Material selection of pressure piping
The practical application environmental conditions in engineering are very complex. The combination of different media, media temperature, media pressure and other operating conditions constitutes countless material selection conditions. In terms of common material selection conditions, it is unrealistic to give the conclusion of material selection one by one here. It is also a problem that various design institutes or engineering companies have been committed to studying. Here, we will change the way, take the material as the main body, and apply metal theory, corrosion theory and engineering theory to determine the use restrictions of various common materials.
In engineering, in addition to determining the material grade, the material standard shall also be determined for the material selection of pressure piping, because different material standards have different requirements for material quality.
Table of Contents
- 1 General restrictions
- 2 Application limitations of common materials
- 3 Service temperature of common materials
When selecting engineering materials, the following principles should be followed first.
Meet the requirements of operating conditions
- a. Judge whether the pipeline is a pressure pipeline and what kind of pressure pipeline it belongs to according to the operating conditions. Due to the different importance of different types of pressure pipelines, the harm degree caused by accidents is different, so the requirements for materials are also different. In general, the requirements of high-class pressure piping (such as class I pressure piping) from material smelting process to final product inspection and test are higher than those of low-class pressure piping.
- b. The selection requirements of materials under operating conditions shall be considered. Different materials have different corrosion resistance to the same corrosive medium. In corrosive environment, materials shall be selected to avoid catastrophic forms of corrosion (such as stress corrosion cracking), while uniform corrosion shall be generally limited to “corrosion resistance”, that is, the maximum annual corrosion rate shall not exceed 0.5mm;
- c. Medium temperature is also an important parameter for material selection. Because the change of temperature will cause a series of performance changes of materials, such as brittleness of materials at low temperature, graphitization and creep of materials at high temperature. Many corrosion forms are closely related to medium temperature, and even the basic conditions of corrosion. Therefore, the material selection of pressure piping shall meet the temperature constraints.
Meet the requirements of material processing technology and industrial production
a. The ideal material should be easy to obtain, that is, it should have good processing and welding properties.
For example, for some corrosive environments, it is undoubtedly economical and applicable to select pressure pipes and components made of carbon steel and stainless steel to replace pure stainless steel materials. However, due to the poor composite process of many manufacturers, problems often occur in use, which limits the application of composite materials, In particular, the composite plate of carbon steel and 0Cr13 is not easy to ensure the on-site welding quality, so it is not dare to use or use it in large quantities in engineering.
b. The application of materials in engineering is serialized and standardized.
Unlike in the laboratory, it can achieve a small number of ideal material applications. Standardizing and serializing materials is convenient for large-scale production and reducing material varieties, which can not only save the investment in design, manufacturing, installation and use, but also greatly reduce the production cost.
Therefore, standard materials should be selected first in the project. For the new materials that must be selected, there should be complete technical evaluation documents, and they can be used only after they have passed the technical appraisal organized by the management department at or above the provincial level.
For materials that must be imported, detailed technical requirements such as specification, performance, material brand, material standard and application standard shall be put forward, and they shall be re inspected according to relevant domestic technical requirements. They can be used only after they are qualified.
Meet applicable and economical requirements
This is a very principled issue, and the actual operation is very complicated. It requires the material engineer to make comprehensive judgment by using the knowledge of engineering, material science, corrosion science and so on. Sometimes such problems can be quantitatively calculated, and sometimes they can not be quantitatively calculated. In general, the following aspects should be considered:
a. Corrosion aspect
- 1) For local corrosion, if other measures (such as process anti-corrosion measures) can prevent or control the occurrence of local corrosion, especially sudden and catastrophic local corrosion, materials with relatively low price can be used. Otherwise, high-grade but expensive materials must be selected.
- 2) For uniform corrosion, in the case of severe corrosion environment, if low-grade but cheap materials are selected, the corrosion rate may be very high, and the materials must be replaced in a short time; The corrosion rate of materials with good corrosion resistance and high price may be small, so as to maintain a long production cycle. For comprehensive technical and economic evaluation, it may be more economical to use high-grade materials at this time. On the contrary, if the corrosion environment is more moderate, low-grade materials are selected at this time. Although the corrosion rate is relatively large, the price is cheap. After economic accounting, it may be more economical to use low-grade materials at this time. In short, this type of material selection should be subject to economic accounting.
