The use of pressure vessels is extremely wide, and the working conditions are very different, so the correct choice of materials in the design process of the vessel is an extremely complex and particularly important work. Many pressure vessels cause accidents is one of the important reasons for improper selection of materials. For example, the use of poor weldability of steel welding pressure vessel, it is easy to produce cracks in the welded joints; some nickel-chromium stainless steel pressure vessel, often due to improper selection of steel or composition, intergranular corrosion, stress corrosion and other forms of damage in use; the choice of ferritic steel manufacturing low-temperature pressure vessels, such as steel transformation temperature higher than the working temperature of the vessel, the vessel work is prone to brittle damage. Therefore, in the selection of pressure vessels with steel, must be based on the working conditions of the vessel (such as wall temperature, pressure, media corrosion, media on the embrittlement of the material and whether it is flammable, explosive, toxic, etc.) to choose the appropriate mechanical properties, physical properties and corrosion resistance of the material, the material selected must also consider the impact of processing (weldability, whether to facilitate processing), and consider its economic rationality and Sources and other circumstances. It should be noted that the pressure vessel with steel, except for a very few castings, the vast majority are rolled, forged, shaped, welded and heat-treated and other processing before being put into use.
1.quality selection principles
① select the pressure vessel with steel must consider the operating conditions of the equipment (such as design pressure, design temperature, the characteristics of the medium), the welding properties of the material, hot and cold processing properties, heat treatment and the structure of the vessel.
② selection of pressure vessels with steel must meet the premise of Article ①, consider the economic rationality. In general, the following provisions are economically reasonable.
a. The required thickness of steel less than 8 mm, between carbon steel and low-alloy high-strength steel, carbon steel plate should be used as far as possible (except for multi-layer container material).
b. In the stiffness or structural design-oriented occasions, should try to use ordinary carbon steel. In the strength design-oriented occasions, according to the pressure, temperature, medium and other restrictions on the use of Q235A, Q235B, 20R (when 20R supply difficulties, can be used 20g), 16MnR and other steel plates.
c. When the required thickness of stainless steel is greater than 12mm, should try to use lining, composite, welded and other structural forms.
d. Stainless steel should not be used as far as possible as the design temperature is less than or equal to 500 ℃ heat-resistant steel.
e. Pearlescent heat-resistant steel should not be used as far as possible as the design temperature is less than or equal to 350 ℃ heat-resistant steel. In must use pearl body heat-resistant steel for heat-resistant or hydrogen-resistant purposes, should be reduced as far as possible, combined with the variety of steel, specifications.
③ this article listed in the selection of various types of steel is the design guidelines, usually should be implemented.
a. Carbon steel for medium corrosive atmospheric pressure, low-pressure vessels, wall thickness is not a medium pressure vessels, forgings, pressurized steel pipes, non-pressurized components and other rigid or structural factors to determine the wall thickness of the occasion.
b. Low-alloy high-strength steel for medium corrosiveness, wall thickness (not less than 8 mm) of the pressurized containers.
c. Pearlescent heat-resistant steel used for high-temperature hydrogen or hydrogen sulfide corrosion, or design temperature in 350 ~ 650 ℃ pressure vessel with heat-resistant steel.
d. Stainless steel for high media corrosion (electrochemical corrosion, chemical corrosion), anti-iron ion pollution, design temperature greater than 500 ℃ or design temperature less than ~ 100 ℃ heat-resistant or low-temperature steel.
e. Does not contain stabilizing elements, and carbon content greater than 0.03% of austenitic stainless steel to be welded or 400 ℃ above the heat processing, should not be used in the environment that may cause intergranular corrosion of stainless steel.
④Steel should meet the requirements of the relevant standards.
⑤ used as equipment flanges, pipe flanges, pipe fittings, hand holes, liquid level meters and other standard parts of chemical equipment steel, should be in line with the national standards of the parts, industry standards for steel technical requirements.
2.economic principles of selection
A large part of the cost of equipment is determined by the price of materials. Therefore, in the selection of materials, should be aware of their prices. In the pressure vessel steel, the price difference of various steel is large. If the price of carbon steel plate Q235-A as 1, the rest of the relative price of the plate has roughly the following relationship, 16MnR for 1.4, 20R (20g) for 1.8, chromium steel (1Cr13, 2Cr13) for 5.1, high alloy steel 0Cr18Ni10 for 14.1.
Of course, the use of inexpensive materials is not necessarily in the economy is reasonable, because expensive materials may have better performance, with it can be made into a thin and light vessel walls, and the use of a longer life, better economic results.
Analysis of the economics of materials can not only look at their prices, but also to see the country's resources. Should use more common easy to take the material, less expensive and scarce materials; more domestic materials, less or no imported materials.
