Volume 1, Number 4 October 2002
TNT Tips:
Positioning a Radiation Source Accurately
Properly locating a radiation source (X-ray or gamma) in a circumferential or girth weld application can be difficult, especially so when dealing with small diameter/heavy wall welds or with extremely large diameter pipe. When emanating radiation is not placed diametrically opposite the area of interest (AOI), the final radiograph may have inconsistent densities. Use this method to accurately position a source.This example uses a 12 in. (300 mm) diameter pipe weld with a circumference of approximately 40 in. (1,000 mm). Start by calculating the circumference (outer diameter x pi or simply wrap a number belt around the item). Then divide the circumference in half. In this example, that would be 20 in. (500 mm). Make a note of this number as our constant, all future source locations will depend on it.
Now decide on the area of interest or view length to use. For this example, we will take five exposures at 8 in. (200 mm) each to keep extremities within code density limits. Next, take the first view which is 0 to 8 in. (0 to 200 mm) and determine the center which is 4 in. (100 mm). Add that number to the constant of 20 in. (500 mm). The result is 24 in. (600 mm) and is the diametrically opposite point on the circumference from the center of the view area. Once the center of the view area has exceeded your constant, subtract it instead of adding it to the constant. Simple - saving time and money.
Richard Gerhard
MQS Inspections, Inc.
Orland Park, Illinois
[ The NDT Technician ]
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