As power plant combustion turbines (CTs) are pushed towards higher thermal efficiencies, increased attention is being given to operating requirements for their fuel gas supply such as the maximum allowable rate-of-change in pressure. It is important to perform detailed analyses for multi-unit plants to ascertain whether pressure transient events, such as those caused by initial trip of one or two combustion turbines, will cause additional combustion turbines to trip off. In this paper, single and dual CT trips were postulated in a near-realistic combined cycle power plant. Predictions of the gas flow behavior, along with propagation and superposition of pressure waves, was carried out using the method of characteristics (MOC) for compressible flows. Specifically, the rate of change in fuel gas supply pressure to each CT was monitored and compared against a typical manufacturer limit of 0.8 bar/s. Instances where simulations showed this threshold exceeded were noted, since such events correspond to automatic valve closure that would shut down one more CT and thereby further reduce plant electrical output.
The overall goal of fuel gas transient analyses is to improve pipeline designs, iteratively when necessary, such that those additional trips are avoided. To that end, this paper presents several simulation cases to illustrate pressure transient phenomena and to show the impact of various pipeline design alterations, some of which caused 40% reductions in the worst pressure rate-of-change during simulations.
PFD analysis of LNG fuel gas supply system for improving combined-cycle power plant safety