Hydrocarbon Processing - January 2021 - 72

Maintenance and Reliability
point calculations. Note: The time windows are arranged so that the measured
events occur at the same spot on the
graph. Each technology advancement
reduces the duration above the target
corrosion rate by improving response to
uncontrolled events. Although these are
specific examples from one system over

FIG. 4. Corrosion event.

FIG. 5. Salting potential analysis.

FIG. 6. Safely increasing distillate yield.

72 JANUARY 2021 | HydrocarbonProcessing.com

time, they are representative of general
improvements seen across multiple systems. The results, again, are lower overall
system corrosion, longer equipment life
and improved production yields.
Dynamic vs. fixed safety margin.
The industry typically seeks to operate

the atmospheric distillation tower upper
section in such a way as to prevent a salt
formation point from occurring inside
the tower. The common method used is
to calculate the salt formation temperature of the system and control the tower
top operating temperature at a fixed safety margin that is higher than that of the
salt formation temperature. Each refiner
decides what safety margin to use, but
generally a 25°F-40°F difference is specified. Based on the number of systems that
experience premature failures and short
equipment life, and the overall cost of
corrosion, this approach does not appear
adequate to effectively prevent ongoing
corrosion, especially the type driven by
transient events.
The issues surrounding the use of a
fixed-temperature safety margin to mitigate ongoing corrosion are two-fold. First,
calculating salt formation temperatures at
fixed intervals leaves gaps that often do
not catch episodic events. Second, using a
fixed margin of safety on a dynamically oscillating parameter leads to control inefficiencies. When the safety margin is insufficient to protect against an uncontrolled
event, corrosion occurs. To illustrate, FIG. 4
shows a system always operating above the
targeted safety temperature margin of 25°F
(red line), but the data inside the red box
identifies a clear corrosion event indicated
by a temporary rise in both corrosion rates
(purple square markers) and iron in the
boot water (green markers). Conversely,
when a system is well controlled and salt
formation temperatures are sufficiently
low, the safety margin becomes larger than
necessary to avoid corrosion, and the reduced yield optimization flexibility results
in operational giveaway.
Together, the advantages of the technology developments described in the
aforementioned sections with parallel
advancements in data handling and quantitative, risk-based analytics have opened
up a new path. They have allowed for
movement beyond fixed-margin temperature targets to dynamic determination
of system corrosion potential and use of
current, safe operating margins. A key
foundation has been the development
of computational engines to process the
large amount of incoming data, evaluate sensitivities and impacts, and reduce
potential actions to a structured priority
list based on a quantitative evaluation of
risk and reward. FIG. 5 charts the over-

http://www.HydrocarbonProcessing.com

Hydrocarbon Processing - January 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - January 2021