Remote Field Eddy Current Testing
Remote Field Eddy Current Technique is based on the transmission of an electromagnetic field through the tube material. The exciter coil generates eddy currents at low frequency in the circumferential direction. The electromagnetic field transmits through the thickness and travels on the outer diameter. A receiver coil that is placed in the remote field zone of the exciter picks up this field. In this zone, the wall current source dominates the primary field directly from the exciter. The separation between the two coils is between two to five times the tube ID.

The RFECT is generally used for detection of wall loss in carbon steel tubes. Pits can be detected using differential coils, however, the reliability for pit detection is limited. Pin holes can be missed by RFECT. Pit detection sensitivity can be improved by reducing the pull speed.

The inspection speed with RFECT is significantly lower than conventional ECT. While conventional ECT can easily be performed at a speed up to 6 ft/sec, RFECT is limited to about 10 inches/sec. Higher pull speeds will miss small defects and increase background noise.

Flaw sizing with RFECT is done using the Voltage-Plane curves (Figure 1). These curves are used to size tube wall loss. The curves relate flaw depth, flaw length, and the flaw circumference to the phase of the remote field signal in the absolute channel. Using these curves an experienced operator is able to determine the circumferential extent (in degrees), percent depth and length of wall loss. Pit depth sizing is performed by developing phase vs. depth curves in the differential channel.

Material permeability is another variable that can influence RFECT inspection. This is especially true in SA179 cold drawn carbon steel tubes where permeability changes along the tube length can easily be mistaken as wall loss and result in false calls. SA 214 hot rolled steels do not present this problem.

Since the RFECT depends on several variables that include frequency, pull speed, bandwidth, material permeability, it is all the more important that the operator be familiar with all these variables to optimize the inspection.

Figure 1. Voltage Plane curves used for sizing wall loss in carbon steel tubes
 

Ultrasonic IRIS

Ultrasonic Internal Rotary Inspection System (IRIS) is based on the principle of measuring thickness using ultrasonic waves. The IRIS probe consists of an ultrasonic transducer that is lined up in the centerline of the tube and incident on a rotating mirror. The mirror reflects the beam in the radial direction as it rotates in the tube. The IRIS probe scans the entire circumference of the tube as it is pulled out of the tube. The IRIS display includes the cross-section of the tube and a C-scan of the tube (see Figure 2).

The IRIS method is mostly used for inspection of carbon steel tubes and is sometimes used in non-ferromagnetic tubes for defect verification. The method is very accurate for thickness measurement as well as detecting ID and OD pits. IRIS can, however, miss pinholes. IRIS is not recommended for detection of cracks. The method is also slow with inspection speeds limited to 3 inches/second (75mm/sec). Because of the inability of maintaining water coupling during the entire tube length, the technique does not result in 100 percent coverage. Some areas can be missed. The inspection also requires good cleaning prior to inspection. Improper cleaning will result in uninspected areas.

One of the limitations of IRIS is the minimum measurable thickness. As the tube gets thinner, the time difference between the ID and OD signals gets smaller. This time difference reaches a limit so that the ID and OD signals cannot be resolved. The minimum level of thickness measurement depends on the tube material (ultrasonic velocity) and surface roughness of tube. In general, for in-service carbon steel tubes, thickness below 0.035 inches cannot be measured. The thickness limit for new (smooth) tubes can be as low as 0.025 inches.

Figure 2. RFECT-IRIS correlation on tube 51-43. RFECT detected wall loss under and adjacent to the END plate (support plate #3) of the feedwater heater. The wall loss is confirmed by IRIS. Nominal wall = 0.083 inches. Minimum wall under the support is 0.033 inches (60% wall loss).

To decide which is better for your application,
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NDE Associates, Inc.
515 Tristar Drive
Webster, TX 77598
Phone: 281-488-8944    Fax: 281-488-8485