Olefins plants employ extreme temperatures which range from 850 oC at the outlet of the pyrolysis furnaces to -165oC in the cryogenic hydrogen-methane separation system. These extreme temperature call for the application of a multitude of material of construction which include, besides the customary carbon steel, the high chrome-nickel alloys of the pyrolysis tubes; the stainless steel of the preheat sections in the pyrolysis furnaces; stainless steel, various nickel alloys, and aluminum in the cryogenic systems; and molybdenum alloy for the methanator which is exposed to high hydrogen partial pressure at elevated temperature.
Chapter covers the selection of materials of construction for process equipment. Many factors have to be considered when selecting engineering materials but for chemical process plant the overriding consideration is usually the ability to resist corrosion. The process designer will be responsible for recommending materials that will be suitable for the process conditions. He must also consider the requirements of the mechanical design engineer; the material selected must have sufficient strength and be easily worked. The most economical material that satisfies both chemical and mechanical requirements should be selected; this will be material that gives the lowest cost over the working life of plant, allowing for maintenance and replacement. Other factors, such as product contamination and process safety, must also be considered. The mechanical properties that are important in the selection of materials are discussed briefly in the chapter.
Material Properties
The most important characteristics to be considered when selecting the material of construction are
1-Mechanical Properties
a) Strength-tensile strength
b) Stiffness-elastic modules (Young’s modulus)
c) Toughness-fracture resistance
d) Hardness-wear resistance
e) Fatigue resistance
f) Creep resistance
2- The effect of high and low temperatures on the mechanical properties.
3- Corrosion resistance.
4-Any special properties required, such as, thermal conductivity, electric resistance, magnetic properties.
5-Ease of fabrication-forming, welding, casting.
6-Avalability in standard sizes-plates, sections, tubes.
7-cost
At low temperature, less than 283k, metals that are normally ductile can fail in brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with a body-centered-cubic (bcc) lattice are more liable to brittle failure than those with a face-centered-cubic (fcc) or hexagonal lattice. For low temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fcc) or aluminum alloys (hex) should be specified.
Recommended Material
Austentic Stainless Steel.
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