Castlelost Flexgen Gas Transient Analysis

• Industry: Power Generation 
• Region: Ireland
• Focus: Engineering a new gas power plant 

Challenge

Castlelost Flexgen is developing a 275 MW OCGT peaking plant (Kiltotan, Oldtown, and Rochfortbridge, ~ 85 km west of Dublin) designed for rapid response to grid demands. Such fast load variations produce transient pressure fluctuations across the gas supply system. The Gas Networks Ireland (GNI) regulator requires up to 5 seconds to react to shifts in flow, which can cause pressure spikes, dips, or gas supply interruption. This introduces risks of turbine alarms, shutdowns, or even triggering of the GNI slam-shut valve.

Maintaining stable turbine inlet pressure near 33 barg is therefore essential under various dynamic operating scenarios.

Solution

Fingleton White conducted a high-fidelity transient gas system analysis using xStream. xStream employs the Method of Characteristics (MOC) to solve the transient equations of gas flow. The fundamentals mass, momentum, and energy equations are solved for each pipe to accurately represent the propagation of transient waves throughout the system.

The full pipeline system—from the GNI regulator through the header down to each Siemens turbine—was modelled for both 5-turbine and 7-turbine operation. The study evaluated steady-state performance and four transient scenarios in each configuration: (1) all turbines tripping simultaneously, (2) all turbines ramping up at the steepest point of startup, (3) a single turbine trip with all turbines running, and (4) a turbine trip with only two turbines running.

The model incorporated regulator response curves, buffer volume calculations, alarm thresholds, turbine operational requirements, and worst-case boundary conditions to validate system robustness.

Results

The analysis demonstrated that the system meets all operational, regulatory, and OEM requirements for both 5-turbine and 7-turbine configurations. No transient event exceeded the GNI slam-shut limit of 38.01 barg. Pressure drops during ramp-ups stayed above the turbine low-pressure alarm threshold of 29 barg. Pressure variations in all trip scenarios were within the allowable transient limit of 1.5 bar.