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-
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-
Other methods have included the use of multiple transducers, which is very demanding
InspecTech has long used special high-
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-
Figure 5 shows the new Array Transducer Assembly for 6-
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-
Confidence in the test even with an imprecise weld location
Detectability of very small flaws
Very easy set-
Fig. 1, 2, 3 & 4.
Response To Standard Defects As Measured Over
A Range Of Transducer Positions
COUNTDOWN SETTING VS. MINIMUM DETECTABLE DEFECT LENGTH
Figure 5 Array Transducer Assembly for 6-