Phased Array Testing for Hydrogen-Induced Cracking in Pressure-Bearing Pipes

Abstract:

Hydrogen blistering (HB) or hydrogen-induced cracking (HIC) may occur in pressure-bearing pipes operating in hydrogen and wet hydrogen sulfide environments due to hydrogen accumulation in steel. Such defects might be easily missed or misjudged as lamination, which is extremely dangerous and could lead to major accidents. However, the use of phased array ultrasonic testing (PAUT) could be a solution to avoid missed testing and misjudgment.

This study introduces technical requirements, testing plan, testing process and testing results of phased array ultrasonic testing, which proves the effectiveness and reliability of PAUT and monitoring for hydrogen-induced cracking.

Keywords: pressure-bearing pipe; Hydrogen-induced?cracking?(HIC); phased array ultrasound; testing

1. Overview

For pressure-bearing pipes operating in hydrogen and wet hydrogen sulfide environment, hydrogen atoms are formed during sulfide corrosion on the steel surface. Hydrogen atoms enter steel and gather at discontinuous places such as inclusion or lamination to form hydrogen molecules that are too large to diffuse out. They continuously focus and generate pressure. When the pressure reaches a certain degree, local deformation may occur, and hydrogen blistering (HB) may be generated. Hydrogen blistering expand at different depths on the steel surface and connect with each other to form cracks, which is called hydrogen-induced cracking (HIC)[1]. See Figure 1.

Usually the conventional inspection methods of pressure-bearing pipes are visual inspection or ultrasonic thickness measurement. Preliminary blistering is not obvious, which may be easily missed by using visual inspection; while ultrasonic thickness measurement is not good for defect characterization, resulting in many HB or HIC defects ?being misjudged as lamination, which is usually considered as an acceptable defect. Therefore misjudging HB or HIC as lamination is dangerous and must be avoided.

2. Difference and hazard between lamination and hydrogen-induced cracking

The defects detected in the in-service hydrogen pressure-bearing pipes and the pressure-bearing pipes under the wet hydrogen sulfide environment, especially pressure-bearing pipes under the medium and low pressure wet hydrogen sulfide environment, such as hydrogen sulfide aqueous solution pipes and liquefied petroleum gas pipes with high hydrogen sulfide content, are easily mistaken as laminated defects generated in the manufacturing process, but HB or HIC and lamination have different harms due to different causes.

Lamination, including ash and inclusions, is a defect formed by porosity or impurities in the steel during the manufacturing process. For some thick-walled pipes, if the stretching is not enough, it may sometimes produce lamination at an angle to the pipe surface. Most of lamination is parallel to the surface of the steel pipe and generally do not expand, which is less harmful.

HB or HIC is caused by the accumulation of gases such as hydrogen in steel pipes and the generation of high pressure. The high pressure in the steel makes the local stress of the steel too large, causing steel cracking, which often continues to expand. HB or HIC will expand even under low applied stress conditions without changing the medium and operating conditions, as shown in Figure 2, which is extremely hazardous.

If HB or HIC parallel to the surface or with a small angle is mistakenly detected as lamination, the inspection result is likely to allow continuous operation. However, with the accumulation of hydrogen molecules, the pressure increases, most of the cracks will expand, and the surface will bulge to form HB or HIC. Moreover, sometimes HB or HIC of adjacent different laminations will connect with each other, further weakening the pressure capacity of LPG tanks and possibly causing more dangerous sulfide stress corrosion cracking (SSCC). In the next inspection cycle, when HB or HIC expands to a certain extent and the pressure bearing capacity is insufficient, it will cause failure of the pressure-bearing pipes and even cause a large amount of medium leakage. If a large amount of hydrogen sulfide or liquefied petroleum gas leakage accident occurs, it will endanger the safety of the surrounding.

3. Technical requirements for PAUT

Hydrogen-induced cracking is very harmful, and conventional testing methods may cause misjudgment, so it is necessary to use a new testing method. PAUT becomes the first choice. With this technology, the multi-channel phased array ultrasonic instrument shall be connected with multi-element probes, transmitting and receiving ultrasonic waves. By controlling different delay times of the transmitting (or receiving) pulses of each element in the transducer array, the phase relationship of the sound wave arriving (or coming) at a certain point in the object is changed, and the focus and beam direction are changed, so as to realize the beam scanning, deflection and focusing of ultrasonic waves. Then, a combination of mechanical and electronic scanning methods is used to achieve imaging, so that in addition to the waveform display, the testing results can also have multiple views such as B/C/D, and by analyzing the high-quality images projected in different directions, the nature of the defect can be identified, and the misjudgment of hydrogen-induced cracking as lamination can be avoided. Figure 3 shows a phased array ultrasonic device inspecting a pressure-bearing pipe.

Phased array testing system relates to ultrasonic, electronic, computer, mechanical and material technologies. There are many parameters set in the equipment, and there are often complex correlation and restriction relations among the parameters. If the parameters are not set properly, grating lobes and artifacts will be produced, which may cause the reduction of signal-to-noise ratio and testing resolution, resulting in defects missing and inaccurate positioning. Only by defining the focusing law and the optimization principle of testing parameters, can we correctly set the parameters such as beam width, scanning range, array element activation, focal length and delay, obtain perfect system characteristics, and give full play to the advantages of phased array ultrasound technology.

