Table of Contents
2.2.1 The time of the start
Even the time of start is not a question of choice. It is important that an eventually prescribed, engine-specific, minimum time interval, it can lie in the region of hours between the shut down of a hot engine and the new start is maintained. A start attempt does not mean a heating up in this sense. The waiting time is necessary, because in the cooling phase, after shut down, due to thermal convection in the engine, casing deformations can arise ( "Ill. 2.2-1 and 2"), that can result in extreme rub with damages of the parts involved (see Chapter 2.4).
Potential consequences are:
- of the turbine.
- Forced overstress at the blades. Weakening of the compressor rotors (decrease of the cross section, overheating).
- Fatigue damage by vibrations.
- Bearing failures.
At the start, a typical, time dependent temperature distribution occurs, after the combustor, in the turbine. Especially noticeable, are the temporary, temperature peaks when ‘igniting the combustor’. These temperature peaks can only be borne for a short time, by the hot parts in the gas stream, without noticeable damage. If these temperature peaks intensify, through the wrong functioning of the governor or the injection system, the false functioning of the monitoring probes or insufficient drainage during a previous start attempt, bigger damages of the hot parts could be induced. These are some reasons for an overtemperature: A surge of the compressor ( "Ill. 3.1.1-2"), the behavior of the control system, the fuel injection , the disfunction of the monitoring system or an insufficient drainage after a preceding start attempt.
