DETECTION AND QUANTIFICATION OF DETRIMENTAL CONDITIONS IN HDPE USING ULTRASONIC PHASED ARRAY

Transcription

1 DETECTION AND QUANTIFICATION OF DETRIMENTAL CONDITIONS IN HDPE USING ULTRASONIC PHASED ARRAY Caleb Frederick Structural Integrity Associates Huntersville, North Carolina U.S.A. ABSTRACT: For over thirty years, the gas distribution industry has installed High-Density Polyethylene (HDPE) piping. More recently, the nuclear power industry has also installed HDPE in both safety-related and non-safety-related systems. Both industries have recognized the benefits of HDPE such as extended service life with minimal maintenance, and cost savings over other materials such as carbon steel for the same application. Due to recent nuclear installations, volumetric examination techniques to ensure butt-fusion joint integrity have been evaluated. This paper investigates development efforts to date using ultrasonic Phased Array to detect and quantify flaws and detrimental conditions in butt-fusion joints throughout the applicable pipe diameters and wall-thicknesses. This technology can be applied to both gas distribution and nuclear power applications. Factors addressed will include pipe diameter, wall-thickness, fusing temperature, interfacial pressure, dwell (open/close) time, and destructive correlation with ultrasonic data. INTRODUCTION: HDPE has experienced a recent increase in interest for nuclear raw water applications. The reason behind this increase is due to extended service life, public safety, and cost savings. Compared to its carbon steel counterpart, HDPE does not rust, rot, or support biological growth. This assures maximum water flow through the system, which contributes to public safety. Additionally, HDPE costs a fraction of steel piping to install and maintain. However, even with the expected benefits, there is a shortage of knowledge and data to support longterm material behavior, critical flaw types and sizes, and their propensity to grow. While testing is ongoing with regards to these issues, this paper will focus on flaw detection within butt-fusion joints. 1

2 To date, the only nuclear utility in the U.S. that performed volumetric examination of HDPE butt-fusion joints as part of their Class 3 essential service water (ESW) installation was Ameren Callaway Plant in Fulton, MO. Since the industry has not yet defined an acceptable/rejectable flaw, Callaway was restricted in their acceptance by removing all joints where any indications were detected. While this is adequate for the time being for first installations, this will likely prove unnecessary after acceptance criteria has been established. Additionally, replacing joints where indications reside will not be feasible for In-Service Inspections (ISI), as this will become a more involved and expensive proposition. Therefore, it is imperative that acceptance criteria based on Fracture Mechanics be established. While developing this ultrasonic Phased Array volumetric examination technique, it is important to first determine what common flaw types and joint conditions must be detected. The expected conditions include lack-of-fusion (LOF) (unbounded), cold fusion (partial bond), inclusions (imbedded) in the joint during field joining such as sand, small gravel, or grass, and voids (small, rounded, volumetric flaws). It is currently unknown what significance these flaws might play on a joints service life. However, preliminary testing is being performed to determine the probability of detection (POD) of these joint conditions and flaw types. This paper will investigate the results to date of these development efforts. BUTT-FUSION JOINING PROCESS: Butt-fusion joints are made through a defined combination of heat and pressure, based on the diameter and wall-thickness of the pipes being joined. As illustrated in Figure 1 below, after the pipe ends are faced to make flush and free of surface contaminants, the pipe ends are heated, then pressed together. During this process, the molten pipe material is pushed outward toward the outside diameter (OD) and inside diameter (ID) surface. The displaced material forms a bead roll-back. Historically, it was believed that performing a visual examination of these beads gave sufficient assurance of the joints quality and integrity. However, this has since been disproven. It is possible to have good roll-back, and still have inferior joint strength. For instance, if the heater temperature it too low the material will not melt and flow properly. But the formation of adequate roll-back can be achieved by compensating with increased inter-facial pressure. This forces outward what material has melted, but then the cooler, solid base material does not fully entangle, if at all. This incorrect joining procedure will give the appearance of good roll-back, while volumetrically having a partial or total lack-of-fusion. It is for this reason that the U.S. Nuclear Regulatory Commission (NRC) is leaning toward requiring volumetric examination for HDPE installations. 2

