Hydrocarbon Processing - November 2021 - 43

Maintenance
and Reliability
W. FAZACKERLEY, Emerson, London, UK
Continuous corrosion monitoring improves
process optimization
Corrosion is one of the most insidiIntrusive
corrosion probes consist of
ous challenges in the oil refining industry.
When it is uncovered too late or a slow leak
has begun, it can lead to catastrophic outcomes.
However, monitoring corrosion
requires resources and vigilance, as well
as the right analytical tools to determine
when and where a problem might arise.
The issue of corrosion has become
more critical in recent years as refineries
opt for less expensive opportunity crudes
to meet forecast production and financial
margins. Many of these cheaper feedstocks
tend to be more corrosive, which
further solidifies the demand for a comprehensive
monitoring program.
Finding the right solution to meet
safety standards for assets and personnel,
while maintaining budgets, requires an
in-depth look at both existing tools and
new options on the market. In addition,
finding that midpoint between corrosion
risk, process data and impact on the plant
can provide a root cause analysis from
which to build a suitable preventative
maintenance process.
In many legacy systems, traditional
methods of corrosion monitoring are used
that do not necessarily provide the data
to best plan production and asset maintenance.
However, more and more facilities
are reexamining those methods as they
consider and implement more efficient
and digitally accessible solutions (FIG. 1).
Traditional corrosion monitoring. Although
several types of instrumentation
have traditionally been used in corrosion
monitoring in oil refineries, two are most
commonly found: intrusive corrosion [or
electrical resistance (ER)] probes and
manual ultrasonic inspection.
an intrusive element with a sacrificial tip
that sits in the process fluid and is (normally)
made from the same material as
the surrounding equipment. As the sacrificial
tip corrodes, its electrical resistivity
changes, which is recorded externally;
these are also increasingly available for
wireless connection. The corrosion of
the sacrificial tip is used to infer the level
of corrosion being experienced by the
surrounding equipment.
Alternatively, manual ultrasonic inspection
is an equally well-established
technique for measuring metal wall thickness.
This technique involves placing a
transducer directly onto the metal surface
and generating ultrasound, which
is transmitted through the metal until it
is reflected off the inside metal surface
(back wall). The reflected ultrasound
signal is recorded, and the time difference
between the sent and reflected signals
provides the measurement of the
wall thickness. While the technique can
be reliable, the completion of a full set of
measurements for a medium-sized refinery
with 80,000-plus corrosion measurement
points is time consuming and labor
intensive, such that the wall thickness at
an individual location may be measured
only every 3 yr-5 yr. This is far from ideal
when monitoring critical pipelines.
In addition, manual ultrasound has
poor repeatability because it is unlikely
that the same technician will measure
at the exact same location that was measured
before. The fluid flowing through
the pipes also cannot be too hot, or it may
damage the transducer or injure the technician
taking the measurement.
A better way to measure. Ultrasound
might be an effective way to measure
FIG. 1. Modern plant operators face a variety of challenges, requiring a tightrope balance
to meeting set operational standards.
Hydrocarbon Processing | NOVEMBER 2021 43

Hydrocarbon Processing - November 2021

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