Transcript Materials and fabrication selection Prof. Dr. Hassan Farag

Materials and fabrication
selection
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 lowcarbon 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 corrosionresistant 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 Alloys
 Nickel exhibits high corrosion resistance to most alkalies. Nickelclad 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 corrosion
due 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 hightemperature 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 heataffected 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.