Eddy current is one of many different non-destructive testing (NDT) methods, utilizing electromagnetism to detect and characterize flaws in conductive materials. By feeding an alternating electrical current into the coils housed within the probe, an oscillating magnetic field is induced into the test material. This magnetic field generates eddy currents in the material itself, creating a magnetic field that opposes the one generated by the probe. When variations in the test material are encountered, the balance between these opposing fields is disrupted, and the resulting impedance change will be detected by the probe and presented as a signal on the instrument to be interpreted by the analyst. The signal's characteristics (phase angle, amplitude, etc.) can then be compared to known defective samples to determine the location, size, shape, and depth of flaws in the material to determine whether corrective action is required.
In chillers and other tube-and-shell heat exchangers, eddy current testing is a rapid, efficient, and accurate method to detect and characterize defects in heat exchanger tubing. The entire circumference of the tubes is inspected, including the outside diameter, allowing detection of defects that would otherwise be impossible to detect with other conventional testing methods. Because defects can also be characterized, operators can understand and hypothesize whether issues pertaining to the tubes' integrity are potentially related to operating procedures, water chemistry and treatment, or other causes. If necessary, individual tubes can be selected for further supplemental testing, such as video borescope inspection or metallurgical analysis, to determine the root causes of defects that developed while the tubes were in service.
When serious abnormalities are found in a tube bundle, they can be selectively replaced, or mechanically plugged off at both ends, effectively removing the tubes from service before they can fail. Some examples of expenses that can be avoided from preventing catastrophic failures include:
- Compressor burnout and electric motor rewind
- Contamination/loss of refrigerants in electric chillers
- Contamination of lithium bromide in absorption chillers
- Retubing and other vessel repair costs
- Unscheduled downtime and/or potential rental chiller costs while repairs are made
Eddy current analysis is highly dependent on the training and field experience of the analyst performing the inspection. The testing method is very effective in preventing tube failures when a qualified analyst can correctly collect and interpret data to provide accurate results. Seeking a well trained and experienced analyst is key when choosing your eddy current testing provider.
