Chris Maharaj

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

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.

ARCMAC Optical Strain Measurement for Creep Monitoring: Recent Improvements in Accuracy and Resolution

Remaining life of power station high pressure steam pipes is heavily dependant upon material creep rates. However, due to the difficulty in monitoring strain in these pipes as a result of the demanding operational conditions, a rugged optical strain gauge system has been developed. The current E.ON UK ARCMAC gauge system has been validated using the UK National Physical Laboratory standard grade extensometer and provides a strain measurement accuracy of 64 micro-strain with an error of <10%. This system uses precision optics, a CCD camera and a light source system to capture images of uniaxial and biaxial optical strain gauges on steam pipes during periodic maintenance. Further developments of the ARCMAC system have included the design, manufacture and validation of an advanced ARCMAC optical measurement system with improved sensor resolution and improved accuracy. Additionally, the methodology of image processing has been studied in order to reduce errors in both the existing and the new ARCMAC systems. Finally, Digital Image Correlation (DIC) has been used alongside ARCMAC gauges to monitor strain fields around welds and defects in steam pipes. Some of these techniques have also been used in a related study into strain monitoring in wind turbine blades.© 2009 ASME

Developments in Combined ARCMAC and Strain Mapping Systems for Creep Measurement

The E.ON UK ARCMAC gauge has been developed to provide for point-to-point biaxial creep strain measurements. Research is continuing to provide for strain field mapping about the ARCMAC point-to-point monitoring system. The aim is to have comprehensive monitoring of creep strain in high temperature steam pipes including bends, joints, welds and other pipe structural features. A requirement is that all creep strain measurements made satisfy the standards of the UK National Physical Laboratory that maintain liaison with the standards authorities in other countries. Creep strain and other monitoring equipment for power-station steam pipes need to be rugged and suitable for the many different locations in which they are to be installed. Appropriate equipment is also required for capturing data at different times from the installed gauges. The use of strain mapping is particularly required when monitoring non uniform sections of pipes. Results to be presented will show the current state-of-art measurement techniques now available.Copyright

Recent Developments in Methods to Study Creep Strain Variations in Power Station Steam Plant

In power station steam plant, experience has shown that portable monitoring equipment is ideal for reliable point-to point measurement of creep strain and also acquisition of strain mapping information. On going laboratory tests have shown positive initial results in that direction; the results being validated against metrology standards. In addition to the monitoring of in-service strain in steam pipes, the study of the effects of strain distribution across welded pipe sections and other known early failure locations on steam pipes and other plant components is growing in interest. An existing strain monitoring method employs the E.ON UK ARCMAC gauge system with a capability of both uniaxial and biaxial strain measurements. For strain mapping, digital image correlation (DIC) is also incorporated and integrated with the ARCMAC system. In an effort to further improve the ARMAC system portability, the use of a digital single-lens reflex (DSLR) camera system is investigated. The scope of a combined ARCMAC/DIC system to provide for both point-to-point and strain mapping of piping features is studied. Identification of a feature using a combination of ARCMAC and DIC techniques is presented here. Additionally, the investigation of an edge-finding technique which can complement the existing ARCMAC system is presented.Copyright

Predicting lifetimes of components in power station engineering plant

Abstract Today, there is an increasing demand for power station plant to be operated cost effectively and to reliably maintain required electrical supply. This requires minimising the risk of having to shut down the plant for emergency repairs. Of paramount importance is plant operating integrity and well prepared and executed maintenance programmes. The present paper reports on the recent developments to the auto-reference creep management and control system used by E.ON UK. This includes achieving biaxial strain measurement with increased resolution and the employment of digital image correlation.

Monitoring Creep Strain in Power Station Engineering Plant

Cost effective and reliable operation of a power station plant and achieving low carbon dioxide emissions can be very dependent on maintenance activities. Planning maintenance to minimise the down-time of the plant is a key cost factor. It is important to have the latest data as to the age and state of the components and parts at the time of plant shut down. Indeed, deciding on the best time to shut down the plant for maintenance can depend upon trends in these data. For steam pipes, the required ageing information is the creep strain rate. These creep strain data obtained at shut down provide key information as to the needed replacement of pipes to maintain reliable plant operation. This paper presents the E.ON UK ARCMAC creep strain measurement systems that are being further developed by Imperial College London with the use of DIC and other techniques.

