Axial Inspection of Corrosion in Pipes

Application

A corrosion inspection solution for axial scanning of pipes using the Olympus HydroFORM^® scanner with customized gasket plates.

Background

Since its introduction to the pipe inspection market, the HydroFORM scanner has proven its worth at tackling corrosion mapping jobs. Thanks to the scanner’s innovative water column concept and the use of phased array ultrasonic techniques, inspection rates and data quality and density have improved immensely in relation to conventional ultrasonic (UT) inspection techniques.

Problem

Ensuring pipe integrity, particularly against corrosion, is a major preoccupation for the petrochemical industry. To make sure that the inspection of pipe systems is reliable and efficient, it is important to have the appropriate tools for the job. When inspecting for corrosion, sometimes only a strip of the pipe needs to be inspected in the axial (longitudinal) direction. For example:

• For smaller pipes that would require overly frequent indexing when scanning circumferentially;
• For pipes with a zone of interest that is concentrated in a specific region on the circumference; or,
• When 360-degree access to the pipe is not possible.

The standard HydroFORM and ChainSCANNER^™ package is a proven solution for circumferential inspections; however, the HydroFORM scanner on its own is not adapted for scanning longitudinally. By using the indexer clicker, or by manually using the software index clicker, it is possible to acquire multiple strips and stack them in the same data file. But, because of beam steering and coupling issues (two aspects that behave quite differently when the probe’s primary axis is sitting tangent to the pipe), this application requires some tweaking.

To ensure that the beams hit the surface at due 0 degrees, steering is required. Also, large-pitch and high-frequency probes, such as the standard 7.5L64-I4, are likely to exhibit side lobes, which disrupt the overall S-scan image and leak energy from the main lobe.

If the gasket’s curvature radius does not match closely enough to the diameter of the pipe, water leakage is also a problem.

Solution

To overcome these hurdles, a more appropriate phased array probe can be used in conjunction with curved gasket plates designed for the HydroFORM scanner.

Kit Components

Problem

This specialized application resembles the regular HydroFORM application, but with a few software and hardware differences. Typically, a linear group with an effective aperture of about 4 mm in width (8 elements with the 5L128-I4 phased array probe) is optimal. In most cases, the focal depth can be set to about 2 mm from the interface.

Figure 1 - Typical setup wizard configuration.

The results in Figure 1 were obtained in the OmniScan wizard using a configuration where all incident beams are perpendicular to the surface. Define the part as an axial tube and follow the steps of the wizard. The circumferential coverage for this configuration (12.75 inches OD) is around 55 mm from first law to last law. It should be noted that the defects shown at the extremities will look slightly smaller than the ones at the center of the probe because of the narrowing distance between the beams as the skew angle increases.

Figure 2 - An optional curved gasket is required to maintain coupling on the pipe.

Diameters between 12 and 20 inches require the use of custom curved gasket plates to ensure proper coupling (see Figure 2). The kit described above comprises one gasket plate (nominal 14 in. OD) for pipes from 12 to 16 inches OD, as well as a second gasket plate (nominal 18 in. OD) for pipes from 16 to 20 inches OD.

While the HydroFORM buggy is sitting on a smooth and regular portion of the pipe, the water column is adjusted using the adjustment knob (see Figure 3) so that all focal laws see the pipe’s interface at 0 mm in true depth. The resulting S-scan shows the interface as being flat. NOTE: Depending on the orientation of the scanner on the pipe’s circumference, the HydroFORM bubble management system may perform differently. It may be necessary to move the scanner quickly back and forth on the pipe to help remove bubbles that are trapped in the water chamber before starting the acquisition.

Figure 3 - S-scan display showing the flat interface.

Sensitivity calibration is recommended to homogenize the amplitude of the focal laws, and time-corrected gain (TCG) can be used to compensate for the attenuation in the material while limiting saturation of the interface echo. Sensitivity calibration is performed on all focal laws with the aid of the calibration wizard. TCG points are added manually on both the OD and the ID (ideally on a calibration block). This section presents the results obtained on a 12 in. OD pipe.

Figure 4 - A 12 in. pipe with a corrosion test patch and some flat bottom holes (FBH).

Figure 5 represents the corrosion patch mapping in the OmniScan flaw detector. A total of 121 focal laws correspond to a circumferential strip of about 55 mm (see Figure 1) with a circumferential resolution of 0.5 mm. Figure 5 - Thickness mapping of the corrosion patch.

Figure 6 shows a 3 mm FBH with a remaining wall of 7.5 mm that has been captured with a very good signal-to-noise ratio. Figure 6 - A 3 mm FBH at 7.5 mm under the surface.

Conclusion

The HydroFORM scanner can be used to perform axial scans, provided that the following conditions are met:

• The smallest OD in the range is 12 inches nominal.
• Two curved gasket plates are used for ODs ranging from 12 inches to 20 inches (both are included in the kit). Beyond 20 inches, the standard HydroFORM gasket plate can be used.
• The proper focal laws are calculated in the Olympus OmniScan MX2 or SX wizard.
• You’re using OmniScan MXU software version 4.4R2 or later.
• The probe that is used has the right pitch and frequency for beam steering according to the pipe diameter.