2. Heat exchangers suffer problems within the tubes that make up
the majority of the useful area within the exchanger unit. Two
of the most common defects that are generally inspected for
are pitting and gradual wall loss.
There are many different corrosion mechanisms that can lead to
either gradual wall loss or pitting, and many contributing
factors that can cause these mechanisms to occur. The
product following through the unit is one of the major
contributing factors, along with the flow rate and its day to day
operating conditions.
3. Often if an exchanger has been out of service, or with standing
water, the potential for microbiologically induced corrosion
(MIC) is present. The presence of water is generally required
for this to occur, though once in effect, can be hard to
remove.
Another common corrosion mechanism is flow accelerated
corrosion (FAC), which is caused by high flow rates eroding
the protective oxide layer from the surface of the metal. The
metal beneath oxidises in order to restore this protective
layer, and a cycle begins, causing continual loss.
4. Erosion corrosion and FAC are often used interchangeably due
to the sometimes unclear mechanisms at play.
Several techniques are available for the inspection of the heat
exchanger tubes, primarily based on ultrasonic and
electromagnetic techniques.
5. IRIS - Perhaps the best of the bunch. IRIS stands for Internal
Rotary Inspection Technique, and relies on ultrasonics to
build a clear picture of what's going on inside the tube. IRIS is
a very precise technique and the go to method for exchangers
that are critical or high risk. Although IRIS is an excellent
technique, there are a couple of drawbacks. The first is
cleanliness. Inspection requires very clean tubes, otherwise
the ultrasound wave has a poor coupling and nothing can be
tested. The second is its cost is generally higher than other
methods, though can depend on the exact nature of the job.
6. Eddy Current - This is another great technique, this time using
electromagnetics. The probe produces an electromagnetic
wave, and changes in the field represent changes in the
material. Eddy Current inspection is slightly less precise than
IRIS, though still highly accurate, and fast. High risk tubes can
then be cleaned and followed up with IRIS.
MagWave - This is another technique based on
electromagnetics, it's very similar to eddy current, however it's
designed to inspect ferromagnetic tubing and thin tubes, by
saturating the tube with a strong magnetic field.
