TECHNICAL REPORT ON CAPACITY BUILDING IN USE OF EDDY CURRENT TESTING EQUIPMENT

Transcription

1 REFERENCE NO. PNRA-CNS-NDT MAY, 2013 TECHNICAL REPORT ON CAPACITY BUILDING IN USE OF EDDY CURRENT TESTING EQUIPMENT PAKISTAN NUCLEAR REGULATORY AUTHORITY P.O. BOX 1912, ISLAMABAD i

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5 Contents 1. INTRODUCTION THE EDDY CURRENT TESTING TECHNIQUE BASIC CONCEPT SKIN EFFECT EDDY CURRENT SENSITIVITY AND DEPTH OF PENETRATION LIFTOFF APPLICATION PRACTICAL WORK KEY FACTORS FOR ET INSPECTION CONDUCTIVITY MEASUREMENT SURFACE FLAW DETECTION CONCLUSION FUTURE TASKS... 6 iv

6 List of Figures Figure 1: Surface probe being used for ET... 2 Figure 2: Eddy Current Depth of Penetration... 2 Figure 3: Practical work being done on the ET equipment... 4 Figure 4: Conductivity Measurement... 5 Figure 5: Surface Flaw Detection... 6 v

7 1. INTRODUCTION Nondestructive testing (NDT) consists of test methods used to examine an object, material or system without impairing its future usefulness. A limited scale NDT laboratory was established in PNRA with a view to train nuclear inspectors of PNRA to enhance the quality of regulatory oversight of nuclear power plants in Pakistan. The following equipments are available in the laboratory: 1. Ultrasonic flaw detector 2. Ultrasonic thickness gauge 3. Portable hardness tester 4. Eddy current test equipment 5. Coating thickness gauge 6. Penetrant testing 7. Universal hardness tester 8. Replica Metallographic Equipment A PNRA task force comprised officers of CNS and SNRS was established to make the equipment functional. This report is submitted after completion of self training on the eddy current test equipment, Phasec-2d. For Phasec-2d, the following team was engaged: 1. Muhammad Rafiq (SE, CNS) 2. Aliullah Jan (SE, CNS) 3. Mansoor Arshad (AE, CNS) 4. Ahsan Shaukat (AE, SNRS) 5. Zafar Abbas (AE, SNRS) The training program comprised of the following activities: 1. Study of ASNT ET Manual 2. Study of Phasec-2d Equipment Manual 3. Practical Work 2. THE EDDY CURRENT TESTING TECHNIQUE 2.1 Basic Concept Eddy Current Testing is one type of NDT and is a powerful method used for flaw detection/evaluation, material characterization, measurement etc. Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil as shown in Figure 1. 1

8 2.2 Skin Effect Figure 1: Surface probe being used for ET Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect. 2.3 Eddy Current Sensitivity and Depth of Penetration The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. This concept is described in the figure 2 below. The depth at which eddy current density has decreased to about 37% of the surface density, is called the standard depth of penetration. Figure 2: Eddy Current Depth of Penetration Since the sensitivity of an eddy current inspection depends on the eddy current density at the defect location, it is important to know the strength of the eddy currents at this location. When attempting to locate flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This helps to assure that the strength of the eddy currents will be sufficient to produce a 2

9 flaw indication. Alternately, when using eddy currents to measure the electrical conductivity of a material, the frequency is often set so that it produces three standard depths of penetration within the material. This helps to assure that the eddy currents will be so weak at the back side of the material that changes in the material thickness will not affect the eddy current measurements. 2.4 Liftoff Liftoff is defined as the distance between the probe and the test sample. The counterpart of lift off for tubular (both internal and external) inspection is called fill factor. As this distance is increased magnetic coupling between the probe and the sample decreases, which results in lesser eddy currents in the test sample. 2.5 Application One of the primary uses of eddy current testing is defect detection when the nature of the defect is well understood. In general, the technique is used to inspect a relatively small area. Since eddy currents tend to concentrate at the surface of a material, they can only be used to detect surface and near surface defects. The main application of ET in the nuclear industry is related to steam generator tube inspection which is a very vital job performed in every RFO. The whole length of 2977 tubes per steam generator (including U- bends) is examined in the process. Flaws like pitting, baffle cuts, magnetite deposits, wall thinning, fatigue or SCC are efficiently detected. Other applications include inspection of closure studs and nuts of RPV for surface flaws like fatigue or creep. 3. PRACTICAL WORK Keeping in view the common applications of ET in the nuclear industry, two exercises were performed using the Phasec-2d equipment for eddy current testing: i. Conductivity Measurement ii. Surface Flaw detection 3.1 Key Factors for ET Inspection Factors such as the type of material, surface finish and condition of the material, the design of the probe, and many other factors can affect the sensitivity of the inspection. Successful detection of surface breaking and near surface cracks requires: a) Knowledge of probable defect type and service history of the component regarding thermal, mechanical and chemical aspects. b) Selection of the proper probe. The probe should fit the geometry of the part, with a fill factor of at least 80% for internal and external probes. c) Selection of a reasonable probe frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at same level of penetration. 3

