Ultrasonic Backscatter and Phased Arrays for Detection of High Temperature Hydrogen Attack

Transcription

1 Ultrasonic Backscatter and Phased Arrays for Detection of High Temperature Hydrogen Attack by Anmol S. Birring, NDE Associates, Inc., Houston, USA Mathieu Riethmuller, Institut de Soudure, Paris, France Koji Kawano, Idemitsu oil Company, Chiba, Japan INTRODUCTION High temperature hydrogen attack (HTHA) is a phenomenon of metal degradation that is well known in the petrochemical industry. HTHA occurs in carbon and low steels exposed to high partial pressure of hydrogen at elevated temperatures. Such damage has occurred over time as very little information was available on the long-term exposure of steels in hydrogen service when the equipment was originally designed. Equipment designed to be safe as per engineering codes has experienced such damage. The damage is caused by the seepage of hydrogen that reacts with metal carbides (MC) to form methane gas. The source of hydrogen is from the flow stream (hydrocarbons) that seeps into the metal causes such damage. This reaction decarburizes the steel, produces micro cracks, and lowers the toughness of steel without necessarily a loss of thickness. MC + 4H = 3M (M:metal) +CH 4 (methane) Detection of hydrogen attack is important to assure safe operation of pressure vessels and piping susceptible to such damage. While several plants have taken a aggressive approach and replaced equipment operating in the susceptible range of HTHA. However, there is still a lot of equipment operating worldwide that may be susceptible to such attack. Most of this includes reactors and heat exchangers that are expensive to replace. Several plants have gone ahead and replaced piping system that are cheaper to replace. Left undetected, HTHA can lead to failure of equipment and major accidents (See Fig 1). 1

2 Figure 1. Secondary damage caused by HTHA failure in a refinery. A fire caused by HTHA failure of a line tipped the column that fell over the pipe rack causing extensive damage. OPERATING LIMITS The operating limit for steels operating in hydrogen environment is given in API 941 (1). Based on prior cases of HTHA incidents, API 941 includes operating limits above which the material may be susceptible to HTHA. These operating limits are based on Hydrogen partial pressure and temperature. Some of the common materials included in API 941 are C-steel, C-0.5 Mo, 1.25 Cr-0.5Mo, 1.0 Cr-0.5 Mo, 2.25 Cr-1.0Mo, etc. Particularly of interest are components made out of C-0.5 Mo Steels whose operating limits have been lowered twice because of unfavorable service experiences. In 1977, the 0.5 Mo curve was lowered by 60º F (33ºC) to reflect plant experiences that involved HTHA of 0.5 Mo steels. However, with additional cases of HTHA that were as much as 200º F (111ºC) below the 1977 curve, the 0.5 Mo curve was removed in The C-0.5 Mo equipment that operates above the C-steel curve should either be replaced or regularly inspected to detect HTHA (see Fig 2). 2

3 Figure 2. API 941 Curves showing that existing C-0.5 Mo steel equipment that is operating above the Carbon Steel Curve should be inspected. The C-0.5 Mo curve was removed in (Curves are approximate) SCHEDULING INSPECTIONS While owners have replaced significant number of C-0.5 Mo piping systems in the plants, there still are quite a number of C-0.5 Mo reactors operating above the carbon steel curve. The reason being the high cost to replace a reactor. Piping is significantly less costly to replace. The C-0.5 Mo equipment operating above the carbon steel curve should be inspected periodically. These inspections can be prioritized based on temperature, hydrogen partial pressure, operating time, thermal history of steel during fabrication, stress, cold work, cladding composition, cladding thickness (3). One such approach is to use a empirical Parameter called as "P v " offered by JSM (Japanese Society of Materials) and developed by the Hydrogen Embrittlement (HE) committee of JPVRC (Japan Pressure Vessel Research Council). In addition to temperature and hydrogen partial pressure, the P v parameter includes the exposure time. The exposure time accounts for the incubation time. The P v parameter is reflected in API 581 (2000), Appendix 1. Reference 3 provides guidelines on prioritizing equipment for C-1/2 Mo equipment in hydrogen service. P v = log P H x 10-4 T (logt + 14) 3

