Chemical Engineering March 2019 - 42

Cover Story
11
10.5
10
9.5
9
8.5
8
Time
FIGURE 3. Ultrasonic-thickness
(UT) measurement data
can be collected and historized
for analysis, just like any
other process variable
The answer depends on the nature of the
corrosive components of the process fluid,
the operating parameters, such as flowrate
or temperature, and the metallurgy of the
pipe and vessel walls. Obviously, the metallurgy
is fixed for all practical purposes, but in
most facilities, it is not consistent across different
areas and process units. There can be
a variety of alloys used in different areas that
have been installed at different times, and
in varying states of health, with respect to
pipe thickness.
Plant operators can work at forestalling
the effects of corrosion in a variety of ways,
including the following:
* Employing a corrosion allowance that
recognizes the inevitability of metal loss
and compensates by simply using thicker
pipe sections in strategic areas.
FIGURE 4. When wall-thickness
measurements are made
manually, there are often too
few data points to provide
useful statistical analysis
12
11.5
11
0.5 mm/yr
10.5
10
9.5
9
8.5
8
1-4-2012
42
14-8-2013
27-12-2014
Time
10-5-2016
22-9-2017
* Establishing safe operating windows to
slow the deterioration process. For example,
operators may have concluded
that metal loss with a specific grade of
crude oil increases significantly beyond
a specific temperature, so the process
UT
is adjusted to remain below the threshold.
Obviously, this requires a great deal
of knowledge about the characteristics
of the crude and its interaction with the
alloys, and it assumes the process will
still work effectively at the adjusted parameters.
Typically, the integrity operating
window is set using theoretical concepts,
which do not necessarily reflect reality.
* Material upgrades are a very effective approach.
Replacing carbon-steel pipe with
an appropriate stainless grade can extend
its life, but it is very costly and cannot
be done easily while a unit is running.
If the next turnaround is not for another
several years, a different approach will
be necessary.
* Corrosion inhibitors can be quite helpful,
but again, these have to be matched with
the characteristics of the process fluid and
the metallurgy to be effective. The additive
designed for the crude or process fluid
being handled currently might not work
with what is slated for next week. It also
adds cost, potentially offsetting the reason
for running the opportunity crude in the
first place.
* Inspections make sense, and can warn
of impending problems if carried out
correctly. However, they can be difficult
to perform, since the operator must access
specific points. Checking an elbow
where there is no catwalk near the top of
a tower that is clad in insulation is not an
easy task. Moreover, manual inspections
using a handheld ultrasonic measuring
device do not have very high repeatability
- remaining within ±1 mm is doing well in
most facilities. Furthermore, the need to
access each location every time a measurement
is desired means that, in practice,
measurements are only taken every
few years, which is not frequent enough
to detect major corrosion events in
many cases.
There is one strategy that can help
make all of these activities more effective,
and it begins with treating corrosion as a
process variable.
Measuring and monitoring metal loss
Process units in a petroleum refinery typically
have hundreds and even thousands of
process instruments measuring temperature,
flow, pressure, level and other attributes.
Operators can watch the readings
in real time, and these readings are usually
recorded in a historian system, so it is possible
to see what a specific value was at a
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2019
UT measurement, mm
UT measurement, mm
http://WWW.CHEMENGONLINE.COM
Chemical Engineering March 2019

Table of Contents for the Digital Edition of Chemical Engineering March 2019
