ULTRASONIC
TESTING OF ERW TUBING
WELD LINE WANDER PROBLEMS ELIMINATED
AT LAST!
Ultrasonic weld testing in
ERW tube mills has been common practice for over 40 years.
Product is tested for defects and flash condition both on the
mill (on-line) and on the finishing floor (off-line).
Over the years ultrasonic systems have improved in performance,
and to-day’s P.C. based systems provide unambiguous results with
simple set-ups and include data storage capability.
The ultrasonic method is
ideal for testing ERW tube, as signal to noise ratios are excellent
(very few false indications) and sensitivity is unaffected by wall
thickness (ID and OD defects are equally discernable). Unfortunately,
although the method is ideal, the application is imperfect.
The imperfection is because
the transducers must operate in the exact relative position to the weld,
as they were when the system was calibrated.
Lack of precision in the transfer from the calibration standard
and the natural “wander” of the weld line on the mill or the
conveyor both conspire to make this condition almost impossible to meet.
The signal from a test notch
can fall by 30dB (50:1) and disappear totally with just 3mm of weld line
shift. This problem is well
known in the welded tube industry and considerable effort has been made
to find a solution over the years.
The “Involute Search
Unit” was an early attempt to compensate for weld line wander.
It was a multi-crystal compound transducer, which was
unfortunately costly and inefficient.
Other methods have included
the use of multiple transducers, which is very demanding on set-up; and
even by the use of phased arrays. These
latter systems can produce sequential scans at varying angles in the
weld zone, but at a repetition rate limited by the material acoustics. The possibility is open for short defects to pass undetected
when the sound beam is focussed elsewhere in the weld zone.
InspecTech has long used
special high-aspect-ratio transducers, which focus the ultrasonic energy
to maximize the beam width through the weld zone, while at the same time
minimize the beam width along the weld length.
These transducers, when used at the maximum possible repetition
rate, have provided good weld wander compensation coupled with excellent
discrimination for small defects. Penetrators
1mm and smaller in length are routinely found using this transducer
technology
Nevertheless high aspect
ratio transducers are not totally immune to weld line shift, especially
in heavy wall products.
InspecTech is pleased to announce a
totally effective solution
to this vexing problem in the form of monoblock array transducers which
require no independent adjustment and therefore easier to use than
conventional systems.
A series of
high-aspect-ratio transducers are mounted as a discrete array, with a
pre-calculated offset which gives just a few dB variation in response over
a very broad range of positions. Unlike
in phased arrays, the transducers are “fired” in unison and at a prf
of several KHz, which ensures total coverage of the weld zone and the heat
affected zone over a broad range of weld line positions.
Figures 1, 2, 3 & 4 illustrate the system’s capability to
capture and display flaw indications within a few dB over a distance range
of ± 1” (25mm). The system
will maintain the ± 1” tested by ultrasonic methods (usually 3” to
24” diameter).
Figure 5 shows the new Array
Transducer Assembly for 6-5/8” pipe weld.
Computer-aided processing of
the ultrasonic data gives the user a conventional A-scan presentation, a
familiar format for most operators.
In order to improve noise
rejection, it is common practice with ultrasonic test systems to
“countdown” a series of defect indications prior to showing the alarm
status for a flaw. Since the
array system consists of discrete transducers all working in unison, the
countdown approach remains perfectly valid as a means of enhancing
performance on line.
The attached Table 1 shows
how the countdown settings used can impact the length of defect, which is
theoretically detectable. In
the area of the table which contains no numbers, the countdown setting has
no influence on defect detectability whatsoever, it becomes an issue of
defect size and orientation only. Therefore,
the equipment operator can use any countdown setting within the blank area
with confidence. Table 1 is
based upon system clock rates close to the maximum useable for this type
of testing without the possibility of wraparound signals.
The discrete array therefore
offers the user 3
brand-new features, which represent important advances in welded,
tube ultrasonic testing:
Confidence in the test even with an
imprecise weld location
And…
Detectability of very small flaws
And…
Very easy
set-up and calibration procedures.
Fig. 1, 2, 3
& 4.
Response To Standard Defects As Measured Over
A Range Of Transducer Positions
TABLE
1
COUNTDOWN SETTING VS. MINIMUM DETECTABLE DEFECT LENGTH
Figure 5 Array Transducer Assembly for 6-5/8” Pipe Weld