NDT Insight - Heat Transfer Solutions
The Lab at Brookes Bell
A state-of-the-art laboratory providing world-class forensic analysis and investigative services.
Critical inspection of non-ferrous heat exchanger tubing
Eddy current testing?(ECT) of tubes was developed as a technique to monitor the condition of tube bundles throughout the life of a heat exchanger unit.?When the tube bundle is in service, periodic inspection is performed to guarantee working life compliance, efficiency and to detect any in-service defects which can compromise the exchangers fitness for service.
A variety of defects can manifest themselves throughout the equipment’s working life such as:
During the inspection of tube bundles, there are several challenges an NDT operator faces, making inspections intricate and complex.?Eddy current test results can be greatly influenced, by the heat exchangers structural components. ?
Features such as tube sheets, support and baffle plates are often made of dissimilar materials to the tubes themselves and therefore have varying permeability (magnetic properties) and conductivity (electrical properties), thus effecting change in the incoming signals the NDT operator will examine.??
Defects inherently form in these areas, owing to geometry and flow characteristics and therefore eddy current signals must be constantly monitored by the tube inspector to ensure accurate results, whilst all the time accounting for spurious signals. The image below is a flow simulation illustrating the turbulence inside a system.
In the heat transfer industry, there are some commonly faced challenges in the inspection and testing process.?For example, test results can vary widely from company to company providing the NDT services, this is partly down to the interpretation skill level of the inspector but also down to the experience and competency of the inspection company as a whole and the reliability of test equipment they use. ?Obtaining reliable eddy-current results requires engaging experienced inspectors with superior data analysis and evaluation skills and equipment that facilitates this
An Inspection Scenario
A straight tube exchanger, featuring a double pass condenser with 1000+ tubes made from cupronickel alloy.?Saltwater flows though the tube side in an open loop and the shell side contains a Freon refrigerant, saltwater creates a high chloride environment which accelerates corrosion at the tubes inside diameter (ID) surface.?The protective 6” plastic insert sleeves, pictured in yellow at the inlet end, are specifically fitted to defend against in-service degradation.?Whilst they are a restriction to the ECT test, they do stop any damage inflicted in these areas.?These can indicate a history of problematic inlet end flow erosion that the heat exchanger has encountered through its service life.
Below, is an in-situ setup using an 128CH Ectane 2 and Magnifi analysis suite:
The Solution
To handle this heat exchanger inspection, The Lab at Brookes Bell’s Tube Inspectors use an Eddy Current Array (ECA) probe, specifically a ‘High-definition Multiplexed ECA or DefHi probe’.?The probe is selected for its sensitivity and resolving power in both circumferential and axial orientations along with its rugged long-lasting design.?The DefHi probe has the capability to detect even the hardest to find defects.?
The set up uses a phenomenon called electronic ‘multiplexing’ to leverage, via timeslots, the physical inputs of the ECA test instrument to accommodate up to 128 ECT ‘channels’ at once, that’s 128 signals all collecting and feeding back information about the component under inspection.?
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The physical process consists of multiple copper pancake coils energising simultaneously, their range of radio frequency signals are combined into one signal over a shared medium or channel by a technique known as modulation, then the signals can be separated back into their original forms in the equipment’s processor for acute analysis using demodulation – for it to work this electronic sequence is performed extremely quickly at rate of milliseconds. The images below show the probe topography and also a simplified version of the multiplexing technique of array signals.?
The DefHi probe does away with many of the downsides associated with conventional tube inspection techniques.?It allows you to detect and size circumferential cracks, a major limitation of conventional ‘ECT Bobbin’ probes used in the past.?Cracks are harmful incipient defects which, if undetected, can cause cross contamination, serious loss in efficiency and worse, downtime.?The DefHi probes also allow for rapid scanning of up to 2-3 feet per second, cutting inspection times and making periodic inspections prudently efficient, without any loss of accuracy.
What can we do with the data?
The data generated during the inspection can be visualised in a ‘C-scan’ or ‘plan view’ as illustrated in the image below.?This advanced capability reduces defect sizing and characterisation constraints through the visual representation of the tubes.?Rather than depending on just a signal waveform, our operators have a much more detailed view to assist with analysis.
Eddy current array technology offers a uniform, high-definition sensitivity to identify defects in any orientation, enabling our operators to discriminate between various complex geometries inside heat exchanger tubing.?The C-scan images can be viewed in both 2D or 3D and we use colour palettes to quickly distinguish between defects.
Another attribute of ‘multi frequency’ ECT inspection is the ability to ‘mix’ and ‘filter’ signals obtained from different frequencies.?What this enables us to do, is to reduce unwanted interference produced by the tube sheets, supports and baffle plates mentioned earlier.?The output of this action is greater phase (angle) separation on the voltage plane (top right box in the above image).?Simplified this means full coverage, being able to clearly discriminate between defects on the outside of the tube with no loss of sensitivity to defects on the inside of the tube.
Electromagnetic waveforms naturally rotate in a helical pattern over time through 360°, signal processing in the Ectane equipment allows a waveform to be decomposed into a linear combination of sinusoids (oscillations) for different frequencies, this way the array coils in an ECA probe provide phase data (the angle) on top of voltage amplitude information for each response i.e. high amplitude gives length to the signal shape in the display as seen in Figure 8 above.?A defects magnitude and depth can then be interpreted in comparison with a calibration tube having defects of a known depth and size, the phase component can be rotated in C-scans to get a defect response of the highest possible definition. ?In real-world testing conditions, ECA probes are capable of detecting and sizing circumferential cracks at the tube sheet as early as 50% wall loss and 30% in the free span area ensuring problems are noticed as early as possible.
3D Modelling and Reporting
Once the inspection is complete, the data is collated in our reporting software.?TubePro transposes the analysed data into meaningful results, by way of highly accurate 2D/3D tube mapping along with complete defect type and sizing reports.?Once parameters are set, the intuitive software communicates with the Magnifi acquisition and analysis suite used to scan the tubes, building the heat exchanger model in real time as interpretation is performed. ?The scans can be encoded for positional data, and heat exchanger tube maps and models can be generated and compared periodically for trend analysis giving asset owners more confidence than ever before in the results of a test.
The Takeaway
When we compare second generation Eddy Current Array (ECA) tube inspection probes and techniques to previous generations utilising ECT bobbin probes, ECA probes enable far superior inspection power, sensitivity and accuracy. We can differentiate easily the various complex geometries inside non-ferromagnetic alloy heat exchangers, enabling precise detection, sizing, and characterising of small-volume circumferential cracks and axial cracks along with wall loss, fretting and pitting.?This move forward, in the inspection of non-ferrous tube bundles, marks a significant improvement in reliability and efficiency of in-service heat transfer systems.?
Arron Jackaman (Technical Lead for NDT services at The Lab)