10 d) Reference standards made of material similar in chemical composition, heat treatment and mechanical working as the component under inspection. Usually, the reference standard is manufactured in the same way as the component under examination. e) ET is applied to materials which are conductive and non-ferromagnetic. Commonly eddy current tested materials are Cu and its alloys, austenitic stainless steels etc. Ferromagnetic materials have to be first magnetized to saturation level and then tested by this method; even then complications may occur. So for ferromagnetic materials, specialized techniques such as RFT and flux leakage are applied. Figure 3: Practical work being done on the ET equipment 3.2 Conductivity Measurement In SNRS Lab, conductivity of an available Copper sample was determined using Phasec-2d. Experiment Setup Equipment Phasec-2d Probe Conductivity Reference Standard Dual Conductivity Standard Frequency 500Hz 2kHz Phase 167 Procedure i. Turned the Phasec-2d ON and connected the conductivity probe ii. Balanced the probe in the air by pressing balance button on top left corner of the equipment iii. Calibrated the equipment using dual conductivity standards, firstly by placing it upon 58.8% and then 8% IACS blocks. System gave a message of calibration completion. iv. Placed given conductivity sample under the probe and system automatically gave its conductivity in terms of %IACS. 4

11 Figure 4: Conductivity Measurement Remarks The equipment was successfully used for conductivity measurement. The result of conductivity measurement is shown in figure 4 above. 3.3 Surface Flaw Detection In SNRS Lab, surface scanning of a metallic strip type specimen was performed using Phasec-2d. Experiment Setup Equipment Phasec-2d Probe Conductivity Reference Standard Dual Conductivity Standard Frequency 500kHz Phase 167 Gain 40 Lift off Settings Automatic (In Conventional method ferrite lift off vertical) Procedure i. Turned Phsaec-2d ON and connected the surface probe. ii. Placed the Probe on sound portion of the standard material and balanced it. iii. The lift off was set horizontal towards left by changing the phase settings of the equipment or by pressing Lift OFF for two seconds which automatically set in the desired direction. iv. Moved the probe along the surface of the sample held firmly against it and noted any indications appearing. v. Indications could be distinguished by their height and phase angle, greater the size of flaw, greater is the indication, and greater the depth of the crack, greater is the phase angle. Remarks All surface flaws present in the specimen strip were successfully detected. The result of Surface flaw detection is shown in figure 5 below. 5

12 Figure 5: Surface Flaw Detection 4. CONCLUSION Two experiments of the standard of NCNDT ET Level-I Certification Course, viz. Conductivity Measurement and Surface Flaw Detection, were successfully completed. As a result of the insight gained into the eddy-current testing, training courses can be devised in the future to impart basic knowledge of ET to inspectors of PNRA which would be useful in monitering ET inspection results at NPPs such as in RFO and major maintanance activities. It may be mentioned that due to the unavailability of reference standards and lack of Phasec-2d-specific advanced training, flaw detection in tubes could not be performed. In this regard, an order has been placed with HMC-3, Taxila for the manufacture of ET tubular standards. However, further training on the Phasec-2d equipment would be required from some local or foreign institutes such as NCNDT (PAEC). 5. FUTURE TASKS The future tasks include specialized trainig on: i. Use of ET for flaw detection in tubes ii. Use of Coating thickness gauge iii. Penetrant testing methodology iv. Use of Universal hardness tester v. Working of Replica Metallographic Equipment 6