Long range (or guided wave) ultrasonic inspection is different from conventional ultrasonic inspection, since a ring of transducers fitted round the outside of the part being inspected sends a sound wave along the length of the component, rather than through it. The component itself (more often than not, a pipe) constrains, or guides, the waves along the pipe wall, hence the term 'guided waves'.
Guided waves are a special case of plate waves in a pipe. The particle displacements are similar, but because the pipe acts as a wave-guide, the pulses can travel over even longer distances, exceeding 100m under some conditions. Figure 1 illustrates the principal wave modes that can be generated in pipe.
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Fig.1. Guided waves in pipes |
The variation of velocity with frequency, wave mode, pipe diameter and wall thickness is illustrated by means of dispersion curves. A simplified illustration showing four wave modes only is shown in Figure 2. These are for a specific pipe diameter and wall thickness. Other diameters and thicknesses will have their own families of dispersion curves.
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Fig.2. Dispersion curves |
It can be seen that the Torsional T(0,1) wave is non-dispersive. Its velocity is constant irrespective of frequency.
The Longitudinal L(0,1) wave is highly dispersive with wide variations in velocity with frequency.
The Longitudinal L(0,2) wave cannot exist at frequencies below about 20 kHz. However, at frequencies above about 40 kHz, the velocity becomes nearly constant with changing frequency. That is the wave becomes non-dispersive.
There is a vast range of flexural waves. Only the dispersion curve for the Flexural F(1,3) mode is shown. This wave can only exist at frequencies above ~25 kHz. Like the L(0,2) wave it becomes almost non-dispersive at frequencies above about 50 kHz.
Dispersive waves are undesirable because the pulse widens as it travels along the pipe and variation in the velocity of the wave makes it difficult to determine the exact position of any reflector along the pipe.
Because their velocity is influenced by wall thickness, guided waves exhibit their most important characteristic for NDT; that of being sensitive to changes in wall thickness. They are therefore sensitive to corrosion or erosion, whether it is on the inside surface or the outside surface of the pipe. They are also sensitive to cracks provided that they present a significant planar reflection transverse to the axis of the pipe.
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Fig.3. Comparison of conventional and guided wave ultrasonics |
The technology can inspect up to 150m of pipe from a single location and provides 100% direct assessment of pipe lengths. The benefits of LRUT include:
- Rapid screening for in-service degradation
- Cost reduction in gaining access to the pipes for inspection.
- Avoidance of removal and reinstatement of insulation or coatings (where present), except for the area on which the transducer tool mounted.
- The ability to inspect inaccessible areas, such as at clamps or sleeved or buried pipes.
- The whole pipe wall is tested, thereby achieving a 100% examination.