MATERIALS AND FABRICATION SELECTION
●In any engineering design;
Selection of available materials of construction combined with the appropriate fabrication technique is important.
●To a new plant or
●In the improvement of an existing plant.
●Why selection of material is important ?
●Availability and cost (See Table 1 for relative cost of some metal plates)
●Because of severe operating conditions such as
1.Higher or lower temperatures
5.Strong acids or strong bases
Materials of Construction;
(Pure metals or alloys)
1.Iron and Steel
●Cost aspects favor their use widely,
●They are used as materials of construction although it is known that some corrossion occurs,
●If this is done, the presence of iron salts and discoloration in the product can be expected,
●They are not suitable for use with dilute acids, but can be used with many strong acids, since a protective coating composed of corrosion products forms on the metal surface
2. Stainless Steel
●There are more than 100 different types of stainless steels,
●They are high chromium or high nickel-chromium alloys,
●They have excellent corrosion-resistance and heat-resistance properties,
●The most common stainless steels (type 302 or type 304) contain approximately 18 % chromium and 8 % nickel and are designated as 18-8 stainless steels,
●Addition of molybdenum to the alloy (type 316) increases the corrosion resistance and high T strength,
●If Ni is not included, the low T brittleness of the material is increased and the ductility and pit-type corrosion resistance are reduced,
●The presence of Cr in the alloy gives resistance to oxidizing agents (see Tables 2, 3, 4 and 5 for types of stainless steels and the common processes where they are used).
3. Hastelloy C
The beneficial effects of Ni, Cr and Mo are combined in Hastelloy C. It is
●expensive but highly corrosion-resistant,
●56 % Ni, 17 % Mo, 16 % Cr, 5 % Fe, 4 % tungsten with Mn, Si, C, P and S making up the balance,
●used where structural strength and good corrosion resistance are necessary under conditions of high Ts (valves, heat exc., piping etc)
4. Copper and Its Alloys
●Cu is relatively inexpensive, possesses fair mechanical strength and can be fabricated easily into a wide variety of shapes,
●Although Cu shows little tendency to dissolve in nonoxidizing acids, it is readily susceptible to oxidation,
●Cu is resistant to atmospheric moisture or oxygen because a protective coating composed of copper oxide is formed on the surface,
●The Cu-oxide is soluble in most acids and thus Cu is not a suitable material of construction when it must contact any acid in the presence of oxygen or oxidizing agent,
●Cu exhibits good corrosion resistance to strong alkalies,
●Cu alloys (brass, bronze, etc.) can exhibit better corrosion resistance and better mechanical properties than pure Cu.
5. Nickel and Its Alloys
●Ni exhibits high corrosion resistance to most alkalies,
●Monel, an alloy of Ni containing 67 % Ni and 30 % Cu, is often used in food industries,
●This alloy is stronger than Ni and has better corrosion-resistance properties than either Cu or Ni.
●The lightness and relative ease of fabrication of Al and its alloys are factors favoring the use of these materials,
●It resists attack by acids because a surface film of inert hydrated aluminum oxide is formed,
●This film adheres to the surface and offers a good protection.
●Pure lead has low creep and fatigue resistance,
●But its physical properties can be improved by the addition of small amounts of silver, copper, antimony, tellerium etc.,
●The excellent corrosion-resistance properties of lead are caused by the formation of protective surface coatings (lead-salts),
●Lead shows good resistance to sulfuric acid and phosphoric acid but not acetic acid and nitric acid.
●Its properties are similar to those of mild steel with the exception that its melting point (2996 C) is higher,
●It is used in the pure form and can be fabricated into many different shapes,
●It is often used for equipment involving contact with HCl.
●It is resistant to alkalies and many hot organic acids,
●Because of its low mechanical strength and high cost, it is generally used only in the form of linings.
