Chemical Engineering February 2016 - 49

Material selection factors
Functionality in the operating environment
is generally a high priority
for material selection of components,
especially when it comes to
the long- and short-term safety and
reliability of the equipment. The following
sections introduce some important
factors that must be considered
in material selection for small,
complex components.
Corrosion resistance. Corrosion is
a major concern that dictates material
selection in many operating environments.
For example, chloride
ions cause stress corrosion cracking
(SCC) in stainless steels, so a material
like titanium is selected because
of its chloride-resistant properties.
These factors are critical because
corrosion in equipment can result in
a number of negative effects, including
fires, explosions and both brittle
and mechanical failures, as well as
the release of hazardous gases, liquids
or vapors.
oxidation is commonly observed
in materials like the stainless-steel
alloys. These metals naturally form
oxide layers for corrosion protection,
which can cause uneven surfaces.
This occurrence requires passivation
treatment to reduce the negative
effects of these oxide layers and
to keep the surfaces smooth and
free of imperfections that can cause
equipment failure.
Exposure to chemicals. Materials
can have adverse reactions to
chemicals in various environments.
Environmental chemical factors can
include high acidity and the presence
of oxygen or aqueous solutions, or
even harsh cleaning or sanitizing
agents. Although the production
environment may be non-corrosive,
the presence of these chemicals can
cause materials to react, so a designer
should consider the risks when
selecting materials.
Temperature resistance. The engineer
should be aware that the mechanical
limits of materials can be
negatively affected by high temperatures,
potentially causing thermal
failure or deformation. Materials are
also selected based on temperature
resistance because the effect
of extreme temperatures on materials
can cause increased corrosion
rates. High-temperature materials include
iron-, nickel- and cobalt-based
metal alloys.
Cost. A major factor in the materialsselection
decision typically involves
the initial cost of the material, the
ease of machining and repair and the
availability of the material. Cost influences
the balance between materials
and machining in the case of material
grades. For example, if an engineer
chooses Inconel as the material
for the product, the grade affects the
machining cost. Since Inconel 600 is
much easier to machine than Inconel
718, the time and cost of machining
can change considerably depending
on which grade is chosen. Figure 1
shows some threaded parts that are
constructed of Inconel 718. However,
note that cost is usually secondary to
the other considerations that involve
consistent and reliable operation of
the part in its application.
Restrictive applications. In most
CPI applications, the selection of the
appropriate material can usually prevent
the negative effects associated
with the operating environment, but
some applications are more restrictive.
due to strict operating-environment
protocols, designing parts for
certain applications, such as nuclear
processing, can be limiting when it
comes to choosing a material. Areas
at risk for fire or explosions, such
as oil-and-gas processing facilities,
are also restrictive when considering
equipment material. Certification
systems, such as the International
Electrotechnical Commission's
(IEC; geneva, Switzerland; www.
iec.ch) globally recognized System
for Certification to Standards Relating
to Equipment for use in Explosive
Atmospheres (IECEx System;
www.iecex.com), may require the
provision of specific material properties
to improve equipment and
personnel safety.
The most common class of materials
chosen for chemical processing
equipment is metal, because metal
materials are typically stronger, more
resistant to fatigue and cracking,
and are typically easier to machine
than other material classes, such as
glasses, polymers and ceramics.
ChemiCal engineering www.Chemengonline.Com february 2016
FIGURE 2. Threaded parts must be able to withstand
a number of harsh conditions in CPI applications,
including exposure to corrosive materials,
high temperatures and pressures and other environmental
restrictions
For small threaded parts (such as
threaded valve connectors) that play
a significant role in the performance
and longevity of chemical processing
equipment, metals are most often
selected for these properties. However,
in some precision equipment,
such as gears, ceramics and other
hard materials are corrosion-resistant
and can achieve equal or better
surface finishes than some metals.
other non-metallic materials, such
as high-performance polymers, are
used in place of metals to reduce the
weight of equipment.
Threaded parts and valves
Threaded parts (Figure 2) and valves
are used in processing equipment in
countless CPI applications, but can
often be overlooked. An understanding
of these complex parts gives engineers
insight into the materials-selection
and machining processes.
Fasteners and threads. Fasteners
and threaded parts can be the
smallest parts in chemical processing
equipment, but are often critical
to the assembly of larger equipment.
These components must be strong,
durable and corrosion-resistant to
ensure that equipment performs
safely and reliably. As mentioned, exotic
metals, including stainless-steel
alloys, titanium and Inconel, which
have greater resistance to harsh
chemicals and can endure very high
temperatures, are examples of desirable
materials for fasteners used in
heat exchangers, exhaust systems,
tanks, vessels and other processing
equipment. Mechanical properties,
such as high strength-to-weight
ratios, will also lead a designer to
choose an exotic metal.
Valves. Valves are highly involved
in chemical processing equipment,
such as the piping systems that
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Chemical Engineering February 2016

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