Chemical Engineering August 2023 - 39

foundational metrics for a project,
will be a durational period for which
the process facility is intended to
operate and the basis from which
operational cycles are calculated
and the length of time in service
for materials is established. Knowing
that length of time permits us
to determine, once we know the
corrosion rate of a material, how
much wall thickness material loss
will possibly occur over the life of a
piping system or a pressure vessel.
This gives the wall thickness that
will be required in order to allow for
the loss of material through corrosion
over the expected facility life
and still have sufficient strength in
the remaining wall thickness to retain
the specified internal pressure
at the end of a facility's lifecycle.
In essence, what needs to happen
in developing specifications for
a project, or for a corporate library
of procedures, specifications and
standards, is to give some forethought
with regard to corrosion allowance
(CA). This is an essential
element in the process of selecting
material. But in building a material
specification, it should be created
in a way in which any number of
conditions (fluid services versus
MoC) can be met. This means that
an attempt should be made in developing
a specification to allow it
to meet the needs of multiple fluid
services, if possible.
In this generalization we will look
at corrosion rates for carbon and
non-alloy steels for non-aggressive
fluids as well as stainless steels and
other alloys selected for use against
aggressive fluids, as follows:
Example 1.
a. Corrosion rate rule of thumb for
non-aggressive fluids (that is, cooling
tower water, chilled water, and
so on) in carbon-steel pipe is to
allow a corrosion rate of 1 mil/yr
(0.001 in./yr) of general corrosion.
This is a nominal value to allow for
the rate of corrosion for this general
type of material. If the rate of
corrosion exceeds this to a point in
which the pipe wall schedule has to
be increased, then you might need
to look at an alloy steel.
b. A corrosion rate for stainless
steels and higher alloys selected for
use against aggressive fluids (that
is, sulfuric acid, NaOH, hydrofluoric
acid and so on) is typically assessed
at 0.000 in.
c. Assume a design basis of a 20year
facility life cycle.
d. The amount of corrosion allotted
for carbon-steel material, based on
1 mil/yr over the 20-year life of the
facility would be 20 x 0.001 in. =
0.020 in. (total expected corrosion
over the life of the facility)
e. The amount of corrosion allotted
for alloy-steel material, over
the 20-year life of the facility would
be 0.000 in., but the specified CA
would, in some cases, be 0.030 in.
to include a margin of safety.
f. Corrosion seldom occurs evenly
throughout a system. It would
therefore be wise for maintenance
within the operating facility to periodically
check for localized corrosion
at specific locations, such
as impingement areas, possible
cavitation areas, downstream from
pressure-reducing valves (PRVs),
and so on.
Based on the suggested corrosion
rate expressed in item d
above, the CA typically applied
to carbon steel would be 0.050
in. And based on the suggested
corrosion rate expressed in item
e above, the CA typically applied
to steel alloys is 0.00 in. to 0.032
in. The reason behind those applied
CA values will be made apparent
after the following example
listing the essential elements of a
selected material:
Example 2.
a. 6 in. NPS A53 Gr B ERW carbonsteel
pipe
b. Sch. 40 has a wall thickness =
0.280 in.
c. Deduct manufacturers tolerance of
12.5% from specified wall thickness
(the manufacturing tolerance is specific
to each material specification)
1) Manufacturers tolerance 0.125
× 0.280 = 0.035 in.
d. Assigned CA value is deducted
from specified wall thickness:
1) CA typically used for carbon
steels = 0.050 in.
e. Total deductions from wall thickness
= 0.035 in. + 0.050 = 0.085 in.
f. 0.280 in. wall thickness. - 0.085
in. total deductions = 0.195 in. reCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
AUGUST 2023
maining wall thickness at the endof-life
cycle
Projecting a remaining wall thickness
of 0.195 in. at the end of the
calculated life span of a facility the
burst pressure, based on that remaining
wall thickness, would be
calculated using Equation (1):Equa%ons
http://WWW.CHEMENGONLINE.COM

Chemical Engineering August 2023

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

Chemical Engineering August 2023 - Intro
Chemical Engineering August 2023 - Cover1
Chemical Engineering August 2023 - Cover2
Chemical Engineering August 2023 - 1
Chemical Engineering August 2023 - 2
Chemical Engineering August 2023 - 3
Chemical Engineering August 2023 - 4
Chemical Engineering August 2023 - 5
Chemical Engineering August 2023 - 6
Chemical Engineering August 2023 - 7
Chemical Engineering August 2023 - 8
Chemical Engineering August 2023 - 9
Chemical Engineering August 2023 - 10
Chemical Engineering August 2023 - 11
Chemical Engineering August 2023 - 12
Chemical Engineering August 2023 - 13
Chemical Engineering August 2023 - 14
Chemical Engineering August 2023 - 15
Chemical Engineering August 2023 - 16
Chemical Engineering August 2023 - 17
Chemical Engineering August 2023 - 18
Chemical Engineering August 2023 - 19
Chemical Engineering August 2023 - 20
Chemical Engineering August 2023 - 21
Chemical Engineering August 2023 - 22
Chemical Engineering August 2023 - 23
Chemical Engineering August 2023 - 24
Chemical Engineering August 2023 - 25
Chemical Engineering August 2023 - 26
Chemical Engineering August 2023 - 27
Chemical Engineering August 2023 - 28
Chemical Engineering August 2023 - 29
Chemical Engineering August 2023 - 30
Chemical Engineering August 2023 - 31
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Chemical Engineering August 2023 - 33
Chemical Engineering August 2023 - 34
Chemical Engineering August 2023 - 35
Chemical Engineering August 2023 - 36
Chemical Engineering August 2023 - 37
Chemical Engineering August 2023 - 38
Chemical Engineering August 2023 - 39
Chemical Engineering August 2023 - 40
Chemical Engineering August 2023 - 41
Chemical Engineering August 2023 - 42
Chemical Engineering August 2023 - 43
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Chemical Engineering August 2023 - 51
Chemical Engineering August 2023 - 52
Chemical Engineering August 2023 - Cover3
Chemical Engineering August 2023 - Cover4
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