As market pressures force tube manufacturers to find ways to increase productivity while adhering to strict quality standards, choosing the best inspection method and support system is more important than ever.While many tube producers rely on final inspection, in many cases manufacturers use testing further upstream in the manufacturing process to detect defective materials or processes early.Not only does this reduce scrap, but it also reduces costs associated with handling defective materials.This approach ultimately translates into higher profitability.For these reasons, adding a non-destructive testing (NDT) system to a factory makes good economic sense.
Many factors—material type, diameter, wall thickness, process speed and method of welding or forming the tube—determine the best test.These factors also influence the choice of features in the inspection method used.
Eddy Current Testing (ET) is used in many pipe applications.This is a relatively low cost test and can be used in thin wall pipe applications, typically up to 0.250 inch wall thickness.It is suitable for magnetic and non-magnetic materials.
Sensors or test coils fall into two basic categories: wraparound and tangential.Encircling coils inspect the entire cross-section of the tube, while tangential coils inspect only the welded area.
Wrap-around coils detect defects in the entire incoming strip, not just the weld zone, and they tend to be more effective when testing sizes smaller than 2 inches in diameter.They are also tolerant of pad drift.A major disadvantage is that passing the incoming strip through the mill requires extra steps and extra care to pass it through the test coil.Also, if the test coil is a tight fit to the diameter, a failed weld can cause the tube to pop open, damaging the test coil.
Tangent coils examine a small portion of the circumference of the tube.In large diameter applications, using tangential coils rather than wraparound coils generally yields a better signal-to-noise ratio (a measure of the strength of the test signal relative to a static signal in the background).Tangent coils also do not require threads and are easier to calibrate outside the mill.The downside is that they only check the weld zone.It is suitable for large diameter pipes and can be used for small sizes if the weld position is well controlled.
Either coil type can test for intermittent discontinuities.Defect testing, also known as void or discrepancy testing, continuously compares the weld to an adjacent portion of the base metal and is sensitive to small changes caused by discontinuities.Ideal for detecting short defects such as pinholes or jump welds, the primary method used in most rolling mill applications.
The second test, the absolute method, found verbose flaws.This simplest form of ET requires the operator to electronically balance the system on good materials.In addition to finding general, continuous changes, it also detects changes in wall thickness.
Using these two ET methods does not have to be particularly troublesome.If the instrument is equipped, they can be used simultaneously with a single test coil.
Finally, the physical location of the tester is critical.Characteristics such as ambient temperature and mill vibration (transmitted to the tube) can affect placement.Placing the test coil close to the solder box gives the operator immediate information about the soldering process.However, temperature-resistant sensors or additional cooling may be required.Placing the test coil close to the end of the mill can detect defects introduced by the sizing or shaping process; however, there is a greater chance of false positives because this location brings the sensor closer to the cut-off system, where it is more likely to detect Vibration during sawing or shearing.
Ultrasonic testing (UT) uses pulses of electrical energy and converts it into high frequency sound energy.These sound waves are transmitted to the material under test through media such as water or mill coolant.Sound is directional; the orientation of the sensor determines whether the system is looking for defects or measuring wall thickness.A set of transducers can create the outline of the weld zone.The UT method is not limited by tube wall thickness.
To use the UT process as a measurement tool, the operator needs to orient the transducer so that it is perpendicular to the tube.Sound waves enter the OD to the tube, bounce off the ID, and return to the transducer.The system measures time-of-flight — the time it takes for a sound wave to travel from OD to ID — and converts the time into a thickness measurement.Depending on mill conditions, this setup can measure wall thickness with an accuracy of ± 0.001 inches.
To spot material defects, the operator positions the transducer at an oblique angle.Sound waves enter from the OD, travel to the ID, reflect back to the OD, and travel along the wall that way.The welding discontinuity causes the sound wave to reflect; it takes the same path back to the sensor, which converts it back into electrical energy and creates a visual display that indicates the location of the defect.The signal also passes through the defect gate, which either triggers an alarm to notify the operator or triggers a paint system that marks the location of the defect.
UT systems can use a single transducer (or multiple single crystal transducers) or phased array transducers.
Traditional UTs use one or more single crystal transducers.The number of sensors depends on the expected defect length, line speed and other test requirements.
Phased array UTs use multiple transducer elements in a body.The control system electronically controls the sound waves without repositioning the transducer elements to scan the weld area.The system can perform a variety of activities, such as detecting defects, measuring wall thickness, and monitoring changes in weld zone cleaning.These inspection and measurement modes can be performed substantially simultaneously.Importantly, the phased-array approach can tolerate some welding drift because the array can cover a larger area than traditional fixed-position sensors.
A third NDT method, Magnetic Leakage (MFL), is used to inspect large diameter, thick walled, magnetic grade pipes.It is ideal for oil and gas applications.
MFLs use a strong DC magnetic field that passes through a tube or tube wall.The magnetic field strength approaches full saturation, or the point at which any increase in the magnetizing force does not result in a significant increase in the magnetic flux density.When magnetic field lines encounter a defect in the material, the resulting distortion of the magnetic flux can cause it to emanate or bubble from the surface.
A simple wire-wound probe passed through a magnetic field can detect such bubbles.As is the case with other magnetic induction 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 tube or pipe.To increase processing speed, this setup uses additional probes (again one array) or multiple arrays.
The rotating MFL unit can detect longitudinal or transverse defects.The differences lie in the orientation of the magnetizing structures and the probe design.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 the two complement each other.ET is suitable for products with wall thicknesses less than 0.250 inch, while MFL is used for products with wall thicknesses greater than this.
One advantage of MFL over UT is its ability to detect less-than-ideal defects.For example, MFL can easily detect helical defects.Defects in such oblique directions can be detected by UT, but require specific settings for the expected angle.
Interested in more information on this topic?The Manufacturers and Manufacturers Association (FMA) has more.Authors Phil Meinczinger and William Hoffmann will provide a full day of information and guidance on the principles, equipment options, setup and use of these processes.The meeting was held on November 10 at FMA’s headquarters in Elgin, Illinois (near Chicago).Registration is open for virtual and in-person attendance.Learn more.
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