Siemens’ new condition based maintenance system uses infrared to determine blade wear and gives operators advanced warning when equipment needs to be repaired or refurbished.
Dr. Hans-Gerd Brummel, Dennis LeMieux, Matthias Voigt, Paul Zombo, Siemens Westinghouse, USA
as turbines have gained increasing popularity in the power industry. Integrated into a combined cycle process, they are able to generate electrical power with an efficiency of nearly 40 per cent in open cycle and nearly 60 per cent in combined cycle mode. To reach these relatively impressive figures, the process temperatures have to be as high as possible, but this is a challenge, as material properties limit the process. For the realisation of the recent gas turbine generation, a combined design strategy had to be applied for the most exposed parts of the hot gas path, which are the first rows of vanes and blades in the actual combustion turbine.
These combined measures include the proper choice of heat resistant alloys, efficient cooling of the vane and blade metal structures, and a thermal insulation layer – referred to as Thermal Barrier Coating (TBC) – to protect the metal components from the direct heat impact of the combustion gases.
Figure 1. Hot gas path of the Siemens gas turbine SGT6-5000F
Although considerable effort is put into keeping these components intact, vanes and blades do not last for the entire life of the gas turbine. They have to be replaced several times during the operational life of the engine, which takes place during scheduled maintenance programmes. There are algorithms in place that determine the proper time for an exchange, but importantly, these calculation methods are only estimates, as they do not take into consideration every detail affecting the actual condition of the hot gas path components. Consequently, there is a possibility that individual blades or vanes may fail before the scheduled exchange date.
Wear and tear
There are three measures which ensure a long component life: material choice, cooling, and TBC. The latter is most affected by wear and tear as this is a ceramic layer sprayed on the vanes and blades. TBC is impacted by erosion, debonding effects and spallations, when parts of the thermal insulation layer chip off, leaving the metal surface unprotected.
A weakened turbine blade, rotating at 3000 or 3600 r/min, carries the potential for severe secondary damage. When the airfoil breaks away from its platform it can destroy the entire turbine. It is extremely difficult to survey those critical components and in particular the condition of TBCs during operation of the engine, due to the high combustion gas temperature and pressure (approximately 1400˚C at a system pressure level of 15 bar), and the high rotation velocity of the blades (about 390 m/s tip velocity for row 1 blades).
Orlando-based Siemens Westinghouse Power Corporation has identified a system to take advantage of the hostile hot environment by combining a high speed infrared camera with a cooled optical system to enable visual access of the core of the engine to obtain the desired information on the rotating blades. It was found that the thermal radiation from the red hot blades provides sufficient energy for an IR camera to obtain images. There are IR cameras available now which have an extremely short integration time in the magnitude of only 1 millionth of a second. The main obstacle, fast blade rotation, which would normally result in a blur on the actual image, could be overcome by literally freezing the movement with this short exposure time.
Other important component needed was an overall supervisory system, which should incorporate all functions to operate/control the monitor, in particular the camera and the blade identification and image triggering system. Siemens decided from the start that any monitor it developed should be able to be incorporated into its global remote monitoring infrastructure. The company also decided the system must be remotely operable and able to automatically evaluate the images taken in terms of detecting defects, and furthermore, capable of transmitting images and evaluation results without human interference, to one of the company’s power diagnostics monitoring centres.
Figure 2. To validate the concept, the entire system was installed on a stationary 60 Hz 200 MW gas turbine
This supervisory system, called BladeInspector, has to identify and select any blade for taking an infrared image. The triggering has to be so sensitive that a variety of view angles can be realised, making the automated control system very versatile. Together with the control functions, which can be utilized remotely, the system must be able to communicate with the standard data acquisition system for Power Diagnostics’ remote monitoring, called WIN_TS, from which the BladeInspector receives additional measured data detailing the engine operation such as electrical output and local exhaust gas temperatures. BladeInspector evaluates the images automatically and sends selected images and evaluation results back to the WIN_TS system, which then transfers these data packages to the power diagnostics centres for further evaluation and central storage.
The Siemens Westinghouse development proposal garnered interest from the US Department of Energy, which co-sponsored the project from 2001 to 2005. After completion of the conceptual phase in 2001, the design of the system was divided into several individual parts:
- access ports on the engine (multiple view angles for row 1 and row 2 blades
- optical system
- camera enclosure including cooling system
- overall supervisory system (BladeInspector).
The actual design work and manufacturing of the components took place in 2002 and 2003. For the realisation of the supervisory system and the development of the automated image evaluation capabilities in particular, Siemens Westinghouse teamed up with Siemens Corporate Research, the US branch of Siemens Corporate Technology.
Most of the individual systems were laboratory tested, but it was not until the installation on a real engine that the overall system was able to prove the validity of the concept by demonstrating that sharp images of a large portion of a gas turbine row 1 blade could be obtained under full load operation.
To validate the concept, the entire system with all sub components was initially installed on a stationary 60 Hz 200 MW class gas turbine at the Siemens gas turbine test centre in Berlin, Germany. The first images were obtained on 27 January 2004, with the quality being better than expected. Even the cooling holes on the blades with a diameter of little more than 1 mm could be clearly seen. Some days later an engine test was conducted, bringing the engine up to maximum load. The black and white insert in Figure 2 shows an infrared image taken during that full load test. This blade is unique because the TBC on the leading edge was artificially removed for blade identification purposes. This can be clearly identified on the picture, proving that TBC spallations can be detected and the blade identification and triggering mechanism works.
After the successful proof of the overall concept and system functionality, the development team moved to another gas turbine, as the test engine in Berlin only runs for several hours at a time. Testing the long term performance of this innovative monitoring system was the next logical step towards the development of a reliable, rugged product. The Empire Stateline commercially operated power plant in Joplin, Missouri, USA was chosen for that purpose.
In contrast to the Berlin test centre, where only two camera ports for row 1 were installed, the engine at Empire Stateline was equipped with a third camera port for the inspection of row 2 blades.
This blade monitoring system has been in operation now for some 4000 hours, providing real-time information concerning the status of the blades. During that period local blade anomalies such as cooling hole blockages, platform gap leakage, thin film cooling deficiencies and TBC defects could be observed.
For a gas turbine manufacturer, the benefits of this innovative monitor come from two areas. Firstly, this system opens up new perspectives for the development of the next gas turbine generation as the core of the engine is now literarily visible. The impact of new engine design features can be immediately verified on the test bed, via the monitor. The system is ready for this application in the current development stage.
Secondly, for customers who make their living from gas turbine operations, the purchase of this type of system or through signing a monitoring contract also has significant benefits. Gaining reliable information on the critical parts of the engine on-line is a major advantage, not only in determining the right time for an exchange (condition based maintenance), but also as a form of insurance against otherwise unpredictable events such as blade failures. For this purpose, the new system still needs some refinement for conversion from a research and development object into a commercial product.