comparison of manufacturing standards
Stainless steel pipeline has various processes summarized above, but the manufacturing standards are different, and their quality is also different. For LNG project, the pipeline conveys ultra-low temperature flammable and explosive media, so the technical quality requirements of stainless steel pipeline are higher. The commonly used standard is “electric fusion welding austenitic chromium nickel alloy steel pipe for high temperature operation” SME A358/A358M and “austenitic stainless steel seamless and welded steel pipe” ASME A312/312M. Although the latter standard has provisions for austenitic stainless steel seamless and welded steel pipes, due to the following reasons, in general, seamless steel pipes are selected to be manufactured according to ASME A312/312M standard, and welded steel pipes are selected to be manufactured with ASME A358/358m, and only one longitudinal weld is allowed. The specific reasons are as follows:
- (1) ASME A358/358M has different classification and description for various welding processes, which is not mentioned in ASME A312/312M;
- (2) The outer diameter of welded pipe in ASME A358 / 358M is stricter than that in ASME A312/312M;
- (3) Both radiographic inspection and magnetic particle inspection are required in ASME A358/358M, but not in ASME A312/312M.
Low temperature impact test
Therefore, under the condition of low temperature impact test, the so-called low temperature impact test of stainless steel pipe is required to be carried out under the condition of low temperature impact test, so as to determine whether the impact test is carried out on the pipeline under the condition of low temperature impact test Low temperature resistance. However, according to part 2 of “pressure piping code for industrial piping” (GB/T 20801-2006), the exemption principle of low-temperature impact test for austenitic stainless steel is described in the material. As long as the requirements are met, the low-temperature impact test can not be carried out.
In general, seamless stainless steel pipes usually meet the specified requirements, and low-temperature impact test is not required. Only welded stainless steel pipes are subject to low-temperature impact test. The test requirements can be in accordance with GB/T20801-2006 or ASME B31.3.
In addition to the above requirements, all stainless steel materials must be delivered in solution annealed condition. The maximum carbon content of materials is 003%, and the maximum sulfur content of welded pipes is 0.015%. In a word, the requirements for stainless steel pipes used in g-fired ultra-low temperature conditions are more stringent than those under other working conditions, mainly to ensure the safe operation of LNG project.
Source: China Stainless Steel Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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