Combustion turbines typically operate at extremely high temperatures, for example, 1500° F. to 2000° F. for steam turbines and 2500° F. to 2900° F. for gas turbines. These high temperatures can cause failure of various components unless the components are protected from the heat. The components include the rotating blades of the turbine, and the vanes for directing gas flow within the turbine. A typical combustion turbine will have three to four rows each of blades and vanes, with approximately 50 to 100 blades or vanes per row, and will typically have approximately 500 total blades and vanes to protect. A commonly used material for vanes and blades is a superalloy such as nickel-cobalt. Other turbine components exposed to these high temperatures include the combustor and the transition. All of these high temperature components are generally insulated by a thermal barrier coating (TBC) so that the turbine can be operated at high temperatures without causing excessive deterioration of these components. A typical TBC comprises yttria stabilized zirconia.
Proper maintenance of turbine engines requires periodic inspection of the turbine components for deterioration or spalling of the TBC and other defects, such as cracks in the underlying components. When spalling or deterioration occurs, stress in the immediate area causes the surface to heat up resulting in further deterioration or spalling of the coating and, eventually, weakening of the superalloy body. In order to prevent turbine failure, it is desirable to monitor the condition of these components and replace or repair them when necessary. Since spalling of the TBC and other defects cause the surrounding areas to heat up, one way to monitor the condition of the TBC is to measure the temperature of the TBC on the blades and vanes.
One common method of measuring temperature relies on detecting the infrared radiation emitted by the TBC surface. Determining the temperature of a turbine component using radiation detected from a site on a body located within the turbine is complicated by the fact that emissivity is difficult to determine. Some current techniques for infrared radiation-based temperature measurements assume that the emissivity of the location being measured is equal to one (i.e. the location is a perfect black body emitter). Other approaches estimate emissivity at the site by measuring the emissivity of a sample composed of the same material as is present in the actual site while the sample is heated to approximately operating temperatures under controlled laboratory conditions. Both the black body assumption and the laboratory estimate often result in inaccuracies because emissivity is a function of temperature, surface type, surface age, and other factors.
In laboratory conditions the inaccuracy introduced by the black body assumption or emissivity estimates do not typically affect temperature measurements because the only radiation, if any, impinging on the measured site is (i) from an external source controlled by the tester, (ii) negligible compared to the emitted radiation, or (iii) both. In contrast, a site on a component of a combustion turbine is surrounded by other surfaces that are emitting ambient radiation at intensities similar to that of the measured site. The lower the emissivity (i.e. the higher the reflectivity), the more the radiation impinging on the site is reflected toward the radiation detector. If a significant, unknown amount of ambient radiation is reflected by the measured site, the temperature reading may not be accurate if, as in the black body assumption, the reflected radiation is treated as though it were emitted from the measured site. Similarly, if the emissivity cannot be determined or eliminated from the calculation, an error may be introduced into the temperature measurement. Thus, there is a need for a method of temperature measurement that can provide accurate temperature measurements that account for ambient radiation that is reflected by the measured site and that are based on the emissivity of the actual measured site.
Another source of error introduced by the black body assumption or emissivity estimates stems from the amount of infrared radiation emitted from the site. The amount of emitted radiance due to the site's temperature is directly proportional to the emissivity of the body. Thus, a significant error in the temperature measurement may result if the black body assumption is used and the emissivity deviates significantly from this assumption. Similarly, the emissivity estimates can be incorrect if the age or condition of the actual surface cause it to have an emissivity that is significantly different from the sample material that is tested. Accordingly, there is a need for a method of temperature measurement that can provide accurate temperature measurements that account for the actual emissivity of the site being measured.
One method for monitoring the condition of turbine parts is disclosed in U.S. Pat. No. 6,072,568. The '568 Patent discloses a nondestructive, off-line method of determining residual stress proximate an intermediate layer in a multilayer TBC system by directing a laser beam through an outer ceramic thermal insulating layer to an intermediate layer. The laser causes a species in the intermediate layer to fluoresce. The amount of fluorescence is detected and compared to control samples to determine stress, not temperature. A major disadvantage of this method is that the measurements are made on a single site on the turbine component. A full analysis of a typical turbine using this method could require at least one measurement for each of the approximately 500 blades and vanes. Thus, there is a need for a method of monitoring the condition of the blades that does not require this excessive number of measurements.
Another method of measuring the condition of turbine parts is disclosed in U.S. Patent Application Publication No. 2004/0179575. The '575 Patent Application Publication discloses a device that detects long wavelength infrared radiation and short wavelength infrared radiation to measure the surface temperatures of the TBC and the underlying substrate, respectively. The '575 method measures temperature without using an external IR emitting source and without accounting for ambient radiation that is reflected by the site. Furthermore, the '575 method does not disclose a method for measuring the temperature of a site based on in situ measurements of the site's emissive properties. Instead, the '575 method relies on the relative values of the detected short wavelength and long wavelength infrared radiation to locate defects in the TBC surface. Thus, there is a need for a method of measuring the temperature of the blades and vanes that is based on the measured site's in situ emissive properties.