Resolution of Creep Strain Measurements Using the ARCMAC Strain Monitoring System

Essential is that creep strain monitoring sensors need to be of a rugged design well able to endure the demanding conditions related to the environment of high-pressure steam pipes and other power plant components. A useful monitoring method employs the E.ON UK ARCMAC gauge system with a capability of both uniaxial and biaxial strain measurements. The choice of monitoring location, for example, can be to welded pipe sections and other known early failure locations on steam pipes and other plant components. The ARCMAC system has been refined over the years to improve the accuracy of measurement. In this paper, recent developments have allowed the strain measurement accuracy to be improved to 64 micro-strain with an error of <10%. Also, recently, high temperature trials of the combined ARCMAC/DIC have commenced and early findings are reported.Copyright

Researching Methods to Study Creep Strain Variations in Power Station Steam Plant

A research programme to investigate different methods of measuring creep strain rate in steam pipes and other power station components is described. One of the studies is to obtain more information of the changing distribution of creep strain around welded joints, changes in cross-section and other features in steam pipes and other power plant components. One method being developed is to use Digital Image Correlation (DIC) together with the E.ON UK ARCMAC gauges. The latter to provide an integrated creep strain rate, with the DIC method revealing the local creep strain rate distribution around the ARCMAC gauges. To validate this approach, laboratory specimens have been prepared that have known faults to ratify the measurement data obtained and its sensitivity to small changes in the studied specimens. Parallel to this laboratory research are investigations as to how best to use the DIC method on working steam pipes and other components of power stations.Copyright

ARCMAC Optical Creep Monitoring: Developments in Image Analysis Techniques and Creep Measurement Validation

Power station steam pipes operate at high temperatures (approximately 570°C) and pressures, and estimation of remaining life is critical in avoiding pipe failure. In estimating remaining life, creep rate for both parent pipe material and weldments is an important metric. A number of optical strain methods have been investigated to measure creep strain, including digital image correlation (DIC) and E.ON’s auto-reference creep management and control (ARCMAC) system. The ARCMAC system measures point to point strain by capturing and analysing images of a pair of inconel gauges. Recently, a modified ARCMAC image capture system has been developed using a DSLR camera, providing higher resolution images and lower costs compared to the existing system. Experimental results from the modified ARCMAC system show reduced accuracy compared to the existing system, likely due to distortion caused by lower quality lens components. However, by calculating and applying a correction for this distortion during image processing, it has been shown that accuracy of the modified system becomes significantly better than the existing system. The processing of ARCMAC images has previously been carried out manually by the user. In order to improve the repeatability and a speed of ARCMAC image processing, a computational code has been developed to carry out the procedure automatically. As well as reducing processing time substantially, the selection of image processing parameters has been standardised, improving accuracy significantly. ARCMAC results from creep tests carried out to simulate in-service conditions and measure creep under controlled conditions are also presented. These creep experiments form part of efforts to develop the ARCMAC system to be used to measure strain on-load rather than during the outage, allowing for more regular readings and more accurate creep rate estimation.Copyright

Validation and Optimization of ARCMAC and Strain Mapping Systems for Creep Measurement

Research is ongoing to validate and optimize the E.ON UK auto-reference creep management and control (ARCMAC) system for high temperature piping creep measurement. In conjunction with the ARCMAC system, research is also being undertaken to incorporate digital image correlation (DIC) into creep strain evaluation, via a rugged high temperature pattern. The aim is to have comprehensive monitoring of creep strain in high temperature steam pipes including bends, joints, welds and other pipe structural features. This paper firstly presents work done into the determination of the best method to capture and process ARCMAC images. It was found that a bi-cubic interpolation resizing function with a combined high light intensity image gives the best results with respect to accuracy and repeatability of image acquisition. Also presented was the testing of a modified (prototype) ARCMAC camera with modified (prototype) ARCMAC gauges. The prototype ARCMAC camera, based on a DSLR camera, is approaching the accuracy of the conventional ARCMAC but an improved lens system would be desirable. For the prototype gauge, it was found that the edge-finding technique using the knife-edge was just as accurate as using the target ball reflection for strain analysis. Both techniques can be used independently to evaluate creep strain. Finally, preliminary high temperature testing results for both the ARMCAC and DIC technique are presented. For the ARCMAC gauge and the DIC high temperature pattern, small changes in strains were observed over time that were not due to sample deformation. Reasons for these small changes in strain were identified and solutions arrived at. All results are very promising and it is foreseen that a combined ARCMAC/DIC tested technique will be applied to plant piping components soon.© 2009 ASME