Reliability enhancement of LNG fuel gas supply system in combined-cycle power plant

Abstract

This article aims to present a technique to enhance reliability of liquefied natural gas (LNG) fuel gas supply system (FGSS) of gas turbine unit in a combined-cycle power plant. In order to prevent unplanned plant shutdowns, the proposed technique is based on the use of failure modes, effects, and diagnostic analysis (FMEDA) to identify and evaluate the effects of possible failure modes, to determine what could minimize the chance of failures, and to design a new safety interlock. The studied FGSS consists of gas analytical system (GAS) in 2-out-of-2 voting scheme, shutdown valve system (SDV) in 1-out-of-2 voting scheme, fuel gas controller (FGC), and engineering/operator workstations. Two automatic GAS subsystems with different measurement methods are installed in the GAS. One of two GAS subsystems uses three gas chromatography analyzers in 2-out-of-3 voting scheme. The gas chromatography results and diagnostic alarm historian are utilized to consider the failure behavior. The reliability models for the GAS subsystem using gas chromatography and the overall FGSS are also included to confirm the effectiveness of utilizing diagnostic information from the gas analyzers. Based on the FMEDA results, the new safety interlock with no additional software and hardware costs can be achieved. The workability of the proposed safety interlock is demonstrated by test results.