NEW TECHNIQUE FOR ONLINE WASHING OF LARGE MECHANICAL-DRIVE CONDENSING STEAM TURBINES

摘要

To compete in today's economic climate, petrochemical plants are strategizing on continuous long-term operation to reduce maintenance costs and increase productivity. This strategy has led some plants to go from eight years between turnarounds to 10 years. For rotating machinery such as mechanical-drive steam turbines, one factor that affects this strategy is heavy deposition on steam turbine internals, caused by impurities in the steam. These impurities result in fouling on the blade and nozzle path surfaces due to contaminated materials such as silica and sodium in the steam. As a result, turbine performance tends to deteriorate gradually. This paper introduces an innovative online washing technique to minimize the impact caused by fouling of the steam path for large multistage condensing steam turbines. This technique, although applied here to extracting-condensing turbines, is also applicable to large condensing turbines. The new technology has water injection nozzles located in the steam chest of the extraction valve rack. The injection nozzles are manifolded to a water supply source, which controls a setpoint temperature, by controlling the water injection rate. The objective is to directly wash off deposits adhering to the blades and nozzles on the low-pressure side with minimal power turndown, and without impacting the turbine's long-term performance. Erosion damage and thermal stress of internal parts such as chest valves and blades due to the injected water had to be taken into consideration. To properly achieve this objective, and considering the potential for damage during the online wash, a new extraction valve box had to be designed. The new design had to consider the effects of optimizing the mixing zone of the steam and water injection to generate a specific particle size, moisture propagation through the condensing section, mechanical deflection of stationary components, and the overall thermodynamic analysis of each stage during the online wash. A prototype model was built and several experiments carried out based on the practical operating condition of actual steam turbines. This paper discusses the evaluations made from the model, by presenting the thermodynamic analysis results, and the finite element analysis (FEA) that was used to evaluate the strength of the internal parts during actual online washing.