FIELD OF INVENTION:
This invention generally relates to the field of Non-Destructive Testing in
particular, magnetic particle inspection of pipe bends in Boiler pressure parts. In
particular, the invention relates to flexible coil magnetic particle inspection on
pipe bends where near to the coil, longitudinal magnetic field is not adequate.
More particularly, the invention relates to an improvement in longitudinal field
adequacy near to the flexible coil by reducing localizes circular magnetic field in
an innovative way of arrangement of coils.
BACKGROUND OF THE INVENTION:
In the field of magnetic particle inspection several types of techniques for the
identification of defects in pipe bends are known in the art. In steam generator
pipe bends of various sizes diameter (323 mm to 1066 mm) and thickness (19 m
to 60 mm) are being used, which are normally prepared in incremental pipe
bending machines by induction heating process. These induction heated bend
pipes are prone to transverse defects in extrados area, where tension is maximum.
To identify those transverse defects, coil magnetic particle inspection is the best
suitable method. Flexible coils with half wave direct current in magnetic particle
inspection generates longitudinal field, which can easily detect transverse defects.
Regarding current selection and field adequacy for large parts due to shape and
size, many standards have given guidelines that the field adequacy shall be
demonstrated. For bigger parts like steam pipe lines, specific on shop trails are
required for the establishment of field adequacy.
Coil magnetic particle testing is done on pipe bends with flexible coils. Electric
current in a coil creates a magnetic field which is more concentrated in the center
of the coil than outside of the coil as shown in figure-1. Stacking multiple coils
concentrates the field even more as shown in figure-2.
As shown in figure-1 and in figure-2 current carrying coil generates longitudinal
magnetic field inside the coil but nearer to the coil circular field dominates the
longitudinal field.

During coil magnetic particle inspection of pipe bends, difficulty was faced by
personnel regarding magnetic field adequacy nearer to the coils. Nearer to the coil
circular magnetic field dominates the longitudinal field as shown in the figure-2.
While inspection, transverse indication is not detectable because of circular field
domination over the longitudinal field nearer to coil. This area shall be identified
and shall be tested again with adequate longitudinal field strength. This re-testing
and identifying such area is practically difficult and time consuming for
personnel. This resulted in additional cycle time also for non-destructive
evaluation.
The drawback of the prior art lead to develop a method for pipe bend magnetic
particle inspection by rearranging coil in an innovative way such that burden of
checking longitudinal field strength near to the coil and re-checking of that area
can be reduced on task performer.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention to propose, an improvement in
longitudinal magnetic field near to the coil by arranging coils in an innovative way
and selecting required current, accordingly.
Another object of the invention is to propose an improved reliability of coil bend
magnetic particle inspection test by eliminating circular field domination over the
longitudinal field near to the coil.
A further object of the invention is to propose an improved flexible coil magnetic
particle inspection method of pipe bends, which eliminates avoidable re-
inspection of same area and reduces the cycle time of non-destructive evaluation
for personnel.
SUMMARY OF THE INVENTION:
According to an aspect of the invention, the circular magnetic field near to the coil
on the surface of pipe bend can be done by displacing the coil away from the
extrados of pipe bend surface as shown in figure- 3, figure-4 and figure-5.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
It is to be noted, however, that the appended drawings illustrate only typical
embodiments of the present subject matter and are therefore not to be considered
for limiting of its scope, for the invention may admit to other equally effective
embodiments. The detailed description is described with reference to the
accompanying figures. Some embodiments of system or methods in accordance
with embodiments of the present subject matter are now described, by way of
example, and with reference to the accompanying figures, in which:
Figure-1 shows a schematic diagram illustrating the principle of current carrying
coil and magnetic field in accordance to the prior art;
Figure-2 shows a schematic diagram illustrating the principle staking of multiple
current carrying coils and magnetic field in accordance to the prior art;
Figure-3 shows a schematic diagram illustrating the innovative arrangement of
current carrying coils in elliptical shape in accordance to the invention;
Figure-4 shows a schematic diagram illustrating the innovative arrangement of
current carrying coil in semicircular shape in accordance to the invention;
Figure-5 shows a schematic diagram illustrating the innovative arrangement of
current carrying multiple coils in semicircular shape in accordance to the
invention.
DETAILED DE DESCRIPTION OF THE PREFERRED EMBODIMENTS :-
In figure-1, “A” is current carrying conductor in coil shape, showing magnetic field
lines “B” due current flowing in coil shaped conductor.
In figure-2, “A” is the pipe bend and flexible multiple coils of current carrying
conductor “B” is wrapped around pipe bend. The multiple coils generate
longitudinal field “D” due current “I” flowing in coils. “C” is the length, where
circular magnetic field is dominating over the longitudinal field nearer to the
flexible coil. This domination length will vary with diameter, thickness and
material of the pipe bend and verifying it practically is not only difficult but also
time consuming.

