It is also a commonly accepted method of checking the wall thickness of pipelines and vessels, which are suspected of being corroded internally, when access is limited to one side of the material. UT is very sensitive to critical defects in a material like cracks, welding defects, porosity, lack of fusion and inclusions, which may weaken the weld. It is also very portable and can be used on a wide range of materials. This method of inspection, though, is very reliant on having an experienced and well-trained Inspector to interpret the indications they come across in field conditions and to determine if the part is satisfactory or if remedial action is required.
UT uses very short duration sound pulses which when induced into a material reflects off different media i.e. air interfaces and inclusions. The time for the reflection from these media are monitored and compared against the known travel speeds for the given material. The careful measurement of these pulse times becomes a measure of the distance the pulse has travelled and these are monitored via signals on a visual display screen.
These signals may represent cracks, back wall echo and lack of fusion (common air interfaces), slag, tungsten and copper (common welding inclusion). The tracking of these signal levels during the inspection enables the inspector to gather information about the size, type and location of the anomalies detected. During wall thickness monitoring of pipelines and vessels where access is limited to one side only, the sound pulse reflects off the inner wall of the component. These signal times become a measure of the distance travelled and any internal erosion/corrosion can be detected due to a reduction in wall thickness (a faster signal time).