Replacement of a lift sensor by a model based lift observer

Due to extreme mechanical stress during bypass operation the lift sensors of the IP (intermediate pressure) bypass station of a major lignite fired power plant frequently broke down. The failure of the sensors resulted in immediate problems handling the pressure control of the IP bypass station and thus the operation of the whole plant. In order to ensure availability of the IP bypass control under the existing stress conditions, an alternative controller concept was developed. Instead of using the directly measured value of the lift, the output of a model based observer is used. This observer approximates the real valve lift by making use of pressure measurements which are readily available. The concept was implemented on site and showed a satisfactory performance.