Regulations are requiring that the emissions from land-based gas turbines be tightly controlled. To limit emissions, the Industrial Trent turbine controls flame temperature through a premixed multi-stage combustor. This requires a multi-path fuel delivery system that provides accurate and repeatable metering of the fuel. There are also requirements that the fuel system quickly stop the flow of fuel and vent appropriate lines in the case of a load rejection or an emergency shutdown.
The gas system is a four-path fuel metering system that incorporates shutoff and vent capabilities in addition to high accuracy multi-path flow metering. One of the fuel paths is dedicated to the ignition torch and is significantly smaller and flow accuracies are less demanding than for the three main metering legs.
The concentration of work was dedicated to providing highly accurate and repeatable flow metering for the three main fuel legs. There was also a drive to minimize cost and provide commonality of parts between each of the three legs. Current fuel schedules require metering through a 20:1 turn down ratio.
Flow through each metering leg is measured in the same manner as flow measurement is performed across an orifice flow meter. The metering valve is a primary element for which the flow characteristics are well known under a large number of valve positions and flow conditions. Analysis and testing has been conducted to define flow accuracy and repeatability. Further work has been conducted to minimize pressure drop across the system and to reduce the number of sensors required.
