ADVANCED NDE TECHNIQUES AND THEIR DEPLOYMENT ON HIGH PRESSURE EQUIPMENT

Transcription

1 Proceedings of the ASME 2012 Pressure Vessels & Piping Conference PVP2012 July 15-19, 2012, Toronto, Ontario, CANADA PVP ADVANCED NDE TECHNIQUES AND THEIR DEPLOYMENT ON HIGH PRESSURE EQUIPMENT Jeffrey P. Milligan Structural Integrity Associates, Inc Vanstory Drive, Suite 125 Huntersville, North Carolina USA Tel: Daniel T. Peters Structural Integrity Associates, Inc Kenneth Drive Rootstown, Ohio USA Tel: Jason K. Van Velsor, PhD Structural Integrity Associates, Inc. 301 Science Park Road, Suite 121 State College, Pennsylvania USA Tel: ABSTRACT 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 the examination of long tubes, such as in tubular low density polyethylene (LDPE) reactors. Effective deployment and delivery of these advanced techniques is critical to the collection and analysis of the inspection data. Common challenges found in high-pressure equipment include access issues such as small diameter deep bores, large and thick section components, weld overlays and examination of thick section welds, complex geometries, and small crack sizes required for detection due to materials and design. Applying these modern techniques in new designs and updating of in-service inspection plans for this high-pressure equipment can lead to a more accurate assessment of the equipment s fitness for continued service, reduced maintenance costs, proper asset management of key capital equipment, and reduced turn-around time. INTRODUCTION Life and condition management of critical assets is key to long term viability in many industries. The high-pressure industry has its own unique sets of challenges. The most common failure mode in this equipment is due to fatigue, which may be assisted by many environmental factors, such as stresscorrosion cracking, corrosion fatigue, and hydrogen assisted cracking, to name a few. Stress relaxation, or room temperature creep, has also been known to occur over time in some highly stressed vessels. Management of these degradation effects throughout the life of the equipment is critical to maximizing the life of these components and plays a key role in 1 Copyright 2012 by ASME

2 making the correct decisions with regard to repair or replacement of equipment. One of the most critical aspects of managing the life of high-pressure equipment is proper condition assessment. Highpressure equipment traditionally use very high-strength materials due to the pressures and high cycles reached during operation. It is common for these high-strength components to have low toughness and high susceptibility to the environmental effects discussed previously. Also, due to the pressures, thick wall sections are common to this type of equipment. In many cases, the need for long-term in-service inspection was not considered during the design. Many high-pressure vessel designs were made with multiple wall layers and inaccessible surfaces, where cracking or corrosion may initiate. Further, due to the extreme pressures in these vessels, manufacturing constraints, and at times local jurisdictional laws, limit the diameter of these vessels while the length can be quite large. This leads to small difficult-to-access bores where cracking may initiate. Further, it is common for the vessels to have experienced yielding in key areas, possibly by design, during the original fabrication or hydrostatic testing, which can produce significant compressive stresses at the surfaces that are most susceptible to cracking. This yielding can also, in some cases, result in subsurface areas of high shear stresses where cracking may initiate prior to free surfaces. Often, only very basic NDE techniques are used for the evaluation of these vessels. Typically, an in-service inspection may include a surface examination such as liquid penetrant testing (PT) or magnetic particle testing (MT) and, in some cases, it may include a volumetric examination such as straight beam or shear wave ultrasonic testing (UT). Radiography is also a possibility for volumetric inspection, but it has its own limitations and potential health hazards. In some cases, where the liner of a vessel is significantly prestressed, the only prescribed techniques include simple measurements of the inner diameters of the vessel to determine if any loss of prestress has occurred. Complete periodic assessment of the entire vessel s condition is key to safe, long-term reliable operation. A comprehensive inspection program should include periodic evaluation of key dimensions in the equipment, and NDE inspection in each area where a potential failure mode may exist. This nondestructive examination should include both surface and volumetric inspection of key areas. In recent years, advancements in both surface and volumetric NDE methods have become more common in industrial applications, such as high-pressure equipment. Surface examinations, which used to be simple PT and MT examinations, can often be replaced with an electromagnetic technique inspection that can provide as good or better sensitivity and quantifiable indication measurement data. Volumetric examinations, which were typically completed with conventional ultrasonic techniques or potentially hazardous radiography, can often be replaced with a phased array ultrasonic approach, including linear phased array (LPA), annular phased array (APA), or two-dimensional (2D) phased array techniques. Phased array ultrasonics has the ability to provide quick, accurate, and sensitive flaw sizing capabilities and visual imagery that is easy to understand, like radiography. Ultrasonic guided wave testing (GWT) also has shown applicability in the high-pressure equipment industry by providing a method for inspection of long tubular sections which may be otherwise inaccessible. An example of this in the highpressure industry would be tubular polyethylene reactors with external cooling jackets. Presently, it is common for these to be operated until failure due to a lack of an effective inspection technique. GWT has the potential to find degradation in these tubes without access to the entire length of tubing. Advanced delivery techniques through the use of robotics have also advanced to allow access to areas of vessels that were previously deemed inaccessible. Robotics are becoming more commonplace and easier than ever to use in complex situations. The need for manual access can be eliminated and often with equal, or greater, sensitivity to traditional hand-held techniques. Finally, another critical aspect of a complete nondestructive evaluation is the ability to accurately identify and document indications, flaws and cracking found in any component of the high-pressure equipment. Advances in computer technology for the acquisition and storage of complex data now allows for complete documentation of the NDE data gathered during the examination. Additionally, the automated delivery systems used for these advanced inspections allow the inspection data to be fully position encoded, which helps the operator and customer visualize the indications and component geometries. Lastly, the digital storage capabilities allow the customer to save all historical inspection data for future reference and assist in monitoring discontinuities over successive inspection intervals. TRADITIONAL NDE APPROACHES For the surface inspection of high-pressure equipment, various NDE methods can be used. Some of the most common methods include liquid penetrant testing (PT), magnetic particle testing (MT), and electromagnetic testing (ET).As with all NDE, each method has benefits and limitations. Liquid Penetrant Benefits: Quick, low-cost examination of large surface areas and complex geometries; high sensitivity to surface discontinuities; can be used on a wide variety of materials; minimal training and skill required. Limitations: Surface preparation is critical since contaminants can hide defects; mechanical operations can smear or peen the surface of a metal and close or reduce the surface opening of existing discontinuities; may be impractical on porous materials; post inspection cleaning is necessary to remove chemicals; visual access is required for evaluation; manual technique and reporting.[1] Magnetic Particle Benefits: Quick, relatively low-cost examination of large surface areas and complex geometries; high sensitivity to surface and some subsurface discontinuities; surface preparation is less critical than PT; minimal training and skill required. Limitations: Only ferromagnetic materials can be inspected; proper alignment of magnetic field and defect is critical; large magnetizing currents are needed for large parts; requires relatively smooth surface; paint or other nonmagnetic coatings may affect sensitivity; demagnetization and post 2 Copyright 2012 by ASME

