Gas Turbine Handbook : Principles and Practices

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208 Gas Turbine Handbook: Principles and Practices Depth of Field (DOF) is the minimum and maximum distance away from the objective lens at which the object is in sharp focus and is a function of the field of view. Fixed-focus type boroscopes do not have adjustable focus control, and objects will be in sharp focus only within a pre-determined range. As the FOV narrows the DOF also narrows. With a boroscope it is possible to see the early signs of oxidation, erosion, corrosion, cracking, and the results of foreign object damage without disassembling the gas turbine. The inspector must be able to distinguish between component conditions that are potentially catastrophic and those that are just insignificant abnormalities. Later in this chapter visual comparisons will be drawn between gas turbine components as seen through a boroscope and the same or similar components as viewed prior to installation or after disassembly. The degree of damage that may be confirmed by way of a boroscope inspection will be discussed in detail. This information can then be used to confirm or refute problems initially identified through gas path or vibration analysis and attributed to a specific section of the gas turbine. One man can boroscope a gas turbine in one hour, whereas over 60 man-hours would be required to teardown a heavy frame unit to achieve the same level of inspection. A BACKUP TO CONFIRMING SUSPECTED PROBLEMS While a boroscope inspection can (and should) be done on a regular basis, it is most effective when it is used to confirm a problem highlighted by either vibration analysis or gas path performance analysis. For example, a shift in turbine temperature profile, with no apparent changes in either turbine or compressor efficiencies, could be indicative of a plugged fuel nozzle or a hole in the combustion liner or transition piece. Boroscoping the combustor will quickly confirm the problem. An increase in turbine temperature could indicate an increase in turbine nozzle area (as a result of FOD or corrosion) or a decrease in compressor efficiency (due to compressor fouling). An inadequate compressor water wash procedure would further cloud the issue by not significantly improving compressor efficiency. This might lead the operator into prematurely removing the gas turbine from service for replacement (aero-derivative) or overhaul (heavy frame). Boroscoping
Boroscope Inspection 209 the gas turbine would help identify the problem area. Compressor vibration could be indicative of a failed compressor blade or imbalance created by contaminant flaking off an airfoil. The failed blade, blade portion, or loose contaminant could be so small that it has an insignificant influence on compressor efficiency. A boroscope inspection will provide the direction to either increase water washing efforts, overhaul the unit, or continue running the unit. ASSESSING DAMAGE TO INTERNAL ENGINE COMPONENTS For convenience in assessing engine health, some manufacturers have installed boroscope ports in strategic locations throughout their engines. Figure 13-3 is a schematic representation of a gas turbine with the typical location of boroscope ports indicated. However, manufacturers are not consistent as to the location of boroscope ports on different engine types and models. Therefore, an inspector must use every opening available or, as in some cases, openings that are easily created by removing an engine component (such as fuel nozzles, pressure or temperature probes, etc.). In general, boroscope ports are located in the combustor and in the turbine. The combustor boroscope port can be used to view the condition of the fuel nozzles, the combustion liner, the first stage turbine nozzles, and the first stage turbine Figure 13-3. Courtesy of United Technologies Corporation, Pratt & Whitney Canada. Schematic cross-section of the FT8 aero-derivative gas turbine with the location of boroscope ports indicated.
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Gas Turbine Handbook: Principles an
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Dedication This book is dedicated t
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Contents PREFACE ..................
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Chapter 13—BOROSCOPE INSPECTION .
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Preface to the Third Edition The ne
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Acknowledgments I wish to thank the
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Figure 2-3. Courtesy of United Tech
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Chart 8-1 Gas Turbine Environments
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Appendix A-2 304 Gas Turbine Handbo
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5. Weather Hood Material __________
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Appendix C-6 Air/Oil Cooler Specifi
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9. Drive Requirements A. Hydraulic
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22. Packaging For Shipment Per Spec
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Parameters Units NH 3 CO H 2 CH 4 C
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.055 Parameters Units 1/0 .88/.12 .
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Koppers Foster Blue Coal Gas Winkle
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Gas Turbine Handbook : Principles and Practices

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