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Eddy Current Testing in the Petrochemical Industry

by A.S. Birring* and G.A. Marshall*

 

Eddy current testing is growing in industry applications. More sophisticated equipment and better technician training are largely responsible. Tougher problems for NDT to solve also contribute to the increased use.


Frank Iddings
Tutorial Projects Editor

 


Figure 1-3
Figure 4-6
Figure 7-9

INTRODUCTION

Eddy current testing has gained significant acceptance in petrochemical plants over the last 10 years. This is primarily due to the introduction of affordable eddy current instruments that have made eddy current testing cost effective in the petrochemical industry. The eddy current technology used in petrochemical plant heat exchanger tests is primarily a scaled down version of eddy current testing for steam generator tubing but at a significantly lower cost.

There are two major types of eddy current tests performed in petrochemical plants. These can be broadly categorized as surface testing and tubing testing. Surface testing techniques use portable and battery powered eddy current systems. The major applications are the detection of surface cracks, clad overlay measurement and wear measurements. Tube testing is done with multifrequency eddy current equipment and is used for testing heat exchanger tubes.

 

EDDY CURRENT TESTING

Principle
The eddy current method is based on the principle of measuring changes in the impedance of an electromagnetic coil as it is scanned over a surface of conductive material (Cecco, 1992). The test is performed by an electromagnetic coil that is placed over a conductive material (Figure 1). An alternating current in the coil produces a magnetic field that is induced in the material. To counter the coil's primary magnetic field, eddy currents are produced in the material. Eddy currents produce a secondary magnetic field H'B to oppose the coil's primary magnetic field HB. When the coil is scanned over a discontinuity, the secondary magnetic field is distorted, thereby changing the loading on the coil. Changes in coil loading directly affect the coil impedance. These changes in coil impedance may be related to the discontinuity.

 

The testing of heat exchangers is the number one application of eddy current testing in petrochemical plants.

Depth of Penetration
Discontinuity detection is limited to the penetration depth of eddy currents. Penetration depth is inversely proportional to the square root of conductivity, frequency and permeability. For most applications, the penetration depth in nonferromagnetic material is limited to approximately 5 mm (0.2 in.). In the case of ferromagnetic materials, such as carbon steel, the penetration depth is extremely shallow because of high permeability. Testing of ferromagnetic material is therefore limited to surface discontinuities only.

 

Instruments
Surface eddy current testing is normally performed with portable eddy current systems. These systems are usually single or dual frequency. Surface eddy current instruments normally cost between $6,000 and $10,000.

Eddy current testing of tubing is done using advanced computer controlled equipment. These systems are multifrequency, with the ability to store data that are acquired at high test speeds. Tubing eddy current instruments normally cost between $30,000 and $50,000.

Surface eddy current testing is performed using probes that include pencil probes, spot probes and cross axis probes. Eddy current testing of tubing is done using bobbin probes.

 

EDDY CURRENT TECHNIQUES
The following are the eddy current testing techniques that are used for petrochemical plant tests.

 

Tube Testing
There are three basic eddy current test techniques for testing tubes. Selection of the techniques depends on the tube material. Birring (2001) provides detailed information on eddy current testing techniques for tube testing.

Conventional eddy current testing is applied when testing nonferromagnetic heat exchanger tubes (ASME, 2001). The test is performed with a bobbin coil that produces an electromagnetic field in the tube. When the probe is pulled across a discontinuity, the electromagnetic field is distorted. This distortion in magnetic field changes the coil impedance that is related to the discontinuity. The eddy current testing method detects pits, wall loss and cracks.

Full saturation eddy current testing is applied in the testing of partially ferromagnetic and thin ferromagnetic heat exchanger tubes. The bobbin probes include a strong magnet that saturates the material magnetically. Once magnetic saturation occurs, testing is done in a manner similar to conventional eddy current testing.

Remote field eddy current testing is applied in the testing of ferromagnetic heat exchanger tubes such as those made of carbon steel. The test is performed with a bobbin coil that transmits an electromagnetic field in the tube. Remote field eddy current testing is limited to detection of large discontinuities and wall loss.

