ECH 158B
Separations and Unit Operations
Materials of Construction
Palazoglu
HUMG, June 2019
Selection of Materials
Success and failure of process equipment
and ultimately the plant operation depends
critically on the materials of construction.
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Selection of Materials
Failures may lead to catastrophic events..
http://www.exponent.com/petrochemical_industry/
http://www.csb.gov/
U.S. Chemical Safety and Hazard Investigation Board (CSB).
Selection of Materials
About 46% of total failures are
caused by corrosion, which is the
most common failure in petrochemical
industry. The second largest failures
in the number of occurrence are
fatigue failures, accounting for 16%.
Liu et al., “Failure Cases Analysis in Petrochemical Industry,” International Journal of Civil and
Environmental Engineering, 12(5), (2018).
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Selection of Materials
Trend is to operate at more severe T & P.
Critical operating scenarios.
How does one choose the “best” material?
Economics play a key role.
Material Ratio (cost/kg metal)/(cost/kg steel)
Flange quality steel 1
304 SS 5
Aluminum 6
Ni-clad steel 8
316 steel 10
Nickel 12
Hastelloy 15
Selection Guide
Economics
Total equipment or materials costs
Installation costs
Maintenance costs
Service life
Replacement cost
Downtime cost
Depreciation, taxes
Interest
Alternative Investment Comparisons
Inflation (remember ECH 158A)
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Selection Guide
Corrosion behavior (Table 10-8, PTW; Table 23.3,
TSBW)
Availability of standard sizes
Ease of fabrication
High and low temperature performance
Physical properties, e.g., thermal
conductivity
Mechanical properties, e.g., stiffness,
toughness, hardness, fatigue resistance
Definitions
Hardness is the measurement of how much a
material resists penetration from a semi-static
force. It is tested using an indenter hardness
machine often (but not solely) by measuring the
size of the indentation after releasing the load.
www.quora.com
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Definitions
The most well known hard material is diamond.
A typical soft material is aluminum, or any
plastic material.
Definitions
• Toughness is the ability of a material to
absorb energy when impacted.
• Materials known to be very tough are
stainless steels and titanium alloys. Materials
known to be very fragile (the opposite of
tough) are ceramics such as glasses or
porcelain.
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Definitions
Toughness usually goes in the opposite
direction of hardness. If a material is very hard,
it is usually very fragile. Diamonds are fragile
even though they are hard. Aluminum is tough
but not hard at all.
Definitions
Stiffness is the tendency of a material to
react with a small deformation when the
material is stressed. It is measured by the
Young's Modulus, which is the angular
coefficient, or slope, of the linear stress-strain
curve.
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Definitions
• Stiff materials are usually ceramics or high
melting point refractory metals such as
diamond, tungsten carbide and osmium. Soft
materials are plastics such as low density
polyethylene (LDPE) and nylon.
• Stiff materials are usually
hard (but there is no direct,
obvious correlation) and more
fragile.
Definitions
Fatigue failure is defined as the tendency of a
material to fracture by means of progressive
brittle cracking under repeated alternating or
cyclic stresses of an intensity considerably
below the normal strength.
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Summary
Materials
A chemical engineer should have a good
working knowledge of materials and their
properties so that better decisions can be
made at the design (and operation) stage.
Metals and Metal Alloys Non-metals
American Iron and Steel Inorganic and
Institute (AISI) has standards organic nonmetals
for steel products.
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Corrosion
Galvanic action resulting from the use of two
dissimilar metals that are in contact through a
conducting liquid can cause one of the metals
(least noble) to dissolve into the liquid and
deposit on the other metal (most noble).
Corrosion
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Corrosion
Galvanic corrosion is a localized mechanism by
which metals can be preferentially corroded.
This form or type of corrosion has the potential
to attack junctions of metals, or regions where
one construction metal is changed to another.
Corrosion
• T dependence
• concentration of agent
• impurities
• physical operation
How does one prevent it?
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Carbon Steel
It is a metal alloy of iron and
carbon. Other elements are
present in quantities too small to
affect its properties. Typically <2.1
w% C.
Used even in corrosive
applications due to low cost.
Strong acids form protective
coating.
Alloying (with Ni, Cr, Si, etc.) helps
increase corrosion resistance,
toughness, hardness.
Stainless Steels
More than 70 types.
Alloyed with Cr, Ni-Cr, Mo
(SS316)
Good machinability.
Corrosion resistance is
enhanced with surface
passivation.
Cast stainless steel alloys.
Table 10-4 in PTW gives
selection guidelines.
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Nickel and its Alloys
High corrosion resistance to
alkalies
Monel (67% Ni, 30% Cu)
often used in food industry
Inconel (77% Ni, 15% Cr)
Hastelloy C: 56% Ni, 17%
Mo, 16% Cr, 5% Fe, 4% Tu, ....
