Heat Exchanger Inspection Techniques and Corrosion Mechanisms
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.
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.
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.
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.
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.
Resources: http://mpgia.com/blog/en/tube-bundle-heat-exchanger/ http://ezinearticles.com/?Heat-Exchanger-InspectionTechniques-and-Corrosion-Mechanisms&id=8141615