Harold R. Simmons

Aerodynamic Instability and Life-Limiting Effects of Inlet and Interstage Water Injection Into Gas Turbines

Gas turbine power enhancement technologies, such as inlet fogging, interstage water injection, saturation cooling, inlet chillers, and combustor injection, are being employed by end users without evaluating the potentially negative effects these devices may have on the operational integrity of the gas turbine. Particularly, the effect of these add-on devices, off-design operating conditions, nonstandard fuels, and compressor degradation/ fouling on the gas turbines axial compressor surge margin and aerodynamic stability is often overlooked. Nonetheless, compressor aerodynamic instabilities caused by these factors can be directly linked to blade high-cycle fatigue and subsequent catastrophic gas turbine failure; i.e., a careful analysis should always proceed the application of power enhancement devices, especially if the gas turbine is operated at extreme conditions, uses older internal parts that are degraded and weakened, or uses nonstandard fuels. This paper discusses a simplified method to evaluate the principal factors that affect the aerodynamic stability of a single-shaft gas turbines axial compressor. As an example, the method is applied to a frame-type gas turbine and results are presented. These results show that inlet cooling alone will not cause gas turbine aerodynamic instabilities, but that it can be a contributing factor if for other reasons the machine s surge margin is already slim. The approach described herein can be employed to identify high-risk applications and bound the gas turbine operating regions to limit the risk of blade life reducing aerodynamic instability and potential catastrophic failure.

Measuring Rotor and Blade Dynamics Using an Optical Blade Tip Sensor

This paper describes an optical measurement system for monitoring combustion turbine blade tips. The sensor measures distance to a blade tip using triangulation of reflected laser light. The system accomplishes triangulation using an optical position sensing device and high speed data acquisition. In this way, it is able to monitor not only average and minimum blade tip clearances, but to monitor the variations of individual blade tip clearances. By appropriate signal processing, it is possible to determine rotor vibration at the probe axial location, variations in shaft DC position, transient losses in blade tip clearance, the potential for tip and seal rubs, vibrations of individual blades in the tangential direction, and rotor torsional vibration at the probe location. Some aspects of blade and torsional vibrations would require more than one probe. The paper presents static calibration data for the measurement system, showing its degree of linearity and range. The paper also presents data obtained on a dynamic blade test rig with tip passing speeds and blade widths comparable to those encountered in high performance industrial combustion turbines. Data from this rig have been processed to show rotor vibration, shift in shaft average position, blade-to-blade tip clearance variation, and variation with speed of minimum blade tip clearance. The measurement system is designed to produce data suitable for use in the monitoring of advanced combustion turbine durability and the diagnosis of turbine functional problems, static and dynamic.© 1990 ASME

Flash Atomization: A New Concept to Control Combustion Instability in Water-Injected Gas Turbines

The objective of this work is to explore methods to reduce combustor rumble in a water-injected gas turbine. Attempts to use water injection as a means to reduce NOX emissions in gas turbines have been largely unsuccessful because of increased combustion instability levels. This pulsation causes chronic fretting, wear, and fatigue that damages combustor components. Of greater concern is that liberated fragments could cause extensive damage to the turbine section. Combustion instability can be tied to the insufficient atomization of injected water; large water droplets evaporate non-uniformly that lead to energy absorption in chaotic pulses. Added pulsation is amplified by the combustion process and acoustic resonance. Effervescent atomization, where gas bubbles are injected, is beneficial by producing finely atomized droplets; the gas bubbles burst as they exit the nozzles creating additional energy to disperse the liquid. A new concept for effervescent atomization dubbed “flash atomization” is presented where water is heated to just below its boiling point in the supply line so that some of it will flash to steam as it leaves the nozzle. An advantage of flash atomization is that available heat energy can be used rather than mechanical energy to compress injection gas for conventional effervescent atomization.