●Glass, carbon, stoneware, brick, rubber, plastics and wood are common examples of nonmetals used as materials of construction,
●Most of them have low structural strength,
●They are often used in the form of linings or coatings bonded to metal supports.
1. Glass and Glassed Steel
●Glass has excellent resistance against corrosion,
●It is brittle and damaged by thermal shock,
●It is suitable for processes which have critical contamination level,
●Glassed steel combines the corrosion resistance of glass with the working strength of steel,
●Its impact strength is 18 times that of safety glass.
2. Carbon and graphite
●Graphite is completely inert to all oxidizing conditions,
●Carbon and graphite have excellent heat transfer in heat exchangers, in pipe and pumps,
●Oxidation T is 350C for carbon and 450C for graphite.
3. Stoneware and Porcelain
●They are about as resistant to acids and chemicals as glass,
●They have greater strength,
●They have poor thermal conductivity and are damaged by thermal shock,
●Porcelain is used to coat steel.
4. Brick and Cement Materials
●Brick-lined construction can be used for many severely corrosive conditions, where high alloys would fail (acidproof up to 900C),
●Cement materials are used with brick,
●They are good against acids, salts and solvents.
5. Rubber and Elastomers
●Rubbers (natural and synthetic) are used as linings or as structural components for equipment,
●Natural rubber is resistant to dilute mineral acids, alkalies and salts but oxidizing media, oils, benzene, and ketones will attack it,
●Widely used rubbers: Chloroprene (neoprene), styrene, nitrile, silicone (polysiloxane), chlorosulfonated polyethylene (hypalon),
●Elastomers are chemicals and high temperature resistant (examples are Fluoroelastomers (Viton A, Kel-F), Polyvinyl chloride elastomers (Korosel)).
●In comparison with metallic materials, the use of plastics is limited to moderate Ts and Ps (e.g., 230C is considered high for plastics),
●They are also less resistant to mechanical abuse and have high expansion rates, low strength and only fair resistant to solvents,
●However, they are lightweight, are good thermal and electrical insulators,
●They are easy to fabricate and install and have low friction factors,
●They have good resistance to weak mineral acids and inorganic salt solutions,
●Tetrafluoroethylene or TFE (Teflon) is one of the most chemical resistant plastics commercially available today, it retains its properties up to 260C.
●Polyethylene is the lowest-cost plastic commercially available,
●Mechanical properties are generally poor above 50C,
●Carbon-filled grades are resistant to sunlight and weathering.
●Acrylonitrile butadiene styrene polymers (ABS) have good resistance to nonoxidizing and weak acids but are not satisfactory with oxidizing acids.
●Nylons resist many organic solvents but are attacked by phenols, strong oxidizing agents, and mineral acids.
●Polyprophylene’s chemical resistance is about the same as polyethylene, but it can be used at 120C.
●This material of construction is fairly inert chemically,
●It is readily dehydrated by concentrated solutions and consequently shrinks badly when subjected to the action of such solutions,
●It also has a tendency to slowly hydrolyze when in contact with hot acids and alkalies.
LOW-AND HIGH-TEMPERATURE MATERIALS
●The extremes of high and low Ts used in many processes creates unusual problems (losing ductility, strength, stress, other mechanical properties etc. of metals) in fabrication of equipment,
●It is important to take these problems into consideration when extreme Ts are used,
●Tables 7 and 8 show the metals and alloys that can be used at high or low temperature process use,
●Table 9 presents information on the corrosion resistance of some common metals, nonmetals and gasket materials,
●Table 10 presents similar information for types of plastics.
SELECTION OF MATERIALS
●An engineer must analyze and understand both the structure of construction materials and the basic process information,
●This knowledge of the process can then be used to select materials of construction in a logical manner,
●A brief plan for studying materials of construction is as follows:
1. Preliminary selection
●Experience, manufacturer’s data, literature, availability, safety aspects,
2. Laboratory testing
●Reevaluation of apparently suitable materials under process conditions,
3. Interpretation of laboratory results and other data
●Effect of possible excess T, P, agitation etc.,
4. Economic comparison of apparently suitable materials
●Materials and maintenance cost,
●Cost of product degredation,
●Liability to special hazards,
5. Final selection
●From this survey choose the suitable material of construction for your purpose; considering ease of fabrication, resistance etc.