3 cleaning is usually necessary; visual access is required for evaluation; manual technique and reporting.[2] Electromagnetic (Eddy Current) Benefits: Real-time detection of surface and near subsurface defects with very high sensitivity; test probe does not have to contact part; can offer quantitative measurement of material properties and discontinuities; minimum surface preparation is required; can be manual or automated with data collection possible. Limitations: Only electrically conductive materials can be inspected; depth of penetration is limited, especially in ferromagnetic materials which may require special treatment to address magnetic permeability; flaw orientation in relation to eddy current flow direction can be critical for detection; inspection training typically more extensive than PT and MT; reference standard required for inspection setup.[3] Volumetric inspection of high-pressure equipment has been commonly performed using ultrasonic testing (UT) or radiographic testing (RT). Each of these methods also has advantages and disadvantages. Ultrasonic Benefits: Nonhazardous, real-time inspection method; able to inspect a wide range of materials, part sizes, and geometries; depth of penetration for flaw detection is superior to other methods; typically only single sided access is required; provides quantitative data for distances and sizes of indications; manual or automated inspection possible with data collection. Limitations: Nearly always requires surface contact with probe and couplant; rough surfaces may prevent effective sound coupling; testing effectiveness may be limited due to extremely thin, geometrically complicated, or exceedingly course grained parts; skill and training required typically more extensive than other common techniques.[4] Radiography Benefits: Virtually any material can be inspected; surface and subsurface defect detectability; able to inspect complex shapes and multi-layered structures; minimum part preparation required. Limitations: Possible radiation hazard to operator and nearby personnel; access to both sides of the structure usually required; orientation of the radiation beam to non-volumetric defects is critical; relatively expensive equipment and personnel investment required.[5] ADVANCED NDE APPROACHES Many of the traditional NDE techniques described above can be replaced or improved by using an advanced NDE approach. Descriptions and benefits of eddy current array, for surface inspections, and phased array ultrasonics, for volumetric inspections, along with delivery systems are discussed next, along with ultrasonic guided wave to inspect long lengths of piping. 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 liquid penetrant inspection and magnetic particle inspection. 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 the 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. Figure 1: a) Flat ECA probe; b) Radiused ECA probe 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 still 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, thereby facilitating data interpretation, and improving flaw detection, sizing, and probability of detection.[6] Encoded eddy current array technology also allows for a permanent record of inspection data to be saved and 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 3 Copyright 2012 by ASME

