Smart owner operators know that solid asset integrity planning is essential to avoiding unplanned shutdowns. They also know how challenging such planning can be. Implementing regular maintenance cycles in ways that minimize disruption and maximize production often requires specialized software, several aspirin, and a high pressure tolerance.
This is where eddy current testing (ECT) can help. ECT is a non-destructive evaluation (NDE) method that can quickly assess the condition of conductive, non-ferrous materials and equipment for safer operations and more accurate integrity planning. Eddy current testing is not only insightful, it is also the fastest, most affordable, and least disruptive NDE method on the market.
What is an Eddy Current?
An eddy current is a rotating current induced in conductive materials with an electromagnetic coil or probe. Sending and receiving conductors in the probe alternate the magnetic current, allowing it to detect variances within the material. When the current encounters a variance, its amplitude fluctuates. This deviation in the current is recorded and measured using eddy current testing equipment and software. The software generates a report, often in real time, indicating abrasions, corrosion, cracks, metal loss and potential failure points.
Eddy current inspection is considered a no-contact NDE method because the probe does not have to physically touch the material to induce the electromagnetic current. This provides a clear advantage when inspecting rough surfaces, sensitive equipment, or components in hazardous or hard-to-access areas. It also requires less site preparation than other methods and no lubrication.
Applications of Eddy Current Testing
Capable of detecting flaws as small as half a millimeter, eddy current examination is designed for surface or near-surface assessments of non-ferrous materials like stainless steel, brass, certain copper-nickel alloys and titanium. It can also be used to
- assess the thickness and integrity of paint and other non-magnetic coatings on conductive materials
- map the flow of liquid through metal pipes
- assess heat treatment conditions
- detect cracks in thin welds
- aid in material identification and sorting
With such useful applications, ECT enjoys broad use across the industrial sector. However, it is particularly well-suited to tube inspections.
Inspecting Heat Exchanger Tubes with Eddy Current
Heat exchangers are the most critical vessels within industrial facilities, as they enable production processes and maximize efficiency. Because they are subjected to extreme internal stresses and temperatures, heat exchangers can accumulate damage quickly, particularly in the tube bundle. Heat exchanger tubes frequently develop
- fouling from sediment and scale deposits
- corrosion and erosion caused by turbulent flow
- fretting from vibration against baffles and tube rings
- metal loss from microbial-influenced corrosion (MIC)
- air pockets caused by improper venting
- warping, cracks or ruptures from thermal fatigue and pressure surges
Industrial heat exchangers can contain up to tens of thousands of tubes per bundle, all enclosed within an insulated steel shell. Rather than taking the exchanger off-stream and removing the bundle to examine the tubes, operators can simply depressurize the exchanger and remove the front-end head to expose the tube ends. Inspectors can then pass an eddy current probe along the length of each tube to detect any issues, including those lurking within U-bends.
While it may sound tedious, this evaluation process is surprisingly quick. Even on the largest heat exchangers, ECT inspections rarely take more than a full working day on site. It is also surprisingly low-cost. The depressurizing and cleaning of the exchanger is, by far, the most expensive part of the process.
Using Your ECT Report to Ensure Asset Integrity
Today’s cutting-edge ECT software provides a 3D, color-coded model of the tube bundle to indicate the condition of each tube. Areas that show significant defects should be addressed right away. Tubes with only minor fouling or corrosion should be cleaned and then monitored across future inspections. Heavily corroded or leaking tubes can be taken out of operation until the next turnaround maintenance cycle with a simple, mechanical plug, causing only a fractional loss in the exchanger’s overall efficiency.
The true value of an ECT report is twofold:
- Identifying issues before they become failures
- Planning effective inspection and maintenance cycles
Indeed, an ECT tube model can tell operators where to look for problems. For example, metal loss on the outside of the tube indicates baffle fretting, which can be resolved by reducing tube vibrations. Excessive warping signifies exceeded pressure or temperature limits. Heavy scale or corrosion points to potential leaking or chemistry problems. Deep, irregular pitting is a tell-tale sign of microbial-induced corrosion.
Every industrial facility has its own operational challenges. Some must regulate pressure and temperature within extremely tight margins, while others frequently experience variances in chemistry. Eddy current testing allows operators to predict when and where potential problems are likely to appear and monitor equipment conditions for smooth, proactive integrity management.
When to Test
To establish a baseline reading for the vessel, operators should order an eddy current test just before a new exchanger is placed into operation. A baseline reading will not only aid in assessing future corrosion, it will also identify any scratches, dents, or design flaws present within the vessel from production and assembly. These defects are prime locations for corrosion to take hold and may further indicate a poor match between the vessel’s design and intended use. Addressing any serious flaws in the material is also safer, easier, and far more cost effective when the vessel doesn’t require draining and cleaning.
After their initial examination, heat exchangers should have regular inspections and maintenance cycles. A risk-based inspection (RBI) can be performed on-stream and should be ordered whenever a decline in the vessel’s performance is noted.
RBI can be used to determine inspection intervals and the type and extent of future inspection/examinations. A RBI assessment determines risk by combining the probability and the consequence of equipment failure.[1]
The frequency of planned inspections will vary by industry and operating conditions. With proper oversight and maintenance, many heat exchangers can operate safely for 25 years or more. In some facilities, that means ordering an RBI inspection every few months; in others, every 1–5 years.
Eddy Current Testing with HMT
HMT’s certified NDE inspectors can help you determine the best inspection methods and frequency for your heat exchanger tubes. We have API-certified, ASNT Level III NDE inspectors specializing in eddy current testing, along with state-of-the-art ECT equipment to evaluate your tubes and vessels. We can even perform inspections remotely for greater cost savings. Contact us today to schedule a tube inspection or to receive expert guidance on integrity planning and maintenance
