Guided wave ultrasonic testing (GWUT) or long-range ultrasonic testing (LRUT) is a non-destructive testing (NDT) method that uses torsional, longitudinal, or flexural ultrasonic waves to detect percent cross-sectional area (%CSA) loss in process, petrochemical, transmission, and other industry piping systems. Depending on the pipe condition the sensitivity can be as low as 1% CSA loss but a more reasonable number is 5% given the wide range of inspection variables which include inside-diameter (ID) surface conditions, outside-diameter (OD) surface conditions, coating, and product conditions. GWUT can also be applied to high temperature piping cross sectional area loss inspections. Using dry-coupled high temperature sensors, GWUT can be applied to pipelines operating in the 60 to 500 decree Celsius range.
High Temperature GWUT Pipeline Sensor
Depending on the temperature of the pipeline to be tested with guided waves, the user may have the option to user piezoelectric or magneto-strictive sensing technology. Since piezoelectric materials de-pole at higher temperatures, magnetostrictive sensor (MSS) guided wave testing is often the most practical solution for LRUT. Where MSS sensor installation is labor some on standard temperature pipelines due to a multi-step epoxy-based installation process, the argument can be made that it is simplified at higher temperatures using the dry-couple sensor design.
Figure 1: High temperature dry-coupled guided wave sensor
The dry-coupled guided wave sensor is installed on the pipeline using a mechanical leveraging design. At high enough clamping forces, the torsional motion generated in the high magnetic permeability cobalt alloy strip is coupled into the pipe OD starting the torsional guided wave generation process. Like conventional ultrasound, guided wave instrumentation also exhibits a main bang or dead zone. The dead zone using high temperature guided wave sensor can be as long as 12 inches about 2-3 time longer than standard temperature guided wave pipeline sensors. Like the standard MSS sensor the dry-coupled sensor also requires the cobalt to be conditioned using a permanent magnet. The objective of this process is to align the magnetic domains in the cobalt strip to maximize the torsional displacement generated in the strip and then the pipe.
Example guided wave data from good and poor conditioned dry coupled sensors are shown below. In the top data, very pure torsional guided wave generation is observed at excellent sign-to-noise ratio (SNR). A narrow main bang, or guided wave dead zone, weld reflections and corrosion are observed at excellent SNR. The bottom GWUT data is typical of a poorly conditioned GWUT dry-coupled sensor. The main bang is wider, SNR is lower, and the GWUT reflections are broader due to flexural wave generation. High temperature guided wave sensors are commonly used on process pipelines over which heat tracer lines are installed. The thin GWUT sensor profile usually slides under the heat tracer line with little to no modification of the tracer line.
Figure 2: High temperature dry-coupled guided wave installation considerations
Heat tracing is one well-known method to keep process media warm. This process involves applying an external heating source to process pipes, tanks, or other vessels. Steam tracing is a form of heat tracing that circulates steam around process pipes to keep the process media within a specific temperature range. Tracer lines are the tubing that carries the steam alongside the process pipe. Both electrical heat tracing and steam tracing are commonly used at process plants. The tracer lines obstruct the GWUT sensor installation but the GWUT dry coupled sensor can be installed under the heat tracer line.
After the guided wave sensor is installed, preliminary data is acquired and sensitivity calibration is performed. It is standard to use 2 or more girth welds for sensitivity calibration and to establish the attenuation in the pipeline (db/m). During the calibration process the weld profile is measured and converted to a known %CSA loss. At the conclusion of the GWUT calibration process, the guided wave amplitude response to a known %CSA reflector and torsional guided wave attenuation are known.
Figure 3 show LRUT data upstream (right) and downstream (left) from the sensor installation position (0 meters). It should be noted that a guided wave sensor array is naturally bi-directional and will inherently excite guided waves directed in both upstream and downstream directions. Guided wave focusing algorithms are applied to first focus data in the forward direction while cancelling out the backward propagating wave. The GWUT focusing algorithm is revered to focus in the backward direction while cancelling out the forward travelling wave. GWUT leakage will always occur in the undesired direction, however. Guided wave bi-directionality is observed at about 3 meters from the sensors in the data shown below. A very strong guided wave reflection is observed at -3 meters and mirrored at +3 meters at lower amplitude. While the LRUT reflection is lower amplitude in the forward direction, 100% cancellation was not achieved. The inability to completely focus presents a secondary issue, namely that there is now a narrow blind zone at the mirrored location.
Figure 3: High temperature dry-coupled guided wave data.
Upon completion of successful GWUT sensor installation, %CSA calibration, and attenuation calibration, guided wave data is relatively straight forward to interpret. A quick analysis of the GWUT data shown above shows that three different girth welds were detected in the downstream direction and one in the upstream direction. Numerous pipe supports were detected in the DS and US directions. An elbow in the upstream direction generated two strong reflections from the leading and trailing girth welds. The practical inspection range in the US direction was limited to 13 meters due to the 90 degree elbow at this location. The DS direction range was approximately 30 meters with a strong weld reflection at 27 meters. An indication from suspected corrosion was observed at -25 meters and follow-up ultrasonic thickness testing was performed in this area.
Summary
This article introduces the reader to high temperature guided wave testing using dry-coupled magnetostrictive sensors. Using dry-coupled high temperature sensors, GWUT can be applied to pipelines operating in the 60 to 500 decree Celsius range. Example guided wave data is from properly and improperly installed sensors are presented for quality control purposes. Finally, example guided wave data from a high temperature pipeline with heat tracers is shared. Excellent LRUT signal-to-noise ratio is achieved.