Table of Contents
2.2.4 The number of starts
As shown, the number of starts is important to the length of life ( "Ill. 2.2-5"). If life limits of individual rotor parts are given, one should examine whether an especially „ gentle“ start procedure from the manufacturer would affirm an appropriate extension of life.
Not only the rotor discs, but also the turbine blades are subject to low frequency fatigue, as a result of higher temperature gradients in the start phase, the so-called thermal fatigue ( "Ill. 3.3-16"). In the case of an individual, instantaneous overheating, e.g. due to a darting flame, one also speaks of thermal shock, recognizable by the expert through special damage marks on the turbine blades. Of interest is the especially high load of the cooled, inner zone of the components ( "Ill. 3.3-14" and "Ill. 3.3-15").
As the general trend of technology, especially with regard to gas turbine design, always leads to higher component load and this is realizable often only through the introduction of new technologies, e.g., thermal barrier coatings ( "Ill. 3.2.3-5"), single crystal blades ( "Ill. 3.3-4"), metal felts for abradables and brush seals ( "Ill. 3.1.2.4-8") one should pay attention, already in the procurement phase, (Chapter 1) to the expected operation performance on well based experience.
Despite all these advantages there are risks, irrespective of the kind of fuel used. An outline to this effect is given below.
Note:
1 normal start equals the life time usage of about ca. 10 - 50 operating hours.