In figure-3, “A” is the pipe bend with diameter “C” and thickness “E” as shown in
side view. A flexible coil “B” wrapped over the pipe bend as shown with the “D”
distance shifting above the extrados of the pipe bend. This distance “D” can be
calculated by using integral of Biot-Savart’s law as per the required longitudinal
field strength on the surface of the pipe bend in such a way that circular field
should not dominate over longitudinal field on the pipe bend surface.
In figure-4, after practical trials of arrangement shown in figure-3, it was found
that the for higher diameter pipe bends intrados part of the coil is not adding
much magnetic field strength on extrados surface of the pipe bend. As the coil
length increases, the electric load and heating of the instrument increases, to
optimize use of the coil length another innovative arrangement was developed as
shown in figure-4. “A” is the pipe bend with diameter “C” and thickness “E” as
shown in side view. A flexible semicircular coil “B” wrapped over the pipe bend as
shown with the “D” distance shifting above the extrados of the pipe bend. This
distance “D” can be calculated by using integral of Biot-Savart’s law as per the
required longitudinal field strength on the surface of the pipe bend in such a way
that circular field should not dominate over longitudinal field on the pipe bend
surface.
In figure-5, after practical trails of arrangement shown in figure-2, it was observed
that to increase the longitudinal field strength staking of multiple semicircular
coils are required. “A” is the pipe bend with diameter “C” and thickness “E” as
shown in side view. The multiple semicircular flexible coils “B” wrapped over the
pipe bend as shown with the “D” distance shifting above the extrados of the pipe
bend. This distance “D” can be calculated by using integral of Biot-Savart’s law as
per the required longitudinal field strength on the surface of the pipe bend in such
a way that circular field should not dominate over longitudinal field on the pipe
bend surface.
Table 1 shows a comparative study between the existing art and the invention.
Comparison data are based on inspection on low alloy carbon steel pipe with
Diameter =660mm and Thickness= 25mm with 2 turns of coil, 200mm apart.

Comparison is done on four parameters. Current was kept same for better
understanding of difference between two methods.


WE CLAIM:
1. A method for coil magnetic particle inspection in pipe bends of a boiler method
comprising the steps of:
- selecting an arrangement of current carrying coils in elliptical shape;
- selecting a suitable current “I”;
- calculating distance “D” for flexible coil shifting away from pipe bend surface;
- selecting an arrangement of current carrying coils in semicircular shape;
- selecting a suitable current “I”;
- calculating distance “D” for flexible coil shifting away from pipe bend surface;
- selecting an arrangement of current carrying multiple coils in semi-circular
shape;
- selecting a suitable current “I”;
- calculating distance “D” for flexible coil shifting away from pipe bend surface;
wherein the calculated distance “D” for flexible coil for each shape is analysed for
further calculation.
2. The method for coil magnetic particle inspection in pipe bends of a boiler
method as claimed in claim 1, wherein the current “I” is such that the circular
magnetic field do not dominate over Longitudinal field on the surface of the pipe
bend.
3. The method for coil magnetic particle inspection in pipe bends of a boiler
method as claimed in claim 1, wherein the overlap for full coverage can be kept
zero.
4. The method for coil magnetic particle inspection in pipe bends of a boiler
method as claimed in claim 1, wherein the distance “D” calculated is such that
circular field does not exceed longitudinal field on the pipe bend surface.

5. The method for coil magnetic particle inspection in pipe bends of a boiler
method as substantially described and illustrated herein with reference to the
accompanying drawings.
6. The method for coil magnetic particle inspection in pipe bends of a boiler
method as claimed in claim 1, wherein the distance “D” is calculated by Biot-
Savart Law.