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

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 it
takes 10 years to achieve complete 100% inspection. So it is likely that some defects remain undiscovered until they are so
large (in excess of 50% of pipe CSA is not uncommon) and the failure probability correspondingly large that the need for
major 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 conventional
MHz frequency compressional and angle beam probes i.e. the type of probes used in existing inspection methods applied
at outage. However, for coverage of the entire weld thickness the angle beam probes must the mechanically scanned
through 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 then
prohibited.
A far better solution is to use a single circumferential array located centrally on a pipe section between two welds phased to
propagate 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 preferred
wave mode, so to provide an array at far less cost than the previous options (1) and (2). At 250MHz, which implies
wavelengths ~15mm, the propagation range is reach the welds at each pipe end, typically up to 6m. Reduction of the
frequency, 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 minimum
detectable defect sizes is only possible with a wave modeling technique such as finite element analysis but it quicker and
useful to obtain order of magnitude estimates using rules of thumb based on lower and upper sensitivity limits. Both the
above theoretical estimates apply strictly only close to the array because they take no account of the depth of the defect
from the array or mode conversion losses along that depth. However as the waves are guided the wave intensity does not
fall off as depth-2 or depth-1 as would 3 or 2 dimensional extensional waves, which indeed is why guided waves are of
longer range, for any given frequency and wavelength. Also mode conversion losses can be minimized by arranging for the
transmitted waves to contain as high a proportion as practically possible of a single non dispersive mode. This depends on
the array design. At the practical operating frequencies used for guide waves in pipes the wave absorption in the pipe
material is low, even at the maximum intended operating temperatures and will thus contribute little towards reducing
defect detection sensitivity. So in conclusion of these considerations the estimated can be credibly applied to the intended
array-weld separation of ~6m. A further factor affecting the practical achievement of the highest feasible sensitivities is the
distribution of wave intensity along a radius from the inner to the outer wall. A distribution with strong maxima at the inner
and outer surfaces is advantageous for simultaneous detection of inner and outer wall creep and fatigue cracks
particular.This illustrate the importance of modeling in the project to determine an optimum short list of modes which might
be use sequentially to achieve the best 100% wall volume coverage.

The ULTRASTEAMLINE project concerns primarily the safety of steam lines in nuclear power plant which operste at a
pressure of 400bar and 350C Weld defects in these lines are responsible for some 10% of current planned reactor outage
time for weld inspection and 10% of forced outage time for weld repairs. The forced outage time implies an inadequacy of
present inspection technique, namely that to limit the planned outage time only 20% of all the welds are inspected at any
one outage so that large defects can remain undiscovered until they cause actual pipe failure. Both inspection and repairs
involve workers being exposed to radiation when attending to the part of the steam lines that are in the containment
building. The project proposes continuous in service monitoring of nuclear steam lines for detecting and monitoring the
growth of defects so that (i) failure of lines through the propagation to failure of undetected defects is avoided and (ii)
workers are not exposed to radiation.
The strongest economic case is basically the reduction in plant outage time i.e. loss product production time that be
achieved by a continuous in service monitoring system for high temperature pipework. A typical nuclear fuel plant with four
steam lines per reactor has up to 4 kilometres of pipe work with some 500 welds carrying superheated steam at pressures
of up to 400 bars and temperatures up to 350 oC. Steam lines in fossil fuel plant have a similar length but operate at
5800C. The extreme pressures produce hoop stresses in a pipe causing the pipe welds to creep continuously in time until
weld creep cracks are generated which, if undiscovered, may grow until the pipe ruptures. The pipes also suffer continuous
cyclic loading through vibrations which produces fatigue cracks in the welds which if undetected are another cause of pipe rupture. Worldwide, failure to detect steam superheated steam pipe cracks results in a catastrophic failure every year or
two with loss of life, appalling injuries, widespread power cuts and massive financial losses for the operators, typically with
a cost impact of 120m ECU per event. A steam plant failure in the EU takes place every 4 years on average in a nuclear or
fossil fuel plant.
WITHIN THE CONSORTIUM the sustained sales will generate 100 jobs (assuming one job per £70kpa turnover).
OUTSIDE THE CONSORTIUM (i) 50 further jobs along the supply chain to PI (ii) From Q1 operator benefits will be ~
£2.5mpa x 483 = £1.2bnpa if the ULTRASTREAMLINE technology became widely adopted by the nuclear sector (iii) The
system will prevent the historic ocurrence every 1-2 years of castastrophic steam line failure events in fossil/nuclear plant
costing an average of £125m each in loss of life and injury compensations, infrastructure renewal and clean up. (v) A part
of all nuclear steam lines is within the containment building where workers have a level of exposure to radiation sufficient to
need continuous monitoring by a dose badge. The ULTRASTEAMLINE will elliminate most of this exposure excepting at
the initial installation of the system to the benefit of the workers' health. (iv) The system will enhance the safety of nuclear
plant thus helping to improve public confidence in this low carbon power source. It is important to enhance this confidence
because there is a gap between the energy that can be provided from the key renewable and low carbon energy sources
(wind, marine and solar), at least in the next 20 years, and consumer requirements dictated by world population growth
projections These benefits are quantified fully in Appendix A.