Chemical Engineering August 2023 - 40

Fluid Service
TABLE 3. EXPECTED RATE OF CORROSION (SS)
MoC
NaOH 50%
Carbon steel
(A53/A53M)
1
2
3
4
5
6
7
8
9
Compatibility at Fluid Composition and Temperature
A to 100% to 125ºF
A 100% to 250ºF
BC 100% 250¬-600ºF
A 100% 600-650ºF
NR 90-100% 750-800ºF
A to 80% to 175ºF
B/NR 70-90% 180-300ºF
B 60-99% 300-500ºF
A to 60% to 212ºF
10
11
12
strength at room temperature
Sµ avg = average actual tensile
strength of test specimens at room
temperature
Sµr = maximum tensile strength of
range of specification at room temp.
Having posited an approach
above in determining the strength
of our piping system at the end of
its lifecycle, we will now revisit the
manner in which the assigned CA
was determined for both the carbon-steel
materials and the alloysteel
materials.
Item " a " under Example 1 above,
points out that the corrosion rate
allotted for carbon steel material in
this discussion is based on 1 mil/yr.
It then goes on in item " d, " under
Example 1, to extrapolate that corrosion
rate out over a 20-year facility
life span as in: 20 × 0.001 in. =
0.020 in. The assumption here is to
conclude that, if you need to assign
a higher corrosion rate than 0.001
in./yr, then you may need to go to
an acceptable steel alloy, nonmetallic
material, or pipe lined with nonmetallic
material. But how do you
know how much of a corrosion rate
to assign to a particular fluid service
as compared to a specific MoC?
As mentioned
earlier
there are
published resources available that
can provide such data. In one such
publication, you will find the legend in
Table 1 to indicate rates of corrosion.
The symbols in Table 1 will indicate
in Tables 2 and 3 the expected
corrosion rate per year based on our
fluid service and the data from the
40
AB 40-70% 212-250ºF
A to 30% to 250ºF
NR 10-60% to 255-345ºF
sources mentioned earlier, namely,
fluid 50% NaOH, 70°F operating
temperature, 95°F design temperature,
80 psig operating pressure and
110 psig design pressure.
In checking the published data for
50% NaOH with a design temperature
of 95°F and a MoC of carbon
steel, we find the expected rate of
corrosion information given in Table
2. Refering to Table 2, we see that
the 50% NaOH solution at the 95°F
design temperature in carbon steel,
line item number 6 within the table
would indicate a possible corrosion
rate of between 0.002 to 0.020
in./yr. Following the old adage that
you should, " plan for the worse and
hope for the best, " we will assume a
corrosion rate of 0.020 in./yr, which
gives us an accumulated corrosion
loss of 0.400 in. at the end of the
20-year facility lifecycle. With that
rate of corrosion, 0.020 in./yr, it
would surpass both the manufacturing
allowance (0.035 in.) and the
specified corrosion allowance within
5 years, which is not even close to
the desired 20-year lifecycle.
I mentioned earlier that, " There are
outlier conditions that may also have
to be considered. " And at this point
I will include one. If you are heattracing
a piping system containing
50% NaOH in which the heat tracing
is set at somewhere between
250°F and 455°F, the heat tracing
will create a high-temperature zone
on the piping. In referring to Table 2,
we can see that once the pipe-wall
temperature exceeds 200°F with
piping that contains 50% NaOH, it
has a detrimental effect on the piping
material. Depending on how
much in excess of the 200°F the
heat tracing is operating at, pits will
begin to form. These corrosion pits
will penetrate the pipe wall within a
matter of a week or two and leak
the NaOH out to the environment.
This is intended to point out the fact
that there could be extenuating circumstances
to consider beyond the
rudimentary fluid compatibility at
design temperature.
The first consideration of carbon
steel was based on cost and the
possibility that it would be acceptable,
which in this case, it does
not. If we go to the next higher
cost value, we might look at a 304
stainless steel. It is one of the less
expensive steel alloys that might
very well do the job. In looking at
Table 3, 304 stainless steel is very
compatible with the 50% NaOH
fluid service at a design temperature
of 95°F. And the stainless material
remains compatible with 50%
NaOH at elevated temperatures up
to 212°F.
In this case, a schedule 10S 304
stainless steel would be a good selection,
from both a cost and compatibility
standpoint for the 50%
NaOH fluid service. But there are
other cost-wise options as well,
such as nonmetallic material. But
the point being made here is in the
selection process itself. And in selecting
a compatible MoC ensure
that you have identified any outlier
concerns, like the heat tracing in
this case, or make certain that there
are no additional concerns. A conversation
with the chemical engineer
would help along those lines.
Such a misstep could have catastrophic
implications weeks or even
years down the road - a misstep
that could be lethal.
In addition to the outlier concern
regarding heat tracing of an
otherwise ambient fluid temperature
described in the above discussion,
ASME B31.3 Appendix
F, paragraph F323.1 General Considerations,
points out a number
of other concerns that might be
considered when selecting piping
material, as follows:
* The possibility of exposure of
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