Chemical Engineering August 2023 - 38

Fluid Service
TABLE 2. EXPECTED RATE OF CORROSION (CS)
MoC
NaOH 50%
Carbon steel
(A53/A53M)
1
2
3
4
5
6
7
8
Working
across
Compatibility at Fluid Composition and Temperature
A/NR 50-100% at 70ºF
NR 50-100% at 100-150ºF
C/NR 50-100% at 150-180ºF
NR 25-100% at 190-800ºF
C/NR 30-50% at 180-200ºF
AB up to 50% to 150ºF
AB up to 22% to 220ºF
A to 2% to 205ºF
international
boundaries with specifications published
by organizations like those
listed above, can be both a little
daunting, confusing, and a little difficult
at times. There is no intended
direct correlation between the various
material specifications and
standards published around the
world. But in these many material
specifications, the chemistry is
typically not specified in exacting
amounts. The chemical compositions
are instead given as a weight
percentage with a maximum value
or a value range, not a specific target
amount. What this does is provide
an environment or situation in
which the chemistry in a specification
from one country can overlap
that of another country. But the
practicality of an exact chemistry
match occurring, in which all of the
chemistry
constituents
between
two specifications from two different
countries will match up, is
not going to happen. Transposing
specifications from those published
by two different countries requires
someone very familiar with the nuanced
effect that the various alloys
have on the chemistry that make
up a steel. This also applies to the
melting temperature, the cooling
process, and post treatment of the
metal manufacturing requirements.
Such determination in transposing
material specifications published by
one organization to that of another
requires an expertise in metallurgy.
Developing MoC specifications
With the wide and varied selection
of steels that are available in the
marketplace, how do you go about
determining which material to use
for a specific application and fluid
38
service? There are many more variables
that go into determining a
proper material for any number of
specific process applications than
are discussed here. But what we
can do is set the stage for a methodology
by providing examples of
things to be aware of. And I say
that because it is not merely a matter
of matching up a metallic material
to a process fluid service. There
are outlier conditions that may also
need to be considered. And you
will see why as we move through
this discussion.
Finding a compatible material
to contain a process fluid service
for piping or equipment is a paring
down process of elimination.
The following outlines key points in
a process that helps to arrive at a
suitable MoC:
1. Find general materials that are
reasonable and compatible candidates
for the applicable process
fluid service chemistry.
2. From the materials selected
in Step 1, further select materials
that can withstand the expected
design pressure and temperature
conditions.
3. From the materials selected in
Step 2, we now have to consider
what I referred to earlier as possible
" outlier conditions. "
4. Assess the cost.
For
Step
1 indicated
above,
what I mean by " reasonable " is to
disregard, right at the start, the
more costly materials, such as the
higher alloy materials. Begin with
the basic carbon steels such as
an ASTM A53 or A106 if they are a
good place to start from. The process
might then lead you to a possible
stainless steel, nickel
alloy,
pipe lined with nonmetallic material,
or nonmetallic material. So, in
order to work through the above
four steps in some meaningful way,
we will set up a sample fluid service
to make some necessary points in
our evaluation. The sample fluid
service will consist of the following
essential elements:
* Fluid: 50% NaOH
* Operating temperature: 70°F
(21°C)
* Design Temperature: 95°F
(35°C)
* Operating pressure: 80 psig
(5.5 barg)
* Design pressure: 110 psig
(7.6 barg)
Fluid service compatibility
The first step is to find compatible
material types for a specific process
fluid service. This will immediately
pare down the overly broad
field of possible candidate materials
to a smaller select group of materials.
To do this I would suggest
referring to sources that have compiled
test results data on the compatibility
of various steel materials
(as well as other metals) with that
of various fluids.
Such sources will provide the
rate of corrosion a material will
sustain while in contact with a specific
fluid at various temperatures.
They typically present the results
by indicating, in various ways, that
the material is acceptable, nominally
acceptable, poor, or not recommended
for application with
the specified fluid at a specified
temperature.
In selecting our material in this
regard, we need parameters upon
which we will make our decision in
selecting a suitable material. Those
parameters include the degree of
corrosion rate we can accept for
the intended service application. So
before starting through the selection
process I will define and explain
those parameters.
Project design basis
To begin with, the design basis of
a project is a procedure that lays
out the essential elements of designing
and constructing a process
facility. In it, along with many other
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
AUGUST 2023
http://WWW.CHEMENGONLINE.COM
Chemical Engineering August 2023

Table of Contents for the Digital Edition of Chemical Engineering August 2023
