Under all circumstances, an engine and its driven equipment(s) must be prevented from operating at a speed above the maximum allowed speed — to ensure the safety of the equipment, plant and its personnel. However, meeting this requirement is particularly challenging for power generation units where the drive train is composed of electric generators driven by free power turbines (i.e. aerodynamically-coupled power turbines), since during load-shed events or circuit breaker failure, full loss of load happens almost instantly.

During these events, usually the Fuel Metering Valve is fully closed by the Engine Control System and the Fuel Isolation Valve is closed by the safety system. But, fuel gas continues to flow to the system during the closing of the valves, and furthermore, the fuel gas trapped in the piping between the valve and the fuel injectors still has enough pressure to flow to the combustion chamber and add energy to the system, which at that point has almost no external load, thus likely to cause an over-speeding of the drive-train.

This paper is to report a dynamic model created for drive-train over-speed predictions. In this model, fuel flow rate to the engine is calculated based on the principle of conservation of mass together with the fuel gas equation of state. The calculated fuel flow rate is then used to find the amount of power supply to the drive train, which in the next step is converted to the torque applied on the shaft. Finally, Newton’s second law is used to determine the angular acceleration and the angular speed. This approach is applied to two different variations of the Industrial RB211 Engine — the DLE (Dry Low Emission) RB211 and Non-DLE RB211 — which have different designs of the fuel gas system and the burners. For both cases, the results using the modeling approach presented in this paper demonstrate around 99% agreement with the actual measured over-speed values recorded during trip events. The model allows studying the drive train speed for different operating conditions and failure cases, and also makes it easy to understand and quantify the effect of fuel gas system parameter variation on drive-train over-speed.

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