It is possible to find heat exchangers (HX) in a wide variety of industrial applications. They are critical to operation and shutting them down is often associated to costly productivity drops.
Heat exchangers are not usually susceptible to defects in their early years. They often begin to suffer increasing failure rates into their fifth to tenth year of service. The tubes inside heat exchangers rest on steel supports (support plates) about 6.4–12.7 mm (0.25 –0.50 in) thick, spaced along their entire length, and tubesheets at both ends. Tubes expand and contract from temperature changes, resulting in erosion. They also vibrate against support plates as fluids flow through them at high velocities (up to 427 m/min or 1400 ft/min), generating circumferential cracking. The localized stress from the tube-to-tubesheet rolling process or swaging also causes circumferential cracking. Tubes inside heat exchangers do not suffer from the same degree of wear, however. As a matter of fact, some may not even show any signs of erosion. Nevertheless, worn down tubes compromise the integrity and reliability of the asset.
Chemical deterioration such as corrosion is equally damaging. It etches support plates, widening the gap between supports and tubes, allowing more vibration. In chiller sections, coolant boiling off the tubes causes outer diameter (OD) pitting. In condensers and other heat exchangers, the physical accumulation of lime, ferrous deposits, and other elements causes corrosion cells that results in inner diameter (ID) pitting. This chemical deterioration can continue until it punch holes through tubes.
As part of asset integrity management (AIM) programs, various non-destructive testing techniques are used to inspect heat exchanger tubes. Some of the most common are:
- Eddy current testing (ECT)
- Remote-field testing (RFT)
- Near-field testing (NFT)
- Partial saturation eddy current (PSEC)
- Magnetic flux leakage (MFL)
- Internal rotating inspection system (IRIS)
They all have advantages and disadvantages, one of the most common for electromagnetic testing techniques being their inability to accurately detect defects under support plates and near tubesheets, which, if left undetected, can lead to contamination, overall lower performance, and, ultimately, a disappointing bottom line.
However, the most widely used shell-and-tube heat exchanger inspection technique is, by far, eddy current testing because it is fast, relatively cheap, and fairly successful in detecting most types of defects found in tubes. But the technique’s high sensitivity to liftoff and ferromagnetic materials, as well as its inability to detect circumferential defects close to support plates and tubesheets contribute to premature tube plugging and not plugging tubes that should be, resulting in lower performance. Furthermore, an industry rule of thumb is that plugging 10–12% of heat exchanger tubes can lead to replacing the entire tube bundle because a higher plug rate involves overall performance drops that prove very costly.
A better inspection solution is therefore necessary to more reliably detect common defects in tubes and circumferential cracking at the tubesheets and support plates.
Developments in eddy current technology have led to the birth of eddy current array (ECA). Applied to tubing inspection, ECA takes the form of the patented DefHi® probe. This probe leverages the power of ECA’s multiplexed elements to sweep the entire inner surface of tubes and accurately detect, size, and characterize common defects in heat exchanger tubing, including circumferential cracking at the support plates and tubesheets.
The coil configuration inside the DefHi probe and their multiplexing pattern enable eddy currents to flow perpendicular to circumferential cracks in tubes. This makes the defects easier to detect and to establish their circumferential length and position—something ECT probes are incapable of. The distance between the transmitter and receiver coils of the probe is also optimized so tube swage signals are flat and easier to isolate. It is also very simple to select an optimal operating frequency for the non-ferromagnetic material of the tubes that generates clearly distinct support plate and tubesheet signals. The multiplexed array coils yield absolute signal responses with amplitude and phase data. In a C-scan, the ability to rotate a signal’s phase component is key to getting high-definition defect responses. The DefHi probe’s design also includes conventional ECT probe data.
Actual testing has demonstrated that the DefHi probe can detect and size circumferential cracks at the tubesheet as early as 50 % wall loss and 30 % in the span area between support plates (blue lines in the above C-scan).
Let’s say you need to inspect a hypothetical tube bundle more than eight years old comprised of 1000–1500, 6 m (20 ft) tubes with eddy current testing (ECT), where you would most likely miss most circumferential cracking, either plugging too many tubes or leaving tubes unplugged that would need to be. So doing, you stand to face unscheduled shutdowns and potentially lose hundreds of thousands of dollars.
So, while more expensive, ECA yields more accurate inspection results, which translate into fewer unscheduled shutdowns, overall higher HX efficiency over a longer period of time, and potentially hundreds of thousands of dollars saved.
The capacity of an ECA probe such as DefHi to discriminate various complex geometries inside heat exchanger tubing allows accurately detecting, sizing, and characterizing small-volume circumferential cracks. This leads to more efficient tube plugging, wiser AIM of HX, and, ultimately, a higher bottom line.
Other benefits of using this probe in inspecting shell-and-tube heat exchangers are:
- It’s fast. Scanning tube ends takes 5 seconds/tube. The probe can also be used at a maximum speed of 1 m/s (40 in/s) for full tube length examinations.
- It can be used to monitor defect evolution. The probe’s ECT bobbin coil can be used as baseline comparison point for previously acquired ECT data.
- Seeing is believing. People without extensive ECT data analysis experience can use C-scans generated with ECA to understand inspection results.