Materials for construction

NON-METALS USED IN MATERIALS CONSTRUCTION

• Glass, carbon, stoneware, brick, rubber, plastics, and wood

• Have low structural strength• Used in the form of linings or coatings bonded

to metal supports. • Glass-lined or rubber-lined equipment has

many applications in the chemical & Biochemical industries.

Glass and Glassed Steel• Glass has excellent resistance• Susceptible to attack only by hydrofluoric acid and hot

alkaline solutions. • Particularly suitable for processes which have critical

contamination levels. • Limitation is its brittleness and damage by thermal shock. • Glassed steel combines the corrosion resistance of glass

with the working strength of steel. • Nucerite - a ceramic-metal composite made in a similar

manner to glassed steel and resists corrosive hydrogen-chloride gas, chlorine, or sulfur dioxide at 650°C.

• Its impact strength is 18 times that of safety glass and the abrasion resistance is superior to porcelain enamel.

Carbon and Graphite• Impervious graphite is completely inert to all but the most

severe oxidizing conditions. • Has excellent heat transfer so are very popular in heat

exchangers, as brick lining, and in pipe and pumps. • Limitation - low tensile strength. • Threshold oxidation temperatures are 350°C for carbon and

400°C for graphite. Stoneware and Porcelain• Resistant to acids and chemicals as glass, but with the

advantage of greater strength. • Poor thermal conductivity and susceptibility to damage by

thermal shock. • Porcelain enamels are used to coat steel, but the enamel

has slightly inferior chemical resistance.

Brick and Cement Materials

• Brick-lined construction can be used for many severely corrosive conditions, where high alloys would fail.

• Acidproof refractories can be used up to 900°C.• A number of cement materials are used with brick.

Standard are phenolic and furane resins, polyesters, sulfur, silicate, and epoxy-based materials.

• Carbon-filled polyesters and furanes are good against nonoxidizing acids, salts, and solvents.

• Silica-filled resins should not be used against hydrofluoric or fluorosilicic acids. Sulfur-based cements are limited to 95°C while resins can be used to about 175°C.

• The sodium silicate based cements are good against acids to 400°C.

Rubber and Elastomers

• Natural and synthetic rubbers are used as linings or as structural components for equipments.

• By adding the proper ingredients, natural rubbers with varying degrees of hardness and chemical resistance can be produced.

• Hard rubbers are chemically saturated with sulfur. The vulcanized products are rigid and exhibit excellent resistance to chemical attack by dilute sulfuric acid and dilute hydrochloric acid.

• Natural rubber is resistant to dilute mineral acids, alkalies, and salts, but susceptible to oxidizing media, oils, benzene, and ketones.

• Chloroprene or neoprene rubber is resistant to attack by ozone, sunlight, oils, gasoline, and aromatic or halogenated solvents.

• Styrene rubber has chemical resistance similar to natural.• Nitrile rubber is known for resistance to oils and solvents. • Butyl rubber’s resistance to dilute mineral acids and alkalies is

exceptional; resistance to concentrated acids, except nitric and sulfuric, is good.

• Silicone rubbers, also known as polysiloxanes, have outstanding resistance to high and low temperatures as well as against aliphatic solvents, oils, and greases.

• Chlorosulfonated polyethylene, known as hypalon, has outstanding resistance to ozone and oxidizing agents except fuming nitric and sulfuric acids. Oil resistance is good.

• Fluoroelastomers (Viton A, Kel-F) combine excellent chemical and high-temperature resistance.

• Polyvinyl chloride elastomer (Koroseal) was developed to overcome some of the limitations of natural and synthetic rubbers.

• It has excellent resistance to mineral acids and petroleum oils.

