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 maximum surge pressure in each case was compared to the pipe’s design pressure of 220 psia (15 bar). Only in the worst-case scenario where both the pumps fail to trip and the vent valves fail to open was this design pressure exceeded (Table 1).

While surge pressure did not exceed the design pressure in most cases, the resulting transient piping forces created major concerns. The imbalanced force would cause stress, deflection, and vibration as the pressure wave transmitted through the system. The worst-case with pumps running and vent valves closed saw forces exceed 12,000 lbf (53 kN) near the transfer valves.

The team exported their force results from Datacor Impulse into ANSYS for a time history analysis and to assess displacement, vibration, and pipe stress (Figure 2).