Materials and fabricationselection

Prof. Dr. Hassan Farag

Introduction

Any engineering design, particularly for a chemical process plant, is only useful when it can be translated into reality by using available materials and fabrication methods.

Thus, selection of materials of construction combined with the appropriate techniques of fabrication can play a vital role in the success or failure of a new chemical plant or in the improvement of an existing facility.

Materials of construction

As chemical process plants turn to higher temperatures and flow rates to boost yields and throughputs, selection of construction materials takes on added importance.

This trend to more severe operating conditions forces the chemical engineer to search for more dependable, more corrosion-resistant materials of construction for these process plants, because all these severe conditions intensify corrosive action.

Fortunately, a broad range of materials is now available for corrosive service.

However, this apparent abundance of materials also complicates the task of choosing the “best” material because, in many cases, a number of alloys and plastics will have sufficient corrosion resistance for a particular application.

Final choice cannot be based simply on choosing a suitable material from a corrosion table but must be based on a sound economic analysis of competing materials.

Metals

Materials of construction may be divided into the two general classifications of metals and nonmetds. Pure metals and metallic alloys are included under the first classification.

Iron and Steel

Although many materials have greater corrosion resistance than iron and steel, cost aspects favor the use of iron and steel. As a result, they are often used as materials of construction when it is known that some corrosion will occur.

If this is done, the presence of iron salts and discoloration in the product can be expected, and periodic replacement of the equipment should be anticipated.

In general, cast iron and carbon steel exhibit about the same corrosion resistance.

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.

Stainless Steels:

There are more than 100 different types of stainless steels. These materials are high chromium or high nickel-chromium alloys of iron containing small amounts of other essential constituents.

They have excellent corrosion-resistance and heat-resistance properties. The most common stainless steels, such as type 302 or type 304, contain approximately 18 percent chromium and 8 percent nickel, and are designated as 18-8 stainless steels.

The addition of molybdenum to the alloy, as in type 316, increases the corrosion resistance and high-temperature strength.

If nickel is not included, the low-temperature brittleness of the material is increased and the ductility and pit-type corrosion resistance are reduced.

The presence of chromium in the alloy gives resistance to oxidizing agents. Thus, type 430, which contains chromium but no nickel or molybdenum, exhibits excellent corrosion resistance to nitric acid and other oxidizing agents.

The types of stainless steel included in the 300 series are hardenable only by cold-working; those included in the 400 series are either nonhardenable or hardenable by heat-treating.

As an example, type 410, containing 12 percent chromium and no nickel, can be heat-treated for hardening and has good mechanical properties when heat-treated. It is often used as a material of construction for bubble caps, turbine blades, or other items that require special fabrication.

Stainless steels exhibit the best resistance to corrosion when the surface is oxidized to a passive state. This condition can be obtained, at least temporarily, by a so-called “passivation” operation in which the surface is treated with nitric acid and then rinsed with water.

Localized corrosion can occur at places where foreign material collects, such as in scratches, crevices, or corners. C&sequently, mars or scratches should be avoided, and the equipment design should specify a minimum of sharp comers, seams, and joints.

Stainless steels show great susceptibility to stress corrosion cracking. As one example, stress plus contact with small concentrations of halides can result in failure of the metal wall.

The high temperatures involved in welding stainless steel may cause precipitation of chromium carbide at the grain boundary, resulting in decreased corrosion resistance along the weld. The chances of this occurring can be minimized by using low-carbon stainless steels or by controlled annealing.

Hastelloy:The beneficial effects of nickel, chromium, and molybdenum are combined in Hastelloy C to give an expensive but highly corrosion-resistant material.

A typical analysis of this alloy shows 56 percent nickel, 17 percent molybdenum, 16 percent chromium, 5 percent iron, and 4 percent tungsten, with manganese, silicon, carbon, phosphorus, and sulfur making up the balance.

Hastelloy C is used where structural strength and good corrosion resistance are necessary under conditions of high temperatures. The material can be machined and is readily fabricated.

