Optical Strain Measurement Techniques to Assist in Life Monitoring of Power Plant Components

Abstract

Sensors for monitoring creep strain in high-pressure steam pipes and other power plant components are subjected to very demanding environmental and operational conditions. It is important that the sensors are of a rugged design and that measurement can be made that only relates to creep movements in power plant components. The E.ON UK auto-reference creep management and control (ARCMAC) optical strain gauges have been designed to have this capability. These optical strain gauges are installed across sections of welded steam pipe and other plant components in locations that provide the best monitoring points to reveal the early onset of failure processes. Reported in this paper are recent developments to improve optical creep strain measurement to achieve a 65 microstrain accuracy level with an error of less than 10%. Also reported are trials of combining optical strain gauges with digital image correlation (DIC) to obtain detailed information of the creep strain distribution around the gauges. The DIC data for known defect geometries have been validated with finite element analysis.

Modelling of creep in Inconel 706 turbine disc fir-tree

“…23. For an original distance or gauge length of 31 mm, the error in the ARCMAC system was found to be less than 10% for measurements of strain greater than 65 me (0.0065%) [20]. Any distance measurement errors due to corrosion is considered negligible as there is very low oxide growth at this region.…”

“…9). A possible on-site technique of measuring the localised high creep strains observed at the lobe regions is through the use of edge-finding technique [19] that incorporates the Auto-reference creep management and control (ARCMAC) camera system [20]. The camera utilises high resolution close-up image capture.…”

A review of non-destructive techniques for the detection of creep damage in power plant steels

“…Small changes in the position of the camera between two images can be tolerated [127]. Laboratory tests have shown that the strain typically accumulated in Cr-Mo-V steel components over a 2-year period of plant operation can be measured with an error of less than 10% [128]. The silicon nitride targets are not affected by the formation of scale on the surface of the component, but they do tend to become dull with time after exposure to high temperatures, so that degradation of image quality is currently an issue [129].…”

Recent Developments in Measuring Creep Strain in High Temperature Plant Components

“…The Auto‐Reference Creep Management And Control (ARCMAC) gauge is a point‐to‐point optical strain gauge that has already been implemented in many UK power stations at selected positions of interest, as seen in Figures and , and has been the subject of ongoing development , . The gauge provides displacement data between two points selected on a test piece.…”