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As market pressures force pipe and pipeline manufacturers to find ways to increase productivity while meeting strict quality standards, choosing the best control methods and support systems is more important than ever. While many tube and pipe manufacturers rely on final inspection, in many cases manufacturers test earlier in the manufacturing process to detect material or workmanship defects early. This not only reduces waste, but also reduces the costs associated with the disposal of defective material. This approach ultimately leads to higher profitability. For these reasons, adding a non-destructive testing (NDT) system to the plant makes good economic sense.

Many factors—material type, diameter, wall thickness, processing speed, and pipe welding or forming method—determine the best test. These factors also influence the choice of characteristics of the control method used.
Eddy current testing (ET) is used in many piping applications. This is a relatively inexpensive test that can be used in thin wall pipelines, typically up to 0.250 inch wall thickness. It is suitable for both magnetic and non-magnetic materials.
Sensors or test coils fall into two main categories: annular and tangential. Circumferential coils examine the entire cross section of the pipe, while tangential coils examine only the weld area.
Wrap spools detect defects across the entire incoming strip, not just the weld zone, and they are generally more effective at inspecting sizes under 2 inches in diameter. They are also tolerant of weld zone displacement. The main disadvantage is that passing the feed strip through the rolling mill requires extra steps and special care before it passes through the test rolls. Also, if the test coil is tight to the diameter, a bad weld can cause the tube to split, resulting in damage to the test coil.
Tangential turns inspect a small section of the circumference of the pipe. In large diameter applications, using tangential coils rather than twisted coils will often give a better signal-to-noise ratio (a measure of the strength of a test signal versus a static signal in the background). Tangential coils also do not require threads and are easier to calibrate out of the factory. The downside is that they only check the solder points. Suitable for large diameter pipes, they can also be used for smaller pipes if the welding position is well controlled.
Coils of any type can be tested for intermittent breaks. Defect checking, also known as zero checking or difference checking, continuously compares the weld to adjacent parts of the base metal and is sensitive to small changes caused by discontinuities. Ideal for detecting short defects such as pinholes or missing welds, which is the primary method used in most rolling mill applications.
The second test, the absolute method, finds the disadvantages of verbosity. This simplest form of ET requires the operator to electronically balance the system on good material. In addition to detecting coarse continuous changes, it also detects changes in wall thickness.
Using these two ET methods shouldn’t be particularly problematic. They can be used simultaneously with one test coil if the instrument is equipped to do so.
Finally, the physical location of the tester is critical. Properties such as ambient temperature and mill vibrations that are transmitted to the tube can affect placement. Placing the test coil next to the welding chamber gives the operator immediate information about the welding process. However, heat-resistant sensors or additional cooling may be required. Placing the test coil close to the end of the mill allows the detection of defects caused by sizing or shaping; however, the probability of false alarms is higher because the sensor is located closer to the cut-off system in this location, where it is more likely to detect vibrations when sawing or cutting.
Ultrasonic testing (UT) uses pulses of electrical energy and converts them into high frequency sound energy. These sound waves are transmitted to the material under test through a medium such as water or mill coolant. The sound is directional, the orientation of the transducer determines whether the system is looking for defects or measuring wall thickness. A set of transducers creates the contours of the welding zone. The ultrasonic method is not limited by the thickness of the pipe wall.
To use the UT process as a measurement tool, the operator needs to orient the transducer so that it is perpendicular to the pipe. Sound waves enter the outside diameter of the pipe, bounce off the inside diameter, and return to the transducer. The system measures transit time—the time it takes a sound wave to travel from the outside diameter to the inside diameter—and converts that time into a thickness measurement. Depending on mill conditions, this setting allows wall thickness measurements to be accurate to ± 0.001 in.
To detect material defects, the operator orients the sensor at an oblique angle. Sound waves enter from the outer diameter, travel to the inner diameter, are reflected back to the outer diameter, and thus travel along the wall. The unevenness of the weld causes the reflection of the sound wave; it returns the same way to the converter, which converts it back into electrical energy and creates a visual display indicating the location of the defect. The signal also passes through defect gates that trigger an alarm to notify the operator, or start a paint system that marks the location of the defect.
UT systems may use a single transducer (or multiple single element transducers) or a phased array of transducers.
Traditional UTs use one or more single element sensors. The number of probes depends on the expected defect length, line speed and other test requirements.
The phased array ultrasonic analyzer uses several transducer elements in a single housing. The control system electronically directs the sound waves to scan the weld area without changing the position of the transducer. The system can perform activities such as defect detection, wall thickness measurement, and tracking changes in flame cleaning of welded areas. These test and measurement modes can be performed substantially simultaneously. It is important to note that the phased array approach can tolerate some welding drift because the array can cover a larger area than traditional fixed position sensors.
The third non-destructive testing method, Magnetic Flux Leakage (MFL), is used to test large-diameter, thick-walled and magnetic pipes. It is well suited for oil and gas applications.
MFL uses a strong DC magnetic field passing through a pipe or pipe wall. The magnetic field strength approaches full saturation, or the point at which any increase in magnetizing force does not result in a significant increase in magnetic flux density. When magnetic flux collides with a defect in a material, the resulting distortion of the magnetic flux can cause it to fly or bubble off the surface.
Such air bubbles can be detected using a simple wire probe with a magnetic field. As with other magnetic sensing applications, the system requires relative motion between the material under test and the probe. This movement is achieved by rotating the magnet and probe assembly around the circumference of the pipe or pipe. To increase the processing speed in such installations, additional sensors (again, an array) or several arrays are used.
The rotating MFL block can detect longitudinal or transverse defects. The difference lies in the orientation of the magnetization structure and the design of the probe. In both cases, the signal filter handles the process of detecting defects and distinguishing between ID and OD locations.
MFL is similar to ET and they complement each other. ET is for products with wall thicknesses less than 0.250″ and MFL is for products with wall thicknesses greater than that.
One of the advantages of the MFL over the UT is its ability to detect non-ideal defects. For example, helical defects can be easily detected using MFL. Defects in this oblique orientation, although detectable by UT, require settings specific to the intended angle.
Want to know more about this topic? Manufacturers and the Manufacturers Association (FMA) have additional information. Authors Phil Meinzinger and William Hoffmann provide a full day of information and instructions on the principles, equipment options, setup, and use of these procedures. The meeting took place on November 10 at the FMA headquarters in Elgin, Illinois (near Chicago). Registration is open to virtual and in-person attendance. To learn more.
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