Miltiadis Kourmpetis

Optical strain monitoring techniques for life assessment of components in power generation plants

Abstract The current paper presents the ongoing development of a combination of two methods for monitoring creep strain in mechanical components of electrical power generation plants. This is to obtain, during plant shut-down maintenance periods, needed data to assess the remaining life of installed steam pipes and other components. Related to this research, but not reported on in the current paper, is the development of monitoring for detection of the onset of fatigue and other failure processes in wind turbine generator blades. The auto-reference creep management and control (ARCMAC) system uses precision optics and a charge-coupled device (CCD) camera for uniaxial and biaxial strain measurement. Digital image correlation (DIC) is employed to obtain strain distribution data about the ARCMAC point-to-point monitored sites. These and other systems are being developed to obtain a more comprehensive range of life assessment data. This is mostly for assessing longevity of steam pipes and other components in power stations that are subject to demanding and hostile operational environments. These permanently installed monitoring systems need to be rugged to withstand the demanding heat and mechanical forces to which they are subjected and of a compact design so they can be sited in difficult-to-access locations. This is one of the advantages of the combined ARCMAC and DIC system that is further being developed. These systems are essential for cost-effective management of power plant operation and maintenance and for achieving reliable continuity of service.

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.

Life Assessment by Monitoring Biaxial Strain-Rates in High Temperature Steam Pipelines

Most of the main coal fired stations in the UK are operating beyond their original design life and it is essential to be able to manage the integrity of the high temperature pipework systems, for both commercial and safety reasons. The fossil fired plant in the UK has operated flexibly for a number of years, which complicates the remanent life assessment of a pipework system. This paper specifically addresses monitoring the creep strain accumulation over time of high temperature pressurized steam pipes in order to better quantify the remanent life and thereby provide improved advice on run/repair/replace issues. Any monitoring instrumentation needs to be rugged and able to endure what is a hostile environment and provide reliable strain measurements in both the longitudinal and circumferential directions of the pipe. This paper presents one monitoring method that has been developed for service use. The monitoring system uses a specially designed strain gauge that carries optical targets, image capture and analysis is used to determine the in-service creep strain rates. Experiences from site installations will be described.Copyright

Monitoring High Temperature Steam Pipes Using Optical Strain Measurement Techniques

Optical strain measurement techniques have been extensively developed in recent years in order to cope in various environments. Power stations and wind turbine blades can provide challenging environments for the use of a measurement technique. There are, however, many installation problems to be overcome. For example, there is the need to have regard for the hostile environment in steam generating plant and the demanding conditions to which wind turbine blades are subjected. Ideally the outputs from individual sensors would be used for continuous remote monitoring. However, measurements can also be useful each time the plant is shut down during a plant outage; which would be used to complement data from existing proven rugged monitoring methods. This paper addresses the monitoring of pressurized steam pipes as to their micro-strain growth related to time in service. This paper presents the progress made in the developing of a ruggedised digital speckle ‘sensor’ and associated image capture system. The effect of subsurface defects in the strain distribution is examined.

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

Digital Speckle Photogrammetry Techniques Applied to Monitoring High Temperature Steam Pipes

Digital Speckle Photogrammetry (DSP) is proving to be a very useful technique for studying, in the laboratory, the distribution of strain about cracks and other defects in stressed specimens. This non-contact technique is able to resolve strain gradients over a small physical area, for example across a weld heat affected zone. The technique has good potential for use as a condition monitoring tool for a variety of components in electrical power stations. In addition this measurement technique could also be applied to monitor the integrity of critical components of newer generation plant, such as wind turbine generator blades. There are, however, many installation problems to be overcome. For example, there is the need to have regard for the hostile environment in steam generating plant and the demanding conditions to which wind turbine blades are subjected. Ideally the outputs from individual DSP sensors would be used for continuous remote monitoring. However, DSP measurements can also be useful each time the plant is shut down during a plant outage; which would be used to complement data from existing proven rugged monitoring methods. This paper describes ongoing work to develop a ruggedised digital speckle ‘sensor’ and associated image capture system.Copyright