Considerations for Measuring Wall Thickness of Tubes & Pipes with the Dakota CX & PCX
Application Overview:
One of the more common applications for ultrasonic thickness gauges is measuring the wall thickness of various types of pipes and tubes, either to check manufacturing tolerances or to assess the extent of wall loss from corrosion or erosion on in-service units. This article will seek to explain some of the more important factors you should consider when choosing a gauge and transducer for pipe and tube inspections to ensure best performance.
Not all pipes are made equally and they’re manufactured in wide variety of sizes, diameters, wall thicknesses and materials to suit the huge number of fluid-conveyance tasks present in industry. In order to deal with different types of pipes, it stands to reason that a UT inspector will need to utilise different types of ultrasonic transducers, as using a transducer that’s unsuited to the pipe type can lead to issues like increased inspection time and measurement inaccuracy.
Measuring Thin Pipes:
For pipes on the lower end of the scale, such as DN6 copper tubes, your wall thickness is usually in the 500 – 800µm (0.5 – 0.8mm) range. This is right on the edge of what it’s possible to measure accurately with a dual element transducer, and if you’re expecting wall loss to occur it’s very possible for the extremely thin walls to produce a ‘double read’, where the gauge reads the second back wall reflection instead of the first, causing you to underestimate spots of extreme wall loss.
For this reason, it’s often much more sensible to use the Dakota PCX Precision Thickness Gauge with a 15MHz delay line transducer on pipes with a wall thickness below 2mm; this setup will allow you to measure steel thicknesses down to 150µm without the potential for a double reading.
Measuring Thick Pipes:
For thicker pipes, especially those made of PVC or ductile cast iron instead of steel or copper, the limiting factor is often attenuation (absorption) of the ultrasonic signal. In order to measure with ultrasound, the pulse needs to make it all the way through the pipe twice and still have enough energy left to trip the gauge’s measurement gate after it returns.
To cope with these more attenuative pipes, it’s often a requirement to move to a larger-diameter or lower-frequency transducer to maximise the sound energy returning to the transducer. For steel pipes this would be the 1/2” diameter dual element 5MHz Dakota CX Corrosion Thickness transducer, and for PVC or Cast Iron we recommend reducing the frequency to 1.00 – 2.25 MHz as these are less easily absorbed by the material.
Probe Rocking Gap:
One caveat to using larger diameter transducers is that on smaller diameter pipes they can suffer from a ‘rocking gap’ between the pipe and transducer as a result of the increased surface curvature. When this gap is very small it doesn’t cause any problems, but as the size of the gap increases you begin to experience a drop-off in both ease and consistency of measurement which can dramatically slow down inspections.
There are two ways of dealing with this issue; the first is to simply change to a smaller transducer (if possible). The second option is to use Dakota’s pipe probe holder, which utilises a spring and housing to increase consistency of placement by pushing the transducer into the optimal orientation.
Full vs. Empty Pipes:
A final consideration is that empty pipes are much easier to measure than full ones as a result of ultrasonic boundary effects. Put simply, if the pipe contains a liquid instead of air or a gas, significantly more ultrasonic energy will pass from the back wall of the pipe into the bore, instead of reflecting back to the transducer.
This means that if you’re specifying your equipment and processes based on testing on an empty pipe that’s not in service, you should pay close attention to whether the in-service units you’ll be called to inspect will be full or not. Unexpected increases in measurement difficulty can slow down an inspection plan quite dramatically.
Dakota gauges used in this application:
For more details on how the Dakota PCX Precision Thickness Gauge can give an advantage to your inspection process.
For more details on how the Dakota CX Corrosion Thickness Gauge can give an advantage to your inspection process.