Carbon steel fabrication requirements

Typically, reactors require some type of catalyst. Reactors with catalyst can be of the fixed-bed style for fiuid-bed types. Fixed-bed reactors are the most common. The feed often enters the reactor at an elevated temperature and pressure. The reaction mixtures are often corrosive to carbon steel and require some type of stainless steel alloy or an alloy liner for protection. If the vessel wall is less than 6 mm, the vessel is constmcted of all alloy if alloy is provided. Thicker reactor walls can be fabricated with a stainless overlay over a carbon steel or other lower alloy base steel at less cost than an all-alloy wall constmction. 

The cost of carbon vs stainless makes carbon steel fabrication worth looking into. It is not a choice for universal application, but for some applications it works well. Under no circumstances would one use carbon steel to fabricate the clean solvent storage tank where there is some air, unless the process design required purging with nitrogen prior to solvent entry. The cost difference between stainless and carbon steel... 

Carbon steel is easily the most commonly used material in process plants despite its somewhat limited corrosion resistance. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. It is also used routinely for the storage of concentrated sulfuric acid and caustic soda [up to 50 percent and 55°C (I30°F)]. Because of its availability, low cost, and ease of fabrication steel is frequently used in services with corrosion rates of 0.13 to 0.5 mm/y (5 to 20 mils/y), with added thickness (corrosion allowance) to assure the achievement of desired service life. Product quahty requirements must be considered in such cases. 

The diameter and materials, specified for the tubes and fins, depend on system requirements. The fins are commonly made from aluminum or copper, but may be fabricated of stainless or carbon steel. Tubes are generally copper, but can be made from almost any material, and they range in size from 5/8- to 1-in. outer diameter. The design of the air-cooled exchanger is such, that individual coils can be removed independently for easy cleaning and maintenance. There are several common design configurations that are commercially available. Each is briefly described below. 

Fabricated of most metals, such as carbon steel, stainless steel, aluminum and others as required. 

Today boiler vessels are usually fabricated from special boiler plate and firebox steels of varying thickness, while their auxiliaries (supplementary equipment) and appurtenances (boiler accessories and instruments, especially those employed for safety reasons) may be produced from any of several different constructional metals, alloys, and other materials, including cast iron, copper alloys, stainless steels, and so forth. Tubes and tube plates may be variously constructed of carbon steel, low-alloy steels, or special alloy steels, with each design providing for particular required levels of thermal and mechanical stress and corrosion resistance. The overall boiler plant system may have a life expectancy in excess of 50 to 60 years, although individual components may need to be replaced periodically during this period. 

The relative cost of equipment made from different materials will depend on the cost of fabrication, as well as the basic cost of the material. Unless a particular material requires special fabrication techniques, the relative cost of the finished equipment will be lower than the relative bare material cost. For example the purchased cost of a stainless-steel storage tank will be 2 to 3 times the cost of the same tank in carbon steel, whereas the relative cost of the metals is between 5 to 8. 

The required heat-transfer area of 19.5 m2 is based on an overall heat-transfer coefficient of 102 W/(m2 K). The best exchanger geometry for this application includes six internal baffles, one shell-side pass and two tube-side passes. The shell is fabricated from standard carbon steel piping of nominal pipe size 30, schedule number 80. The 112 tubes required are each 1.83 m long and 38.1 mm (1.5 in.) o.d. (BWG 12). The tubes must be fabricated from stainless steel type 250 for reasons of temperature tolerance. 

The shell is constructed from carbon steel and will be fabricated from standard pipe of nominal size 30, schedule number 80. The 112 tubes required are 1.83 m (6ft) lengths and standard BWG 12. The tubes are made from stainless steel type 250 as recommended in the Australian Design Code AS1548 Design of Boilers and Pressure Vessels. 

The refinery operators were in the process of switching feed from the D drum to the C drum when a 45-degree elbow in the feed line ruptured. Investigators later determined that the 6-inch-diameter elbow was made of carbon steel instead of the 5-percent chrome alloy steel required by specifications. It seems that this section of piping was fabricated and installed in 1963. The mistake in piping fabrication was discovered 20 years later. The extent of the fire damage was such that the central unit was down for a period approaching a year and the adjacent units were each down for a few weeks. [21]... 

To stress relieve or postweld heat treat carbon and low-alloy steels, they are typically heated to 1100°F to 1350°F (595°C to 730°C) for extended time, followed by air coohng. The minimum time is specified by the relevant engineering code, and the temperature must be less than the lower transformation temperature of the steel, which is the lowest temperature at which austenite starts to form, for example, 1333°F (720°C) for plain carbon steels. In order to avoid degrading the required mechanical properties of a heat treated alloy, subsequent fabrication heat treatment temperatures, such as those for stress relief and PWHT, must not exceed the tempering temperature (discussed in the next section). 

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