Cascade Testing and CFD Applied to Gas Turbine Performance Improvement with Compressor Cleaning

With the growing interest in life cycle costs for heavy-duty gas turbines, equipment operators are investigating the trade off between performance improvements and associated maintenance costs. One of the key factors leading to performance degradation during plant operation is compressor fouling especially in harsh environments. These results from the adherence of dust or sand particles mixed with small oil droplets to compressor blade surfaces. The result is a reduction in compressor pressure ratio and an overall loss in mass flow, compressor efficiency and, therefore, overall power output. To restore this power loss, an increased fuel flow rate with the consequent higher firing temperature (TET) is necessary. This will seriously reduce the creep life of the HP turbine blades. The research described here is divided into three sections: The first section focuses on the simulation and diagnostic analyses of a typical clean and fouled industrial gas turbine. The analysis tool is a performance code (Turbomatch) developed at Cranfield University. The performance degradation was based on the following engine which is currently operating in the Libyan Desert, using field data available from the company. A single shaft industrial gas turbine (11 MW) engine The study comprised a simulation of the "clean" engine performance of a typical small gas turbine used for power generation. The study also examined the economic impact of compressor fouling on operating costs due to increased fuel flow rate and the reduced creep life of the HP turbine. Additionally, the study included an economical analysis of various types of compressor online washing techniques. The real engine data of a Sulzer Type 7 gas turbine (11MW) have been collected from a Libyan oil field over a period of four months without compressor washing. Corrected to standard ambient conditions the results are then compared with the clean engine performance. A case study shows that as a result of compressor fouling, the compressor efficiency decreases about 2.5%, and Heat Rate increases by 7% and power output falls by 10%. Consequently, the cost of power losses in a one year period is around $600,000. The second section involves high fidelity CFD (two dimensional and three dimensional) simulations of the fouling mechanism of the ninth stage of an axial compressor. This HP9 stage was tested in No. 3 Compressor Test Facility at Rolls-Royce. The comparison between the experimental data and the ANSYS CFX is shown in chapter 4. This research contains a description of the numerical analyses carried out on the HP9 test case to include the effects of variable amounts of roughness both in terms of spatial distribution and equivalent grain size. The examination of the effects of adding roughness to selected regions of the rotor blade suction side has shown the relative importance of the leading edge end on the blade suction side over other regions of the blade. The relative impact was quantified in terms of the wake velocity defect, magnitude and wake width. However when the radial distribution is considered the differences between clan and roughened blades is less apparent excepting for the case where the entire suction side of the blade is roughened. The pressure distribution around the blades was also examined. This part of the work has shown the benefits of flow analysis using CFD in a very small region of the flow field. In this case close the analysis is confined to the leading edge and trailing edge of a compressor blade. It also demonstrates clearly the disadvantages of fouling in reducing pressure rise through increases of total pressure loss and therefore reduced efficiency. It has been established that the leading edge and the concave surfaces of the rotor blade are the most sensitive regions to the fouling. It is emphasised that experimental verification of this trend would be very difficult to achieve with a cascade rig because both rotor and stator would need static pressure tapings very close to one another at the leading edge of the blade. This would be extremely difficult to achieve in practice. In the third section experimental work has been undertaken on a cascade facility in the Gas Turbine Laboratory. This details measurements of the cascade the pressure losses as a result of increasing levels of fouling on the blades. In addition, further experiments are undertaken to recover performance loss due to fouling through a washing technique. This has facilitated a rigorous estimation of the magnitude of compressor stage inefficiency caused by fouling. By observing the actual distribution of fouling in a typical compressor, it is now possible to estimate more accurately the overall impact of the fouled performance on overall compressor efficiency. Furthermore an experimental study has been carried out based on a need in practice for online compressor washing; an investigation was undertaken by using a different spray nozzle pressure. The detergent used for the washing experiments was a water based cleaner diluted with demineralised water in a ratio of 1: 4. The fouled compressor efficiency after cleaning with the current project washing scheme has been improved by 2.2%.