A Look at Traditional NDE Inspection Methods While other NDE methods have been around for ages, they do not deliver the ideal inspection. The ideal should be fast, accurate, and easy to use. This has led to the accepted process of sampling. The most well-known inspection method for heat exchangers is eddy current testing (ET). In this method, a probe is physically pushed through each tube and pulled back. The probe contains one or more coils, whose electrical impedance is affected by the surrounding tube. Eddy current testing is relatively accurate, with the ability to inspect 30–60 tubes per hour, but it is heavily dependent on tube wall material, which is a twofold limitation: It cannot be used to inspect ferromagnetic materials at all, and requires a “calibration standard” for any tube it can be used on. Additionally, ET depends heavily on the subjective interpretation of the technician. A study conducted by MTI and EPRI, for example, showed that a skilled technician detected 87% of the faults in a heat exchanger mockup, whereas another technician detected only 50% of the faults on the same tubes. Another well-known NDE method is based on ultrasound. Ultrasound is a form of reflectometry: Sending a wave through a medium, and recording any reflections from discontinuities in the medium. Adapting this method to tube inspection is quite involved: Once more, a probe is invasively inserted down the tube. The probe creates an ultrasonic beam parallel to the tube axis, which hits a spinning 45-deg mirror. As the probe moves down the tube, it scans the tube wall in a spiral. This method is known as an internal rotating inspection system (IRIS). The IRIS method, although accurate, does have drawbacks. The need for good resolution dictates a narrow beam, which in turn entails a very slow pull rate if the spiral scan of the tube is to provide full coverage. Filling the tubes with water, without air bubbles, is messy and time consuming. An internal rotating inspection system also requires 26 Inspection Trends / January 2011 cleaning the tubes down to the metal, which is time consuming and costly, and must take place before inspection begins. Finally, the accuracy of IRIS reduces with the thickness of the tube walls, and it cannot be used below a thickness of 0.9 mm. Benefits of APR Following are some of the main benefits of APR, along with a comparison to other NDE methods. • It is not material dependent. Tube material does not affect APR. It works equally well on metal (ferromagnetic or not), graphite, composite, and plastic. This is in contrast to eddy current, which cannot work on ferromagnetic tubes. • It does not require calibration standards.With APR, reflections are detected by subtracting each measurement from a baseline that represents a faultless tube, so there is no need to calibrate on known faults. Eddy current testing requires a calibration standard, which is a sample tube with carefully manufactured defects, for every type of material, diameter, and wall thickness. • It provides straightforward analysis. It offers a straightforward method for detecting and sizing faults of tubes, comparing the peak reflections to sets of predetermined thresholds. Software is available to easily perform this comparison, and can scan through the measurements from hundreds of tubes in seconds, automating data analysis. Eddy current readings must be analyzed by a highly trained technician, a lengthier and more subjective process. • It enables full inspection of all types of tubes. Acoustic waves propagate easily through U-bends, coils, and spirals, which are often overlooked in testing or usage due to their historical difficulty to inspect with invasive methods. By enabling reliable and easy testing of these different tubes, APR is helping companies to opt for more energyefficient tubes in their systems. Summary Today’s emphasis on preserving the environment brings the importance of well-maintained tubes, integrity testing, and inspection to a new meaning. Industry should closely examine its energy consumption, efficiency, and increased need to quickly and efficiently maintain, inspect, and refurbish facilities. Acoustic pulse reflectometry technology is enabling both small and large enterprises to increase efficiency, safety, and inspection speed. By eliminating sampling, unnecessary and costly downtime of mission-critical equipment can be avoided. With APR, it is not necessary to judge the integrity of 100% of the tubes based on a choice selection of tested tubes. Instead, 100% of the tubes can be tested quickly and at less cost. DR. NOAM AMIR (noam@acousticeye.com) is chief technical officer, AcousticEye, Leusden, Netherlands. Want to be an Inspection Trends Advertiser? For information, contact Rob Saltzstein at (800) 443-9353, ext. 243, or via e-mail at salty@aws.org.
Inspection Trends - January 2011 - Winter
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