3 Pipe-ends before facing Displaced molten material Completed joint Faced ends before heat and pressure Bead roll-back FIGURE 1: ILLUSTRATION OF THE BUTT-FUSION JOINING PROCESS SENSITIVITY LEVELS ACHIEVED: When selecting an ultrasonic transducer frequency for detection of a specific size flaw, the rule-of-thumb is that the flaw must be 1/2 the wavelength () in order to be detected. This is calculated using the formula v/ƒ where v is the acoustic velocity of the material being inspected and ƒ is the frequency of the sound wave that is induced into the material. By calculating the for HDPE having a longitudinal velocity of ~.092in./µs (2330m/s) and a transducer frequency of 1.5MHz (used for deep penetration of wall-thickness >2.25in. or 57.15mm), the resulting equals.061in (1.55mm). The current side-drilled hole (SDH) that is used for establishing the calibration sensitivity level is.008in. (.2mm) diameter. By dividing.008in. into a.061, this calculates out to detecting the.008 in. diameter SDH with ~1/8 of the. Such extraordinary levels of sensitivity and detection are due to the way in which the crystallization and entanglement occur, and therefore the resulting HDPE has negligible material noise. This key characteristic contributes to high resolution and detection of small flaws even in heavy-wall, attenuative material. Figures 2 and 3 below help illustrate the high level of sensitivity that has been achieved. This.008 in. (.2mm) diameter drill bit was used to place the SDHs in the calibration block. FIGURES 2 AND 3:.008in (.2mm) DIAMETER DRILL BIT 3

4 Figure 4 below shows the theoretical refraction and coverage based on a fixed probe (index) position. As shown in Figure 5 below, HDPE provides a very high signal-to-noise ratio (SNR). This means that the background noise level in HDPE is very low. This is important as it allows the ultrasonic response from potential flaws to stand out from the surrounding noise of the material. When compared to that of certain steel applications with a much higher background (material) noise level such as stainless or cast, it is acceptable in these cases have a SNR as low as 2:1 (2 being the response from the calibration reflector (SDH, notch, etc.), and 1 being the background noise. Therefore detection in these materials with a higher background noise level are limited to detection of larger flaws. HDPE however has a SNR of ~100:1. This further supports the fact that HDPE material aids in ultrasonic inspectability. FIGURE 4: BEAM SIMULATION SHOWING PROBE PLACEMENT AND THEORETICAL COVERAGE FIGURE 5: ULTRASONIC RESPONSES FROM THE.008in. (.2mm) DIAMETER SDHs IN THE CALIBRATION BLOCK RESOLUTION: Resolution is important as this allows the examiner to better characterize and determine the extents of a flaw, as well as the proximity of indications to each other. Future acceptance criteria will likely define the proximity limits. If indications are adequately separated, then it could be determined acceptable. However, if indications are in too close proximity, then it could be ruled as one larger flaw, therefore rejectable. Better resolution will allow the examiner to make this determination. As shown in Figure 6, to test resolution, three.008in. (.2mm) diameter SDHs were clustered together with.125in. (3.17mm) separation. These SDHs were intentionally placed outside of the fusion line in order to test the capability of examining through the joint, but still detecting and resolving beyond the joint. See results in Figures 7 and 8 below. FIGURE 6: THREE CLUSTERED.008 in. (.2mm) DIAMETER SDHs WITH.125 in. (3.17mm) SEPARATION 4

