Many advances in Non-Destructive Examination (NDE) have occurred in recent years. Some of these are becoming common in typical industry applications and are slowly migrating their way into niche industries, such as high-pressure applications. These advanced NDE techniques include the use of Linear Phased Array (LPA) ultrasonic examination for volumetric examination and Eddy Current Array (ECA) technology for surface examination. Advancements in ultrasonic Guided Wave Testing (GWT) also show promise for specific applications involving long tubes, such as in tubular LDPE reactors.
Complete periodic assessment of a high pressure vessel’s condition is key to safe, long-term reliable operation. Structural Integrity provides a comprehensive inspection program that analyzes high pressure equipment and identifies critical areas where a potential failure mode may exist. Using advanced NDE techniques, we are able to overcome some common challenges found in high-pressure equipment, like access issues of small diameter deep bores, large and thick section components, weld overlays and examination of thick section welds, complex geometries, and the requirement to detect small crack sizes due to equipment design and materials used.
ADVANCED NDE APPROACHES
Many traditional NDE methods, such as liquid penetrant testing (PT), magnetic particle testing (MT), eddy current testing (ET), radiographic testing (RT) and conventional single-element ultrasonic testing (UT) can be replaced or improved by using an advanced NDE approach.
Eddy Current Array (ECA)
The eddy current array inspection approach provides a surface, or near-surface, inspection of electrically conductive materials, such as stainless steel, and is therefore well suited for inspection of many high-pressure equipment materials. ECA provides many advantages over other surface NDE techniques such as PT and MT. Some of these advantages include increased speed of inspection, digital data storage for a permanent record, depth sizing, no chemical waste, and ECA can be done remotely using an automated scanner.
Eddy current inspection is an NDE method that utilizes the principle of electromagnetism, specifically electromagnetic induction. When an alternating electric current is applied to a conductor, such as a copper wire (coil or probe), a magnetic field develops in and around the coil. When this coil is brought close to a conductive material, such as stainless steel, the coil’s changing magnetic field generates current flow in the conductive material. The induced current flows in closed loops called eddy currents. Changes in the flow of these eddy currents, like disruption by a flaw, can be detected and quantified on the eddy current instrument display.
Eddy current array technology provides the ability to electronically drive multiple eddy current coils, which are placed side by side as an array in the same probe assembly. Each individual eddy current coil in the probe produces a signal relative to the phase and amplitude of the structure below it. This data is referenced to an encoded position and time, and can be represented graphically as a C-Scan image. The ECA probe can be designed to be flat or contoured to fit a specific geometry (Figure 1). Some probes are sold as flexible arrays that can fit multiple contours. The size, frequency, and amount of coils in the array probe will be dependent on inspection requirements like material type, critical flaw size, and part geometry. The capability of the eddy current array acquisition system will also dictate the amount of coils available for inspection.
Compared to conventional single-coil eddy current technology, eddy current array technology drastically reduces inspection time because a large area can be scanned in a single probe pass while maintaining high resolution. ECA also reduces the complexity of mechanical and robotic scanning systems required to inspect a specific surface geometry or surface area. When used with an encoded scanning system, it provides real-time C-Scan image of the inspected region. This facilitates data interpretation, and improves flaw detection, sizing, and probability of detection.1 Encoded eddy current array technology also allows for a permanent record of inspection data that can be referred to for future inspections.
Typical for flaw detection, a reference standard calibration block is needed in order to normalize the individual coils of the eddy current array probe and setup a comparison of known flaw sizes. Figure 2 shows an ECA C-Scan display next to the impedance plane display, or Lissajous, and under that is a strip chart display. An image of a radiused reference standard can also be seen in Figure 2. The C-Scan display shows a top down view of the encoded scan area with colors representing signal amplitude in volts. Further analysis of any position on the C-Scan display can be done using the impedance plane (Lissajous) or strip chart displays. Signals from individual coils can be analyzed to determine accurate measurements of voltage amplitude and phase angle in order to categorize and quantitatively size indications.
Linear Phased Array (LPA) Ultrasonics
The linear phased array inspection approach provides a volumetric inspection of materials and is therefore well suited for inspection of potential cracking on many high-pressure components. LPA ultrasonic technology utilizes an array transducer (probe) that contains multiple transducer elements, as opposed to the single element of conventional pulse-echo transducers. Each element of an array probe can be utilized as a transmitter and/or receiver, and when each transducer element is pulsed sequentially with small, precise timing delays imposed one to the next, ultrasonic beam steering and focusing can be varied and controlled. A linear array consists of a number of linear elements arranged in a single row, or in a two-dimensional pattern. Array probes are available in a variety of shapes, sizes, number of elements and frequencies. All these parameters are important in determining the steering and focusing capabilities of the probe.2
Linear phased array technology provides the ability, by proper phasing, to steer the ultrasonic beam through a series of different angles covering a sector typically over a range of 60º, depending on wave mode and array parameters. The linear array is used primarily to influence beam direction, electronically focus the beam, or a combination of the two. A true spatial representation of the linear array data requires that the data be presented in polar coordinates. The amplitudes of the waveforms, plotted sequentially at each digitization point along each waveform, are typically presented in colors so the presentation provides instant recognition of the position of a reflector as well as its significance in terms of reflection amplitude. These plots have become known as sectorial scans, or S-scans, because they represent sectors of the cross-section of the component in the plane of the beam.