It is used in the form of valves, piping, heat exchangers, and various types of vessels. Other types of Hastelloys are also available for use under special corrosive conditions.

Copper and its Alloys:Copper is relatively inexpensive, possesses fair mechanical strength, and can be fabricated easily into a wide variety of shapes. Although it shows little tendency to dissolve in nonoxidizing acids, it is readily susceptible to oxidation.

Copper is resistant to atmospheric moisture or oxygen because a protective coating composed primarily of copper oxide is formed on the surface. The oxide, however, is soluble in most acids, and thus copper is not a suitable material of construction when it must contact any acid in the presence of oxygen or oxidizing agents. Copper exhibits good corrosion resistance to strong alkalies, with the exception of ammonium hydroxide.

At room temperature it can handle sodium and potassium hydroxide of all concentrations. It resists most organic solvents and aqueous solutions of organic acids

Copper alloys, such as brass, bronze, admiralty, and Muntz metals, can exhibit better corrosion resistance and better mechanical properties than pure copper.

In general, high-zinc alloys should not be used with acids or alkalies owing to the possibility of dezincification. Most of the low-zinc alloys are resistant to hot dilute alkalies

Nickel and its AlloysNickel exhibits high corrosion resistance to most alkalies. Nickel-clad steel is used extensively for equipment in the production of caustic soda and alkalies. The strength and hardness of nickel is almost as great as that of carbon steel, and the metal can be fabricated easily.

In general, oxidizing conditions promote the corrosion of nickel, and reducing conditions retard it. Monel, an alloy of nickel containing 67 percent nickel and 30 percent copper, is often used in the food industries.

This alloy is stronger than nickel and has better corrosion-resistance properties than either copper or nickel. Another important nickel alloy is Inconel (77 percent nickel and 15 percent chromium). The presence of chromium in this alloy increases its resistance to oxidizing conditions.

Aluminum:

The lightness and relative ease of fabrication of aluminum and its alloys are factors favoring the use of these materials.

Aluminum resists attack by acids because a surface film of inert hydrated aluminum oxide is formed.

This film adheres to the surface and offers good protection unless materials which can remove the oxide, such as halogen acids or alkalies, are present.

Lead: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, or tellurium.

Lead-clad equipment is in common use in many chemical plants. The excellent corrosion-resistance properties of lead are caused by the formation of protective surface coatings. If the coating is one of the highly insoluble lead salts, such as sulfate, carbonate, or phosphate, good corrosion resistance is obtained.

Little protection is offered, however, if the coating is a soluble salt, such as nitrate, acetate, or chloride. As a result, lead shows good resistance to sufuric acid and phosphoric acid, but it is susceptible to attack by acetic acid and nitric acid.

Galvanic Action between Two Dissimilar Metals

When two dissimilar metals are used in the construction of equipment containing a conducting fluid in contact with both metals, an electric potential may be set up between the two metals.

The resulting galvanic action can cause one of the metals to dissolve into the conducting fluid and deposit on the other metal.

As an example, if a piece of copper equipment containing a solution of sodium chloride in water is connected to an iron pipe, electrolysis can occur between the iron and copper, causing high rates of corrosion.

As indicated in Table 6,iron is higher in the

electromotive series than copper, and the iron pipe will

gradually dissolve and deposit on the copper. The farther apart the two metals

are in the electromotive series, the greater is the

possible extent of corrosiondue to electrolysis.

NONMETALS:

Glass, carbon, stoneware, brick, rubber, plastics, and wood are common examples of nonmetals used as materials of construction. Many of the nonmetals have low structural strength.

Consequently, they are often used in the form of linings or coatings bonded to metal supports.

For example, glass-lined or rubber-lined equipment has many applications in the chemical industries.

Glass and Glassed Steel:

Glass has excellent resistance and is subject to attack only by hydrofluoric acid and hot alkaline solutions. It is particularly suitable for processes which have critical contamination levels.