4 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. 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. A typical S-scan image is provided in Figure 3. In this case, the image represents a calibration block containing a stack of side drilled holes, with a drawing of the block shown at the left and the representative S- scan image to the right. Phased array imaging allows the operator, and customer, the ability to better visualize the geometries of the parts and locations and sizes of all reflections that occur ultrasonically. Figure 2: ECA C-Scan, Impedance Plane (Lissajous), and strip chart display of a reference standard with different size electrical discharge machined (EDM) notches 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.[7] 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 such that the Figure 3: Linear Phased Array (LPA) ultrasonic S-scan image of a series of side drilled holes down a calibration block with angle sweep of 30º to 85º One advantage of linear phased array technology includes detection 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 4 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 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 inches (18.0 inch sound path) from the probe inspection position, as seen in Figure 5. Figure 4: 5L32 probe beam simulation at 40º in carbon steel 4 Copyright 2012 by ASME

5 0.04 in. Wire EDM (subsurface) Figure 5: S-scan (35º-55º refracted shear wave beam angles) of 0.04 inch wire EDM notch, inches deep (18.0 inch sound path at 45º) in carbon steel An advantage of using these advanced NDE techniques is the ability to successfully detect and size small flaws in areas that traditional NDE techniques would be limited. One example of this would be the inspection of threads in pressure vessel closures. Figure 6 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, which typically work well, but they offer no depth sizing information. The flaw depth sizing advantage of LPA can be observed in Figure 6. Artificial defects are shown to be easily detected and sized between inch and inch deep in expected cracking locations. Figure 6: LPA Ultrasonic Images of Buttress Thread EDM notches Guided Wave Testing (GWT) Long-range guided wave testing of piping has been successfully utilized in the energy industries 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 dominant cause of indications. The primary advantage of GWT is the 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.[8] Figure 7 shows a stillframe 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. 5 Copyright 2012 by ASME

6 Propagation Direction Figure 7: Still-frame from a finite-element animation showing an ultrasonic guided wave propagating down a length of a pipe. 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. Consequently, they are generally run to failure. 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 thickwalled (~2in) pipe is shown in Figure 8. From the data it is seen that welds and supports have distinct high amplitude reflections 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. 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 Inner Vessel Automated Scanner (IVAS), shown in Figure 9. The IVAS was designed for inspecting the inside surfaces of a large high-pressure vessel and utilizes 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 inches; however, the maximum resolution of the MCDU is 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. Figure 8: Long-range guided wave scan of a ~2 thick steel pipe showing indications from welds and supports for over 130ft of piping 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 Figure 9: Inside Vessel Automated Scanner holding ECA probe SUMMARY Advancements in NDE technology have led to more frequent use of phased array ultrasonics, eddy current array, and 6 Copyright 2012 by ASME

7 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. REFERENCES 1. ASNT, 1996, Nondestructive Testing Handbook, second edition: Vol. 10, Section 3 Liquid Penetrant Testing, American Society for Nondestructive Testing, Columbus, OH. 2. ASNT, 1996, Nondestructive Testing Handbook, second edition: Vol. 10, Section 8 Magnetic Particle Testing, American Society for Nondestructive Testing, Columbus, OH. 3. ASNT, 1996, Nondestructive Testing Handbook, second edition: Vol. 10, Section 7 Electromagnetic Testing, American Society for Nondestructive Testing, Columbus, OH. 4. ASNT, 1996, Nondestructive Testing Handbook, second edition: Vol. 10, Section 10 Ultrasonic Testing, American Society for Nondestructive Testing, Columbus, OH. 5. ASNT, 1996, Nondestructive Testing Handbook, second edition: Vol. 10, Section 5 Film Radiography, American Society for Nondestructive Testing, Columbus, OH. 6. Lafontaine, G., and R. Samson. Eddy Current Array Probes for Faster, Better and Cheaper Inspections, NDT.net, Oct 2000, Vol. 5 No R/D Tech, 2004, Introduction to Phased Array Ultrasonic Technology Applications, pg. 7-18, R/D Tech, Quebec, Canada 8. Rose, J. L.,1999. Ultrasonic Waves in Solid Media. Cambridge University Press, Cambridge, UK. 7 Copyright 2012 by ASME