One advantage of linear phased array technology includes inspection of small indications over long metal sound paths. A feasibility study was performed using a 32 element, 5 MHz linear array probe (commonly referred to as 5L32), which allowed for focusing of the ultrasonic sound beam to over 9.5 inches deep (12.4 inch sound path distance) in a low-alloy carbon steel mock-up block. Figure 3 shows a beam simulation of this 5L32 transducer at a 40º refracted shear wave angle. As can be seen, the greatest amount of sound energy is focused near 6.5 inches deep in the part, however the -6dB focal zone stretches to over 9.5 inches deep. This focusing capability allows for excellent sensitivity and detection of a 0.04 inch wire EDM notch located 8.25 inches deep in the mock-up block. The focusing capability of the 5L32 also provides clear detection from a 0.04 inch wire EDM notch that is at a depth of 12.75 inches (18.0 inch sound path) from the probe inspection position, as seen in Figure 4.
An advantage of using these advanced NDE techniques is the ability to successfully detect and size small flaws in areas where traditional NDE techniques would be limited. One example of this would be the inspection of threads in pressure vessel closures. Figure 5 is an example of a buttress thread setup, similar to what may be used in a vessel closure. Traditional methods for crack detection at the thread roots would commonly be a surface exam using either PT or MT. These methods typically work well, but they offer no depth sizing information. The flaw depth sizing advantage of LPA can be observed in Figure 5. Artificial defects are shown to be easily detected and sized between 0.010 inch and 0.020 inch deep in expected cracking locations.
Guided Wave Testing (GWT)
Long-range guided wave testing of piping has been successfully used in the energy industry for over 10 years to inspect long lengths of piping for corrosion and other damage. Considered a screening technology, guided waves are capable of detecting changes in acoustic impedance, which is affected by variations in local material properties, changes in stiffness, and by changes in the cross-sectional area (CSA) of the pipe. When inspecting for corrosion or circumferential cracking, variations in CSA are the primary cause of indications.
The primary advantage of GWT is its ability to inspect long sections of pipe from a single sensing position. Ultrasonic guided waves utilize lower frequency activation to generate waves that are guided by the boundary of the structure and capable of traveling great distances.3 Figure 6 shows a still-frame from a finite-element model showing the propagation of a guided wave down the length of a pipe. Other advantages include 100% volumetric coverage of the inspected length, the ability to inspect inaccessible piping, and axial and circumferential location and extent classification capabilities.
Guided wave testing has much potential for the inspection of tubular high-pressure polyethylene reactors with external cooling jackets. Because the cooling jackets prevent access to a majority of the tube surface, traditional NDE methods cannot be used on these thick-walled components. As a result, they are generally allowed to run until they fail. With GWT, the entire tube can be screened for corrosion or circumferential/oblique cracking using a transducer collar placed on the end of the tube, prior to the start of the cooling jacket. An example of the GWT of a thick-walled (~2 in.) pipe is shown in Figure 7. From the data ,it is seen that welds and supports are clearly identified over a test length of more than 130ft. The signal-to-noise ratio (SNR) of the scan is excellent, demonstrating the potential for the inspection of other thick-walled components, such as polyethylene reactors.
Computerized Data Acquisition and Scanners
A major advantage of the advanced NDE techniques are the computerized data acquisition and data storage capabilities. NDE inspections are highly reproducible when inspecting with automated (moved by encoded motor-controlled drive unit) or semi-automated (encoded movement by hand) scanning systems. Encoded scanning systems offer speed and versatility when inspecting large areas, as well as precise movement and adjustment of data collection resolution when dealing with complex geometries. Having a digital permanent record can provide baseline inspection data, and assist in monitoring discontinuities over successive inspection intervals. This permanent, digital inspection data can help calculate growth rates of discontinuities and plan repair or replacement activities. Accurate NDE inspection data is an important tool for implementing Risk Based Inspection Programs, Fitness for Service Analysis and remaining useful life programs. Additionally, sophisticated data analysis software can be used to assist in flaw sizing and interpretation, and in some cases 3-D image presentation of defects.
An example of an automated encoded scanner for large inspection areas is the custom- made Structural Integrity Inner Vessel Automated Scanner (SIIVAS), shown in Figure 8. The SIIVAS was designed for inspecting the inside surfaces of a large high-pressure vessel and uses four axes of motion to accurately position an ultrasonic phased array probe or eddy current array probe on any inside vessel surface. The vertical and rotational axes are encoded and use a motor control drive unit (MCDU) to control the position, speed, and zero point. The third and fourth axes are controlled remotely and used to orient the probe for scanning and maintaining contact on the vessel walls. Both encoded axes have a resolution down to 0.0001 inches; however, the maximum resolution of the MCDU is 0.001 inches. The scanner is also equipped with a vision system consisting of two cameras with LED lights, time stamp and recording capabilities.
This type of automated scanning can be very effective for the inspection of the bottom radius area in deep bore vessels. These deep bore areas can experience high stresses and corrosion, which need to be inspected and monitored. These types of automated delivery systems are effective in accessing difficult-to-reach areas of a vessel and provide the ability for surface and volumetric inspection of previously inaccessible areas.
Advancements in NDE technology have led to more frequent use of phased array ultrasonics, eddy current array, and ultrasonic guided wave testing in industrial applications, specifically high-pressure equipment inspection. The use of encoded scanners and computerized data acquisition and analysis programs provide reliable detection, sizing and permanent recording of critical inspection data. These capabilities can improve comprehensive inspection programs which will ultimately lead to better life and condition management of high-pressure equipment critical assets when done by trained NDE technicians.
- Lafontaine, G., and R. Samson. “Eddy Current Array Probes for Faster, Better and Cheaper Inspections”, NDT.net, Oct 2000, Vol. 5 No.10.
- R/D Tech, 2004, Introduction to Phased Array Ultrasonic Technology Applications, pg. 7-18, R/D Tech, Quebec, Canada.
- Rose, J. L.,1999. Ultrasonic Waves in Solid Media. Cambridge University Press, Cambridge, UK.