ECONOMICS IN SELECTION OF MATERIALS
●First cost (purchased) of equipment or material often is not a good criterion when comparing alternate materials of construction for process equipment.
●Any cost estimation should include the following items:
1.Total equipment or material costs,
5.Replacement costs (installed cost /estimated life)
Table 11 presents “Alternative investment comparison of three different materials A, B and C”.
FABRICATION OF EQUIPMENT
●An engineer should be acquinted with the methods for fabricating equipment and the problems involved in the fabrication should be considered when equipment specifications are prepared.
●Generally, the following steps are involved in the complete fabrication of major pieces of equipment such as tanks, autoclaves, reactors, towers, heat exchangers etc.
1.Layout of materials,
2.Cutting to correct dimensions,
3.Forming into desired shape,
●Establish the layout of various components,
●Allowances (tolerances) must be made for losses caused by cutting, shrinkage due to welding or deformation caused by the various forming operations.
Several methods can be used for cutting the laid-out materials to the correct size
a) Shearing: It is the cheapest method and is satisfactory for relatively thin sheets
b) Burning: It is often used for cutting metals.
●Carbon steel is easily cut by an oxyacetylene flame,
●Stainless steels and nonferrous metals that do not oxidize readily can be cut by a method known as powder or flux burning.
●e.g., an oxyacetylene flame is used and powdered iron is introduced into the cut to increase the amount of heat and improve the cutting characteristics.
●It can be used to cut metals that are in the form of flat sheets,
●It is expensive, however, it is used only when the heat effects from burning would be detrimental.
●After cutting, the next step is to form them into a desired shape,
●This can be accomplished by various methods, such as by rolling, bending, pressing, spinning on a die etc,
●When the shaping operations are finished, the different parts are assembled and fitted for fastening,
●The fitting is accomplished by use of jacks, hoists, wedges and other means.
●Riveting can be used for fastening operations,
●But electrical welding is far more common and gives superior results,
●The quality of weld is very important, because the ability of equipment to withstand pressure or corrosive conditions is often limited to the conditions along the weld.
The most common types of welding:
a) Manual shielded-arc process:
●An electrode approximately 14 to 16 in. Long is used and an electric arc is maintained manually between the electrode and the material being welded,
●The electrode melts and forms a filler metal, while, at the same time, the work material fuses together,
●A special coating is provided on the electrode,
●This protects the metal from surrounding air until the metal has solidified and cooled below red heat.
b) Submerged-arc process:
●It is commonly used for welding stainless steels and carbon steels when an automatic operation is acceptable,
●The electrode is a continuous roll of wire fed at an automatically controlled rate,
●The appearance and quality of the submerged-arc weld is better than that obtained by shielded-arc process,
●However, the automatic process is limited to main seams or similar long-run operations.
c) Heliarc welding process:
●It is used for stainless steels and most of the nonferrous materials,
●A stream of helium or argon gas is passed from a nozzle in the electrode holder onto the weld, where the inert gas acts as a shielding blanket to protect the molten metal,
●A filler rod is fed into the weld, but the arc in the heliarc process is formed between a tungsten electrode and the base metal.
●All welded joints can be tested for imperfections by X rays examination of main seams,
●Hydrostatic tests can be conducted to locate leaks,
●Helium probe test may be used to check for very small leaks.
6. Heat treating
●It may be necessary to heat treat the equipment to remove forming and welding stresses, restore corrosion-resistance properties to heat affected materials and prevent stress corrosion conditions.
●This process involves preparing the equipment for final shipment. (e.g., sandblasting, polishing, painting, coding, etc.)