Field probes advance NDE turbine inspection beyond state-of-the-art

Eddy currents (ECs) are electrical currents induced in a conductor by reaction with a magnetic field. They are circular in nature, with paths oriented perpendicular to the direction of the applied magnetic field.

In general, during EC testing, the varying magnetic fields are generated by an alternating current (ac) flowing through a coil positioned immediately adjacent to the conductor, around the conductor, or within the conductor. The ECs induced in the conductor can vary in magnitude and distribution in relation to attributes of the part being inspected, including: electrical conductivity; magnetic permeability; geometry; and homogeneity.

To yield useful information from eddy current inspection, one must be able to isolate and examine those areas of the specimen (a turbine blade, for example) consistently and without variation. Therefore in order to gain meaningful, accurate readings and to determine the condition of the component, the eddy current generating conductor must remain at the same exact distance and angle from the surface of the specimen under inspection.

This distance must be maintained for every subsequent specimen inspected because in a component of infinite dimensions, the intensity of the ECs will decrease as distance from the coil or surface of the part increases.

In practice, EC strength drops off so rapidly that at a relatively short distance from the coil the currents are negligible and become undetectable by conventional instrumentation. In all instances, the physical size of the conductor housing must allow the probe or coil to be placed consistently in the same position relative to the area being inspected. This effect or response due to geometry changes and coil-to-part spacing is called lift-off.

Changes in lift-off can also result from surface roughness, slight contour changes, probe wobble, probe bounce, and inconsistencies in the thickness of nonmetallic coatings, such as paint, primer, and anodic coating. This can be a problem because the magnitude of these impedance changes can exceed and mask the response from a relatively large crack or flaw in a particular component.

When performing manual EC inspections with a hand-held surface probe, it would be virtually impossible to maintain the probe at the same angle and distance to the part as the probe passes across its surface. To overcome this, EC probe manufacturers developed and fabricated geometry-specific conductor housings designed to conform to the specific shapes.

This technique has proven very effective in identifying flaw indications when the inspection zone is identified and targeted ahead of time. Unfortunately, that also means numerous geometry-specific probes are required to accomplish inspections of complex machines or structures like highly stressed aircraft frames, turbine blades and disk components.

Reduced inspection time

Over the past ten years, a great deal of work has been done to expand the coverage of EC probes and reduce inspection time. To this end, the aircraft industry has developed fixtures that allow the placement of several EC coils within a single fixture (multicoil arrays) designed, according to standard practice, to match a part's specific geometry.

Take a turbine disk slot for example; due to the nature of this highly stressed area it is very important that each disk slot be inspected along the pressure faces of both sides. Through the use of computer controls, the inspecting technician can be assured that all surfaces are scanned with a very high degree of reliability.

Although this technique is very sensitive and effective, it still requires multiple probes, fixtures, extensive personnel training and expensive inspection devices that can exceed $1 million in cost. Using this inspection equipment and technique can boost inspection times to as much as 12 hours per turbine disk. This all contributes to increased downtime, inspection time and related costs.

Traditional EC examination of turbomachinery components employs fixture and coil designs to match the inspection zone's geometry. These sensors and inspection techniques have shown their effectiveness in detecting very small cracks. Nevertheless, when applying EC technology to the inspection of turbine components in the industrial power generation setting, time factors and other cost-related issues and constraints have placed a premium on increasing zone inspection coverage and accuracy while decreasing inspection time.

Innovation

Working with its EC vendor, Siemens Power Corporation, based in Milwaukee, Wisconsin, has applied significant investment dollars and other resources to develop the extended field probe (EFP) for use in the power generation industry. With one pass of the coil, these innovative tools can inspect much larger areas of a given component thus outdating what had been accepted as the industry standard.

The EFP utilizes an EC coil with its axis parallel to the substrate. The coil is shaped to fit the broach slot geometry of an engine disk or blade root. A driver coil is positioned between two matched receiver coils wired in the differential mode and wound to operate over the range of 1 to 6 MHz. To ensure correct coil placement in relation to the angle of the broach slot, the coil is positioned accordingly in a Delrin shoe oriented parallel to the edge.

With this method, the coil is oriented perpendicular to the standard split driver coil and therefore the magnetic field and associated ECs generated by the inspection subject's material differ significantly.

The sensitivity of the shape coil drops off dramatically in relation to flaw orientation as a result of the current flow. So in the area where current density is highest, flow is generally in the direction perpendicular to the coil axis. Thus flaws or cracks oriented parallel to the coil axis obstruct the largest portion of current flow and therefore produce the highest signals. This type of flaw orientation generally occurs in the dovetail slots or turbine disk.

Siemens has been using this new technology for around three years, and as a result of the experience gained on Siemens turbines, has been able to reduce by 50 per cent the necessary NDE time to complete a major inspection while enhancing inspection sensitivity. This became apparent on a recent gas turbine major inspection outage.

The standard Siemens NDE procedure requires that an EC and fluorescent penetrate (FPI) level 4 inspections are performed on the stage 4 rotating blade roots. This requires two people and takes two days to complete the FPI portion and an additional day to complete the EC inspection. On the recent major inspection outage in question, the inspections were completed, but with no flaws or cracks identified during the procedure.

