The weld is divided into zones typical1y 1-3mm high and beam angles are selected to optimise response off the fusion face of the weld bevel. See Figure 1.
 Fig 1: Showing 6-zone configurations for three different weld bevel profiles |
ID surface notches and 2-3mm diameter flat bottom holes typically provide targets on which signal amplitude and travel-time-in-a-gate are set. These targets are machined in a project specific section of pipe and arranged along the theoretical weld bevel profile. See Figure 2.
 Fig 2: Illustrating the tandem probe configuration used to obtain a signal from a 2mm diameter flat bottom hole positioned in the centre of a zone (Fill 2 of a 3 fill zone used for this example). |
The calibration piece is secured in a section of pipe so that the probes can be arranged symmetrically on either side of the theoretical weld centreline. The same guide band as used by the mechanised welding is used to mount the scanning apparatus on. By means of a strip-chart type presentation the operator can easily assess the amplitude and relative positions of the targets in the calibration piece.
When an acceptable calibration is achieved the system is ready to test a weld. The probe array is positioned at the top of a weld, the couplant switched on and the data acquisition readied to collect information when probe motion begins. Scanning is typically carried out at 50-100mm/s. The operator monitors couplant quality and temperature changes as the scan progresses. A 48 inch diameter pipe weld can be scanned in less than a minute (that is near 4m of weld!).
When the scan is completed the operator reviews the data to ensure it is good then give the OK to remove the probe array and to move on to the next weld. Prior to scanning the next weld the operator evaluates the scan results to the customers' acceptance criteria, marks any relevant indications and assigns a status of accept or reject to the weld within 2-3 minutes of completion of the scan. This speed of inspections provides opportunity for process control of the welding. A hard copy can be printed as the customer's permanent record or all of the data can be stored to magnetic or optical storage devices.
Evaluation is principally based on an intuitive strip-chart type presentation. Amplitude is indicated by a solid line and sound path in a gated region is indicated by solid shading. Some form of go/no-go is often included indicating where amplitude signals exceed a threshold. When assembled symmetrically on a chart one can imagine the weld split down the middle and laid out with the root zones in the middle and moving outwards to the top Fill zones. See Figure 3
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Fig 3: Strip-Chart Presentation: Showing a flaw in Fill 2 on the "downstream side" (second column from right). Little "interaction" is seen on the adjacent fill zones (Fill1 and Fill 3) so the flaw height can be estimated as a single zone. The low amplitude signal on the hp1-us channel (5th from the left) is a result of beam interaction with the root.
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Maintaining zone discrimination is a critical aspect of the technique. Acoustic velocity variations from 3100m/s to 3400m/s were found to exist in the steels from various pipe mills. To ensure this is not a potential problem the new code requires that calibration blocks be made from project specific pipe. As well, the effect of temperature on the acoustic velocity in refracting wedges can be significant. The code provides guidance on when to control this parameter.