HTHA occurs in low-alloy carbon steels that are subjected to high pressure and high temperature, typically above 205°C (400°F). In this environment, hydrogen can dissociate and react with carbides in the steel to form methane. These methane molecules are larger than hydrogen atoms and therefore cannot diffuse through the steel. Instead, they build up in the grain boundaries, where they can, over time, cause surface decarburization, microcracks, voids and eventually large fractures.
Pressure vessels at refineries, petrochemical plants, hydrogenation plants and other facilities where hydrogen-containing fluids are processed are especially susceptible to HTHA. These large vessels are assembled using long runs of longitudinal and circumferential welds with significant heat-affected zones (HAZ), a common area for HTHA to occur.
Early on, HTHA can be hard to detect with conventional ultrasonic testing alone because the methane voids are so small, usually less than 0.01 mm (0.0004 inch). That’s much smaller than most ultrasonic wavelengths. For this reason, it is necessary to use a combination of advanced inspection techniques, specifically phased-array ultrasonic testing (UT), time-of-flight diffraction (TOFD) and high-resolution total focusing method (TFM).
If you are new to ultrasonic testing or you contract inspection services and are unfamiliar with TOFD or TFM, it is important to understand the basic concepts of these techniques. You will be better equipped to work with your testing service on an inspection plan that delivers early detection and better characterization of HTHA.
Phased-Array UT and TOFD
Time-of-flight diffraction is a rapid and robust ultrasonic technique for initial screenings of base material and welded regions. In a TOFD system, the technician places ultrasonic probes on opposite sides of a weld. One probe acts as a transmitter, emitting an ultrasonic pulse into the material; the other is a receiver.
Instead of measuring only for high-amplitude soundwaves that reflect off the back of a component, the TOFD method calculates the response time of low-amplitude waves that are diffracted by the tips of discontinuities in the material, such as microcracks and other voids. The simultaneous use of phased-array UT and TOFD can produce significant advantages.
The technician can use a two-sided phased-array UT examination with standard shear-wave phased-array probes to detect planar and surface-breaking flaws and TOFD to accurately locate and size embedded flaws. This combination can detect all welding flaw types and provide reliable through-wall sizing in a single pass, which increases probability of detection and improves the productivity of the inspection team simply by reducing the number of scans and manipulations that need to be done.
At a minimum, the scanning device required for the simultaneous encoded application of phased-array UT and TOFD needs to support four search units (two phased-array UT probes and one TOFD pair). As the thickness of the material becomes larger, however, the TOFD inspection needs to be done in different depth zones and therefore requires more TOFD probes and larger probe-center separation to cover the full volume of interest. In order to maximize the efficiency of the inspection crew, which often has to inspect thin-wall and heavy-wall components in the same shift, the scanner should allow the flexibility to support a wide range of thicknesses.
A similar technique using TULA probes (TOFD ultra-low angle) is well-suited for inspections of thicker base materials. Like regular TOFD, increased grain noise (short indications) and clustering (beehive) in A-scan signals are indicative of early-stage HTHA.
TOFD and TFM
Combining TOFD with high-resolution “live” TFM can leverage the benefits of both techniques for HTHA inspections.
A TFM algorithm sums the elementary A-scan signals from all elements in the array to generate a frame of pixels, where each pixel is computed using a dedicated focal law. High-resolution TFM frames can be used for “live” interpretations, or they can be stored for each position of the probe for processing similar to a “dynamic” merge view in regular phased array.
In theory, ideal focusing is achieved in each point of the frame, but the focusing capability of this technique still depends on the acoustic wavelength and total aperture of the array. TFM has the ability to conserve ideal focusing over a much larger region as long as the region of interest is in the near-field of the active aperture.
For thicker components, a large aperture is required to be able to focus in the complete region of interest. For thinner components – for instance, to inspect for HTHA or corrosion on components between 5 and 15 mm thick – it can be an advantage to use a smaller aperture to generate the TFM frames. Only the probe elements close to the defects will adequately contribute to the imaging in these thin components.
This is where it is useful to have a phased-array UT instrument like Zetec’s TOPAZ64, which has the capability to perform a “sliding” TFM with a partial aperture of the probe – for instance, 16 or 24 elements out of 64. This sliding TFM is the equivalent of the electronic linear scan, but the moving aperture is used to generate a totally focused signal.
It can also reduce the wedge echoes when performing TFM with the probe on a 0-degree LW wedge. As with any weld inspection that uses phased array, probes need to be positioned at the optimal beam angles for detecting the damage, either in the first-half skip or after skipping off the back wall.
Because advanced ultrasonic inspection techniques produce digital results and have a high degree of repeatability, changes in degradation can be recorded and compared over time.
Historical records are important. There have been a number of changes to regulations and industry guidance to reduce the risk of HTHA over the years, including post-weld heat treatment (PWHT) and recommendations from the U.S. Chemical Safety Board to replace carbon-steel process equipment that operates above 205°C (400°F) and greater than 50 Psa hydrogen partial pressure.
Many “legacy” pressure vessels constructed to different standards remain in service today, so it is important to incorporate nondestructive testing for HTHA and other types of wear, damage and defects that can occur throughout decades of use in severe environments.
Well-trained technicians using the right combination of ultrasonic techniques and advanced technology can do their part to detect and monitor HTHA damage while improving plant reliability and worker safety.
All photos/graphics provided by Zetec Inc.