Stainless Steel Pipe Fittings

Application of stainless steel pipe in petroleum and petrochemical industry

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1 Application of Stainless Steel in Petroleum and Petrochemicals
The most commonly used classification of stainless steel is classified according to the structure of the steel and can generally be divided into ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, duplex stainless steel, and precipitation hardened stainless steel. In petroleum and petrochemical applications, austenite stainless steel, ferritic stainless steel, and duplex stainless steel account for a large proportion.

Ferritic stainless steel generally has a Cr content of between 13% and 30% and a C content of less than 0.25%. In general, ferritic stainless steels have lower corrosion resistance than austenitic stainless steels and duplex steels, but are higher than martensitic stainless steels. However, due to its lower production cost compared to other stainless steels, it has a wide range of applications in the areas of chemical and petrochemical applications where corrosion resistance and strength requirements are not high.

Martensitic stainless steel generally has a Cr content of between 13% and 17%, and a high C content of between 0.1% and 0.7%. It has higher strength, hardness and wear resistance, but lower corrosion resistance. It is mainly used in petroleum and petrochemical fields in environments where corrosive medium is not strong, such as components requiring higher toughness and impact loads, such as turbine blades, bolts, and other related parts.

Austenitic stainless steels have Cr contents between 17% and 20%, Ni contents between 8% and 16%, C contents generally below 0.12%, and the austenite transformation area is mainly expanded by the addition of Ni elements, thus at room temperature. Under the austenite structure. Austenitic stainless steels are superior to other stainless steels in terms of corrosion resistance, plasticity, toughness, processability, weldability, and low temperature performance. Therefore, their application in various fields is the most extensive. Their use accounts for approximately all stainless steels. About 70% of the volume. In the petroleum and petrochemical field, austenitic stainless steels have a greater advantage for strong corrosive media and low temperature media.

Duplex stainless steel is developed on the basis of single-phase stainless steel. Its Ni content is generally about half that of austenitic stainless steel, which reduces the cost of the alloy. Austenitic stainless steel has excellent corrosion resistance and high overall performance. It solves the shortcomings of poor corrosion resistance of ferritic and martensitic stainless steels and insufficient strength and wear resistance of austenitic stainless steels. In the petroleum and petrochemical field, it is mainly used in offshore oil platforms that are resistant to seawater corrosion, acidic components and equipment, and particularly in components that are resistant to pitting corrosion.

Precipitated-strengthened stainless steels mainly obtain high-strength properties through precipitation strengthening mechanism. At the same time, they sacrifice their own corrosion resistance. Therefore, they are used less in corrosive media and are generally used in petrochemical machinery mining and other industries.

2 Application of Stainless Steel Pipe in Petroleum and Petrochemicals
In the past 20 years, both stainless steel pipes and welding pipes have been greatly improved in terms of production technology. Stainless steel pipes produced by some domestic manufacturers have reached the level that can completely replace imported products, and have achieved localization of steel pipes.

In petroleum and petrochemical industries, stainless steel pipes are mainly used in pipeline transportation systems, including high-pressure furnace pipes, piping, petroleum cracking pipes, fluid transportation pipes, and heat exchange pipes. Requires stainless steel to perform well under wet and sour service conditions.

2.1 The application of stainless steel seamless pipe with large diameter thick wall high pressure hydrogen
In order to meet the requirements for processing low-quality crude oil and meeting environmental protection requirements, domestic refining companies continue to optimize the processing structure of the refinery equipment and adjust the product structure. High-pressure hydrogenation units such as hydrocracking and hydrotreating have developed rapidly in recent years, and the processing capacity of the equipment has been improved. It is also constantly improving. Hydrogen pipelines are characterized by a large diameter and a thick wall.

For the selection of high-pressure hydrogen-imparting materials, TP321/H, TP347/H, etc. are generally used at home and abroad as the material for high-pressure hydrogen-infiltrating pipes because of their high-temperature and high-pressure working conditions. Both stainless steel materials are stable due to the addition of Ti, Nb, etc. Chemical elements have high temperature corrosion resistance and high temperature mechanical properties.

