Basic Principles of the Eddy Current Inspection Technique

1. Basic Principles of the Eddy Current Inspection Technique Presentation to the 2014 Feedwater System Reliability User Group Workshop Presented By: Steven Schaefer Authored By: Steven Schaefer & C. Dan Spake January 20-23, 2014

2. INTRODUCTION • What is the Eddy Current Technique? • The interaction of an electromagnetic field generated by an AC current running through a coil and induced into a conductive material • What affects the eddy currents to cause signals? • The eddy current are disrupted by changes of material conductivity, permeability and dimensional variations. • How are damage and non-damage signals differentiated? • The signal response is compared to known defects in the calibration standard(s) which provide different characteristics • How is depth of damage estimated? • The known defects are used to create correlation curves to compare with the responses identified during analysis • The signal’s phase angle and/or amplitude is plotted along the data correlation curves

3. INTRODUCTION • How are damage types of differentiated? • Tubing material • Location along the tube and/or in the heat exchanger • Fluid type inside & outside of the tubes • Flow rates – high, low, stagnant • Energy source – flow induced vibration or erosion • Contaminants – chemical, sand, silts, debris, clams, mud, foreign material • How is eddy current inspection applied to HX tubes? • Sample size – could be schedule or cost driven • Historical information for trending or baseline documentation • Known or suspected leak(s) • Condition assessment or life extension analysis

4. Basic Eddy Current Principles When an electrical current flows through a conductor an associated magnetic field is produced around the conductor. Using the Right Hand Rule you can determine the direction of magnetic field around the conductor.

5. Basic Eddy Current Principles A coil or solenoid concentrates a magnetic field inside it. The greater the number of turns the solenoid has, the greater the intensity of the resulting magnetic field.

6. Basic Eddy Current Principles We discussed a magnetic field is produced when a current flows through a conductor or coil. If we apply an AC current to coil #1 a changing magnetic field will be produced. This changing magnetic field has the ability to induce voltage into other conductors or coils within it’s field. This current flow in coil #2 produces its own magnetic field that induces a voltage in coil #1. Since the magnetic field created by the current in either coil can induce a voltage into the other, they are said to have mutual inductance.

7. Basic Eddy Current Principles Now, let’s replace the second coil with a conductive test specimen. When the specimen is placed near the coil so that the alternating magnetic field intersects it, a current is induced in the specimen by mutual inductance. The eddy currents induced in the specimen also produce their own alternating magnetic field which opposes the field of the coil. The eddy currents magnetic fields likewise induce a current in the coil. If the path of the eddy currents are disrupted by a physical flaw in the specimen, the eddy current magnetic fields also change inducing a change in the coil. The eddy current technique uses the effect of electromagnetism and induction to characterize physical properties of a conductive material under test.

8. Basic Eddy Current Principles If we apply an AC current to the inductive circuit shown here and measure the voltage across and current through the coil, the sine waves shown will be displayed. Note the voltage and current are out of phase. When the voltage reaches its maximum values, the current is at zero; therefore, the voltage (E) leads the current (I) by 90°.

9. Basic Eddy Current Principles It is impossible to design a purely inductive circuit because resistance is always present, even in the wire itself. Resistance must be considered when we discuss total opposition to current flow in an AC circuit. If we measure the AC voltage across and the current through the resistor in the circuit below, the sine waves are similar to the ones shown here would be similar. The voltage and current are exactly in phase with each other through a resistor as there is no inductive effect.

10. Basic Eddy Current Principles This illustration shown indicates that a coil offers opposition to the flow of AC current due to inductive reactance (XL) and resistance (R). The inductive reactance is a function of the applied AC frequency and the coil’s inductance while the wire that makes up the coil offers opposition to current flow just like resistance in a simple DC circuit. Together, resistance and inductive reactance offer a total opposition to AC current flow in a coil. This opposition to AC current flow in a coil is called impedance (Z). The total impedance of an AC current is the sum of the inductive reactance and resistance.

11. Basic Eddy Current Principles Fundamental Principles of eddy current testing can be stated as follows: • An inspection coil powered by AC current is placed on or near a conductive test specimen inducing the primary magnetic field into the specimen. • Small circulating electrical currents are induced in the specimen. • These eddy currents produce a secondary magnetic field that opposes the primary magnetic field. • The secondary magnetic field causes a change in the primary magnetic field. • These changes in the primary magnetizing field cause a change in the impedance of the coil. • This change in impedance can be detected and displayed by the eddy current instrument.

