Air separation unit expander

Special pressure-bearing equipment generally bears internal pressure, that will cause tensile stress inside the shell, which is called working stress. The working stress is directly proportional to the pressure and diameter, and inversely proportional to the vessel wall thickness. The axial stress is half of the circumferential stress. Therefore, for cylindrical vessels, the stress of longitudinal weld is twice that of girth weld. Because the geometric shape of the spherical shell is symmetrical to the spherical center, and the axial stress is equal to the circumferential stress in numerical value. Therefore, the wall thickness of the spherical vessel can be reduced by half as much as that of the cylindrical vessel under the same pressure and diameter.

Lean alloy steel is often used in special pressure-bearing equipment. The alloying elements often added include manganese, silicon, chromium, nickel, molybdenum, vanadium, boron and rare earth elements. Most of silicon and manganese melt into ferrite, only a small amount into cementite, which are commonly used strengthening elements in lean alloy steel. The amount of chromium and nickel is generally small, and the main role of chromium and nickel is to increase the undercooling degree of austenite and refine the microstructure. Part of chromium is dissolved in ferrite and some in cementite, which can improve the stability of cementite. Molybdenum is dissolved in ferrite slightly, and has the effect of solid solution strengthening. It is a strong carbide forming element, which can improve the high temperature strength of steel and improve the temper brittleness.