A chief drawback is its brittleness and damage by thermal shock. On the other hand, glassed steel combines the corrosion resistance of glass with the working strength of steel.

Nucerite is 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.

Rubber and Elastomers:

Natural and synthetic rubbers are used as linings or as structural components for equipment in the chemical industries. 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 oxidizing media, oils, benzene, and ketones will attack it. 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 resistantto 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. Also, FEP can be extruded on conventional extrusion equipment, while TFE parts must be made by complicated “powdered-metallurgy” techniques.

Polyethylene is the lowest-cost plastic commercially available. Mechanical properties are generally poor, particularly above 50°C and pipe must be fully supported.

Carbon-filled grades are resistant to sunlight and weathering.

Low- and high temperature materials

The extremes of low and high temperatures used in many recent chemical processes has created some unusual problems in fabrication of equipment.

For example, 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

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.

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

The chemical 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:

In making an economic comparison, the engineer is often faced with the question of where to use high-cost claddings or coatings over relatively cheap base materials such as steel or wood.

For example, a column requiring an expensive alloy-steel surface in contact with the process fluid may be constructed of the alloy itself or with a cladding of the alloy on the inside of carbon-steel structural material.

Other examples of commercial coatings for chemical process equipment include baked ceramic or glass coatings, flamesprayed metal, hard rubber, and many organic plastics.

The durability of coatings is sometimes questionable, particularly where abrasion and mechanical-wear conditions exist.

As a general rule, if there is little economic incentive between a coated type versus a completely homogeneous material, a selection should favor the latter material, mainly on the basis of better mechanical stability.

When these factors are considered, cost comparisons bear little resemblance to first costs.

Table 11 presents a typical analysis of comparative costs for alternative materials when based on return on investment. One difficulty with such a comparison is the uncertainty associated with “estimated life.” Well-designed laboratory and plant tests can at least give order-of-magnitude estimates. Another difficulty arises in estimating the annual maintenance cost.

This can only be predicted from previous experience with the specific materials. Table 11 could be extended by the use of continuous compounding interest methods as outlined in Chaps. 7 and 10 to show the value of money to a company above which (or below which) material A would be selected over B, B

Fabrication of equipment

Fabrication expenses account for a large fraction of the purchased cost for equipment.

A chemical 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, such as tanks, autoclaves, reactors, towers, and heat exchangers:

• Cutting:

Several methods can be used for cutting the laid-out materials to the correct size. 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 is often used for cutting metals.

This method can be employed to cut and, simultaneously, prepare a beveled edge suitable for welding. Carbon steel is easily cut by an oxyacetylene flame.

The 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 ~7u.x 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. However, sawing is expensive, and it is used only when the heat effects from burning would be detrimental.

• Forming:

After the constructional materials have been cut, the next step is to form them into the desired shape. This can be accomplished by various methods, such as by rolling, bending, pressing, bumping (i.e., pounding), or spinning on a die.

In some cases, 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.

The most common type of welding is the 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. A special coating is provided on the electrode.

This coating supplies a flux to float out impurities from the molten metal and also serves to protect the metal from surrounding air until the metal has solidified and cooled below red heat.

The type of electrode and coating is determined by the particular materials and conditions that are involved in the welding operation

A 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. The appearance and quality of the submerged-arc weld is better than that obtained by an ordinary shielded-arc manual process; however, the automatic process is limited in its applications to main seams or similar long-run operations.

• Hefiurc welding:

is used for stainless steels and most of the nonferrous materials. This process 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. In some cases, fastening can be accomplished by use of various solders, such as brazing solder (mp, 840 to 905°C) containing about 50 percent each of copper and zinc; silver solders (mp, 650 to 870°C) containing silver, copper, and zinc; or ordinary solder (mp, 220°C) containing 50 percent each of tin and lead

Screw threads, packings, gaskets, and other mechanical methods are also used for fastening various parts of equipment.

• 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 1/1.5 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.