5 Threedistinctreflectors canbeobservedinthedata. Evenwithanincreased soundpath,threedistinct reflectorscanstillberesolved. FIGURE 7: RESOLUTION FROM THE NEAR- SIDE OF THE JOINT. FIGURE 8: RESOLUTION FROM THE FAR-SIDE OF THE JOINT. FLAW TYPES: Due to the way in which butt-fusion joints are made in HDPE, a few distinct flaw types are possible. Figure 9 below illustrates these types of flaws, which will be addressed later in this paper. Lack-of-Fusion (LOF) Cold-Fusion (partially bonded) Inclusion (embedded) Void (round, volumetric) DETECTION RESULTS: Flaw detection in HDPE (as in all materials) requires an impedance mismatch between the material being inspected and that of the flaw. Acoustic Impedance (Z) is the product of the density (P) and acoustic velocity (v) of the material, Z = P(v). This formula helps determine the amount of sound energy that is transmitted, and that which is reflected between two media. The greater the impedance mismatch, the more acoustic energy is reflected at the interface. More energy reflected from the second media, translates to increased likelihood of detection. A crack, void, or LOF are considered to be an air interface which has a Z of ~ kg/m²sx10. When placed in HDPE with a Z of 2.36 kg/m²sx10, this creates a significant mismatch (difference) of kg/m²sx10. By comparison, wood having a Z of 1.57 kg/m²sx10, when placed in HDPE, produces a much lesser impedance mismatch of.79 kg/m²sx10. For this reason (theoretically), embedded wood is more difficult to detect than a crack, void, or LOF in HDPE. FIGURE 9: ILLUSTRATIONS OF POTENTIAL FLAW TYPES IN HDPE BUTT- FUSION JOINTS 5

6 VOIDS: Development efforts to date have focused primarily on fabrication and examination of cold fusion and LOF samples. Phased Array has shown promise in detection and resolution of voids, partial fusion, and LOF conditions. However, these efforts are on-going and fully quantified testing has not yet been completed. As described above, voids have proven the easiest condition to detect because they represent an almost ideal reflector, much like the SDHs placed in a calibration block. Figure 10 below is from a thin-wall butt-fusion joint that was fabricated in the field. The ID bead geometry responses are strong and always present. However, the fusion zone is free of indications. By comparison, Figure 11 shows the same ID bead responses, in addition to a low-amplitude indication in the fusion zone. Figures 12 and 13 below are destructive confirmation of the presence of small, accumulated voids in the fusion line. Fusionzone IDbeadresponse Indicationinfusionzone FIGURE 10: ULTRASONIC RESPONSES FROM A GOOD BUTT-FUSION JOINT. THE ID BEAD GEOMETRY RESPONSE IS ALWAYS PRESENT. FIGURE 11: LOW-AMPLITUDE INDICATION DETECTED IN THE FUSION ZONE, IN ADDITION TO THE ID BEAD RESPONSES. FIGURE 12: DESTRUCTIVE ANALYSIS CONFIRMS THE PRESENCE OF SMALL VOIDS IN THE FUSION LINE. 6 FIGURE 13: ZOOMED VIEW OF DESTRUCTIVE ANALYSIS IN FIGURE 12.

