On line and global structural health monitoring of high temperature steam lines

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Presently ultrasonic inspection of steam line welds involve taking off the pipe lagging and erecting scaffolding at outages.The time and expense involved in these processes mean that only 20% of the welds are inspected at any one outage and ittakes 10 years to achieve complete 100% inspection. So it is likely that some defects remain undiscovered until they are solarge (in excess of 50% of pipe CSA is not uncommon) and the failure probability correspondingly large that the need formajor and expensive repair becomes immediate and essential.The most obvious approach to 100% weld inspection, at first sight is to place close to the weld a ring array of conventionalMHz frequency compressional and angle beam probes i.e. the type of probes used in existing inspection methods appliedat outage. However, for coverage of the entire weld thickness the angle beam probes must the mechanically scannedthrough one skip distance normal to the weld line. Also fluid coupling is required with a limited lifetime at high temperatures.Alternatively permanent high temperature adhesives, of uncertain lifetime can be used but mechanical movement is thenprohibited.A far better solution is to use a single circumferential array located centrally on a pipe section between two welds phased topropagate a guide wave mode. By the use of longer wavelengths the number of transducers in the array, evenly spaced,can be lowered to 8 or even as little as 4 whilst still being able to insonify the entire pipe welds fairly evenly with a preferredwave mode, so to provide an array at far less cost than the previous options (1) and (2). At 250MHz, which implieswavelengths ~15mm, the propagation range is reach the welds at each pipe end, typically up to 6m. Reduction of thefrequency, for example down to 25kHz, wavelength ~ 150 mm, would increase the range to cover several pipe lengths,proportionality reducing the equipment costs of 100% inspection, at the expense of reduced inspection sensitivity(increased minimum detectable defect size). Accurate prediction of defect reflection coefficients and hence minimumdetectable defect sizes is only possible with a wave modeling technique such as finite element analysis but it quicker anduseful to obtain order of magnitude estimates using rules of thumb based on lower and upper sensitivity limits. Both theabove theoretical estimates apply strictly only close to the array because they take no account of the depth of the defectfrom the array or mode conversion losses along that depth. However as the waves are guided the wave intensity does notfall off as depth-2 or depth-1 as would 3 or 2 dimensional extensional waves, which indeed is why guided waves are oflonger range, for any given frequency and wavelength. Also mode conversion losses can be minimized by arranging for thetransmitted waves to contain as high a proportion as practically possible of a single non dispersive mode. This depends onthe array design. At the practical operating frequencies used for guide waves in pipes the wave absorption in the pipematerial is low, even at the maximum intended operating temperatures and will thus contribute little towards reducingdefect detection sensitivity. So in conclusion of these considerations the estimated can be credibly applied to the intendedarray-weld separation of ~6m. A further factor affecting the practical achievement of the highest feasible sensitivities is thedistribution of wave intensity along a radius from the inner to the outer wall. A distribution with strong maxima at the innerand outer surfaces is advantageous for simultaneous detection of inner and outer wall creep and fatigue cracksparticular.This illustrate the importance of modeling in the project to determine an optimum short list of modes which mightbe use sequentially to achieve the best 100% wall volume coverage.