 

Surface Testing
Surface eddy current testing is used for the detection of surface cracks in both nonferromagnetic and ferromagnetic materials. The method is very sensitive in detecting tight cracks. Calibration is performed on electrical discharge machined notches as shown in Figure 2. In addition to detection, the method can accurately measure crack depth in nonferromagnetic materials. These materials include stainless steels and high temperature nickel chromium alloys. When sizing cracks, the eddy current test frequency is selected so that the depth of penetration is greater than the expected crack depth. Some common probes used for surface eddy current testing are spot probes, X-point probes and pencil probes.

Both surface eddy current testing and liquid penetrant testing are surface test techniques used for the detection of surface breaking cracks. Eddy current testing has advantages over liquid penetrant testing in certain applications:

  • eddy current testing is significantly more sensitive than liquid penetrant testing in the detection of tight cracks (for example, stress corrosion cracks in stainless steels)
  • eddy current testing can measure crack depth in nonferromagnetic materials, whereas liquid penetrant testing does not have this capability
  • eddy current testing can test through paint coatings
  • for testing in small areas, eddy current testing is much faster; liquid penetrant testing is slow because of the long dwell times.

 

Clad Overlay Measurement
This method is applied for measurement of nonferromagnetic material clad over a ferromagnetic base (for example, stainless steel clad over carbon steel) as shown in Figure 3. A change in clad thickness changes the impedance of the eddy current coil. This change in impedance is related to the clad thickness. Eddy current testing is a fast and reliable method for this application. Testing is done with spot probes.

 

Coating Thickness Measurement
The method is based on the principle of measuring the liftoff of the eddy current test probe over the surface. Portable eddy current machines are available for this application. Testing is done with spot probes.

 

Wall Loss Measurement
The eddy current test method can measure wear or wall loss in nonferromagnetic materials. The method is applicable if the depth of penetration is greater than the material thickness. Varying the frequency can control eddy current depth of penetration. Lowering the frequency can increase depth of penetration.

 

PETROCHEMICAL PLANT APPLICATION

Heat Exchangers: Pitting, Cracks, Wall Loss
The testing of heat exchangers is the number one application of eddy current testing in petrochemical plants. Heat exchangers include condensers, general petrochemical plant exchangers, feedwater heaters, air coolers and lube oil coolers. Both conventional and remote field eddy current testing are used (Figure 4). Conventional eddy current testing is used for testing nonferromagnetic tubing, such as stainless steel, copper nickel alloys and titanium. Remote field eddy current testing is applicable for testing ferromagnetic tubing, such as carbon steel and nickel. Conventional eddy current testing is a fast, reliable and accurate method for detecting discontinuities in tubing. Tests can be done at pulling speeds of up to 1.8 m/s (6 ft/s). Remote field eddy current testing is limited to the detection of larger discontinuities and test speed is limited to approximately 0.3 m/s (1 ft/s). A specialized version of conventional eddy current testing is full saturation. This technique is applicable for thin ferromagnetic tubes such as SA-268 steel in condenser tubes and partially ferromagnetic tubing materials such as nickel copper alloy, SA-789 steel and SA-790.

A heat exchanger test report is compiled by making a tube map and superimposing the eddy current test results for each tube. The colors represent the discontinuity depth range. Figure 5 shows an example of the tubesheet map report.

 

Heat Exchangers: Cracks under Tubesheet
A unique problem in heat exchangers can be leaks in the tubesheet roll. This can occur when overrolling of the tubes under the tubesheet causes circumferential cracking. In addition, there can be circumferential cracks just behind the tubesheet. Such cracks can easily be missed by bobbin probes and require a special technique. Cracks under the tubesheet are detected using surface probes. These probes can be simple handheld surface probes or motorized probes similar to the ones used for bolt hole testing in the aircraft industry. Figure 6 shows eddy current testing under the tubesheet using the motorized rotating probe.

 

Vessels
Two major applications of eddy current testing in vessels are crack detection and clad thickness measurement.

Surface crack detection and measurement is performed on the inner diameter surface of stainless steel vessels (Figure 7). The technique is used as an alternate to liquid penetrant testing as it can detect tight cracks and is significantly faster. Surface testing can include both the base metal and the weld. Base metal testing can be done using spot probes or pencil probes. These probes, however, cannot be used in the weld areas because of signals produced from liftoff. In such a case, special probes such as crosspoint send/receive probes are used.

Clad thickness measurement is performed on carbon steel vessels with stainless steel or high temperature nickel chromium alloy clad. Cladding can wear from erosion, thereby exposing carbon steel to direct chemical attack. Eddy current testing is a rapid and accurate technique for measuring loss of stainless steel clad over the carbon steel shell. Several hundred measurements can be taken per hour. Portable eddy current equipment is used for such tests.

