Basic Principles for Material Selection of Spherical Storage Tank

Spherical tank is a structural form of pressure vessel, so it is the same as pressure vessel in the basic requirements of material selection. Compared with other pressure vessels, the spherical tank has its special characteristics: firstly, the volume of the spherical tank is large, and the length of the weld seam should be reduced as much as possible; the steel plate must have sufficient width, length and thickness; secondly, the safety requirements of the spherical tank are higher The occurrence of safety failure, which has great impact and hazard, requires higher plasticity and toughness reserves of the material; the third spherical tank is field welded, with many welds, high local stress, and high weldability requirements for the welding material.

Because of the inherent characteristics of spherical tanks, special requirements must be put on the materials. The selection of spherical tanks should mainly consider the use conditions of the spherical tanks (such as design temperature, design pressure, structural characteristics, etc.), the welding performance of the materials, and the Manufacturing process and assembly welding requirements and economic rationality. Spherical tank steel should be calm steel, and boiling steel should never be used. The following problems should be solved.

(1) Intensity level

Strength mainly refers to the yield strength ReL and tensile strength Rm of a material. At present, there are international classifications based on yield strength as a material level and also tensile strengths as a material level classification. For example, Q 345R and Q370R are classified by yield strength. CF62, 610U2, 610F are classified according to tensile strength.

For spherical tanks of the same specification, the higher-level materials are used, the thickness of the spherical shell plate is thinner, and the weight of the spherical tank is lighter, but the higher-level materials have higher unit prices, are more difficult to form and group weld, and have poor welding performance Therefore, the selection of materials should be considered from the aspects of the volume of the spherical tank, the nature of the storage materials, and the thickness of the spherical shell plate.

(2) toughness index

Toughness is an important indicator to ensure that the material avoids cracks and rapid fractures. It is especially important for spherical tanks. There are many toughness indicators, including V-notch impact test, drop weight test (NDT temperature), wide plate tensile test considering "turning temperature", COD and K1C considering "fracture toughness", etc. Due to the variety of toughness indicators, they cannot be unified due to different test methods. However, countries around the world practically associate various toughness indicators with the absorbed energy of the V-notch impact test in order to achieve simple and convenient purposes. Spherical tanks should have a high toughness reserve for safety reasons.

(3) Plasticity index

Spherical tanks must undergo various deformations during the forming process. The good plasticity of steel is a prerequisite for the manufacture of spherical shell plates. It must have a certain plasticity, that is, a certain elongation (δ) and sectional shrinkage (ψ ).

(4) Solderability

Weldability indicators of steel are as follows.

①Carbon equivalent CE The formula of carbon equivalent recommended by the International Welding Institute (IIW) for low alloy steel is

Generally, the CE of low-alloy high-strength steel with tensile strength of 610 MPa is controlled to be less than or equal to 0.40.

② The crack sensitivity index Pc carbon equivalent has a certain relationship with the hardness of the heat-affected zone of welding. However, a large number of research results have pointed out that the possibility of crack occurrence is not comprehensive enough. Crack sensitivity index Pc. Pc is calculated as follows:

Where δ is the thickness of the steel plate, mm;

H-1 H content in the weld, mL / 100g.

The applicable range of formula (7-2) is: wc = 0.07% ~ 0.22%, ωMn = 0.40% ~ 1.40%, ωSi = 0 ~0.6%, ωCu = 0 ~ 0.5%, ωCr = 0 ~ 1.2%, ωv = 0 ~ 0.12%, ωNi = 0 ~ 0.05%, ωB = 0 ~ 0.005%, δ = 19 ~ 50mm, H = 1.0 ~ 50mL / 100g. A large number of test results show that the probability of crack occurrence is high under the condition of PC> 0.35, and the probability of crack occurrence is small when PC <0.30 (of course, the effect of preheating should also be considered). From PC, the preheating temperature to prevent cracks and its empirical relationship are

Where T is the preheating temperature to prevent cracks in the Y-shaped groove restraint test, ℃.

③ Crack sensitivity coefficient Pcm The crack sensitivity index Pc includes the two items of plate thickness and the amount of diffusible hydrogen in the weld metal, which has little relationship with the material. Therefore, these two items are omitted when considering the welding crack of the plate. After cancellation, the crack sensitivity coefficient Pcm was induced. Pcm is essentially a formula for calculating carbon equivalent, and is especially suitable for high-strength steels with a yield strength greater than 390 MPa.

The research on crack sensitivity coefficient in Japan is very deep. According to the requirement of Pcm ≤ 0.20, low crack sensitivity steel has been studied, and it has been widely used in spherical tanks.

(5) Economical

Various special requirements are put on the steel for spherical tanks, which will certainly increase the cost economically. The price of steel for spherical tanks accounts for a large proportion of the total investment in spherical tanks. Therefore, in the selection of spherical tank materials, economics must be a key consideration.

(6) Corrosion resistance

The corrosion damage caused by storage materials to the spherical tank is very serious. Corrosion will damage the surface of the spherical tank, and cause corrosion pits, grooves and even cracks. Some even erode into the metal and change its structure, which deteriorates the mechanical properties of the steel. It may seriously cause the spherical tank to fail due to corrosion, or even cause a major accident.

There are many reasons for corrosion, so there are many types of corrosion, such as general corrosion, intergranular corrosion, pitting corrosion, alkali embrittlement, hydrogen embrittlement, stress corrosion, and corrosion fatigue. It should be noted that in recent years, H2S stress corrosion has become the main form of corrosion damage of spherical tanks, so the problem of corrosion resistance must be considered from the selection of materials. If H2S is contained in materials, steels with lower strength levels should be selected as much as possible