October 31, 2008
Industrial gas turbines operate in many different environments, ranging from industrial sites to offshore locations to desert and arctic areas. In spite of the highly sophisticated air cleaning systems available today, ingression of air borne particles into the gas turbine compressor is inevitable, especially particles that can readily travel through the filter mesh, such as those in the micron and sub micron size range. As a result, fouling of air path surfaces in the turbine compressor occurs, leading to performance loss and increased fuel consumption. Because of this, compressor cleaning is an important aspect of equipment maintenance as it affects output, life cycle costs and revenue.
Worldwide field experience has clearly demonstrated that axial flow compressor deterioration during operation accounts for the major portion of gas turbine related performance loss from the new and clean condition. Of the total gas turbine performance loss approximately 70% can usually be attributed to compressor deterioration due to blade fouling. Fouling of axial flow compressor blades is generally attributed to airborne particulate in the sub-micron to 10-micron size range and this will be the major source of fouling. Another possible source of compressor fouling is oil leakage from the compressor rotor inlet bearing. The chances are, therefore, that all gas turbine operators will be faced, at one time or another, with performance losses due to compressor fouling and consequently with the need to clean the compressor.
Scottish & Southern Energy (SSE) is a major energy supply company in the UK serving customers from more than 150 sites across the UK, operating over 10,000 MW of generation capacity. SSE are the UK’s second largest generation business, with ownership interest in over 100 thermal and renewable power stations. SSE are the UK’s leading generator of renewable energy, owning and operating almost half of the UK’s total renewable generation capacity, including the UK’s largest wind farm.
The gas fired SSE Keadby power plant, based in England operated 2 x General Electric (GE) Fs9FA gas turbines each generating 255.6 MW (iso), the combined total generation is 720 MW. Since the plant start-up, more than 10 years ago, gas turbine compressor cleaning has been performed using the standard wash system offering from GE for both online (fired) compressor washing and offline (on-crank) compressor washing. The online wash system has had limited use because of a partial ban by GE on online compressor cleaning due to potential compressor blade erosion and liberation as a result of poor online wash injection processes. Offline washing using the GE system has its own set of issues and in 2003 SSE issued a tender to implement improvements to the current wash system.
The GE offline wash system problems are as follows:
- Flooding of the exhaust duct resulting in premature damage and failure of the exhaust bellows.
- Flooding of the lower half fuel nozzle pig tails – 2 x 8 hours of labour time
- High volume of wash system waste fluid requiring local storage and disposal
- Wash delivery system very labour intensive and many components obselete
The prime objective of the SSE Keadby tender were to resolve the listed issues above while improving the overall washing efficiency of the system and utilise as much equipment as possible from the existing wash system.