Plastics• In comparison with metallic materials, the use of plastics is limited to relatively

moderate temperatures and pressures (230°C is considered high for plastics).• Plastics are also less resistant to mechanical abuse and have high expansion rates,

low strengths (thermoplastics), and only fair resistance to solvents. • However, they are lightweight, are good thermal and electrical insulators, are easy

to fabricate and install, and have low friction factors.• Generally, plastics have excellent resistance to weak mineral acids and are

unaffected by inorganic salt solutions-areas where metals are not entirely suitable. Since plastics do not corrode in the electrochemical sense, they offer another

advantage over metals: most metals are affected by slight changes in pH, or minor impurities, or oxygen content, while plastics will remain resistant to these same changes.

• One of the most chemical-resistant plastics commercially available today is tetrafluoroethylene or TFE (Teflon).

• This thermoplastic is practically unaffected by all alkalies and acids except fluorine and chlorine gas at elevated temperatures and molten metals. It retains its properties up to 260°C.

• Chlorotrifluoroethylene or CFE (Kel-F) also possesses excellent corrosion resistance to almost all acids and alkalies up to 175°C. FEP, a copolymer of tetrafluoroethylene and hexafluoropropylene, has similar properties to TFE except that it is not recommended for continuous exposures at temperatures above 200°C.



















Wood

Fairly inert chemically, 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 low and high temperatures used in many industrial processes has

created some unusual problems in fabrication of equipment.• Some metals lose their ductility and impact strength at low temperatures,

although in many cases yield and tensile strengths increase as the temperature is decreased. It is important in low temperature applications to choose materials resistant to shock.

• Minimum Charpy value of 15 ft . Lbf is specified at the operating temperature. For severe loading, a value of 20 ft . lbf is recommended.

• Ductility tests are performed on notched specimens since smooth specimens usually show amazing ductility.

• Among the most important properties of materials at the other end of the temperature spectrum are creep, rupture, and short-time strengths.

• Stress rupture is another important consideration at high temperatures since it relates stress and time to produce rupture.

• Ferritic alloys are weaker than austenitic compositions, and in both groups molybdenum increases strength.

• Higher strengths are available in Inconel, cobalt-based Stellite 25, and iron-base A286.

• Other properties which become important at high temperatures include thermal conductivity, thermal expansion, ductility, alloy composition, and stability.

• Strength and mechanical properties become of secondary importance in process applications, compared with resistance to the corrosive surroundings.

• All common heat-resistant alloys form oxides when exposed to hot oxidizing environments.

• Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film.

• Thus, mild steel is seldom used above 500°C because of excessive scaling rates. Higher temperatures require chromium.

GASKET MATERIALS• Metallic and nonmetallic gaskets of many different

forms and compositions are used in industrial equipment.

• The choice of a gasket material depends on the corrosive action of the chemicals that may contact the gasket, the location of the gasket, and the type of gasket construction.

• Other factors of importance are the cost of the materials, pressure and temperature involved, and frequency of opening the joint.

SELECTION OF MATERIALS• Engineer responsible for the selection of materials of construction must have a thorough understanding of

all the basic process information available. • 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, special literature, general literature, availability, safety aspects,

preliminary laboratory tests 2. Laboratory testing Reevaluation of apparently suitable materials under process conditions 3. Interpretation of laboratory results and other data Effect of possible impurities, excess temperature, excess pressure, agitation,and presence of air in

equipment Avoidance of electrolysis Fabrication method 4. Economic comparison of apparently suitable materials Material and maintenance cost, probable life, cost of product degradation, liability to special hazards 5. Final selection

ECONOMICS IN SELECTION OF MATERIALS• First cost of equipment or material often is not a good economic criterion when

comparing alternate materials of construction for chemical process equipment. Any cost estimation should include the following items:1. Total equipment or materials costs2. Installation costs3. Maintenance costs4. Estimated life5. Replacement costs

FABRICATION OF EQUIPMENT• Fabrication expenses account for a large fraction of the purchased cost for equipment. An

engineer, therefore, should be acquainted with the methods for fabricating equipment, and the problems involved in the fabrication should be considered when equipment specifications are prepared.

