ARTICLE MT-21
INSERVICE PULSED EDDY CURRENT TECHNIQUE FOR
CORROSION SCREENING
MT-2110 SCOPE
This Article specifies requirements for inservice pulsed
eddy current (PEC) examination of components constructed
from ferromagnetic materials for detection of
corrosion or erosion or both.
When specified by the invoking documents, the PEC examination
described in this Article shall be used together
with Article MT-1. Terms used in this Article are defined
in Article MT-1, Mandatory Appendix I, MT-I-121.13.
MT-2120 GENERAL
(a) PEC examination, as described in this Article, is an
examination of a low alloy carbon steel or cast iron component.
PEC examination is usually performed through a
cover. PEC examination is used to find areas with inservice
anomalies (typically corrosion or erosion) either
internally or externally.
(b) The PEC examination system consists of a probe
that contains a device for generating a (primary) magnetic
field and a device for detecting the secondary field
emanating from the component, resulting from eddy currents
generated by the primary field. The device used for
detection may be the same as the device used for generation
of the primary magnetic field.
(c) In common PEC examinations, the primary field is a
direct current (DC) magnetic field that is switched to generate
eddy currents. The time between switching, i.e., the
time for which the DC magnetic field is on, is called the excitation
pulse. Due to switching of the magnetic field, eddy
currents are generated. After switching the magnetic
field, the eddy currents will decay. The secondary magnetic
field is recorded and displayed on a graph. In a successful
examination, the characteristics of the shape of the
decay curve are extracted. These characteristics correspond
to the wall thickness of the component.
(d) PEC examination is usually performed as a relative
measurement, where the system is calibrated on a reference
spot on the component.
(e) PEC examination provides an estimation of volume
of material under a footprint area. Basic measurement
will yield a result roughly proportional to the average
wall thickness in the area in which eddy currents are generated.
The basics of PEC system operation are described
in Nonmandatory Appendix A to this Article.
MT-2121 WRITTEN PROCEDURE REQUIREMENTS
MT-2121.1 Requirements. PEC examination shall be
performed in accordance with a written procedure that
shall, at a minimum, contain the requirements listed in
Table MT-2121.1-1. The written procedure shall establish
a single value, or range of values, for each requirement.
MT-2121.2 Technical Feasibility Evaluation. Due to
the properties of PEC examination, a technical feasibility
evaluation shall be performed. This technical feasibility
evaluation shall comprise the following, as a minimum:
(a) an evaluation of the ability of PEC examination to
penetrate the cover. This shall be performed with theoretical
or practical methods.
(b) an evaluation of the ability of PEC examination to
detect anomalies sufficiently for the intended purpose
of the examination. This may be performed theoretically
or practically. The evaluation shall include a call level, a
level over which indications are recommended for followup
examination. The minimal detectable defect, as determined
in the technical feasibility evaluation, shall be
indicated.
(c) a trial measurement on the component. The test
measurements at the reference location (see MT-2151)
may be the same as the trial measurement, provided the
measurement results correspond to the results of (a)
and (b). Correspondence shall be reported.
MT-2121.3 Changes to Requirements. A change of a
requirement in Table MT-2121.1-1 identified as an essential
variable from the specified value, or range of values,
shall require revision of the technical feasibility evaluation.
A change of a requirement identified as a nonessential
variable from the specified value, or range of values,
does not require revision of the technical feasibility evaluation.
All changes of essential or nonessential variables
from the value, or range of values, specified by the written
procedure shall require revision of, or an addendum to,
the written procedure or scan plan, as applicable.
MT-2122 SCAN PLAN
A scan plan shall be developed. The scan plan shall address,
as a minimum, the items listed under scan plan in
Table MT-2121.1-1. Extent, location, and coverage of testing
shall be graphically shown, e.g., on isometric drawings
of piping or engineering drawings of the component, noting
testing locations
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