Chemical Engineering March 2019 - 43

specific time in the past. This is valuable information in
trying to figure out the causes of upsets and improving
overall efficiency. Corrosion, quantified as actual metal
loss measured in mils (one mil is equal to 1/1,000
of an inch) or micrometers (µm), can be treated the
same way.
Wireless wall-thickness measurement sensors (Figure
2) can be installed throughout a unit on pipe and
vessel walls to monitor metal thickness continuously.
Data from these sensors can be transmitted over a
wireless network to automation systems and process
historians. Engineers can use these data to make decisions
in real time, and the data can also be historized
(Figure 3), just like data from a temperature or pressure
transmitter or sensor, and retrieved for analysis.
The ability to watch metal loss over time
can provide even deeper insight into the
process, and assist with other corrosionmitigation
strategies
Just as a pressure transmitter can warn when the
equipment has exceeded a safe value, a thickness
monitor can trigger an alarm when wall thickness has
reached a safety threshold. This is an important function,
but the ability to watch metal loss over time can
provide even deeper insight into the process, and assist
with other corrosion-mitigation strategies.
The variability of metal loss
With years of data collected using continuous wallthickness
monitoring in many facilities, one point has
emerged time and again: metal loss is not a linear
process. Experience has shown how it is common to
see 80% of the metal loss during 20% of the time, or
even 90% in 10% of the time. This accelerated metal
loss happens when the corrosive elements really dig
into the metal due to the presence of particularly aggressive
corrosive elements that are unaffected by the
inhibitors in use.
Metal loss is not a fast process, so observing it requires
time and precision. Permanently mounted wireless
ultrasonic-thickness sensors can achieve a repeatability
of 10 µm, which is the equivalent of 0.000394 in.,
so it is possible to detect very small changes over time.
It is also possible to identify the variability in the rate of
loss, a major improvement as compared to infrequent
periodic inspections.
A hypothetical example. A well-trained inspector has
taken two readings, one year apart, at exactly the same
place with a handheld ultrasonic thickness-measuring
device. Assuming that the device has a higher degree
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Chemical Engineering March 2019

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