To use phased array ultrasonic system correctly, inspectors need to combine, optimize and comprehensively configure the parameters of phased array ultrasonic equipment by using engineering design knowledge and engineering practice experience of nondestructive testing, and analyze signals and images and judge defects according to the tissue characteristics of inspected materials. Generally, the following jobs are required:

(1)? The acoustic field characteristics of phased array ultrasonic probe are studied, and the differences between phased array ultrasonic probe and conventional ultrasonic probe are clarified.

(2)? Optimization of excitation frequency. The main lobe height and side lobe in focused sound field directly affect the resolution of phased array ultrasonic imaging and the generation of artifacts. This method can improve the focused sound field to some extent, increase the intensity of focused sound and improve the signal-to-noise ratio, thus improving the imaging quality.

(3)? Optimization of focusing law parameters. By studying the focusing principle of phased array ultrasonic technology and the characteristics of focused acoustic beam, the principle and realization method of dynamic focusing technology are clarified. The optimization principles and methods of focusing law parameters such as focusing position, focusing mode and scanning mode are proposed to improve the imaging resolution and signal-to-noise ratio of phased array ultrasound.

(4)? Optimization of instrument setting parameters. The physical significance, optimization principles and methods of pulse amplitude, pulse width, testing, filtering, gain, averaging, sampling rate and pulse repetition frequency are studied.

(5)? Phased array ultrasonic calibration and compensation technology is adopted to calibrate the echo amplitude equalization of reflectors at different depths, and to realize the acoustic path distance-amplitude compensation, so that the testing sensitivity at different depths is consistent.

(6)? Probe: The setting and optimization of probe parameters in phased array ultrasound system is also very important, such as probe type, frequency, wave mode and probe size. Probe type and waveform shall be selected according to the testing position, geometric size and shape of the testing object. Probe frequency has great influence on resolution and attenuation. Choosing appropriate probe frequency can improve ultrasonic penetration and defect resolution. The selection of probe geometry parameters such as length, width, spacing and number of array elements plays a decisive role in the directivity of phased array sound field. Reasonable selection of probe geometry parameters is the basis of improving imaging resolution, sensitivity and signal-to-noise ratio.

4. PAUT technology

Phased array ultrasonic instrument shall have product quality certificate or qualified certification documents, at least have multi-channel ultrasonic transmission and reception, automatic data acquisition and recording, multi-view display and other functions, the main performance indicators shall comply with ISO 18563 Performance and inspection of phased array ultrasonic equipment Part 1: Technical requirements for instruments.

The main performance indicators of a phased array ultrasonic probe shall comply with ISO 18563 Performance and testing of phased array ultrasonic equipment Part 2: Technical requirements for probes.

The scanning device shall ensure the smooth operation of the probe during scanning. The position of the probe can be adjusted. The sound beam is perpendicular or nearly perpendicular to the direction of common defects of the workpiece to be inspected. It can record the movement track and position of the probe in real time with the phased array ultrasonic detector.

The equipment used in this example is SIUI brand, in which the instrument model is SIUI Synscsan2 and the probe model is SIUI 5.0L-0.5-10. The cosmetic design of the instrument is shown in Figure 4.

The contrast test block is made in-house and made of the same material as the workpiece acoustic performance. Its thickness matches the wall thickness of the measured pressure-bearing pipe. Its overall size can represent the characteristics of the workpiece and meet the scanning requirements of the scanning device. Due to the large curvature radius of this pressure-bearing pipe, a planar contrast test block is used, which has 12 standard reflectors, i.e. Φ2 flat-bottomed holes with different depths, covering all testing ranges. See Figure 5 for the appearance of the test block.

Testing sensitivity should be set prior to testing, and contrast test blocks are generally used for sensitivity settings. If sensitivity is set by using standard reflector on contrast test block, set reflected signal amplitude to 50% of full screen height, and pay attention to material grain noise not higher than 5% of full screen height. In addition, the surface compensation in this example is 0dB.

In order to enlarge the scanning range and optimize the scanning effect, we use a 64-element linear array probe with a frequency of 5MHz. The wedge thickness matches the wall thickness of the pressure pipe, so that the reflected signals in the whole wall thickness range are between the wedge interface waves, the wedge sound velocity is 2337m/s, and the wedge angle is 0 degrees.

In terms of instrument setting, the scanning depth should match the wall thickness of the pressure-bearing pipe, which is 30mm in this case; The material type is low carbon steel, the shear wave speed is 3230m/s; The longitudinal wave velocity is 5900m/s; The pulse voltage is 20V and the pulse width is 200ns. Repeated frequency 1100Hz; scan type linear scan; The scanning waveform is longitudinal wave, the focal depth is 15 mm, the angle is 0 degrees; and the aperture is 32. The test results are shown in Figure 6.