Aerodynamic Instability Effects on Compressor Blade Failure: A Root Cause Failure Analysis

Diagnosing the root cause of compressor blade failures by high cycle fatigue (HCF) is elusive for gas turbines with a long history of successful operation; long operating times preclude design, fabrication, and material defect issues that are usually associated with short term failures. Long exposure time might implicate erosion or corrosion issues for compressor failures. An investigation of 3rd stage stator vane and 4th stage rotor blade failure in a Frame 7 gas turbine revealed that aerodynamic excitations associated with mild compressor instability (undetected by installed sensors) was the most probable cause. A Blade Vibration Audit (BVA) approach exploits information collected and compared from independent sources: 1) Fracture mode details and expected failure stress levels estimated in metallurgical examination; 2) Aerodynamic excitations due to internal airfoil wakes, rotating stall, and flutter applied to vibratory stress response data obtained by modal testing to estimate relative operating stress levels; 3) Design margins deduced from successful operating experience, which establish a base line for comparison with the excitation sources considered likely. Diagnosis of HCF failure causes must produce results that match observations of the specific failures in three ways: 1) The airfoils must be in resonance for sufficient time for fatigue to occur, 2) The vibratory stress must be greater at the failure location than elsewhere, and 3) The causal effect must significantly increase the stress at the failure location to be more than elsewhere. The analysis showed that the 3rd stator vane failed at 2/3 span, which led to 4th blade failure due to subsequent adverse aerodynamic excitation and impact damage. The most likely cause of the 3rd stator vane failure was a combination of resonant excitation from excessive wakes of the downstream rotor blades and flutter associated with mild intermittent surge.Copyright

Toward Developing a Probabilistic Methodology for Predicting High-Cycle Fretting Fatigue in Aero-Engines

This paper reports the results of an investigation focused on identifying the necessary steps required to develop a probabilistic fracture mechanics-based methodology for treating high-cycle fretting fatigue in military engine disks. The current methodology based on finite-element method (FEM) modeling, analytical contact stress analysis, and probabilistic fracture mechanics for analyzing low-cycle fretting fatigue is highlighted first. Incorporation of high-frequency vibratory stress cycles into a composite mission profile containing mostly low-cycle stresses requires the use of the Campbell diagram and the need to identify the mode shape, frequency, and forcing function for blade excitation induced by stator wake, flutter or rotating stall. Forced response computation methods for addressing these phenomena in the literature are reviewed to assess their applicability for integration with a contact stress analysis and a probabilistic fracture mechanics life-prediction code. This overview identifies (1) a promising path for combining vibratory stress computation, FEM structural modeling, contact stress analysis, and probabilistic fracture mechanics for treating high-cycle fretting fatigue at the attachment region of engine disks, and (2) a new approach for treating high-cycle fretting fatigue due to vibratory stresses separately from low-cycle fretting fatigue at various positions of a fan-speed profile.Copyright

Aerodynamic Instability and Life Limiting Effects of Inlet and Interstage Water Injection Into Gas Turbines

Gas turbine power enhancement technologies such as inlet fogging, interstage water injection, saturation cooling, inlet chillers, and combustor injection are being employed by end-users without evaluating the potentially negative effects these devices may have on the operational integrity of the gas turbine. Particularly, the effect of these add-on devices, off-design operating conditions, non-standard fuels, and compressor degradation/fouling on the gas turbine’s axial compressor surge margin and aerodynamic stability is often overlooked. Nonetheless, compressor aerodynamic instabilities caused by these factors can be directly linked to blade high-cycle fatigue and subsequent catastrophic gas turbine failure; i.e., a careful analysis should always proceed the application of power enhancement devices, especially if the gas turbine is operated at extreme conditions, uses older internal parts that are degraded and weakened, or uses non-standard fuels. This paper discusses a simplified method to evaluate the principal factors that affect the aerodynamic stability of a single shaft gas turbine’s axial compressor. As an example, the method is applied to a frame type gas turbine and results are presented. These results show that inlet cooling alone will not cause gas turbine aerodynamic instabilities but that it can be a contributing factor if for other reasons the machine’s surge margin is already slim. The approach described herein can be employed to identify high-risk applications and bound the gas turbine operating regions to limit the risk of blade life reducing aerodynamic instability and potential catastrophic failure.Copyright