- 3) For the same corrosive environment, if high-grade materials are selected, the corrosion may be local corrosion with high risk, while the corrosion when low-grade materials are selected may be uniform corrosion with large corrosion rate. At this time, low-grade materials should be considered and supplemented with other anti-corrosion measures.
b. Material standard and manufacturing
There is no one-to-one correspondence between the category of pressure piping system and material standards and manufacturing requirements, which requires the material engineer to apply relevant knowledge for comprehensive consideration. In many material standards and manufacturing standards, there are several options for users to confirm.
1) Some of these options are general items. When the user does not specify them, the manufacturer will do it according to its own habits.
For example, the supply length and supply status of steel pipes belong to this kind of project.
2) Some items are additional inspection items. These inspection items are not necessary and can only be done by the manufacturer when required by the user. In other words, users can add several inspection items according to different service conditions, so as to better control the internal quality of materials. However, putting forward these special requirements means that the product price increases, and the cost of some inspection items such as radiographic inspection is very high. How to add these additional inspection items should be comprehensively considered in combination with service conditions and product price. Sometimes it is difficult to grasp this scale.
c. Application of new materials and new processes
Actively adopting new materials and supporting the development and application of new materials and processes can effectively reduce construction investment and meet the requirements of production processes for materials.
- For example, aluminized carbon steel is used instead of stainless steel to resist the corrosion of sulfur and organic acids;
- Replace the pure stainless steel with the composite of carbon steel and stainless steel;
- Replace seamless steel pipe with seamed steel pipe with guaranteed welding quality; wait.
Application limitations of common materials
There are two kinds of commonly used cast iron: Malleable Cast Iron and nodular cast iron.
- 1) Used in pressure or non pressure pipes with medium temperature of – 29 – 343 ℃;
- 2) It shall not be used for conveying combustible fluid pipes with medium temperature higher than 150 ℃ or gauge pressure greater than 2.5MPa;
- 3) It shall not be used to transport toxic media under any temperature and pressure conditions;
- 4) It shall not be used under the condition of cyclic change of temperature and pressure or vibration of pipeline.
In fact, malleable cast iron is often used for non pressurized valve handwheel and underground pipeline; Ductile iron is often used in valve bodies in industrial pipelines.
Ordinary carbon steel
a. Boiling steel
- 1) It shall be limited to the condition that the design pressure is ≤ 0.6MPa and the design temperature is 0 – 250 ℃;
- 2) It shall not be used for pipes with flammable or toxic fluids;
- 3) It shall not be used in LPG medium and stress corrosion environment.
b. Killed steel
- 1) It is limited to the design temperature of 0 – 400 ℃;
- 2) When used in the environment sensitive to stress corrosion cracking, the body hardness and weld hardness shall not be greater than HB200, and 100% NDT shall be carried out for the body and weld.
c. Boiling steel and killed steel for pressure pipes
1) The carbon content shall not be greater than 0.24%.
2) GB700 standard gives four commonly used grades of ordinary carbon structural steel, namely: Q235A (F, b), Q235B (F, b), Q235C and Q235D. Its scope of application is as follows:
- Q235-A.F steel plate: design pressure P ≤ 0.6MPa; The service temperature is 0 – 250 ℃, and the thickness of steel plate is ≯ 12mm;
- It shall not be used in pipelines with flammable, medium, high or extremely hazardous media.
- Q235-A steel plate: design pressure P ≤ 1.ompa; The service temperature is 0 – 350 ℃; Steel plate thickness ≯ 16mm;
- It shall not be used in pipelines with liquefied petroleum gas, highly toxic or extremely hazardous media.
- Q235-B steel plate: design pressure P ≤ 1.6Mpa; The service temperature is 0 – 350 ℃; Steel plate thickness ≯ 20mm;
- It shall not be used for steel pipes with high and extremely hazardous media;
- Q235-C steel plate: design pressure P ≤ 2.5MPa; The service temperature is 0 – 400 ℃; Steel plate thickness ≯ 40mm.