After these initial inspections, an EFP was fabricated and re-inspection of the entire section was carried out. A Delrin shoe, machined to fit the blade root profile, positioned a coil large enough to provide an affective EC field over the entire inspection zone. The inspection zone was scanned thoroughly, but with only one pass of the EFP, therefore ensuring that the entire blade root was inspected.

All stage 4 blades were re-inspected with the EFP in approximately one hour by one person. During the procedure, six blades were identified with suspect indications. Subsequently, the flaws the indications identified on five of the blades were removed by mild sanding/buffing.

On the sixth blade, however, the indicated flaw could not be removed by sanding because it was not found to be on the surface of the part. This blade was inspected again with a level 6 FPI and an even more powerful traditional EC probe. In both cases the flaw could not be found with these traditional methods. This was followed-up with an acid etch procedure which revealed, under a 30x stereo zoom microscope, the flaw that the EFP had identified.

Now with the flaw located visually, a second level 6 FPI was performed. The penetrate was allowed to dwell on that area for approximately 12 hours. This test was followed again by a scan with the more sensitive probe. As before, the flaw could not be identified. The blade was broken open and a crack 1.9 mm long and 1.2 mm deep was finally brought to light.

Judging by the nature and extent of the follow-up test procedures, Siemens inspection technicians came to the conclusion that the only method to identify this indication was via the ultra-sensitive EFP.

It is apparent that the EFP has improved sensitivity and can reduce inspection time. But an inherent problem with this type of EC probe, as well as other probes, is the wobble/lift-off effect and the fact that each probe must designed and fabricated to fit a particular application.

Siemens and its EC vendor began experimenting with coil/probe configurations to reduce this effect. The effort led to the development of a totally new EC probe concept called the flexible extended field probe (FEFP). Laboratory and subsequent field trials proved that this innovative design has been able to eliminate much of the probe wobble and lift-off problems encountered with traditional probe designs.

The FEFP probe allows a technician to inspect the blade root or disk slot of several different types of blades or disks with just one probe.

This FEFP also uses proven EFP technology but has the added advantage of limiting the lift-off effect. The net effect is a quicker, more accurate and lower-cost inspection routine. Further testing and application has shown that one FEFP will fit and inspect not only Siemens' stage 1 V84.2 rotating blades, but will also provide the same thorough and accurate inspection on GE Frame 7 and Westinghouse 501 series gas turbines.

FEFP up close

As with the EFP, the FEFP has a driver coil positioned between two matched receiver coils wired in differential mode and wound to operate over a range of 1 to 3 MHz. The coil is placed along a flexible membrane and housed in a Delrin holding device that is designed to locate and set atop the inspection surface.

The flexible membrane allows the coil to change with the dimensional aspects of the part while keeping lift-off to a minimum.

An inspection using the probe is accomplished by positioning the probe onto a calibration piece containing an electro discharged notch (EDM) of 0.762 mm x 3.81 mm x 0.0762 mm. Once calibration is completed, the probe can be placed upon the inspection surface. A spring-loaded outrigger is used to position the flexible sensor in the slot of the blade or disk, which ensures proper and repeatable sensor coverage. The spring-loaded outrigger secures the flexible EC sensor and prevents it from drooping or changing as the sensor exits the inspection area of the part.

A loading platform is used to align the probe to ensure proper outrigger deployment. The loading platform has an internal slot to protect the flexible coil assembly when not deployed and allows the coil assembly to move up and down in the slot for deployment.

The loading platform also has stabilizing slide rods to properly guide the coil block assembly as the operator loads the EC probe. Springs are mounted on the side rods to allow the coil block assembly to move up and down within the loading platform.

Once the probe is in the correct position, the operator applies downward pressure on the loading platform. The coil is deployed and the flexible coil extends and fills the inspection area. The probe is then scanned along the inspection surface while the technician monitors the EC display.

With one 5 to 7 s scan, the disk or blade slot is thoroughly inspected. Once this is accomplished, the probe is removed and the spring retracts the coil inside the housing for protection. To continue the inspection, the probe is moved to the next slot and the process repeated.

In the event a different blade or slot is selected for inspection, small adjustment devices are provided that permit the technician to control and properly deploy the FEFP and ensure the inspection area has 100 per cent coverage.

In fact, the presence of these adjustment devices is precisely why these probes can be used on multiple parts with varying geometry.

The major advantages of the FEFP include:

one probe will fit numerous parts

reduced costs by eliminating redundant EC probe development and fabrication expenses

significantly reduced operator training time

reduced inspection times (approximately 50 per cent)

100 per cent of the inspection surface is inspected with only one pass of the probe

reduced need to have multiple geometry-specific probes and standards

in the event of probe failure, almost any FEFP will be able to continue the inspection

two FEFPs will replace 16 standard EFPs used to inspect Siemens V84.2, V84.3 and V64.3 gas turbines.

As a result of this breakthrough, Siemens can inspect almost any turbine blade and or disk on site with little or no pre-inspection development expense.

Both new and experienced hardware can be inspected, and Siemens has estimated that it is able to inspect as many as four rows of turbine disk blade slots or blade roots in approximately four hours, resulting in a time savings of approximately four days over previous state-of-the-art NDE inspection techniques.