Transcript Basic Eddy Current Principles

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
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
2
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
3
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.
4
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.
5
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.
6
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.
7
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°.
8
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.
9
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.
10
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.
11
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.
12
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.
13
Basic Eddy Current Principles
14
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.
15
Basic Eddy Current Principles
Typical ASME / Combo Calibration Standard
16
Basic Eddy Current Principles
Calibration Standard showing 100% Through Wall Hole,
60% OD & 20% OD flaws in 4 frequencies
17
Basic Eddy Current Principles
Eddy Current Data Correlations
Signal Phase Angle vs. Wall Loss
300kHz
150kHz
18
75kHz
Basic Eddy Current Principles
Good Tube Support Plate
19
Basic Eddy Current Principles
Vibrational Wear at Tube Support Plate
Approximate Wall Loss 13%
20
Basic Eddy Current Principles
Tube with OD Damage in Free Span
Approximate Wall Loss 30%
21
Basic Eddy Current Principles
Tube with OD Damage in Free Span
Approximate Wall Loss 86%
22
Basic Eddy Current Principles
Tube with Micro-Biological Induced Pits (MIC)
Approximate Wall Loss 100%
23
Basic Eddy Current Principles
Tubesheet map showing leakers and tubes
inspected
24
Basic Eddy Current Principles
Picture of area of leakers from vibrational failure
25
Basic Eddy Current Principles
Video probe picture of tube showing complete failure from
vibrational wear at Tube Support
26
Basic Eddy Current Principles
Drawing of FWH TSP spacing showing are of failures between
15 & 19 feet from tubesheet.
27
Basic Eddy Current Principles
Condenser neck FWH bundle replacement in progress
28
Basic Eddy Current Principles
Close up of bottom rows of U-bend tubes
29
Basic Eddy Current Principles
Scrap bin a couple days later
30
Basic Eddy Current Principles
Showing U-bend end with approximately 8 rows removed
31
Basic Eddy Current Principles
Final ET Inspection Results Tubesheet Map
of a 4 pass U-Tubed HX
32
Basic Eddy Current Principles
Cad drawing showing areas of vibrational damage
33
Basic Eddy Current Principles
Manufacturers ‘as built’ drawing showing
tube support plate and U-tube details
34
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
35
Basic Eddy Current Principles
Eddy Current Analysis record of tube # P1-3-4
showing tube contact at only even numbered TSPs
36
Basic Eddy Current Principles
Eddy Current Analysis record of tube # P2-8-6
showing tube contact at all 10 TSPs
37
Basic Eddy Current Principles
Eddy Current Analysis record of tube # P1-4-2
showing full contact at even & edge of odd TSPs
38
Basic Eddy Current Principles
Eddy Current Analysis record of tube # P1-4-2
showing full contact at odd & edge of even TSPs
39
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
40
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
41
For Additional Information
Steven Schaefer
C. Dan Spake
Manager, BOP Services
ET Level III QDA, ASNT ET III
Anatec Division
Project Manager
ET Level III QDA
Anatec Division
Phone: 949-498-3350
Mobile: 860-885-4695
[email protected]
Phone: 949-498-3350
Mobile: 704-477-1414
[email protected]
Darren Howe
Chris J. Speas
Vice President
ET Level III QDA
Anatec Division
V. P. - Business Development
ET Level III QDA, ASNT ET III
Anatec-LMT
Phone: 949-498-3350 x202
Mobile: 949-300-2173
[email protected]
Phone: 949-498-3350 x302
Mobile: 602-885-3350
[email protected]
42
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