High quality carbon steel
High quality carbon steel is the most widely used carbon steel in pressure piping, and the corresponding material standards are:
GB/T699, GB/T8163, GB3087, GB5310, GB9948, GB6479, etc. These standards put forward different quality requirements according to different service conditions. Their common use restrictions:
- a. The possibility of alkali embrittlement shall be considered when transporting alkaline or caustic media, and manganese steel (such as 16Mn) shall not be used in this environment;
- b. When working in the environment with stress corrosion cracking tendency, post weld stress relief heat treatment shall be carried out, and the weld hardness after heat treatment shall not be greater than hb200. Welds shall be subject to 100% non-destructive testing. Manganese steel (such as 16Mn) should not be used in the environment with stress corrosion cracking tendency;
- c. When working in uniform corrosive medium environment, economic evaluation shall be carried out according to corrosion rate, service life, etc. If the calculation results prove that the selection of carbon steel is appropriate, sufficient corrosion allowance shall be given, and take corresponding other anti-corrosion measures;
- d. When carbon steel, carbon manganese steel and manganese vanadium steel work at 425 ℃ and above for a long time, their carbides may be transformed into graphite. Therefore, the maximum working temperature shall not exceed 425 ℃ (the boiler specification stipulates that the temperature is 450 ℃);
- e. During hydrogen operation, the possibility of hydrogen damage shall be considered.
- f. Carbon steel with carbon content greater than 0.24% shall not be used for welded pipes and components;
- g. Low temperature impact toughness test shall be conducted when it is used at – 20 ℃ and below;
- h. Carbon steel materials used under high-pressure hydrogen and alternating load should be refined outside the furnace.
Chromium molybdenum alloy steel
The commonly used chromium molybdenum alloy steel material standards include GB9948, GB5310, GB6479, GB3077, GB1221, etc., and the use restrictions are as follows:
- a. When carbon molybdenum steel (c-0.5mo) works at 468 ℃ for a long time, its carbide tends to convert into graphite, so its maximum long-term working temperature is limited to no more than 468 ℃;
- b. When working in uniform corrosion environment, economic accounting shall be carried out according to corrosion rate and service life, and sufficient corrosion allowance shall be given;
- c. During hydrogen operation, the possibility of hydrogen damage shall be considered;
- d. When working in high temperature H2 + H2S medium environment, its service conditions shall be determined according to Nelson curve and couper curve;
- e. Avoid using in the environment with stress corrosion cracking;
- f. When working in the temperature range of 400-550 ℃ for a long time, the problem of preventing tempering brittleness shall be considered.
- g. Chromium molybdenum alloy steel should generally be smelted in electric furnace or refined outside the furnace.
Stainless heat resistant steel
The standards of stainless and heat-resistant steel materials commonly used in pressure pipelines mainly include GB/T14976, GB4237, GB4238, GB1220, GB1221, etc. The common use restrictions are as follows:
- a. When Ferritic and martensitic stainless steels containing more than 12% chromium work for a long time in the temperature range of 400-550 ℃, it should be considered to prevent tempering brittle failure at 475 ℃, which is manifested in the embrittlement of materials at room temperature. Therefore, when applying the above stainless steel, its bending stress, vibration and impact load shall be reduced below the sensitive load, or it shall not be used at a temperature above 400 ℃;
- b. When austenitic stainless steel passes through the temperature range of 540 – 900 ℃ during heating and cooling, it shall be considered to prevent intergranular corrosion. When there is corrosive medium with strong reducibility, stable (containing stabilizing elements Ti and Nb) or ultra-low carbon (C < 0.0.03%) austenitic stainless steel shall be selected;
- c. Stainless steel may have stress corrosion cracking and pitting corrosion when contacting wet chloride. Contact with wet chloride shall be avoided, or the chloride ion concentration in material and environment shall not exceed 25 × 10-6；
- d. When the service temperature of austenitic stainless steel exceeds 525 ℃, its carbon content shall be greater than 0.04%, otherwise the strength of the steel will decrease significantly.
Service temperature of common materials
Table.3-1 service temperature of common metal materials
Service temperature / ℃
Low carbon austenitic stainless steel (018CrNi9、0Cr17Ni12Mo2、0Cr18Ni19Ti)
Ultra low carbon austenitic stainless steel (00Cr19Ni10)
Ultra low carbon austenitic stainless steel (00Cr17Ni14Mo2)
Source: Network Arrangement – Piping System Supplier: www.epowermetals.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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