Over the last decade, commercialization of Guided Wave Ultrasonic Testing (also referred to as Long Range UT) has provided industry with a powerful new technique for ascertaining the integrity of piping systems. The major advantage of the technique is it's capability to screen inaccessible piping over long distances from a single exposed location. The information developed allows the user to identify the locations in a pipe that have suffered potentially injurious corrosion and gain an understanding of the significance of the damage.
Guided Wave Ultrasonic Testing (GWUT) provides this and other advantages when compared to more conventional non-destructive testing (NDT) techniques. As with all NDT techniques, though, GWUT has limitations that must be kept in mind by the user to properly understand and utilize the results. This paper will discuss these advantages and limitations to assist the user in utilizing this important inspection technology.
Until the 1950s, detection of internal features in piping could only be accomplished with radiography. At that time the ultrasonic method of non destructive testing (UT) was developed and is now one of the most widely used methods to detect thickness of piping and define the extent of corrosion, erosion or other forms of metal wastage. Ultrasonic waves are mechanical waves, and in conventional testing normally have a frequency range from 1 MHz to 25 MHz. For measurement of wall thickness the pulse-echo technique utilizing longitudinal (compression) waves is generally employed. These waves are generated by an oscillating crystal, travel through the material under test at a known velocity, and when reflected off the far surface travel back through the material and are detected by a receiving probe I.
Although this method is widely used and well accepted, there are disadvantages with compression wave ultrasonic wall thickness testing. A major limitation is only the thickness of the material directly under the probe is determined. Although computerized automated ultrasonic scanning systems are now available that are capable of measuring the thickness of material in many thousands of locations in a given area (often referred to as corrosion mapping), again, the wall thickness is determined only at spots that are directly under the probe or transducer. Therefore, direct access to one surface of the pipe to be inspected is required. Furthermore, the transducer must be coupled to the pipe to eliminate air gaps, and the contacted surface of the material being tested must therefore be relatively smooth.
These limitations compromise the usefulness of compression wave ultrasonic testing to determine the condition of long lengths of piping, particularly piping that is difficult to access (insulated, in elevated pipe racks or drops down the sides of tall vessels, directly buried or encased under road crossings, and so forth). This problem created the impetus to develop other techniques that can provide a more cost effective solution for inspecting long lengths of piping.
