Heat exchangers suffer from problems within the tubes that make up most of the useful area within the exchanger unit. Two of the most common defects that are generally inspected are pitting and gradual wall loss.

There are many different corrosion mechanisms that can lead to gradual wall loss or pitting, and many contributing factors that can cause these mechanisms to occur. The product that follows through the unit is one of the main contributing factors, along with the flow rate and your daily operating conditions.

Often times, if an exchanger has been out of service or with standing water, there is the possibility of microbiologically induced corrosion (MIC). Usually the presence of water is required for this to happen, although once in effect it can be difficult to remove.

Another common corrosion mechanism is flow accelerated corrosion (FAC), which is caused by high flow rates that erode the protective oxide layer on the metal surface. The metal underneath rusts to restore this protective layer and begins a cycle that causes continuous loss.

Erosion corrosion and FAC are often used interchangeably due to the sometimes unclear mechanisms at play.

There are several techniques available for the inspection of heat exchanger tubes, mainly based on ultrasonic and electromagnetic techniques.

IRIS: perhaps the best of the bunch. IRIS stands for Internal Rotary Inspection Technique and relies on ultrasound to create a clear image of what is going on inside the tube. IRIS is a very accurate technique and the gold standard for exchangers that are critical or high risk. Although IRIS is an excellent technique, there are some drawbacks. The first is cleanliness. The inspection requires very clean tubes, otherwise the ultrasound wave is poorly coupled and nothing can be tested. The second is that their cost is generally higher than other methods, although it may depend on the exact nature of the work.

Eddy Currents – This is another great technique, this time using electromagnetism. The probe produces an electromagnetic wave and changes in the field represent changes in the material. Eddy current inspection is slightly less accurate than IRIS, although it is still very accurate and fast. The high-risk tubes can then be cleaned and followed with IRIS.

MagWave – This is another electromagnetism-based technique, it is very similar to eddy current, however it is designed to inspect ferromagnetic tubes and thin tubes, saturating the tube with a strong magnetic field.