At present, domestic large-caliber thick-walled hydrogen-producing pipelines are mainly produced by hot perforation + cold rolling/cold drawing.

The hot-perforated + cold-rolled/cold-drawing tubes are superior to the steel tubes produced by other methods in that they have a good surface, high dimensional accuracy, and uniform wall structure.

For high-pressure hydrogen-free steel pipes, due to the special nature of the working medium, the requirements for the steel pipe raw materials are relatively high. Therefore, the design requirements for the high-pressure hydrogen-producing pipes are: S ≤ 0.015%, P ≤ 0.030%, Non-metallic inclusions A Classes B, C, and D are not higher than 1.5. Ultrasonic inspection is required for the finished tube, and the artificial contrast defect is not more than 5% of the nominal wall thickness of the tube.

2.2 Application of Stainless Steel Welded Pipe for Low Temperature LNG
Due to the development of society, people’s awareness of environmental protection has increased and more and more attention has been paid to clean energy. LNG is a clean and efficient energy source and plays an important role in the production and life of the people. Therefore, LNG receiving stations and LNG carriers have mushroomed. LNG is to cool gaseous natural gas to -162°C under normal pressure and condense it into a liquid. Therefore, the pipeline for LNG transportation must have high low temperature performance.

For low-temperature LNG conveying pipes, ultra-pure, low-carbon, low-sulfur, and low-phosphor stainless steels are mostly used at home and abroad. In recent years, the dual-grade stainless steels are very popular among LNG users, among which TP304/304L, TP316/316L and other applications are particularly extensive. Double-certified steel not only has L-level corrosion resistance and low-temperature properties, but also has high mechanical properties.

At present, domestic and foreign mainstream welding stainless steel tubes for low temperature LNG are generally processed by using automatic unit welding forming process, UOE forming process, and JCO forming process.

The automatic welding unit is a fast, efficient and automatic welding pipe production method used in the case of not thick wall thickness. At present, most of the rollers are used to form the plate, and then the welding and heat treatment are performed online. Some welding units also Integrated advanced technology such as on-board ultrasonics for plates, on-line weld ultrasonics, and automatic weld seam tracking technology can provide high-efficiency manufacturing operations for LNG long-distance pipeline manufacturing and reduce the production deadlines of manufacturers.

UOE molding technology is currently the most widely used, most mature, and most recognized quality of a low-temperature LNG welded pipe production process, the main process technology has been established.

The JCO molding process is a fresh molding process in recent years. This molding technology is an organic combination of step-wise pre-bending and pipe NC bending.

For the LNG stainless steel welded pipe, because its use environment is in a low temperature environment of -162°C, it is necessary for the LNG tube to have a high low temperature impact performance. At present, most design institutes, research institutes, and manufacturers require LNG tubes to have low-temperature impact performance of not less than 80 J, and lateral expansion volumes of not less than 0.38 mm according to ASME B 31.3.

For stainless steel welded pipes, as the weak link of the pipe, the quality of the weld directly affects the safety of the pipeline and even the pipeline. The welding coefficient is one of the important factors for evaluating the quality of the weld. For low temperature LNG welded pipes, the welding coefficient is Ej = 1.0 and the welded joint must be a full penetration welded joint. After welding joints are completed, all welds must undergo 100% ray inspection. The welds must have no defects such as incomplete penetration, no weld inclusion, no undercut, and no cracks to ensure the stability of welded joints at low temperatures.

3 Outlook
Petroleum pipelines are the bulk of consumption of stainless steel pipes. Stainless steel pipes play an important role in equipment manufacturing, oil recovery, oil refining, and transportation in the oil industry. In recent years, the state has increased the development of petroleum resources. At the same time, as the world’s largest net oil importing country, as the rigid oil demand increases, the oil-related industries will further develop, and the demand for stainless steel pipelines will continue to increase. increase. In 2018, the Chinese steel industry has shown signs of recovery. Domestic stainless steel pipe leading companies have increased their cooperation with relevant oil pipelines between PetroChina and Sinopec to increase market share. At the same time, domestic steel pipe enterprises have also started activities with foreign oil companies to push China’s stainless steel pipe manufacturing to the world platform.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (

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