Valves, piping, heat-
exchanger tubing
Aluminum and its Alloys
• Lightness and ease of
fabrication make aluminum
an attractive construction
material.
• Resists acid attacks by a
passive aluminum oxide
layer.
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Aluminum and its Alloys
When alloyed with small amounts of other
metals such as Cu, Zn, Mg and with Si,
Aluminum becomes stronger (and can be
made even stronger than steel). For
example, Duralumin (Dural) is an alloy of Al,
Cu, Mn and Mg, with Al ca 94%.
Copper and its Alloys
Oxidizes easily
Readily fabricated,
and fairly inexpensive
Over 400 alloys
Some alloys show
better corrosion
resistance
https://copperalliance.org.uk/about-copper/copper-alloys/
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Copper and its Alloys
Alloys: brass (copper and zinc), bronze
(copper and tin), admiralty (copper, zinc
and tin)
Used in food industry / brewing
HEX tubes, high k
Low temperature use
Inorganic Nonmetals
Glass
Porcelain, stoneware
Brick, concrete
Low structural strength
Often used as linings
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Organic Nonmetals
• Plastics (e.g., PVC,
HDPE, TFE) have excellent
resistance to weak acids.
• Polyethylene is cheap.
• Carbon and Graphite –
inertness and favorable
heat transfer properties.
• Rubbers and elastomers –
vulcanized products offer
good chemical resistance
Organic Nonmetals
Vulcanization is a rubber curing process
involving high heat and addition of sulfur or
other equivalent curatives. It is a chemical
process in which polymer molecules are linked
to other polymer molecules by atomic bridges
composed of sulfur atoms or carbon to carbon
bonds.
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Organic Nonmetals
The result is that the springy rubber molecules
become cross-linked to a greater or lesser
extent. This makes the bulk material harder,
much more durable and also more resistant to
chemical attack. It also makes the material
surface smoother and prevents it from sticking
to metal.
Fabrication
Materials are fabricated, according to
specifications and related codes, into the
various equipment and parts shapes.
ASME Codes
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Fabrication
The layout of metal is the
procedure of measuring and
marking material for cutting,
drilling, or welding.
One uses shop drawings,
sketches, and blueprints to
obtain the measurements
required to fabricate the job
being laid out.
Fabrication
The next step is cutting. Metal is cut through
the use of specialized tools, including oxy-fuel
and plasma torches.
Other methods include sawing, shearing,
chiseling, and numerical control cutters like
lasers, mill bits and water jets.
http://www.fsm.co.uk/capabilities/sheet-metal-fabrication/
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Fabrication
Forming/bending follows the cutting step. It
provides the final shape of the part or the
equipment.
Main tool is the CNC (Computer Numerical
Control) machine.
https://www.wilkelaser.com/cnc-forming-fabrication-services.html
Fabrication
For the fastening/joining step, screws, rivets,
welding, adhesives, folding, are some of the
commonly used sheet metal parts joining
techniques. Selection depends on the
material, its thickness and application.
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Fabrication
Testing involves certified personnel carrying
out integrity tests on the fabricated material
using methods like ultrasonic, magnetic
particle, dye penetrant, radiography, and visual
inspection.
Fabrication
There are four heat treatment methods.
1. Hardening is performed to improve the hardness
of a metal, resulting in a tougher and more durable
product. The metal is heated above its critical
transformation temperature and then cooled quickly
via quenching. Hardening is often followed by
tempering of the metal to alleviate internal stresses.
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Fabrication
2. During Annealing, metals are heated to a specified
temperature and held there for a prolonged period of
time. They then undergo cooling, which can be fast or
slow, depending on the type of material and desired
characteristics. This process refines the grain
structure of the material, making it easier to work
with.
Fabrication
3. Quenching involves rapid cooling of a metal.
Ferrous alloys harden when quenched, while non-
ferrous alloys tend to become softer. Cooling can be
conducted using air or nitrogen. Most often liquids like
water or oil are used. Due to the rapid rate of cooling,
quenching builds up stresses which must be relieved
by tempering.
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Fabrication
4. Tempering alleviates the stresses built up in the
material during other heat treatment processes. Metals
are reheated at a low temperature to attain a desired
combination of strength, hardness and durability.
Tempering time and temperature are controlled to
produce the desired material properties.
Fabrication
Various finishing methods are available
depending on the desired surface properties:
• Black oxide finish
• Peening
• Plating
• Powder coating/painting
• Vibratory finishing
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What have we covered?
Wide variety of materials available.
Process conditions (T, flow behavior, etc.)
limit selection.
Economics determine viability.
Metals vs nonmetals.
Fabrication issues (material properties)
need to be considered.
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