7 COLD FUSION: A cold fusion condition is perhaps the most concerning joint condition as the cocrystallization process occurs, though not completely. While there is currently no consensus in the HDPE community of the true definition of cold fusion, some believe this occurs at a molecular level. The theory behind this is that only a percentage of the molecules entangle, giving the joint some strength, but not all entanglement occurs. Therefore, the joint might pass a short-term pressure test, but could fail prematurely during its service life. From an NDE perspective, there must be a scientifically-supported definition of this condition, and representative samples fabricated before detection and quantification efforts begin in earnest for this condition, as it is currently unknown when this condition is detected, or what it looks like ultrasonically. LACK-OF-FUSION: In order for detection to be possible, there must be a source of scatter. This scattered energy is returned to the Receiver probe for detection. When correlating the ultrasonic and destructive data, it was determined that complete LOF conditions would appear very similar to that of a good joint. See Figure 13. The highest coverage angles typically receive a high-amplitude response at the near-surface, as there is a partial bond (scatter source), and these high angles are most perpendicular with that of the fusion line. See Figure 14. But as the angles decrease to cover the mid-wall portion of the pipe, the amplitude response will decrease to the point of no detection of the fusion line. Then the lower coverage angles could detect the portion of the joint at the far-surface, as there is again a partial bond. After further analysis, it was determined that due to the surface of the joint face being very smooth (like a mirror), the ultrasonic beams would skip off the joint face, down to the ID surface, and not return to the Receiver probe. As a result, this condition was for the most part, going undetected. It was based on this discovery, that a supplemental technique was developed. The technique believed best-suited for detection of this condition is Tandem Phased Array. As illustrated in Figure 15, this requires two probes placed one in front of the other. The Transmitter probe in the back transmits a sweep of angles providing the required coverage. These angles are skipped off the ID surface, then upward to cover the fusion line. If the joint if fully bonded, the sound will pass through the fusion with no sound returned to the Receiver probe. See Figure 16. However, as illustrated in Figure 17, if there is an unbonded fusion line, the sound will skip off the joint face, back to the Receiver probe for detection. See Figure 18. 7

8 BondedatODbead andnearsurface portionofwall CompleteLOF. Notice mirror surface BondedatIDbead andfarsurface portionofwall FIGURE 14: DESTRUCTIVE CONFIRMATION OF A LOF JOINT CONDITION. FIGURE 15: HIGH-AMPLITUDE RESPONSE FROM HIGH ANGLES AT THE NEAR-SURFACE. NO INDICATION OF A LOF IN MIDWALL. FIGURE 16: IF NO LACK-OF-FUSION (UNBONDED) CONDITION IS PRESENT, NO SOUND WILL BE REFLECTED TO THE RECEIVER PROBE FIGURE 17: ULTRASONIC RESPONSE FROM FULLY BONDED FUSION 8

9 FIGURE 18: IF A LACK-OF-FUSION (UNBONDED) CONDITION IS PRESENT, SOUND WILL REFLECT TO THE RECEIVER PROBE FIGURE 19: ULTRASONIC RESPONSE FROM UNBONDED FUSION INCLUSION: Depending on environmental conditions and attention paid to eliminating the opportunity for inclusion during field installation, contaminants such as grass, dirt, small rocks, and sand can be unintentionally embedded in the fusion joints. This may result in poor joint strength, and ultimately lead to premature failure of the joint, all depending on the service conditions of the piping system and the amount and dispersion of the contaminants around the wall of the fusion joint. Figure 20 below is of data taken from a test sample that was fabricated with dirt placed in the fusion joint after the heat cycle, prior to applying interfacial pressure. While these contaminants were easily detected, it is unknown at this time what effect these contaminants have in the joints performance. This sample, along with other samples that were fabricated with inclusions under controlled joining conditions as part of a comprehensive study will be destructively tested and analyzed, then correlated with the ultrasonic results to better understand and quantify the data. These results will be thoroughly documented in a later report. FIGURE 20: ULTRASONIC RESPONSE FROM DIRT IN THE FUSION JOINT 9

10 CONCLUSION: Ultrasonic Phased Array continues to be at the forefront for volumetric examination of HDPE butt-fusion joints due to its proven detection and resolution capabilities. However, there is much to learn regarding flaw types and critical size. NDE development efforts must continue to optimize examination techniques, and quantify detection capabilities. Tandem Phased Array in particular has proven capable of detecting LOF conditions, but further testing is required to quantify the detection threshold. Based on testing performed and knowledge gained to date, there likely will not be one technique capable of providing a total solution for volumetric examination of butt-fusion joints in HDPE. Therefore, these complimentary techniques must be optimized to achieve the best results attainable. Caleb Frederick Development Engineer Structural Integrity Associates, Inc Vanstory Drive, Suite 125 Huntersville, North Carolina U.S.A. Telephone: Fax: cfrederick@structint.com 10