Study On Unit Load Fluctuation Caused By Steam Turbine Governing Valve Unstability

Considering the development of national energy requirements and energy-saving emission reduction,since 2008, shutting down small thermal power units have been required in china. But with the development of electric power in the whole world, there are still many countries and regions in a state of low electric power, and vigorously need small and medium units, such as 50MW class units. On the other hand, in addition to electricity generation, 50MW unit is also widely used for industrial steam of owned power plants in metallurgy, chemical industry, sugar industry; district heating and many other fields. 50MW units have a strong competitive advantage in cogeneration. Therefore, in order to ensure 50MW units safety, stable running, and the further expansion in the foreign markets, study 50MW units has obvious significance.Since 2005, with the super-critical, ultra-supercritical 600MW units putting into operation more and more, and the development of 1000MW units'technology, 50MW class units have become smaller and smaller in China's electric power equipment, and it gradually runs from base load to peaking load. Thus, load fluctuation caused by steam valve gradually emerges in 50MW class steam turbine. Load fluctuation in 50MW class units is the representation which the steam valve and its control is unsuitable to the power development in China, and it consequentially exists.In this paper, through the analysis of steam turbine governing valve structure, researches found that: valve unload large and having pre-inlet valve are the reasons which may cause the major valve floating up and down, and consequentially cause load fluctuation. Reducing the pre-inlet valve stroke is an effective method of solving stem vibration and load fluctuation.Secondly, this paper by studying the relation between pre-inlet valve stroke and valve lift force, come to a conclusion: With the pre-inlet valve stroke decreases, the valve lifting force is gradually increasing. Thus reducing pre-inlet valve stroke may cause valve lifting force increase. In order to ensure the valve can be opened, it is recommended only to cancel the last two pre-inlet valves'stroke.Thirdly, this paper researches from the perspective of valve control, and draw a conclusion: achieving the valve management function is a good way to solve load fluctuation at valve points of 50MW class units. At the same time, the adoption of high-pressure fire-resistant oil can solve the big valve lifting force problem caused by reduction or cancellation of the pre-inlet valve stroke. Moreover, this paper shows that inappropriate valve overlap is one of the reasons which may cause load fluctuation.Finally, the valve management function in low-pressure oil system is discussed.