On Linearizing the Steam Mass Flow Relative to the Controller Output

In order to develop efficient control and to predict a steam turbine’s power output precisely, it is desirable to have a linear relationship between controller output, RA, and steam mass flow. Unfortunately, steam mass flow through a turbine is not only determined by the control valve stroke but also by the pressure in front of these valves. Even at constant pressure the relation between valve stroke and steam flow through the turbine is extremely non linear. The complexity is increased by the fact that a turbine is generally operated by two or four control valves which do not necessarily work parallel over the complete operational range. Additionally, the pressure in front of the control valve changes with the admitted steam mass flow. A counterbalancing method was developed that allows to include individual process data, such as pressure in front of the valves and at the turbine inlet in dependence of steam mass flow and valve throttle characteristics, as well as process engineering constraints, like valve staggering, for example. The developed method is applicable to new steam power plants as well as to retrofits. With the developed method it is also possible to predict individual valve strokes at valve testing or at deviating exterior conditions. The later feature is extremely useful for retrofit applications. Firstly, the correctness of the implementation with respect to the ‘old’ set-up can be verified, and then, secondly, the characteristics of the retrofitted components and thermodynamic conditions, respectively, can be substituted. The developed method was successfully applied for several power plants. A comparison of predicted data and commissioning data will be provided.Copyright © 2011 by ASME