12. Basic Eddy Current Principles An inside –coil or bobbin coil consists of several turns of wire wrapped around a cylindrical form. This probe type is passed through the inside diameter of the test specimen such as tubes or bores. The bobbin coil’s axis is parallel to the longitudinal axis of the test object; therefore the coils magnetic field is also parallel to the coils axis. The magnetic field induces eddy currents that flow parallel to the coils windings.

13. Basic Eddy Current Principles The distance that eddy currents penetrates into the material is called the depth of penetration. The depth where the eddy current density is equal to 37% of the density at the surface is referred to as standard depth of penetration. When the conductivity of a material is known, the following equation can be used to determine the standard depth of penetration.

14. Basic Eddy Current Principles

15. Basic Eddy Current Principles Bobbin probes are commonly used in the inspection of installed HX tubing. The probe is pulled through the inside diameter of the tubing.

16. Basic Eddy Current Principles Typical ASME / Combo Calibration Standard

17. Basic Eddy Current Principles Calibration Standard showing 100% Through Wall Hole, 60% OD & 20% OD flaws in 4 frequencies

18. Basic Eddy Current Principles Eddy Current Data Correlations Signal Phase Angle vs. Wall Loss 300kHz 150kHz 75kHz

19. Basic Eddy Current Principles Good Tube Support Plate

20. Basic Eddy Current Principles Vibrational Wear at Tube Support Plate Approximate Wall Loss 13%

21. Basic Eddy Current Principles Tube with OD Damage in Free Span Approximate Wall Loss 30%

22. Basic Eddy Current Principles Tube with OD Damage in Free Span Approximate Wall Loss 86%

23. Basic Eddy Current Principles Tube with Micro-Biological Induced Pits (MIC) Approximate Wall Loss 100%

24. Basic Eddy Current Principles Tubesheet map showing leakers and tubes inspected

25. Basic Eddy Current Principles Picture of area of leakers from vibrational failure

26. Basic Eddy Current Principles Video probe picture of tube showing complete failure from vibrational wear at Tube Support

27. Basic Eddy Current Principles Drawing of FWH TSP spacing showing are of failures between 15 & 19 feet from tubesheet.

28. Basic Eddy Current Principles Condenser neck FWH bundle replacement in progress

29. Basic Eddy Current Principles Close up of bottom rows of U-bend tubes

30. Basic Eddy Current Principles Scrap bin a couple days later

31. Basic Eddy Current Principles Showing U-bend end with approximately 8 rows removed

32. Basic Eddy Current Principles Final ET Inspection Results Tubesheet Map of a 4 pass U-Tubed HX

33. Basic Eddy Current Principles Cad drawing showing areas of vibrational damage

34. Basic Eddy Current Principles Manufacturers ‘as built’ drawing showing tube support plate and U-tube details

35. Basic Eddy Current Principles Eddy Current Analysis record of tube # P4-14-12 showing vibrational wear at TSP #9 & #7 and Tube extending beyond TSP #9

36. Basic Eddy Current Principles Eddy Current Analysis record of tube # P1-3-4 showing tube contact at only even numbered TSPs

37. Basic Eddy Current Principles Eddy Current Analysis record of tube # P2-8-6 showing tube contact at all 10 TSPs

38. Basic Eddy Current Principles Eddy Current Analysis record of tube # P1-4-2 showing full contact at even & edge of odd TSPs

39. Basic Eddy Current Principles Eddy Current Analysis record of tube # P1-4-2 showing full contact at odd & edge of even TSPs

40. SUMMARY • What is the Eddy Current Technique? • The interaction of an electromagnetic field generated by an AC current running through a coil and induced into a conductive material • What affects the eddy currents to cause signals? • The eddy current are disrupted by changes of material conductivity, permeability and dimensional variations. • How are damage and non-damage signals differentiated? • The signal response is compared to known defects in the calibration standard(s) which provide different characteristics • How is depth of damage estimated? • The known defects are used to create correlation curves to compare with the responses identified during analysis • The signal’s phase angle and/or amplitude is plotted along the data correlation curves

41. SUMMARY • How are damage types of differentiated? • Tubing material • Location along the tube and/or in the heat exchanger • Fluid type inside & outside of the tubes • Flow rates – high, low, stagnant • Energy source – flow induced vibration or erosion • Contaminants – chemical, sand, silts, debris, clams, mud, foreign material • How is eddy current inspection applied to HX tubes? • Sample size – could be schedule or cost driven • Historical information for trending or baseline documentation • Known or suspected leak(s) • Condition assessment or life extension analysis

42. For Additional Information

43. Basic Principles of the Eddy Current Inspection Technique Presentation to the 2014 Feedwater System Reliability User Group Workshop Presented By: Steven Schaefer Authored By: Steven Schaefer & C. Dan Spake January 20-23, 2014