Tubular Inspection

Download Inspectioneering Journal (May-June 2007; Volume 13, #3) PDF

Although there are a variety of heat exchanger and boiler tube inspection technologies available to the industry, there is no single non-destructive testing technique that can be applied to inspect all of the various tube materials, design configurations and applications such as:

• Condenser Testing
• Fin Fan Tube Testing
• Twisted Tube Testing
• Boiler Tube Testing
• Reformer Tube Header Testing
• Defect Characterization

Selecting the NDT technique therefore becomes an important decision based on each different scenario as well as the defect types expected. With many years experience in this arena, TechCorr personnel are able to select the correct tool for each individual project.

Eddy Current
This technique is based on measuring the impedance of a coil as the electromagnetic field interacts with the material. The coil (probe) is inserted into the tube and pulled through at roughly 3 ft / second. The impedance changes relate to type and size of defects. Although this technique is fairly quick, it only works on non-ferromagnetic materials including but not limited to stainless steel, brass, copper-nickel alloys and titanium.

Remote Field Testing
This technique is based on the transmission of an electromagnetic field through the tube material. The eddy current works at low frequency in the circumferential direction. The electromagnetic field flows through the thickness and travels on the outer diameter. This magnetic field is measured and reported through the proprietary vendor software. The Remote Field Eddy Current Technique is effective when inspecting carbon steel tubes however wall loss sensitivity is limited where pitting is present. The technique is also has limitations to the speed at which the probe may be moved through the tubing.

Magnetic Flux Leakage
The MFL technique is based on creating a magnetic field using a coil type and hall element. These magnets saturate the tube and the magnetic field is then fluxed when defects are identified. The technique is limited to ferromagnetic materials and the pull through speed must remain constant. Severe anomalies can be missed if pull speed is not consistent since voltage output is directly proportional to the rate of flux change.

Internal Rotary Inspection Services
Probably one of the most accurate techniques for inspecting carbon steel tube bundles, this technique is limited as it requires the tubing to extremely clean. The technique is based on the principles of measuring thickness using ultrasonic waves generated by the IRIS probe through an ultrasonic transducer which is lined up in the center line of the tube along with a rotating mirror. The ultrasonic beam is reflected by a mirror which rotates circumferentially, inspecting the entire tube as it is pulled out. The method is extremely accurate for measuring wall thickness as well as detecting internal and external pits. However like every tool it has it’s limitations including difficulty to locate cracks or pin holes.