4 T= Operating Temp, K t = hours in Service P H2 = Hydrogen Partial Pressure Kg/cm 2 A chart is presented in Reference 3 whereby one can evaluate the expected HTHA in C- 0.5 Mo Steels. The critical value of P v depends whether the evaluation is done for base metal, HAZ with PWHT or HAZ without PWHT. Base Metal, P v Critical = 5.8 HAZ with PWHT, P v Critical = 5.6 HAZ without PWHT, P v Critical = 4.9 Some of the factors that are important for scheduling inspections are as follows: operating conditions (partial pressure of hydrogen and temperature) relative to the operating limits provided in API 941 (1). slow cooled C-1/2 Mo steels have less resistance to hydrogen attack than normalized steels (2). post-weld heat treated welds are less susceptible to hydrogen attack compared to the welds that are not heat treated. A general discussion of HTHA prediction is given in reference 4. ULTRASONIC INSPECTION METHODS HTHA can occur in both the base metal and the weld HAZ. The base metal and weld HAZ should therefore be inspected for hydrogen attack. Base metal attack as shown in Fig 3 is detected by using a combination of ultrasonic back-scatter and velocity measurements (5,6). Hydrogen attack increases the ultrasonic backscatter and reduces the ultrasonic velocity in the material. In particular, HTHA increases the ratio of S-wave to L-wave velocities. In addition, HTHA also attenuates the higher frequencies because of scattering (7). The ultrasonic backscatter technique was first applied at the Chevron Richmond Refinery in 1989 by A. S. Birring (6). Details of the UT techniques for HTHA inspection are given in references 5 and 6. One has to understand that that while hydrogen attack affects velocity-ratio, backscatter and the frequency of the ultrasonic signal, other material anomalies can influence these ultrasonic parameters as well and give a false call. Micro-cracks, voids, inclusions and grains in the material scatter ultrasound and have frequency dependent affects (8, 9). There have been cases in the industry where inspectors have either missed HTHA or incorrectly called HTHA. Figure 4 shows ultrasonic scattering from HTHA and ultrasonic reflections from stringers. Stringers in steel are nonmetallic inclusions in the original ingot that became elongated through rolling process into, long, continuous or semi-continuous inclusions. Ultrasonic reflections from stringers can be incorrectly misinterpreted as HTHA especially when stringers are close to the ID surface. Such miscalls can cause the plant operators to loose confidence in the technology. Application 4

5 of the ultrasonic techniques for HTHA detection requires a skilled ultrasonic technician with a good understanding of the mechanism of HTHA and how it affects the propagation and scattering of ultrasonic waves. Ultrasonic inspection for this application is therefore not straightforward and requires logical test methodology to detect HTHA. Figure 5 shows backscatter obtained from a 65 mm thick reactor. Figure 3. Microcracks at the grain boundaries scatter ultrasound (a) Metal damaged by HTHA scatters ultraound. (b) Microcracks at the grain boundaries. Figure 4. Detection of HTHA using ultrasonic scattering. (a) HTHA initiates from the ID (b) the one-dimensional RF signal shows scattering from HTHA (c) Phased Array display of HTHA (d) stringers in steel can result in miscalls (e) RF signals from stringers (f) the two dimensional phased array display improves resolving stringers from HTHA. 5

6 Figure 5. Application of ultrasonic backscatter for detection of HTHA. The ultrasonic backscatter is ID connected. Tests on a 65 mm thick C-0.5 Mo Reactor with significant HTHA. Transducer Frequency is 5.0 MHz. Weld HAZ attack is detected using angle beam techniques. The two most commonly used approaches are ultrasonic shear wave technique and Time of Flight Diffraction (TOFD) and. Since the HTHA cracking is in the weld HAZ is extremely fine, shear wave inspection is done at a very high sensitivity. One difficulty in the detection of Weld HAZ cracking is that base metal HTHA next the HAZ will scatter ultrasound thereby reducing the sensitivity of inspection. Figure 6 shows a display of TOFD showing areas with micro-cracking simulating HTHA. Further verification of HTHA is done by surface replication. 6

7 Figure 6. TOFD detects three areas of micro-cracking in a plate. APPLICATION OF ULTRASONIC PHASED ARRAYS Ultrasonic Phased arrays common in the medical industry have recently been introduced for non-destructive testing at a affordable cost. Phased array systems are available that are portable, battery powered and can be used in the plant environment. This technology holds promise for detection of microcracks produced by HTHA. Additionally, by looking at the image pattern one can evaluate whether the reflections are produced from HTHA or from stringers in steel. HTHA damage will always be connected to the ID and will be quite uniform. On the other hand, signals from material discontinuities will be isolated. Phased array technology was applied on several reactors during a planned inspection in a refinery. The thickness of the reactors is approximately 65 mm. The results are shown in Figures 6, 7 and 8. The data was taken with the Omniscan using a 5MHz transducer with 16 elements. Figure 6 shows the image from an area with no HTHA and no material anomalies. The image is clear with no artifacts. Figure 7 is the data taken from an area with material anamolies. These artifacts are discrete and can be resolved accordingly. Figure 8 is data taken from an area with HTHA. The image clearly shows ID connected backscatter signals that are typical of HTHA. These tests have shown the potential of 7

8 phased arrays for detection of HTHA. The two dimensional Phased Array display is a significant improvement over the one-dimensional RF display in Figure 5 for HTHA interpretation. Figure 6. Location on Reactor with no material anomalies and no HTHA. Instrument is Omniscan Sectorial scan from -15º to +15º with focal law calculated every 0.2º. Reactor thickness is 65 mm. 8