• Many of the design and fabrication details for equipment are governed by various codes, such as the ASME Codes. These codes can be used to indicate definite specifications or tolerance limits without including a large amount of descriptive restrictions. For example, fastening requirements can often be indicated satisfactorily by merely stating that all welding should be in accordance with the ASME Code.

• The exact methods used for fabrication depend on the complexity and type of equipment being prepared. In general, however, the following steps are involved in the complete fabrication of major pieces of chemical equipment:

1. Layout of materials2. Cutting to correct dimensions3. Forming into desired shape4 . Fastening5 . Testing6 . Heat-treating7 . Finishing

Layout• Layout of the various components on the basis of detailed instructions prepared by the fabricator.• Flat pieces of the metal or other constructional material involved are marked to indicate where

cutting and forming are required. • Allowances losses caused by cutting, shrinkage due to welding, or deformation caused by the various

forming operations.• After the equipment starts to take shape, the location of various outlets and attachments will

become necessary. • If tolerances are critical, an exact layout, with adequate allowances for deformation, shrinkage, and

losses, is absolutely essential.

Cutting• Shearing is the cheapest method and is satisfactory for relatively thin sheets. • The edge resulting from a shearing operation may not be usable for welding, and the sheared edges

may require an additional grinding or machining treatment.

• Burning can be employed to cut and, simultaneously, prepare a beveled edge suitable for welding.

• Carbon steel is easily cut by an oxyacetylene flame - heat effects on the metal are less than those involved in welding.

• Stainless steels and nonferrous metals that do not oxidize readily can be cut by a method known as powder or flux burning.

• An oxyacetylene flame is used, and powdered iron is introduced into the cut to increase the amount of heat and improve the cutting characteristics.

• The high temperatures involved may affect the materials, resulting in the need for a final heat-treatment to restore corrosion resistance or removal of the heat-affected edges.

• Sawing can be used to cut metals that are in the form of flat sheets.• Sawing is expensive, and it is used only when the heat effects from burning would be

detrimental.

Forming• Accomplished by rolling, bending, pressing, bumping (i.e., pounding), or spinning on a die. • Heating may be necessary in order to carry out the forming operation. Because of work

hardening of the material, annealing may be required before forming and between stages during the forming.

• 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. When the fitting is complete and all edges are correctly aligned, the main seams can be tack-welded in preparation for the final fastening.

Fastening• Riveting can be used for fastening operations, but electric welding is far more common and

gives superior results. • The quality of a weld is very important, because the ability of equipment to withstand

pressure or corrosive conditions is often limited by the conditions along the welds. • Although good welds may be stronger than the metal that is fastened together, design

engineers usually assume a weld is not perfect and employ weld efficiencies of 80 to 95 percent in the design of pressure vessels.

• Manual shielded-arc process in which 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.

• Submerged-arc process 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 arc is submerged in a granulated flux which serves the same purpose as the coating on the rods in the shielded-arc process.

• Hefiurc welding is used for stainless steels and most of the nonferrous materials, can be carried out manually, automatically, or semiautomatically.

• 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. As in the shielded-arc and submerged-arc processes, 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.

Testing• All welded joints can be tested for concealed imperfections by X rays, and code

specifications usually require X-ray examination of main seams. • Hydrostatic tests can be conducted to locate leaks. Sometimes, delicate tests, such

as a helium probe test, are used to check for very small leaks. Heat-treating• After the preliminary testing and necessary repairs are completed, 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.

• A low-temperature treatment may be adequate, or the particular conditions may require a full anneal followed by a rapid quench.

Finishing• The finishing operation involves preparing the equipment for final shipment.• Sandblasting, polishing, and painting may be necessary. • Final pressure tests at 1.5 to 2 or more times the design pressure are conducted

together with other tests as demanded by the specified code or requested by the inspector.

Documents

Materials for construction