Date: November 3, 2010

Graphitization Tube Study

Author: Shawn Gowatski

Mirant provided 6 boiler tubes to TesTex that were suspected of containing graphitization. Four

of the tubes were removed from Bowline Station and two were removed from the Pittsburg Station.

The Balanced Field Electromagnetic Technique (BFET) was used to examine the tube samples. Curved

OD HawkeyeDP probes along with a 45˚ Hawkeye DP probe were used with the ProdigyII Electronics

(serial #09-3118-04)

Balanced Field Electromagnetic Technique Fundamentals:

The Balanced Field Electromagnetic Technique was developed to enhance the signal responses

produced from smaller defects such as cracks. The technology was originally used to detect cracking

from the outside surface to inspect welds in DA Tanks, pressure vessels, tube stubs, and any other areas

suspected to have cracking. In this technique, electromagnetic coils are wound and arranged in a

balanced state. This balanced state is achieved by placing coils in both the “x” and “z” geometries at

zero potential to each other. With the excitation coil in the “x” geometry and the sensor coil in the “z”

geometry, a differential signal is produced over defected areas. In detail, the alternating current

produced by the excitation coil is uniform and undisturbed when no defects are present. Conversely, the

current is interrupted when a defect is present and forced to travel around it in a distorted fashion. It is

this state of distortion that causes the coils to become unbalanced and thus producing an indication for

the user that signals a defect. This distortion signal response can be measured and a crack depth can be

calculated by comparison to a calibration standard. The sizing of graphitization will be extremely

difficult but some type of ranking is possible through the scanning some collected tube samples with

graphitization. At minimum, the defects found can be ranked or classified according to the signal

strength of the defect. Another key to the balanced field electromagnetic technique is the ability to

eliminate liftoff (and/or probe wobble) and noise from the signal. This is accomplished through a

special algorithm in which these unwanted elements are rotated away from the main signal.

With the technology based on electromagnetics, a polished surface is not required. Quality

readings can be acquired through coatings such as paint, epoxy, and rubber. The actual probe does not

need to be in contact with the test piece. Sometimes the probe is placed in a small cart with the probe

base set slightly above the test piece. The probe can be pulled quickly across the test specimen at a

speed up to 1 foot per second. The BFET method can test different types of metal by adjusting the test

frequencies which range from 100HZ to 30,000HZ.

During an inspection, the data is viewed in real-time on a computer screen. This allows defects

to be found and located during the data collection process which allows a plant to quickly prove-up any

suspect defects and to schedule repairs without having to wait for the completion of the job.

The picture above shows various OD HawkeyeDP probes used to test the samples. The handle on the probe can be modified

to fit the probe into tight areas.

The picture above shows the 45˚ HawkeyeDP.

Findings:

All six tubes were scanned initially with the 45˚ HawkeyeDP probe. This probe design allows

circumferential and axial cracks to be detected. The circumferential HawkeyeDP probes were used to

prove-up any defects found and also to test around the welds on the two Pittsburg samples. The tests

were performed at 100Hz. A total of 8 discontinuities were detected. Each area containing a

discontinuity was marked with a square and labeled by a number. Some of the discontinuities gave a

stronger response than others. TesTex did not have any calibration standards to size the responses from

these discontinuities. The discontinuities are graded in a relative ranking from the strongest to weakest

signal and are listed below. Indications 6, 7, and 8 are on the two Pittsburg tubes and the remaining

indications are on the Bowline samples.

Indications 3, 6, and 8 showed the strongest signal response.

Indications 1 and 5 gave a moderate signal response.

Indications 2, 4, and 7 gave a small but distinguishable signal response.

A Metallurgical evaluation was performed on all eight indications and graphitization was found in each

area. The table below lists the results of the metallurgical evaluation.

Area Number Station Damage Present

1 Bowline Chained graphitization through section

2 Bowline Chained graphitization through section

3 Bowline Chained graphitization through section

4 Bowline Chained graphitization through section

5 Bowline Chained graphitization through section

6 Bowline Chained graphitization through section

7 Pittsburg Chained graphitization at OD with scattered graphite nodules

throughout

8 Pittsburg Band of graphite midwall with scattered graphite nodules throughout

The table above shows the metallurgical evaluation results.

Waveforms and pictures for each indication follow.

The waveform above is from indication 1 that was collected The waveform above is from indication 1 that was collected

using a 45˚ HawkeyeDP. using a circumferential HawkeyeDP.

The Balanced Field Electromagnetic Technique (BFET) waveform is displayed in (5)

different windows. The bottom right window is the raw data. The bottom left window shows the data

processed. The middle left window is a simulated C-scan. The top left window is a zoomed in view of

the data and the top right window shows a color-coded 3D view of the data. Any sharp crack-like

defects will cause the waveform to rise up/down sharply.

The picture above is a traverse section from Indication 1 with chained graphitization at the 11 and 1 o’clock positions.

The picture above is from Indication 1 showing chained graphitization at higher magnification.

The picture above is from Indication 1 showing chained graphitization at higher magnification.

The waveform above is from indication 2 that was collected The waveform above is from indication 2 that was collected

using a 45˚ HawkeyeDP. using a circumferential HawkeyeDP.

The first picture above is from Indication 2 showing chained graphitization through wall and the second picture is from

Indication 3 showing chained graphitization through wall.

The waveform above is from indication 3 that was collected The waveform above is from indication 3 that was collected

using a 45˚ HawkeyeDP. using a circumferential HawkeyeDP.

The waveform above is from indication 4 that was collected The waveform above is from indication 4 that was collected

using a 45˚ HawkeyeDP. using a circumferential HawkeyeDP.

The first picture above is from Indication 4 showing chained graphitization through wall and the second picture is from

indication 5 showing chained graphitization through wall.

The waveform above is from indication 5 that was collected The waveform above is from a visible crack that was collected

using a 45˚ HawkeyeDP. using a circumferential HawkeyeDP.

The waveform above is from indication 6 that was collected The waveform above is from indication 6 that was collected

using a 45˚ HawkeyeDP. using a circumferential HawkeyeDP.

The first picture above is from Indication 6 showing chained graphitization through wall and the second picture is from

indication 5 showing chained graphitization near the OD.

The waveform above is from indication 7 that was collected The waveform above is from indication 8 that was collected

using a circumferential HawkeyeDP using a circumferential HawkeyeDP.

The pictures above show the graphite nodules as polished from Indication 8.

The picture above shows indications 1 and 2.

The picture above shows indications 3 and 4.

The picture above shows indication 5. There was some uneven scale on the surface, so the location of the signal was

sandblasted.

The picture above shows the location of indications 6 and 7. The area around the weld was sandblasted due to uneven scale

on the surface.

The picture above shows the location of indication 8. The area around the weld was sandblasted due to uneven scale on the

surface.