|
Abstract:
Submerged Are Welded Pipes (SAW) are gaining increasing
importance in our country due to enhanced requirements
from the Petro-Chemical Sector.
This
paper discusses the technique used for Ultrasonic
inspection of the parent material and the weld,
through automated system.
Various
components of the inspection system, test procedure,
calibration of equipment, documentation of test
results and typical applicable inspection codes
are highlighted.
INTRODUCTION
The
process of testing a Saw Pipe starts from testing
its raw material i.e. plates, for lamination. Lamination
along its linear edges is more critical since it can
result in a very poor welded region.
Once the pipe is welded and the process of fabricating
is completed (after pressure testing) ultrasonic inspection
is performed to test the weld for defect like cracks,
porosity, slag and lack of fusion.
Ultrasonic Inspection for pipes can he carried out
on-line as well as off-line. Mainly on line inspection
is carried out for volumetric type defects. The biggest
advantage of on-line testing is that welding process
of pipe can he monitored and controlled accordingly.
Among the NDT Techniques used, Ultrasonic inspection
is usually employed for testing the plates, used to
fabricate the pipes and the welds of the finished
pipes. Gamma or x-ray Radiography is usually carried
out for spot cheeks of the welded portion and on the
pipe edges, which may have remained untested by Ultrasonic.
Magnetic Particle or manual ultrasonic testing is
carried out to inspect the beveled edges of the finished
pipe.
ULTRASONIC INSPECTION OF RAW MATERIAL PLATES
Plate surface is scanned from one surface,
either to cover 100% of the plate area or in parallel
grids and complete along its linear edges for detecting
lamination, Ref. Fig. 2.
Due to the nature of defects, normal beam dual crystal
probes are used.
The inspection code would usually determine size and
frequency of the probes as well as the extent of scanning
area to be covered and defect acceptance limit for
plates.
No of probes used is determined by the inspection
code requirements, testing speeds and commercial implication.
For grid scanning, number of probes are located at
a fixed distance from each other, along the full width
of the plate Ref, Fig. 3. The plate, while moving
on the conveyor, passes under the probes. Water is
used as couplant and probes are housed in specially
designed jerk free and protective holders to prevent
excessive wear and tear and to facilitate their mounting.
For covering the full plate surface, the fixed probes,
as mentioned above are mounted on an oscillating device,
thereby traversing the plate perpendicular to its
linear axis, in to and fro movement.
The plate surface should be fairly even, smooth and
free from dirt and scales, which otherwise can effect
the test results.
Reliability of testing is ensured by electronic circuits
which constantly test the working of
probes and give an alarm indication should even one
of them does not perform normally.
The components of plate testing system consist
of:
- Conveyor to transport the plate.
- Probe boom to mount the probes.
- Oscillating device (in case of full surface testing)
- Probes with probe holder
- Water circulating system.
- Multi channel test electronics.
- Audio /visual alarm.
- Paint markers.
- Location sensors and interlocks.
- Strip Chart Recorder for time versus flaw or decoupling
event recording.
- Microprocessor for system control & documentation
of test results.
Testing systems operator role is limited to calibration
of the equipment and overseeing the routine working,
since in normal working with use of electronic markers,
defects would trigger an audio visual alarm. Paint
markers can spray the location as well as printed
reports of the defect locations are possible.
ULTRASONIC INSPECTION OF WELDS
Due to the nature of defects expected,
angle beam probes are used to scan weld seam. They
scan the weld region by transverse ultrasonic waves
which are transmitted at a predetermined angle and
dual crystal normal probes are used to test the
heat affected zone area near the weld seam.
The probes are positioned away from the weld as
shown in fig. 4.
Like in the plate testing, the probes are suspended
from a convenient device. Probe holders are provided
with wear plated to prevent damage to them. Water
is circulated in an even flow to maintain a uniform
coupling layer.
The pipe under test is placed on a moving trolley,
which passed under the probe boom Fig. 5. The probes
are precisely located with reference to the weld
center line. In case of weld skew, the probe boom
has to be corrected manually or through automated
process. Auto weld seam tracking system is used
to maintain the constant distance between weld seam
and probes to get reliable test result at weld skew
area.
Probe boom is designed in such a way that probes
can be set easily in required test configuration
on different sizes of pipes. To test spirally welded
pipes there is a provision to swivel the probe carriage
in helix angle of spiral seam so that probes can
be placed on either side of pipe weld seam at their
best weld scanning position. To test spirally welded
saw pipes; lateral and circular synchronized moment
is given to pipes such that weld seam should pass
through the expected centre fine of probe rows.
The probes are employed in a basic configuration
known as the '1+1+X' arrangement to scan for longitudinal
and transverse defects and H.A.Z. area as well to
cheek the performance of the probes by constantly
checking the coupling between a pair of probes,
Ref. Fig. 6.
Probe pair 1 & 3, 2& 4 test for longitudinal
defects while operating in the pulse echo mode.
They also test for coupling between them, when one
of the probe in each pair acts as a transmitter
and the other as a receiver.
Probe pair 5 & 7, 6 & 8 test for transverse
defect and operate in pulse reflection mode i.e.
one of the probe in each pair is a transmitter and
the other is a receiver.
To test heat affected zone area near weld seam a
pair of dual crystal probes are provided.
Probe configuration can be selected as per test
requirement.
i) K+ H.A.Z.Mode.
ii) I+I+K + H.A.Z. Mode.
iii) I+I+X + H.A.Z. Mode
The above arrangement is achieved by specially configured
electronics. There is no cross interference between
the probes due to use of multiplexer, which operates
one scan at a time, in a sequential operation.
In On-line weld testing system prebathing of weld
seam is carried out to reduce the seam temp. to
about 40-50ºC from welding temp., as this test
is carried out at just 4-5 mtr. distance from welding
junction.
They depend entirely on the nature of pipe, testing
codes and the customers requirement. The welded
region should be uniform, free from rust and weld
splatter.
The weld testing system consist of:
- Trolley to position and transport the pipe
- Probe boom with X Y & Z axis swiveling movement
- Weld seam prebathing system
- Auto weld seam tracking system
- Testing bay, Guide rails to facilitate movement
of trolley.
- Probe, probe holders, water coupling system.
- Multi channel test & control electronics.
- Audio/visual alarm.
- Paint markers.
- Strip Chart Recorder for time versus flaw or decoupling
event recording
- Microprocessor for system control & documentation
of test results.
CALIBRATION
It is essential that prior to carrying
out the test on plates or welds, samples with simulated
defects are used to calibrate the test system. They
are also used to periodically check the performance
of test system.
Generally defects are simulated in accordance to
the inspection codes.
Area of pipe, which is left, untested and bevel
ends of pipe are tested by manual ultrasonic flaw
defector.
Inspection Codes:
Few e.g. of international codes applicable are as
follows
A) Plates
ASTM A435
ASTM A578
B) Pipes
ESI 98-5
ASTM 273
API - 5L
The above specified codes are just a few, from many,
standard practices employed internationally.
Due to individual needs there may be many modifications
to suit one's requirements.
Last but not the least the persons operating the
test system should have the necessary proficiency
in the Ultrasonic NDT Technology, having successfully
completed training as per internationally accepted
codes, along with specialized skills in operating
the system.
With the use of automated
system better test reliability, increased throughput,
documented test results and testing economy are
achieved. |