Remote Condition Monitoring of a Peaking Gas Turbine

This paper illustrates how software and hardware for telecommunications and data acquisition enable cost-effective monitoring of peaking gas turbines using personal computers. It describes the design and evaluation of a system which transmits data from each start-up and shutdown over 1,500 miles to a monitoring computer. It presents system structure, interfaces, data content, and management. The system captures transient sequences of acceleration, synchronization, loading, thermal stabilization, steady operation, shutdown and cooldown; it yields coherent sets of speed, load, temperature, journal eccentricity, vibration amplitude, and phase at intervals appropriately spaced in time and speed. The data may be used to characterize and identify operational problems.© 1991 ASME

Effects of Stator Flow Distortion on Rotating Blade Endurance: Part 2—Stress Analysis and Failure Criteria

Blade vibrations, with the possibility of failure, is one of the major factors controlling the reliability of compressors and turbines. The prospects of encountering high alternating stress environments in blades make efficient turbomachine operation a very challenging task. In many cases the compressor or turbine functions through a wide range of load, flow, temperature, and speed which affect blade vibration, thus the stress environment continuously changes as the operating conditions changes. Any flow disturbance upstream of the rotating blades and some disturbances downstream will produce repetitive wake pulses that excite the blades. Resonance occurs with any coincidence of repetitive pulses with structural natural frequencies of rotating blades or impellers resulting in substantial amplification of alternating stresses. Most OEM design practices control vibratory stresses by avoiding resonance with expected stator sources; those excitations that cannot be avoided are designed with sufficient endurance to prevent failure. Thus three aspects of rotor/ blade design affect reliability: 1) aerodynamic excitation level and frequency, 2) structural response and resonance margins, and 3) selection and control of materials, coatings and their fabrication process to withstand the service environment. The main objective of this study is to develop a mathematical model to simulate the stresses in the rotating blade row that evaluates all three aspects of design to assess long term endurance.This is a two part paper on high cycle fatigue (HCF) failure analysis procedure of rotating blades and impellers. Part 1 [1] discusses aerodynamic excitation caused by stator vane and its role in generation of blade vibration. Here comprehensive computational fluid dynamics (CFD) is used to get a better understanding of the stator-rotor flow interactions at different operating conditions. The results of the aerodynamic simulations are order related excitation spectrum that can be applied to the stress/pulsation relationship defined in this part of the paper.This paper, Part 2, discusses an empirical dynamic stress model developed by impulse testing, assessing material endurance strength, and evaluation of criteria for failure by HCF.Copyright

Tools for Diagnosing Case Deflections and Alignment on a Power Utility Combustion Turbine

This paper discusses development, installation, and analysis of instrumentation systems for reliably measuring casing, thermal distortion, and alignment deviation on a large combustion turbine in power utility service. A variety of redundant measurement systems were installed to document casing distortion during the cool-down period after firing. The operating principles of each measurement system are described and presented with the rationale developed for installing and locating sensors. A vertical deflection sensor used for casing bow and bearing misalignment measurements is highlighted in the paper to illustrate its potential for use in other investigations. Additional sensors used include an array of shaft proximity probes, blade tip proximity probes, thermocouples, and axial growth probes, blade tip proximity probes, thermocouples, and axial growth probes. A measurement system for casing ovalization was developed using LVDTs mounted from a thermally stabilized ring. An automated data acquisition system was developed and installed to facilitate the recording of turbine cool-down events over the complete operating season without the need for constant on-site attention. Preliminary results define the turbine cylinder bow and ovalization response during the cool-down event following normal unit operation and correlated casting distortions with thermal gradients.

Turning Gear Operation: Its Influence on Combustion Turbine Rotor Eccentricity and Starting Dynamics

This paper presents an experimental program which investigates how turning gear operation influences thermal distortion and start quality of a power generation combustion turbine. It presents results which quantify how taking a turbine off turning gear under hot or cold conditions influences rotor distortion. It further shows how time on turning gear reduces rotor eccentricity. This paper uses available unbalanced sensitivity data and rotor eccentricity measured on turning gear to estimate the margin for vibration trip on start-up. The paper discusses how rotor eccentricity data can be independently obtained with limited additional non-intrusive instrumentation, and how the data can help guide turning gear operational strategies for different utility load profiles.Copyright