9 Figure 7. Location on Reactor with material anomalies such as stringers. Note the discrete and isolated reflectors. Instrument is Omniscan. Sectorial scan from -15º to +15º with focal law calculated every 0.2º. 9

10 Figure 8. Location on Reactor with HTHA. HTHA damage is connected to the ID surface. The A-scan corresponding to this image is shown in Fig 5. Phased Array instrument is Omniscan. Reactor thickness is 65 mm. The first tests conducted by Phased Array have shown that this methodoly can detect and quantify HTHA damage. Moreover, visualization of the image makes interpretation easier than analyzing a the A-scan. We plan to further improve the Phased array inspection process by increasing the image resolution. This will done by reducing focal spot size and increasing the number of focal law. Also image contrast will be improved by using B&W displays instead of color. PLANT EXPERIENCE Several NDT service companies offer services for detection of HTHA in petrochemical plants. They either conduct backscatter recognition or backscatter frequency analysis for HTHA detection. The technicians are trained on written procedures and practical tests on a limited number of samples. Some technicians may not even have any training on samples with HTHA. Because of this reason, the reliability of these techniques is not known. A case in point is inspection of a C-0.5 Mo reactor that was tested by the author in The reactor was placed in service in 1972 and was operating above the C-steel curve. An inspection done in 2003 by another company concluded there was no HTHA in the reactor. The 2004 inspection by the author on the same reactor found backscatter depth to be about 40 percent of the thickness (see Figure 5). Three locations were flagged with possible HTHA and were recommended for follow-up replication. Replication confirmed HTHA at all the three locations. Further metallurgical, visual and magnetic particle tests from the reactor ID showed very high levels of HTHA. The results of these tests concluded that the reactor was operating at the end of its life and must be removed from service immediately. The reactor was never returned to service and retired after the inspection. This experience showed the effectiveness of the inspection in detecting serious HTHA damage and removing the reactor out service before causing a failure. This test also showed how another inspector missed HTHA at its last stage. One can therefore conclude that the reliability of the ultrasonic techniques is subjective and depends on the inspector s interpretation. Based on this experience, we recommend that plant owners carefully select inspectors to conduct critical inspections. An unreliable inspection is worse than no inspection as it gives a false hope of structural integrity. Wherever possible replication should be done to complement ultrasonic techniques. CONCLUSIONS Inspection of C-0.5 Mo Equipment operating above the C-steel curve requires reliable NDT method for detection of HTHA. Such inspection is done using the ultrasonic backscatter technique. This paper has introduced an additional tool for assessment of HTHA. Tests performed on a C-0.5 Mo reactor showed that the phased array technology successfully discriminated between normal base metal, base metal with stringers and base metal with HTHA. The phased array image clearly identifies HTHA and presents it in the 10

11 sectorial scan. The two dimensional Phased Array display is a significant improvement over the one-dimensional RF display for HTHA interpretation. Preliminary assessment has shown the approach to be very promising. Optimization of the method will further improve the reliability. REFERENCES 1. Steels for Hydrogen Service at Elevated Temperatures and Pressure in Petroleum Refineries and Petrochemical Plants, API Recommended Practice 941, Sixth Edition, American Petroleum Institute, T. Ishiguro, H.Yamamoto, K. Kawano, et al, Metallurgical Effect on Hydrogen Attack Damage in C-½Mo Steels, Proceedings, 1996 ASME/ICPVT Pressure Vessels and piping Conference, July, Hattori, K. and Aikawa, S., "Scheduling and Planning Inspections of C-0.5Mo Equipment Using the New Hydrogen Attack Tendency Chart", PVP vol 239/MPC-vol 33, Serviceability of Petroleum Process and Power Equipment, ASME, G. R. Prescott, "History and basis of Prediction of Hydrogen Attack of C-1/2 Mo Steel", Material Property Conference, Vienna, Oct 19-21, A. S. Birring, et al. "Method and Means for Detection of Hydrogen Attack by Ultrasonic Wave Velocity Measurements" US Patent, 4,890,496, January 2, A. S. Birring and K. Kawano, " Ultrasonic Detection of Hydrogen Attack in Steels", Corrosion, March, W. David Wang, Ultrasonic Detection, Characterization, and Quantification of Localized High Temperature Hydrogen Attack in Weld and Heat-affected Zone, Service Experience in Fossil and Nuclear Power Plants, PVP-Vol. 392, edited by J. Pan., ASME, New York, 1999, pp V. K. Varadhan, V. V. Varadhan and L. Adler, Frequency Dependent Properties of Materials Containing a Distribution of Pores and/or Inclusions, Nondestructive Methods for Material Property Determination, edited by C. O. Ruud and R. E. Green, Plenumm Press, N.Y, R.H. Latiff and N.H. Fiore, Ultrasonic Attenuation and Velocity in Two-Phase Microstructure, J. Acoustical Society of America, v 57, No. 6, Part II, June 1975,