Testing is performed using a scanning device with an encoder, and the position sensor is calibrated prior to testing. The calibration mode shall enable the error between the displacement displayed by the testing equipment and the actual displacement less than 1% when the scanning device moves a certain distance; Scanning increment refers to the sampling interval between scanning signals during scanning. During testing, signals shall be collected according to scanning increment. Scanning increment setting is related to the thickness of pressure-bearing pipe. In this example, scanning increment is 0.26mm.

5. Testing and monitoring processes

Temperature effect must be considered when testing in-service pressure-bearing pipes. It should be ensured that the test is carried out within the specified temperature range. If the temperature is too low or too high, effective measures should be taken to avoid it, and if it cannot be avoided, its impact on the test results should be evaluated. The temperature difference between calibration and actual test shall be controlled within 20℃. The surface temperature range of pressure pipe is 0~50℃ when a conventional probe and couplant are used. Outside this temperature range, special probes or couplant may be used, but calibration and verification shall be performed on a contrast test block at the temperature of use. Effective and suitable medium for in-service pressure-bearing pipe is adopted as ultrasonic couplant, and the couplant selected is mainly considered to ensure stable and reliable testing within a certain temperature range. The couplant used for the actual test shall be the same as that used for calibration.

Iron filings, oil dirt and other impurities shall be removed from the moving area of the probe. The testing surface shall be smooth for easy scanning of the probe. The surface roughness Ra value shall not be less than 6.3um. Before testing, determine and mark the testing area, mark the scanning path according to the testing area and probe on the pressure-bearing pipe, including the scanning starting point and scanning direction, and draw a reference line on the pressure-bearing pipe to ensure the movement track of the probe. Re-examine the found defect parts or determine the key parts. The testing area can be reduced to the corresponding parts and re-marked.

Depth calibration shall be carried out by using contrast test block before testing, and it shall be ensured that the depth measurement error is not greater than 1% or 0.5mm of the thickness of pressure-bearing pipe, whichever is larger.

During scanning, it shall be ensured that the error between the moving track of the probe and the proposed scanning path does not exceed 10 mm. Hydrogen-induced cracking testing of pressure-bearing pipe is a base metal testing in a large range, so scanning in sections will be performed. The overlapping range of each scanning area is at least 20 mm. Pay close attention to the amplitude during scanning. If the interface wave is obviously reduced, the material grain noise changes or the coupling is suspected to be poor, such region should be scanned again.

At the end of the test or during the test, when the test equipment is started or stopped or the parts are replaced, the test system shall be reviewed. Since the contrast test block is used for initial setting and calibration, the same test block shall be used for review. If the parameters of initial setting and calibration deviate from each other during review, it shall be corrected or retested.????? ??

6.?Hydrogen-induced Cracking Test Results

Phased array ultrasonic equipment was used to inspect and monitor pressure-bearing pipes in service under hydrogen and wet hydrogen sulfide environment, and many hydrogen-induced cracks were found. Generally, the displayed A-type echo is uneven and fluctuates irregularly, sometimes fluctuates in a large range, indicating that the reflection surface of the defect is irregular. The B-scan images can be visually observed for the panoramic reflection amplitude and depth of the defect in the acoustic beam coverage area of the probe, providing basis for the qualitative determination of the defect. The actual defect and testing result are shown in Figure 7.

When phased array ultrasound is used for inspection, the range, depth, quantity and direction of hydrogen-induced cracking can be determined by A/B/C multi-view. If it is used for monitoring, the change of hydrogen-induced cracking can be observed at any time.

Some suspected HIC were found by phased array ultrasound testing, with no blistering, no cracking angle or very small angle, which was confirmed as HIC by dissection. Such defects, macroscopically no abnormalities, cannot be found through visual inspection, and it is not easy to be found through other testing such as X-ray, magnetic particles or penetration. It is also likely to be misjudged as lamination by using ultrasonic thickness measurement. Practice proves that PAUT is more accurate and reliable. Phased array ultrasonic inspection can also detect all pressure-bearing pipes that may have HB or HIC before installation of pressure-bearing pipes, and find small defects that do not exceed the standard. During the use of pressure-bearing pipes, it can be used for in-service inspection or key area monitoring to find HB or HIC. Because impurities such as MnS in steel are easy to accumulate hydrogen, it is advisable to select low sulfur steel. If raw materials contain defects that do not exceed the standard, such as Class I minor defects, accurate records should be made. After continuing the test of new defects or such original small defects, the defect changes can be determined, and new HB or HIC can be found immediately.

When defects in pressure-bearing pipes are found by the inspector, he should not only judge according to the regulations or codes/standards, but also conduct in-depth research, use phased array ultrasonic and other advanced testing methods to timely make the decision whether it is HB, HIC or other defects, make accurate judgment on the safety of the pressure-bearing pipes, take measures or use phased array ultrasonic monitoring to track change of defects at any time, and therefore prevent accidents from happening.

References:

[1] Zheng Jiegen. Hydrogen sulfide corrosion and protection of LPG spherical tank [J]. Petrochemical equipment technology. 2005, 26(6): 56-57.