Measuring and testing – Vibration – By mechanical waves
Patent
1997-09-10
2000-11-21
Noori, Max
Measuring and testing
Vibration
By mechanical waves
G01N 924
Patent
active
061486728
DESCRIPTION:
BRIEF SUMMARY
The present invention is concerned with the inspection of elongate members generally, but especially pipes, rods, beams, struts and other structural members in order to ascertain their condition, especially whether or not they are subject to fatigue, corrosion or erosion. The invention will be particularly described with reference to its industrial application for inspecting pipes but this is not to be construed as a limit of its application.
The inspection of pipe condition for such deleterious effects as corrosion, erosion or fatigue is frequently complicated by the need to shut down plant, the provision of obscuring lagging and the difficulty in imaging the interior of the pipe. Existing techniques for the inspection of pipes include removing any lagging to inspect the pipe visually, and then employing ultrasonic inspectoscopes or the like to look for defects developing on the inside of the pipe. An alternative technique involves mounting an ultrasonic or eddy current probe on a sledge which is then towed along the inside of the pipe. As with the aforementioned external approach every point along the pipe length must be scanned by the probe resulting in very long inspection times.
It is an object of the present invention to provide an apparatus for inspecting the condition of a pipe which alleviates the aforementioned disadvantages of the prior art.
According to one aspect of the present invention there is provided apparatus for inspecting a pipe comprising at least one ring of angularly spaced wave exciters adapted to propagate one of: condition of the pipe.
Preferably the wave will be an axisymmetric Lamb wave.
The excitation assembly is preferably composed of a first excitation ring which can be secured to the exterior of the pipe wall and controlled to propagate axi-symmetric Lamb waves along the pipe.
The first excitation ring propagates many modes of axi-symmetric Lamb waves in both the forward and reverse directions along the pipe. The signals received by the receiving means using such a simple excitation assembly are extremely difficult to analyse usefully. Accordingly the assembly may advantageously include second and third excitation rings, each of a construction similar to the first excitation ring. The first, second and third excitation rings cooperate to suppress all but a single axi-symmetric mode propagating in a single "forward" direction. If only a single extra excitation ring is included in the excitation assembly the reverse direction propagating waves or the unwanted modes can be suppressed by cooperation of the excitation rings in the assembly. Thus the pipe can be inspected by means of a single axi-symmetric mode Lamb wave propagating in one direction thereby greatly simplifying the analysis of the waveform received by the receiving means.
The axi-symmetric unidirectionally propagating Lamb wave can be received by a receiving means of similar construction to the excitation assembly at a position longitudinally remote from the excitation assembly. However a more convenient and economic receiving means can be provided by wave exciter/transducers of the excitation assembly. In this preferred latter case the waveform received will be a reflection from a structure in the pipe wall. The reflected wave will have been altered by any structure in the pipe wall; such structures may include cracks or deformations which can thus be detected by analysis of the received waveform.
It will be appreciated that under some circumstances the apparatus according to the invention may be used as a permanent installation for frequent or continuous inspection of the pipe condition. However, the apparatus can be adapted to be readily mounted and dismounted for intermittent pipe inspection.
The preferred operating frequency of the Lamb wave will depend upon the size of the pipe but will ordinarily be in the range 10 kHz to 500 kHz. Preferably the fastest propagating mode of Lamb wave will be selected for the inspection.
The excitation of Lamb waves has previously been achieved by the use of exciters comprisi
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Alleyne David Nathaniel
Cawley Peter
Chan Che Wan
Imperial College of Science, Technology of Medicine
Noori Max
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