Table of Contents
2.2.3 Operating performance in the start phase
The operation performance, during start, is definitely determined by the temperature and speed-dependent clearances in the area of the blade tips ( "Ill. 3.1.2.4-1") and labyrinths ( "Ill. 3.1.2.4-5").
Unfavorable start procedures can partly lead to big clearances and, consequently, big leakage loss ( "Ill. 3.1.1-3"), resulting in undesired states of operation such as a lack of performance, Surge (‘stall’, "Ill. 3.1.1-2"), in the compressor and/or overheating of hot parts. In order to avoid stall, bleed valves in the compressor region are used, providing an operation favorable pressure distribution, through a temporary removal of air. The wrong functioning of the bleed valves can lead to unpermitted vibrations in the compressor, inducing fatigue failure at the next blade.
A too quick acceleration of the gas turbine can, depending on the type, lead to surge in the compressor, or the turbine gets stuck. This indicates a decreased amount of combustion and cooling air, by a higher amount of fuel, with the danger of extreme temperatures and corresponding damages. On the other hand, a too slow run up, which is not according to specifications, is also problematic, as there are