 

Piping Systems: Surface Crack Detection and Measurement
Stainless steel is susceptible to stress corrosion cracking when exposed to chlorides and moisture. One source of chlorides is insulation with more than an acceptable level of chlorides. Eddy current testing is a highly effective technique for the detection of stress corrosion cracking on the outer surface of stainless steel piping. The technique detects tight cracks that can be missed by liquid penetrant testing. Eddy current testing will also detect cracks that are just below the surface and within the eddy current skin depth. Rapid scanning of pipe surface is done manually using eddy current spot probes.

 

Bellows: Surface Crack Detection
Cracks in bellows are caused by stress corrosion. Both stainless steel and nickel chromium bellows are tested for the detection of inner and outer diameter surface cracks (Figure 8). Testing is done with a surface probe that is placed between the convolutions of the bellows and scanned in the circumferential direction. Eddy current frequency is adjusted to obtain full penetration so that both inner and outer diameter cracks can be detected.

 

Turbine and Compressors
Eddy current is used for detecting surface cracks in turbine and compressor components, including disks and blades. The technique is used for surface crack testing of blades, disks and rotor bore. The applications include crack detection in the disks, at the trailing edges of blades and steeples of blades (Figure 9). While magnetic particle testing can also be used for this application, eddy current testing is more sensitive for detection of tight cracks in turbine blade steeples.

 

Wear Measurement
A special application of eddy current testing for gas turbine blades is wear measurement. Gas turbine blades wear with time from erosion. Excessive erosion can expose the cooling holes in the blades. Eddy current testing is used to effectively measure the thickness of the material over the cooling holes and indirectly measure the wear. Normally, a remaining wall of less than 0.5 mm (0.02 in.) is a cause for repair or rejection.

 

TRAINING AND CERTIFICATION OF EDDY CURRENT TECHNICIANS

The above discussion shows that eddy current testing is a sophisticated and technically challenging technology. It is therefore very important that the technicians performing it understand the techniques and are properly trained in the application of the technology. Normally, an eddy current technician conducting tests is required to be certified as a Level II per SNT-TC-1A (ASNT, 2001). However, since Level II is an employer based certification, there is wide variation in the quality of technicians and their skills. When improperly trained technicians make mistakes, it can give the technology a bad name. For example, a technician trained only for crack detection will not be able to measure clad thickness without additional training. There have been several cases where companies have stopped using eddy current technology altogether after having bad experiences from poorly trained technicians. Therefore, it is very important that an experienced instructor train and test technicians performing eddy current testing.

Recognizing this problem, some companies using eddy current testing have instituted their own practical tests, especially for the testing of heat exchanger tubes. Only inspectors who qualify on the tests can work in these facilities. At present, there are four such companies that require inspector testing in addition to the basic Level II certification. Inspectors are tested on mock up bundles that contain artificial and service induced discontinuities in heat exchanger tubes. Technicians are tested on these bundles to determine their performance. Performance is measured either as pass/fail criteria or as the candidate's score in terms of probability of detection.

 

CONCLUSION
This paper broadly presented the application of eddy current testing in the petrochemical industry. The applications included testing of heat exchanger tubes, vessels, bellows and turbines. While these are the major applications, there are several more areas where this technology can be applied. The paper also emphasized the importance of using technicians who are properly trained and competent in this technology.

 

REFERENCES
American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, Section V, Article 8: Eddy Current Examination of Tubular Products, New York, ASME, 2001.

American Society for Nondestructive Testing, Recommended Practice No. SNT-TC-1A, Columbus, Ohio, ASNT, 2001.

Birring, A.S., "Selection of NDT Techniques for Heat Exchanger Tubing," Materials Evaluation, Vol. 59, 2001, pp. 382-391.

Cecco, V.S., "Eddy Current Inspection," Nondestructive Evaluation and Quality Control, Vol. 17, 1992, pp. 164-194.

 

 

*  NDE Associates, Inc., 515 Tristar, Webster, TX 77598; (281) 488-8944; fax (281) 488-8485; e-mail <nde@nde.com>.

 

Copyright © 2003 by the American Society for Nondestructive Testing, Inc. All rights reserved.

 
 
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