Over the past years, Alstom gas turbines have been protected against icing based on a set of ambient temperature and relative humidity limits. These limits were derived mainly from operational and fleet experience. In recent times, the potential for optimizing these limits arose as they were observed to be too conservative. It is recognized that lowering the icing limits by a better understanding of the formation of condensate ice offers an opportunity for engine performance optimization while simultaneously ensuring adequate protection of the engine hardware. However, the level to which the original limits could be extended has not been known and this necessitated the setting up of a dedicated project to address the issue. This paper presents part of the results of the work done within this project and addresses how the new limits have been derived based on the thermodynamics of ice accretion at stationary and rotating surfaces of the compressor. The theory of ice accretion on the variable inlet guide vane (VIGV) and compressor blade surfaces as the intake air is expanded through the GT inlet system presented in this paper covers the process of condensation of moist air, the solidification of the condensate and the accumulation of the sub-cooled water condensate on surfaces with temperatures below 0°C. Using a state-of-the-art gas turbine modelling environment, relevant thermodynamic quantities including static and velocity components up to the first rotating plane of the compressor have been used to quantify the amount of condensate in the intake air at the first compressor rotating plane at various ambient conditions of temperature and humidity and at various engine operation modes (base load and part load operation). Empirical in-house relations for surface temperatures have been used to estimate the VIGV and the surface temperature of the first blade of the compressor. The theoretical results obtained have been validated on a heavy-duty gas turbine engine. Based on the confirmation of the theoretical results with engine data, the presented method can accurately be used to determine the anti-icing limits for a gas turbine. The approach is a generic one and is therefore applicable to all compressor designs for stationary gas turbines.

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